\r\n\tThis book intends to cover a wide range of important areas of heat pump technology starting from fundamentals, theories, performance to advanced applications. It will also present a state of the art in research and development of heat pumps, their performance and impact in various sectors like social, economic and environment.\r\n
\r\n\tThe book will be of great interest to engineers, researchers, graduate students, and manufacturers whose study/work/business involves heat pumps.
Probiotic foods are a group of functional foods with growing market shares and large commercial interest . Probiotics are live microorganisms which when administered in adequate amounts confer a beneficial health benefit on the host . Probiotics have been used for centuries in fermented dairy products. However, the potential applications of probiotics in nondairy food products and agriculture have not received formal recognition. In recent times, there has been an increased interest to food and agricultural applications of probiotics, the selection of new probiotic strains and the development of new application has gained much importance. The uses of probiotics have been shown to turn many health benefits to the human and to play a key role in normal digestive processes and in maintaining the animal’s health. The agricultural applications of probiotics with regard to animal, fish, and plants production have increased gradually. However, a number of uncertainties concerning technological, microbiological, and regulatory aspects exist .
Probiotics are live microbes that can be formulated into many different types of products, including foods, drugs, and dietary supplements. Probiotic is a relatively new word that is used to name the bacteria associated with the beneficial effects for the humans and animals. The term probiotic means ‘‘for life’’ and it was defined by an Expert Committee as ‘‘live microorganisms which upon ingestion in certain numbers exert health benefits beyond inherent general nutrition’’ . FAO/WHO Expert Consultation believes that general guidelines need to provide to how these microorganisms can be tested and proven for safety and potential health benefits when administered to humans.
Lactobacillus and Bifidobacterium are most commonly used probiotics in food and feed (Table 1). Other microorganisms such as yeast Saccharomyces cerevisiae and some Escherichia coli and Bacillus species are also used as probiotics. Lactic acid bacteria (LAB) which have been used for food fermentation since the ancient time, can serve a dual function by acting as food fermenting agent and potentially health benefits provider. LAB are GRAS (general recognized as safe) with no pathogenic, or virulence properties have been reported. For the use of LAB as probiotics, some desirable characteristics such as low cost, maintaining its viability during the processing and storage, facility of the application in the products, resistance to the physicochemical processing must be considered.
|Lactobacillus species||Bifidobacterium species||Others|
|L. acidophilus||B. adolescentis||Bacillus cereus|
|L. amylovorus||B. animalis||Clostridium botyricum|
|L. brevis||B. breve||Enterococcus faecalisa|
|L. casei||B. bifidum||Enterococcus faeciuma|
|L. rhamnosus||B. infantis||Escherichia coli|
|L. crispatus||B. lactis||Lactococcus lactis subsp. cremoriss|
|L. delbrueckii subsp. bulgaricus||B. longum||Lactococcus lactis subsp. lactis|
|L. fermentum||Leuconostoc mesenteroides subsp. dextranicum|
|L. gasseri||Pediococcus acidilactici|
|L. helveticus||Propionibacterium freudenreichiia|
|L. johnsonii||Saccharomyces boulardii|
|L. lactis||Streptococcus salivarius subsp. thermophilus|
|L. paracasei||Sporolactobacillus inulinus a|
Characteristics of probiotics will determine their ability to survive the upper digestive tract and to colonize in the intestinal lumen and colon for an undefined time period. Probiotics are safe for human consumption and no reports have found on any harmfulness or production of any specific toxins by these strains [7, 8]. In addition, some probiotics could produce antimicrobial substances like bacteriocins. Therefore, the potential health benefit will depend on the characteristic profile of the probiotics. Some probiotic strains can reduce intestinal transit time, improve the quality of migrating motor complexes , and temporarily increase the rate of mitosis in enterocytes [10, 11].
The most common probiotics are Lactobacillus and Bifidobacterium. In general most probiotics are gram-positive, usually catalase-negative, rods with rounded ends, and occur in pairs, short, or long chains . They are non-flagellated, non-motile and non-spore-forming, and are intolerant to salt. Optimum growth temperature for most probiotics is 37°C but some strains such as L. casei prefer 30 °C and the optimum pH for initial growth is 6.5-7.0 . L. acidophilus is microaerophilic with anaerobic referencing and capability of aerobic growth. Bifidobacterium are anaerobic but some species are aero-tolerant. Most probiotics bacteria are fastidious in their nutritional requirements [12, 13]. With regard to fermentation probiotics are either obligate homofermentative (ex. L. acidophilus, L. helvelicas ), obligate heterofermentative (ex. L. brevis, L. reuteri), or facultative heterofermentative (ex. L. casei, L. plantarum) . Additionally, probiotics produce a variety of beneficial compounds such as antimicrobials, lactic acid, hydrogen peroxide, and a variety of bacteriocins [15, 16]. Probiotics should have the ability to interact with the host microflora and competitive with microbial pathogens, bacterial, viral, and fungal .
Probiotic research suggests a range of potential health benefits to the host organism. The potential effects can only be attributed to tested strains but not to the whole group of probiotics. Probiotics have shown to provide a diverse variety of health benefits to human, animal, and plans. However, viability of the microorganisms throughout the processing and storage play an important role in transferring the claimed health effects. Therefore, the health benefits must be documented with the specific strain and specific dosage .
Probiotics display numerous health benefits beyond providing basic nutritional value . These evidences have been established by the scientific testing in the humans or animals, performed by the legitimate research groups and published in peer-reviewed journals [16, 18]. Some of these benefits have been well documented and established while the others have shown a promising potential in animal models, with human studies required to substantiate these claims . Health benefits of probiotic bacteria are very strain specific; therefore, there is no universal strain that would provide all proposed benefits and not all strains of the same species are effective against defined health conditions .
Probiotics have been used in fermented food products for centuries. However, nowadays it has been claimed that probiotics can serve a dual function by their potentially importing health benefits. The health benefit of fermented foods may be further enhanced by supplementation of Lactobacillus and Bifidobacterium species . L. acidophilus, Bifidobacterium spp. and L. casei species are the most used probiotic cultures with established human health in dairy products, whereas the yeast Saccharomyces cerevisiae and some E. coli and Bacillus species are also used as probiotics .
Several studies have documented probiotic effects on a variety of gastrointestinal and extraintestinal disorders, including prevention and alleviation symptoms of traveler’s diarrhea and antibiotic associated diarrhea , inflammatory bowel disease , lactose intolerance , protection against intestinal infections , and irritable bowel syndrome. Some probiotics have also been investigated in relation to reducing prevalence of atopic eczema later in life , vaginal infections, and immune enhancement , contributing to the inactivation of pathogens in the gut, rheumatoid arthritis, improving the immune response of in healthy elderly people , and liver cirrhosis.
In addition, probiotics are intended to assist the body’s naturally occurring gut microbiota. Some probiotic preparations have been used to prevent diarrhea caused by antibiotics, or as part of the treatment for antibiotic-related dysbiosis. Although there is some clinical evidence for the role of probiotics in lowering cholesterol but the results are conflicting. Probiotics have a promising inhibitory effect on oral pathogens especially in childhood but this may not necessarily lead to improved oral health . Antigenotoxicity, antimutagenicity and anticarcinogenicity are important potential functional properties of probiotics, which have been reported recently. Observational data suggest that consumption of fermented dairy products is associated with a lower prevalence of colon cancer, which is suggested that probiotics are capable of decreasing the risk of cancer by inhibition of carcinogens and pro-carcinogens, inhibition of bacteria capable of converting pro-carcinogens to carcinogens .
Probiotics which are traditional idea in the human food have been extended to animals by developing fortified feed with intestinal microbiota to benefit the animals. The microflora in the gastrointestinal tracts of animals plays a key role in normal digestive processes and in maintaining the animal’s health. Probiotics can beneficially improve the intestinal microbial balance in host animal. Commercial probiotics for animal use are claimed to improve animal performance by increasing daily gain and feed efficiency in feedlot cattle, enhance milk production in dairy cows, and improve health and performance of young calves  and in improving growth performance of chickens . Probiotics can attach the mucosal wall, adjust to immune responses , and compete the pathogenic bacteria for attachment to mucus [31, 32]. Probiotics provide the animal with additional source of nutrients and digestive enzymes [33, 34]. They can stimulate synthesis vitamins of the B-group and enhancement of growth of nonpathogenic facultative anaerobic and gram positive bacteria by producing inhibitory compounds like volatile fatty acids and hydrogen peroxide that inhibit the growth of harmful bacteria enhancing the host’s resistance to enteric pathogens [32, 35]. Probiotics stimulate the direct uptake of dissolved organic material mediated by the bacteria, and enhance the immune response against pathogenic microorganisms [36, 37]. Finally, probiotics can inhibit pathogens by competition for a colonization sites or nutritional sources and production of toxic compounds, or stimulation of the immune system.
The more beneficial the bacteria and fungi are, the more “fertile” the soil is. These microorganisms break down organic matter in the soil into small, usable parts that plants can uptake through their roots. The healthier the soil, the lower the need for synthetic herb/pesticides and fertilizers.The concept that certain microorganisms ‘probiotics’ may confer direct beneﬁts to the plant acting as biocontrol agents for plants. The plant probiotic bacteria have been isolated and commercially developed for use in the biological control of plant diseases or biofertilization . These microorganisms have fulfilled important functions for plant as they antagonize various plant pathogens, induce immunity, or promote growth [38-40]. The interaction between bacteria and fungi with their host plants has shown their ability to promote plant growth and to suppress plant pathogens in several studies [41-44].
Today an increase in knowledge of functional foods has led to develop foods with health benefits beyond adequate nutrition. The last 20 years have shown an increased interest among consumers in functional food including those containing probiotics. The presence of probiotics in commercial food products has been claimed for certain health beneﬁts. This has led to industries focusing on different applications of probiotics in food products and creating a new generation of ‘probiotic health’ foods. This section will summarize the common applications of probiotics in food products.
Milk and its products is good vehicle of probiotic strains due to its inherent properties and due to the fact that most milk and milk products are stored at refrigerated temperatures. Probiotics can be found in a wide variety of commercial dairy products including sour and fresh milk, yogurt, cheese, etc. Dairy products play important role in delivering probiotic bacteria to human, as these products provide a suitable environment for probiotic bacteria that support their growth and viability [45-48]. Several factors need to be addressed for applying probiotics in dairy products such as viability of probiotics in dairy [19, 48], the physical, chemical and organoleptic properties of final products [49-51], the probiotic health effect [52, 53], and the regulations and labeling issues [4, 54].
Among probiotics carrier food products, dairy drinks were the first commercialized products that are still consumed in larger quantities than other probiotic beverages. Functional dairy beverages can be grouped into two categories: fortified dairy beverages (including probiotics, prebiotics, fibers, polyphenols, peptides, sterol, stanols, minerals, vitamins and fish oil), and whey-based beverages . Among the probiotic bacteria used in the manufacture of dairy beverages, L. rhamnosus GG is the most widely used. Owing to L. rhamnosus GG acid and bile resistance , this probiotic is very suitable for industrial applications. Özer and Avnikirmaci have reported several examples of commercial probiotic dairy beverages showing that L. acidophilus, L. casei, L. rhamnosus, and L. plantarum as most applied probiotics .
Several factors have been reported to affect the viability of probiotic cultures in fermented milks. Acidity, pH, dissolved oxygen content, redox potential, hydrogen peroxide, starter microbes, potential presence of flavoring compounds and various additives (including preservatives) affect the viability of probiotic bacteria and have been identified as having an effect during the manufacture and storage of fermented milks [19, 48, 57]. Today, a wide range of dairy beverages that contain probiotic bacteria is available for consumers in the market including: Acidophilus milk, Sweet acidophilus milk, Nu-Trish A⁄B, Bifidus milk, Acidophilus buttermilk, Yakult, Procult drink, Actimel, Gaio, ProViva, and others .
Probioticts such as Lactobacillus and Bifidobacterium strains grow weakly in milk due to their low proteolytic activity and inability to utilize lactose [47, 57]. These bacteria also need certain compounds for their growth which is missing in milk [19, 58, 59]. To improve growth and viability of probiotics in dairy beverages various substances have been tested in milk. Citrus fiber presence in fermented milks was found to enhance bacterial growth and survival of probiotic bacteria in fermented milks . Addition of soygerm powder has shown certain positive effects on producing fermented milk with L. reuteri. Soygerm powder may release important bioactive isoﬂavones during fermentation that could protect L. reuteri from bile salt toxicity in the small intestine . Other substances include fructooligosaccahrides (FOS), aseinomacropeptides (CMP), whey protein concentrate (WPC), tryptone, yeast extracts, certain amino acids, nucleotide precursors and an iron source were also documented [59, 63, 64]. Additionally, the selection of probiotic strains and optimization of the manufacturing conditions (both formulation properties and storage conditions) are of utmost importance in the viability of probiotic bacteria in fermented milk [47, 65].
Yogurt is one of the original sources of probiotics and continues to remain a popular probiotic product today. Yogurt is known for its nutritional value and health benefits. Yogurt is produced using a culture of L. delbrueckii subsp. bulgaricus and
Streptococcus salivarius subsp. thermophilus bacteria. In addition, other lactobacilli and bifidobacteria are also sometimes added during or after culturing yogurt. The probiotic characteristics of these bacterial strains that form the yogurt culture are still debatable. The viability of probiotics and their proteolytic activities in yoghurt must be considered. Numerous factors may affect the survival of Lactobacillus and Bifidobacterium spp. in yogurt. These include strains of probiotic bacteria, pH, presence of hydrogen peroxide and dissolved oxygen, concentration of metabolites such as lactic acid and acetic acids, buffering capacity of the media as well as the storage temperature [19, 66, 67].
Although yogurt has been widely used as probiotics vehicle, most commercial yogurt products have low viable cells at the consumption time [19, 68]. Viability of probiotics in yogurt depends on the availability of nutrients, growth promoters and inhibitors, concentration of solutes, inoculation level, incubation temperature, fermentation time and storage temperature. Survival and viability of probiotic in yogurt was found to be strain dependant. The main factors for loss of viability of probiotic organisms have been attributed to the decrease in the pH of the medium and accumulation of organic acids as a result of growth and fermentation. Among the factors, ultimate pH reached at the end of yogurt fermentation appears to be the most important factor affecting the growth and viability of probiotics. Metabolic products of organic acids during storage may further affect cell viability of probiotics . The addition of fruit in yogurt may have negative effect on the viability of probiotics, since fruit and berries might have antimicrobial activities. Inoculation with very high level of probiotics with attempts to compensate the potential viability loss, might result in an inferior quality of the product. The present of probiotic was found to affect some characteristics of yogurt including: acidity, texture, flavor, and appearance . However, encapsulation in plain alginate beads, in chitosancoated alginate, alginate-starch, alginate-prebiotic, alginate-pectin, in whey protein-based matrix, or by adding prebiotics or cysteine into yogurt, could improve the viability and stability of probiotics in yogurt [70-79].
Yogurt and milk are the most common vehicles of probiotics among dairy products. However, alternative carriers such as cheese seem to be well suited. Cheeses have a number of advantages over yogurt and fermented milks because they have higher pH and buffering capacity, highly nutritious, high energy, more solid consistency, relatively higher fat content, and longer shelf life [80, 81]. Several studies have demonstrated a high survival rate of probiotics in cheese at the end of shelf life and high viable cells [45, 48, 82, 83]. Probiotics in cheese were found to survive the passage through the simulated human gastrointestinal tract and significantly increase the numbers of probiotic cells in the gut . However, comparing the serving size of yogurt to that of cheese, cheese needs to have higher density of probiotic cells and higher viability to provide the same health benefits. Cheese was introduced to probiotic industry in 2006 when Danisco decided to test the growth and survival of probiotic strains in cheese . At that time, only few probiotic cheese products were found on the market. The test showed that less than 10% of the bacteria were lost in the cheese whey. Based on the process, a commercial probiotic cheese was first developed by the Mills DA, Oslo, Norway. Nowadays, there are over 200 commercial probiotic cheeses in various forms, such as fresh, semi-hard, hard cheese in the marketplaces. Semi-hard and hard cheese, compared to yogurt as a carrier for probiotics, has relatively low recommended daily intake and need relatively high inoculation level of probiotics (about 4 to 5 times). Fresh cheese like cottage cheese has high recommended daily intake, limited shelf life with refrigerated storage temperature. It may, thus, serve as a food with a high potential to be applied as a carrier for probiotics.
Other dairy products including quark, chocolate mousse, frozen fermented dairy desserts, sour cream, and ice cream can be good vehicles of probiotics. Quark was tested with two probiotic cultures to improve its nutrition characteristics and the results showed that probiotics can ensure the highest level of utilization of fat, protein, lactose, and phosphorus partially in skimmed milk . Chocolate mousse with probiotic and prebiotic ingredients were developed . Probiotic chocolate mousse was supplemented with L. paracasei subsp. paracasei LBC 82, solely or together with inulin and the results showed that chocolate mousse is good vehicle for L. paracasei . Sour cream was investigated as probiotic vehicle and the results showed that using sour cream as a probiotic carrier is proved feasible . Ice creams are among the food products with high potential for use as probiotic vehicles. Cruz and others have reviewed the technological parameters involved in the production of probiotic ice creams . They have pointed several factors that need to be controlled, including the appropriate selection of cultures, inoculums concentration, the appropriate processing stage for the cultures to be added, and the processing procedures and transport and storage temperatures. They concluded that probiotic cultures do not modify the sensory characteristics of the ice-creams and frozen desserts also these products hold good viability for probiotics during the product storage period.
Dairy products are the main carriers of probiotic bacteria to human, as these products provide a suitable environment for probiotic bacteria that support their growth and viability. However, with an increase in the consumer vegetarianism throughout the developed countries, there is also a demand for the vegetarian probiotic products. Nondairy probiotic products have shown a big interest among vegetarians and lactose intolerance customers. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the U.S. National Institutes of Health, about 75% of the world population is lactose intolerant. The development of new nondairy probiotic food products is very much challenging, as it has to meet the consumer’s expectancy for healthy benefits [89, 90]. Granato and others have overview of functional food development, emphasizing nondairy foods that contain probiotic bacteria strains . From their review, some nondairy probiotic products recently developed are shown in Table 2.
Fermentation of vegetables has been known since ancient time. Fermented vegetables can offer a suitable media to deliver probiotics. However, it shows that the low incubation temperature of vegetable fermentation is a problem for the introduction of the traditional L. acidophilus and Bifidobacterium probiotic bacteria. Probiotic of L. rhamnosus, L. casei and L. plantarum are better adapted to the vegetable during fermentation . Nevertheless, when the temperature is adjusted at 37ºC, probiotic bacteria grow quite rapidly in plant-based substrates .
|Fruit and vegetable based||Vegetable-based drinks|
|Fermented banana pulp|
|Many dried fruits|
|Green coconut water|
|Cranberry, pineapple, and orange juices|
|Grape and passion fruit juices|
|Probiotic banana puree|
|Nonfermented fruit juice beverages|
|Soy based||Nonfermented soy-based frozen desserts|
|Fermented soymilk drink|
|Soy-based stirred yogurt-like drinks|
|Cereal based||Cereal-based puddings|
|Yosa (oat-bran pudding)|
|Mahewu (fermented maize beverage)|
|Wheat, rye, millet, maize, and other cereals fermented probiotic beverages|
|Boza (fermented cereals)|
|Millet or sorghum flour fermented probiotic beverage|
|Other nondairy foods||Starch-saccharified probiotic drink|
|Probiotic cassava-flour product|
|Dosa (rice and Bengal gram)|
To develop new probiotic vegetable products, many studies have been carried out. The suitability of carrot juice as a raw material for the production of probiotic food with Biﬁdobacterium strains was investigated . Kun and others have found that Biﬁdobacteria were capable of having biochemical activities in carrot juice without any nutrient supplementation . Yoon and others studied the suitability of tomato juice for the production of a probiotic product by L. acidophilus, L. plantarum, L. casei and L. delbrueckii. They reported that the four LAB were capable of rapidly utilizing tomato juice for cell synthesis and lactic acid production without nutrient supplementation and pH adjustment . Yoon and others also tested the suitability of cabbage to produce probiotic cabbage juice and suggested that fermented cabbage juice support the viability of probiotics and serve as a healthy beverage . The viability of various bifidobacteria in kimchi was investigated under various conditions and the results show the acceptable levels of probiotics in kimchi . In addition, sauerkraut-type products such as fermented cabbage, carrots, onions, and cucumbers based on a lactic fermentation by L. plantarum could be good probiotic carrier. Yoon and others have evaluated the potential of red beets as substrate for the production of probiotic beet juice by four strains of lactic acid bacteria and all strains were capable of rapidly utilizing the beet juice for the cell synthesis and lactic acid production . However, traditional methods of production might result in inactivation of the probiotic cultures and the use of probiotics in fermented vegetables would require low temperature storage of the products .
Moreover, soybean has received attention from the researchers due to its high protein and quality. Soymilk is suitable for the growth of LAB and bifidobacteria [100, 101]. Several studies have focused on developing fermented soymilk with different strains of LAB and Bifidobacteria to produce a soymilk product with improved health benefits [62, 101-103]. Soymilk is now known for their health benefits such as prevention of chronic diseases such as menopausal disorder, cancer, atherosclerosis, and osteoporosis, therefore, soymilk fermented with bifidobacteria may be a unique functional food [62, 104]. In probiotic soy products, fermentation by probiotics has the potential to (1) reduce the levels of some carbohydrates possibly responsible for gas production in the intestinal system, (2) increase the levels of free isoflavones, which has many beneficial effects on human health, and (3) favor desirable changes in bacterial populations in the gastrointestinal tract. Supplementing soymilk with prebiotics such as, fructooligosaccharides (FOS), mannitol, maltodextrin and pectin, was found to be a suitable medium for the viability of probiotic bacteria .
Nowadays, there is increasing interest in the development of fruit-juice based probiotic products. The fruit juices contain beneficial nutrients that can be an ideal medium for probiotics [106, 107]. Fruit juices have pleasing taste profiles to all age groups and they are perceived as being healthy and refreshing. The fruits are rich in several nutrients such as minerals, vitamins, dietary fibers, antioxidants, and do not contain any dairy allergens that might prevent usage by certain segments of the population [107, 108]. Those characteristics allow the selection of appropriate strains of probiotics to manufacture enjoyable healthy fruit juice. However, the sensory impact of probiotic cultures would have different taste profiles compared to the conventional, nonfunctional products. The different aroma and flavors have been reported when L. plantarum was added to orange juices which consumers do not prefer. But if their health benefits information is provided the preference increases over the conventional orange juices. Different attempts have been made to reduce the sensations of unpleasant aromas and flavors in probiotic fruit juice. Luckow and others reported that the perceptible off-flavors caused by probiotics that often contribute to consumer dissatisfaction may be masked by adding 10% (v/v) of tropical fruit juices, mainly pineapple, but also mango or passion fruit .
To develop probiotic fruits, many studies have been carried out. The suitability of noni juice as a raw material for the production of probiotics was studied by Wang and others and found that B. longum and L. plantarum can be optimal probiotics for fermented noni juice . Suitability of fermented pomegranate juice was tested using L. plantarum, L. delbruekii, L. paracasei, L. acidophilus. Pomegranate juice was proved to be a suitable probiotic drink as results have shown desirable microbial growth and viability for L. plantarum and L. delbruekii . Optimized growth conditions of L. casei in cashew apple juice were studied. L. casei has shown suitable survival ability in cashew apple juice during 42 days of refrigerated storage. It was observed that L. casei grew during the refrigerated storage and cashew apple juice showed to be suitable probiotic product . Tsen and others reported that L. acidophilus immobilized in Ca-alginate can carry out a fermentation of banana puree, resulting in a novel probiotic banana product with higher number of viable cells . Kourkoutas and others reported that L. casei immobilized on apple and quince pieces survived for extended storage time periods and adapted to the acidic environment, which usually has an inhibitory effect on survival during lactic acid production .
Cereal-based probiotic products have health-benefiting microbes and potentially prebiotic fibers. The development of new functional foods which combine the beneficial effects of cereals and health promoting bacteria is a challenging issue. Nevertheless, cereal-based products offer many possibilities. Indeed, numerous cereal-based products in the world require a lactic fermentation, often in association with yeast or molds. Cereals are good substrates for the growth of probiotic strains and due to the presence of non-digestible components of the cereal matrix may also serve as prebiotics [114, 115]. Due to the complexity of cereals, a systematic approach is required to identify the factors that enhance the growth of probiotic in cereals . Champagne has listed number of cereal-based products that require a lactic fermentation, often in association with yeast or molds. We have found it useful to include part of these products in Table 3.
|Adai||India||Cereal, legume||Pediococcus spp., Streptococcus spp., Leuconostoc spp.|
|Anarshe||India||Rice||Lactic acid bacteria|
|Aya-bisbaya||Mexico||Rice||Lactic acid bacteria|
|Bhatura||India||Wheat||Lactic acid bacteria, yeasts|
|Burukutu||Nigeria||Sorghum, cassava||Lactic acid bacteria, Candida spp., S. cerevisiae|
|Llambazi, lakubilisa||Zimbabwe||Maize||Lactic acid bacteria, yeasts, molds|
|Injera||Ethiopia||Sorghum, tef, corn, millet, barley, wheat||L. plantarum, Aspergillus spp., Penicillium spp., Rhodotorula spp., Candida spp.|
|Milk (yoghurt), wheat||L. casei, L. plantarum, L. brevis, B. subtilis, B. licheniformis, B. megaterium, yeasts|
|Sorghum, millet||Lactobacillu. spp., L. brevis, L. fermentum, E. faecium,|
Acetobacter spp., S. cerevisiae
|Togwa||Tanzania||Maize, sorghum||L. plantarum, L. brevis, L. fermentum, L. cellobiosus|
P. pentosaceus, W. confusa, S. cerevisiae, C. tropicalis
A multitude of fermented cereal products have been created, but only recently probiotic microorganisms involved in traditional fermented cereal foods have been reported. Strains of L. plantarum,Candida rugosa and Candida lambica isolated from a traditional Bulgarian cereal-based fermented beverage exhibited probiotic properties, being resistant up to 2% bile concentration, which enables them to survive bile toxicity during their passage through the gastrointestinal system . More studies are being done to demonstrate that cereals are suitable substrates for the growth of some probiotic bacteria. Rozada-Sa´nchez and others have studied the growth and metabolic activity of four different Bifidobacterium spp. in a malt hydrolisate using four Bifidobacterium strains with the aim of producing a potentially probiotic beverage . The study has reported potential use for malt hydrolysate as probiotic beverage with the addition of a growth and yeast extract. Angelov and others have used a whole-grain oat substrate to obtain a drink with probiotics and oat prebiotic beta-glucan. They have found that viable cell counts reached at the end of the process were about 7.5×1010 cfu/ ml. Also the addition of sweeteners aspartame, sodium cyclamate, saccharine and Huxol (12% cyclamate and 1.2% saccharine) had no effect on the dynamics of the fermentation process and on the viability of the starter culture during product storage . Charalapompoulos and others have done experiments with different cereals to determine the main parameters that need to be considered in the growth of probiotic microorganisms, defining them as follows: the composition and processing of cereal grains, the substrate formulation, the growth capability and productivity of the starter culture, the stability of the probiotic strain during storage, the organoleptic properties and the nutritional value of the final product . They reported that many cereals supported the growth of probiotics with some differences. Malt medium supported the growth of all examined strains (L. plantarum, L. fermentum, L. acidophilus and L. reuteri) better than barley and wheat media due to its chemical composition. Also, wheat and barley extracts were found to exhibit a significant protective effect on the viability of L. plantarum, L. acidophilis and L. reuteri under acidic conditions (pH 2.5).
Oat is often used in studies of cereal fermented by probiotic bacteria. Several studies have evaluated the potential of oat as substrates for the development of a probiotic product. Kedia and others have explored the potential of using mixed culture fermentation to produce cereal-based foods with high numbers of probiotic bacteria. In this study, LAB growth was enhanced by the introduction of yeast and the production of lactic acid and ethanol were increased in comparison against pure LAB culture. They have fermented whole oat ﬂour with L. plantarum along with white ﬂour and bran in order to compare the suitability of these substrates for the production of a probiotic beverage. Those substrates were found to enhance probiotic viability at the end of fermentation above the minimum required in a probiotic product . Martensson and others have studied the development of nondairy fermented product based on oat . Yosa is a snack food made from oat bran pudding cooked in water and fermented with LAB and Bifidobacteria. It is mainly consumed in Finland and other Scandinavian countries. It has a texture and a flavor similar to yogurt but it is totally free from milk or other animal products. It is lactose-free, low in fat, contains beta-glucan and it is suitable for vegetarians . Yosa is therefore considered a healthy food due to its content of oat fiber and probiotic LAB, which combine the effect of beta-glucan for cholesterol reduction and the effect of LAB benefits to maintain and improve the intestinal microbiota balance of the consumer.
Other cereals and cereal components that can be used as fermentation substrates for probiotics have been studied. Survival of probiotics in a corn-based fermented substrate was reported . Autoclaved maize porridge was fermented with probiotic strains (grown separately): L. reuteri, L. acidophilus and L. rhamnosus for 24h at 37 ◦C. All strains examined showed good growth in maize porridge with added barley malt. Probiotic fermented maize products could have a good world-wide acceptance, since maize fermentation induces fruity flavors in traditional Mexican foods. Prado and others have summarized some of the international cereal based probiotic beverages including: Boza made from wheat, rye, millet and other cereals in Bulgaria, Albania, Turkey and Romania, Bushera made from sorghum, or millet flour in Western highlands of Uganda, Mahewu (amahewu) made from corn meal in Africa and some Arabian Gulf countries, Pozol made from maize in the Southeastern Mexico, and Togwa made from maize flour and millet malt in Africa .
Normally sourdoughs are the cereal products fermented by LAB cultures. However, baking will kills most probiotic bacteria and only probiotics which synthesize a thermostable bioactive compound during leavening can be of use in bread making. Different studies have shown the ability of human derived strains of L. reuteri to resist simulated gastric acidity and bile acid, and also to grow well in a number of cereal substrates [89, 116]. In this perspective, L. reuteri has potential use in bread making due to reuterin synthesis . The L. reureri cells might be inactivated by heating, but the bioactive compound might remain active. Probiotic Bacillus strains could better adapt to bread making due to their spore-forming characteristics.
Probiotic applications are restricted to fermented meats, such as dry sausages. The idea of using probiotic bacteria in fermenting meat products has introduced the idea of using antimicrobial peptides, i.e. bacteriocins, or other antimicrobial compounds as an extra hurdle for meat products. Meat starter culture was defined as preparations which contain living or resting microorganisms that develop the desired metabolic activity in the meat . LAB are the most common used starter culture in meat which produce lactic acid from glucose or lactose. As meat content of these sugars are low, sugar is added at 0.4–0.7% (w/w) for glucose and 0.5–1.0% (w/w) for lactose to the sausage matrix . Some LAB strains such as L. rhamnosus GG are not able to utilize lactose, therefore, the starter culture properties have to be taken into account for successful applications. From pentoses, such as arabinose and xylose, meat starter LAB produce both lactic acid and acetic acid . As indicated in commercial catalogues LAB strains currently most employed in meat starter cultures are L. casei, L. curvatus, L. pentosus, L. plantarum, L. sakei, Pediococcus acidilactici and Pediococcus pentosaceus .
LAB have been used for dry sausage manufacturing process since 1950s in order to ensure the safety and quality of the end product. Dry sausages are non heated meat products, which may be suitable carriers for probiotics into the human gastrointestinal tract . Dry sausage is made from a mixture of frozen pork, beef and pork fat with the addition of sugars, salt, nitrite, and nitrate, ascorbates and spices. The raw sausage material is stuffed into casing material of variable diameters and hung vertically in fermentation and ripening chambers for several weeks. Salt, nitrite, and added spices are the main contributors in the inhibition of different bacteria on the surface of the sausages. Lactic acid bacteria and staphylococci used as starter cultures to ferment the sausage. Salt decreases the initial water activity inhibiting or at least delaying the growth of many bacteria while favoring the growth of starter LAB and starter staphylococci. During the first day of fermentation the growth of microbes in sausage material uses up all the oxygen mixed in the sausage matrix during the chopping. After few days of fermentation, LAB decrease the pH to about 5.0 which acts as a hurdle for several Gram-negative bacterial species [126, 127]. The presence LAB in the food suggests that bacteriocins may be active in the human small intestine against food pathogens as long as they are able to survive the environment of gastrointestinal tract . Likewise, probiotic strains with antimicrobial effects on food act similarly and therefore might be more successful than commonly used food fermenting bacteria. It could be concluded that dry sausage is suitable carrier for probiotics. However, human clinical studies are needed before the final answer concerning the health promoting effects of probiotic dry sausage.
Some traditional Indian fermented fish products such as Ngari, Hentak and Tungtap have been analyzed for microbial load . LAB were identified as Lactococcus lactis subsp. cremoris, Lactococcus plantarum, Enterococcus faecium, L. fructosus, L. amylophilus, L. coryniformis subsp. torquens, and L. plantarum. Most strains of LAB had a high degree of hydrophobicity, indicating that these microorganisms have a probiotic potential.
Probiotics applications have been extended from human applications to diversity of agricultural application. Agricultural applications include animal and plants.
Probiotics, with regard to animal applications, were defined as live microbial feed supplements beneficially improve the intestinal microbial balance in host animal . They have been approved to provide many benefits to the host animal and animal products production. They are used as animal feed to improve the animal health and to improve food safety with examples of the application in poultry, ruminant, pig and aquaculture.
The microflora in the gastrointestinal tracts of poultry plays a key role in normal digestive processes and in maintaining the animal’s health. Some feed additives can substantially affect this microbial population and their health promoting effects. Recently, concerns about some unwanted harmful side effects caused by antibiotics  has grown in many countries, so that there is an increasing interest in finding alternatives to antibiotics in poultry production. Probiotic has provided a possible natural alternative to antibiotics in poultry production to produce foods of reliable quality and safety . In addition, the application of probiotic to chicken feed was shown to increase the internal and external quality of eggs. Addition of probiotic to chicken feed increased egg weight shell thickness, shell weight, albumen weight, and specific gravity and decreased shape index . Farm animals are often subjected to environmental stresses which can cause imbalance in the intestinal ecosystem and could be a risk factor for pathogen infections. Applications of probiotics in feed have decreased the pathogen load in the farm animals. Feeding probiotic LAB and yeast to calve was found to promote the growth and suppress diarrhea in Holstein calve . Gaggia and others have reviewed the applications of probiotics and prebiotics in animal feeding that can introduce to safe food production . Probiotics has been used to intervene in decreasing pathogen load and in ameliorating gastrointestinal disease symptoms in pigs. Beside the in vitro test to identify the best potential probiotics, several studies are conducted in vivo utilizing different probiotic microorganisms. Most of the studies showed a beneficial role of improving the number of beneficial bacteria, decreasing the load of pathogens, stimulating the immune cell response towards pathogens in comparison to control, and increasing defensive tools against pathogenic invasion. In contrast, some authors reported an enhancement of the course of infection or a partial alleviation of diarrhea.
Applications of probiotics in aquaculture generally depend on producing antimicrobial metabolites and their ability to attach to intestinal mucus. Aeromonas hydrophila and Vibrio alginolyticus are common pathogens in fish, however, addition of probiotics strains (isolated from the clownfish, Amphiprion percula) were found capable to prevent the adhesion of these microbes to fish intestinal mucus and to compete with the pathogens . Feeding probiotics to shrimp was found to reduce disease caused by Vibrio parahaemolyticus in shrimp . Balcazar and others have reviewed the use of probiotics for prevention of bacterial diseases in aquaculture .
A strong growing market for plant probiotics for the use in agricultural biotechnology has been shown worldwide with an annual growth rate of approximately 10%. Based on the mode of action and effects, the plant probiotics products can be used as biofertilizers, plant strengtheners, phytostimulators, and biopesticides . Berg has reported several advantages of using plant probiotics over chemical pesticides and fertilizers including: more safe, reduced environmental damage, less risk to human health, much more targeted activity, effective in small quantities, multiply themselves but are controlled by the plant as well as by the indigenous microbial populations, decompose more quickly than conventional chemical pesticides, reduced resistance development due to several mechanisms, and can be also used in conventional or integrated pest management systems . Plant growth promotion can be achieved by the direct interaction between beneficial microbes and their host plant and also indirectly due to their antagonistic activity against plant pathogens. Several model organisms for plant growth promotion and plant disease inhibition are well-studied including: the bacterial genera Azospirillum [44, 135], Rhizobium , Serratia , Bacillus [138, 139], Pseudomonas [140, 141], Stenotrophomonas , and Streptomyces  and the fungal genera Ampelomyces, Coniothyrium, and Trichoderma . Some examples of commercial products that have plant probiotics are listed in Table 4.
Several mechanisms are involved in the probiotics-plant interaction. It is important to specify the mechanism and to colonize plant habitats for successful application. Steps of colonization include recognition, adherence, invasion, colonization and growth, and several strategies to establish interactions. Plant roots initiate crosstalk with soil microbes by producing signals that are recognized by the microbes, which in turn produce signals that initiate colonization [43, 51]. Colonizing bacteria can penetrate the plant roots or move to aerial plant parts causing a decreasing in bacterial density in comparison to rhizosphere or root colonizing populations . Furthermore, in the processes of plant growth, probiotic bacteria can influence the hormonal balance of the plant whereas phytohormones can be synthesized by the plant themselves and also by their associated microorganisms .
|Microorganism||Name of the product||Plant pathogens, or pathosystem||Company|
|Ampelomyces quisqualis M-10||AQ10 Biofungicide||Powdery mildew on apples, cucurbits, grapes, omamentals, strawberries, and tomatoes.||Ecogen|
|Azospirillum spp.||Biopromoter||Paddy, millets, oilseeds, fruits, vegetables, sugarcane, banana||Manidharma Biotech|
|Bacillus subtilis GB03||Kodiak||Growth promotion; Rhizoctonia and Fusarium spp.||(Gustafson); Bayer CropScience|
|Bradyrhizobium japonicum||Soil implant||Soy bean||Nitragin|
|Bacillus pumilus GB34||YiedShield||Soil-born fungal pathogens||(Gustafson); Bayer CropScience|
|Coniothyrium minitans||Contans WG, Intercept WG||Sclerotinia sclerotiorum, S. minor||Prophyta Biologischer Pflanzenschutz|
|Delftia acidovorans||BioBoost||Canola||Brett-Young Seeds Limited|
|Phlebiopsis gigantea||Rotex||Heterobasidium annosum||E~nema Biologischer Pflanzenschutz|
|Pseudomonas chlororaphis||Cedomon||Leaf stripe, net blotch, Fusarium sp., sot blotch, leaf spot, etc. on barley and oats||BioAgri AB|
|Streptomyces griseoviridis K61||Mycostop||Phomopsis spp., Botrytis spp., Pythium spp.,Phythophora spp.||Kemira Agro Oy|
|Trichoderma harzianum T22||RootShield, PlantShield T22, Planter box||Pythium spp., Rhizoctonia solani, Fusarium spp||Bioworks|
|Pseudomonas spp.||Proradix||Rhizoctonia solani||Sourcon Padena|
Besides these mechanisms, probiotic bacteria can supply macronutrients and micronutrients. They metabolize root exudates and release various carbohydrates, amino acids, organic acids, and other compounds in the rhizosphere . Bacteria may contribute to plant nutrition by liberating phosphorous from organic compounds such as phytates and thus indirectly promote plant growth . Furthermore, probiotic can reduce the activity of pathogenic microorganisms through microbial antagonisms and by activating the plant to better defend itself, a phenomenon termed “induced systemic resistance” [146, 147]. Microbial antagonism includes the inhibition of microbial growth, competition for colonization sites and nutrients, competition for minerals, and degradation of pathogenicity factors [38, 43]. In Japanese composting, at least three groups of compositing bacteria were used individually, or in combination. The following species were used: Bacillus bacteria groups, Lactic acid bacteria groups and Actinomycetous groups. These bacteria species can protect plant products from cropping hazards. They do this by expelling against various bad worms and insects, such as nematodes with potatoes and some types of insects with soybeans and maize. They are also effective in controlling fungi such as powdery mildew, downy mildew, phythium (damping off with many plants), plasmodipophora brosscae (club-root with the cabbage Jamily); Crucijert1e (plants. and fusarium of wilt with tomato and banana) .
From a technological standpoint, Champagne has listed many challenges in the development of a probiotic food product including: strain selection, inoculation, growth and survival during processing, viability and functionality during storage, assessment the viable counts of the probiotic strains particularly when multiple probiotic strains are added and when there are also starter cultures added, and the effects on sensory properties . Champagne has focused in his chapter on three of these challenges: inoculation, processing and storage issues. Other challenges such as: maintaining of probiotics, diversity and origin of probiotics, probiotic survival and being active, dealing with endogenous microbiota, and proving health benefits have also been discussed . This section will focus on the viability and sensory acceptance as we have found these are the most important challenges to ensure transferring the health benefits and the commercial success.
Probiotics have been proved to provide many health benefits. However, the claimed health benefits can’t be achieved without high number of viable cells. Many probiotic bacteria have shown to die in the food products after exposure to low pH after fermentation, oxygen during refrigeration distribution and storage of products, and/or acid in the human stomach [150, 151]. Probiotic products need to be supplemented with additional ingredients to support the viability throughout processing, storage, distribution, and gastrointestinal tract to reach the colon. Several reports have shown that survival and viability of probiotic bacteria is often low in yogurt. The efficiency of added probiotic bacteria depends on dose level and their viability must be maintained throughout storage, products shelf-life and they must survive the gut environment . Several studies have focused on the effect of adding certain compounds to enhance the probiotic viability. Many evidences have shown that inulin, oligosaccharides, and fructooligosaccharides (FOS) have good impacts on the probiotics viability. However, the effect of these compounds are strain specific. Martinez-Villaluenga and others have examined the influence of raffinose on the survival of Bifidobacteria and L. acidophilus in fermented milk. The results showed that retention of viability of Bifidobacteria and L. acidophilus greater in fermented milk with raffinose . Supplementing probiotic products with FOS, mannitol, maltodextrin and pectin were found to provide a suitable viability for probiotic bacteria . Inulin and FOS were found to support the growth and viability of L. acidophilus but did not significantly affect growth and viability of Bifidobacterium and L. casei . During food formulation step several things need to be considered such as the composition (nutrients, antimicrobials), structure (oxygen permeability, water activity) and pH of the food matrix, and possible interactions with starter microbes in fermented food matrices. Growth of probiotics in non-fermented foods is not desirable (due to possible off flavor formation), but their growth during the production of fermented foods can lower process costs and increase the adaptation of probiotics leading to enhanced viability. The starter microbes in fermented foods can sometimes inhibit probiotics but they can also enhance their survival by producing beneficial substances or by lowering the oxygen pressure. In beverages the most important factor affecting probiotic viability is probably the pH. Shelf-stable beverages typically have pH values below 4.4 to ensure their microbial stability and this low pH value combined with long storage periods is very demanding for most probiotic strains, especially those representing bifidobacteria. The packaging material should be a good oxygen barrier to promote the survival of especially anaerobic probiotic bacteria (bifidobacteria) . Transportation and storage temperature is an important determinant of the shelf-life; with increasing temperatures viability losses can occur rapidly .
The viability and survival of probiotics are strain specific. To maintain the viability of very sensitive strains, encapsulation is often the only option, especially microcapsulation that do not affect the sensory properties of the food produced. Microencapsulation technologies have been developed and successfully applied using various matrices to protect the bacterial cells from the damage caused by the external environment . Overall microencapsulation improved the survival of probiotic bacteria when exposed to acidic conditions, bile salts, and mild heat treatment . The immobilization of probiotics using microencapsulation may improve the survival of these microorganisms in products, both during processing and storage, and during digestion [157, 158].
Some probiotic bacteria, such as the spore-forming bacteria, GanedenBC30 provides better viability and stability, making it an ideal choice for product development, compared to other probiotic bacteria strains, such as L. acidophilus and bifidobacteria. This spore safeguards the cell’s genetic material from the heat and pressure of manufacturing processes, challenges of shelf life and the acid and bile it is exposed to during transit to the digestive system. GanedenBC30 can withstand manufacturing processes. and survive through high temperature processes such as baking and boiling, low temperature processes such as freezing and refrigeration and high pressure applications like extrusion and roll forming. GanedenBC30 requires no refrigeration and can be formulated into products to have up to a two-year. Once it is safely inside the small intestine, the viable spore is then able to germinate and produce new vegetative cells or good bacteria .
Probiotic foods must show, at least, the same performance in any sensory test as conventional foods. In most probiotic foods sensory tests are aiming to determine acceptance of the products, without, obtaining details concerning the addition of the probiotics to the food and their interaction with the consumer. Therefore, it is important to development sensory tests for probiotic foods that can be accompanied by specific sensory analyses. Sensory testing must cover all characteristics with regard to change over time during storage. Some studies have reported the possibility of obtaining similar, or even better, performance with probiotic products as compared to conventional products such as: functional yogurt supplemented with L. reuteri RC-14 and L. rhamnosus GR-1 , chocolate mousse with added inulin and L. paracasei , curdled milk with inulin, and L. acidophilus , and milk fermented with B. animalis and L. acidophilus La-5, and supplemented with inulin .
Sensory methodology will allow obtaining important data for developing the probiotic foods. In most cases the developed products need to match similar commercial products in parallel. In general, metabolism of the probiotic culture can result in the production of components that may contribute negatively to the aroma and taste of the food product, probiotic off-ﬂavor. For example, acetic acid produced by Biﬁdobacterium spp. can result in a vinegary flavor in the product, prejudicing the performance in sensory assessments.
Masking is one technique that has been used to reduce the off flavors in foods and it has been performed successfully through the addition of new substances or flavors to reduce the negative sensory attributes contributed by probiotic cultures. The addition of tropical fruit juices, mainly pineapple, but also mango or passion fruit, might positively contribute to the aroma and flavor of the final product and might avoid the identification of probiotic off-flavors by consumers . The influence of exposure has been identified in many consumer studies [91, 163] that the frequency of exposure to a food stimulus is increased, food stimuli have been shown to be better liked. Therefore, repeated exposure and increased familiarity to sensory off-flavors, may influence consumer attitudes in a positive way, therefore increasing willingness to consume probiotic juices. Nonsensory techniques have proven useful in enhancing the sensory quality of products, such as providing consumers with health benefit information associated with probiotic cultures. Health information has been shown to be a vital tool in the consumer acceptance of a variety of probiotic food products [164-166]. Finally, microcapsules of probiotics may help prevent the off flavor of cultures .
Dairy based products containing live bacteria are the main vehicles of probiotics to human. Non-dairy beverages would be the next food category where the healthy bacteria will make their mark. Microencapsulation technologies have provided the necessary protection for probiotics and moved them outside the pharmaceutical and supplemental use to become food ingredients.
The word “nano” comes from the Greek for “dwarf ”. A nanometer is a thousandth of a thousandth of a thousandth of a meter (10-9 m). Nanoparticles are usually sized below 100 nanometers which will enable novel applications and benefits. Nanotechnology of probiotics is an area of emerging interest and opens up whole new possibilities for the probiotics applications. Their applications to the agriculture and food sector are relatively recent compared with their use in drug delivery and pharmaceuticals. The basic of probiotic nanotechnology applications is currently in the development of nano-encapsulated probiotics. The nanostructured food ingredients are being developed with the claims that they offer improved taste, texture and consistency. Applications of nanotechnology in organic food production require precaution, as little is known about their impact on environment and human health. Some recent food applications of nanotechnology, safety and risk problems of nanomaterials, routes for nanoparticles entering the body, existing regulations of nanotechnology in several countries, and a certification system of nanoproducts were reported [168, 169]. Currently, no regulations exist that speciﬁcally control or limit the production of nanosized particles and this is mainly owing to a lack of knowledge about the risks . Nanoencapsulation is defined as a technology to pack substances in miniature using techniques such as nanocomposite, nanoemulsification, and nanoestructuration and provides final product functionality and control the release of the core . Encapsulation of food ingredients may extend the shelf life of the product. Nanoencapsulation of probiotic is desirable technique that could deliver the probiotic bacteria to certain parts of the gastrointestinal tract where they interact with specific receptors . These nanoencapsulated probiotic bacterial may also act as de novo vaccines, with the capability of modulating immune responses .
Microencapsulation with alginate can be applied to many different probiotic strains and results show better survival than free cells at low pH of 2.0, high bile salt concentrations, and moderate heat treatment of up to 65 ◦C . Microencapsulation may prove to be an important method of improving the viability of probiotic bacteria in acidic food products and help deliver viable bacteria to the host’s gastrointestinal tract. Furthermore, microencapsulation appeared to be effective in protecting cells from mild heat treatment and thus could stimulate research in functional food products that receive a mild heat treatment . The microencapsulation allows the probiotic bacteria to be separated from its environment by a protective coating. Several studies have reported the technique of the microencapsulation by using gelatin, or vegetable gum to provide protection to acid-sensitive Bifidobacterium and Lactobacillus [172-176].
With the revolution in sequencing and bioinformatic technologies well under way it is timely and realistic to launch genome sequencing projects for representative probiotic microorganisms. The rapidly increasing number of published lactic acid bacterial genome sequences will enable utilizing this sequence information in the studies related to probiotic technology. If genome sequence information is available for the probiotic species of interest, this can be utilized, e.g. to study the gene expression (transcription) profile of the strain during fermenter growth. This will enable better control and optimization of the growth than is currently possible. Transcription profiling during various production steps will allow following important genes for probiotic survival during processing (e.g., stress and acid tolerance genes) and identifying novel genes important for the technological functionality of probiotics .
Increasing knowledge of genes important for the technological functionality and rapid development of the toolboxes for the genetic manipulation of Lactobacillus and Bifidobacterium species will in the future enable tailoring the technological properties of probiotic strains. However, before wide application of tailored strains in probiotic food products, safety issues are of utmost importance and have to be seriously considered for each modified strain .
Depending on intended use of a probiotic (drug vs. dietary supplement), regulatory requirements differ greatly. If a probiotic is intended for use as a drug, then it must undergo the regulatory process as a drug, which is similar to that of any new therapeutic agent. An Investigational New Drug application must be submitted and authorized by the Food and Drug Administration before an investigational or biological product can be administered to humans. The probiotic drug must be proven safe and effective for its intended use before marketing . In the United States, probiotic products are marketed to a generally healthy population as foods or dietary supplements. For dietary supplements, premarketing demonstration of safety and efficacy and approval by the Food and Drug Administration are not required; only premarket notification is required. The law allows that in addition to nutrient content claims, manufacturers of dietary supplements may make structure/function or health claims for their products. The ‘‘health claims’’ must be defensible when placed under the scrutiny by the controlling authorities. Efforts are being made to establish meaningful standards or guideline for probiotic products worldwide (Table 5). The Joint Food and Agriculture Organization of the United Nations/World Health Organization Expert Consultation on Evaluation of Health and Nutritional Properties of Probiotics developed guidelines could be used as the global standards for evaluating probiotics in food that could lead to the substantiation of health claims.
|Organization||Region of impact||Action|
|Food Agriculture Organization (FAO)/|
World Health Organization (WHO)
|Worldwide||Developed guidelines for the evaluation of probiotics in foods.|
|International Dairy Federation||Worldwide||Has begun working on methods to determine certain functional and safety properties outlined in the FAO guidelines for the evaluation of probiotics in food.|
|European Food and Feed Culture Association||Europe||Developed guidelines for use of probiotics in foods.|
|Codex Standard for Fermented Milks|
(Codex Stan 243-2003)
|Worldwide||Among other composition stipulations, this standard specifies minimum numbers of characterizing and|
additional labeled microbes in yoghurt, acidophilus milk, kefir, kumys and other fermented milks.
|National Yogurt Association||USA||Petition under consideration by the FDA which would change the standard of identity of yoghurt, including the requirement of minimum levels of live cultures in yoghurt, but not specifically levels for any additional probiotic cultures.|
|International Scientific Association for|
Probiotics and Prebiotics
|Worldwide||Industry Advisory Committee and Board of Directors to consider method validation and establishment of laboratory sites to assess microbiological content of probiotic products.|
The uses of probiotics and their applications have shown tremendous increase in the last two decades. Probiotics can turn many health benefits to the human, animals, and plants. Applications of probiotics hold many challenges. In addition to the viability and sensory acceptance, it must be kept in mind that strain selection, processing, and inoculation of starter cultures must be considered. Probiotics industry also faces challenges when claiming the health benefits. It cannot be assumed that simply adding a given number of probiotic bacteria to a food product will transfer health to the subject. Indeed, it has been shown that viability of probiotics throughout the storage period in addition to the recovery levels in the gastrointestinal tract are important factors [3, 48, 83]. For this purpose, new studies must be carried out to: test ingredients, explore more options of media that have not yet been industrially utilized, reengineer products and processes, and show that lactose-intolerant and vegetarian consumers demand new nourishing and palatable probiotic products.
Rising energy demand due to population growth has led to the rapid consumption of fossil fuels and serious environmental problems . Currently, most of the world’s energy comes from fossil fuels, which will eventually lead to its predictable depletion. The decline of fossil energy reserves and the urgency to reduce greenhouse gas emissions to alleviate climate warming is forcing us to seek a cleaner, more renewable, and sustainable alternative energy source [2, 3]. Hydrogen is considered as a future ideal energy carrier to replace fossil fuels due to its high gravimetric energy density and zero carbon emissions [4, 5, 6]. But the achievement of this clean energy scheme largely depends on economically efficient hydrogen production technologies. At present, the industrial production of hydrogen is mainly realized by the reforming of hydrocarbon steam in fossil energy or coal through reaction to fossil fuels under the control of steam, which is not only expensive but also causes large emissions [7, 8]. Therefore, the use of renewable energy to produce hydrogen is considered, despite challenges stand in the way [7, 9].
In recent years, solar energy has attracted much attention as the largest renewable energy source on the planet. If solar energy can be effectively used, it will provide a continuous supply of energy for future energy [10, 11]. However, the vision of solar power to provide a significant portion of the global infrastructure is far from being realized. The main challenge comes from not having a cost-effective way to store solar energy. Solar water splitting is a prospective, environmentally friendly, and sustainable method to achieve this beautiful vision [10, 12, 13]. There are three types of solar water decomposition systems, photovoltaic electrolysis (PV-E), photochemical (PC) systems, and photoelectrochemical (PEC) cells, as shown in Figure 1. PV-E is achieved by connecting the photovoltaic cell and water electrolyzer. The advantage of this strategy is its solar-hydrogen conversion efficiency of more than 10%, but it is still too expensive compared to traditional hydrogen production methods [14, 15, 16, 17, 18]. The maturity of PV-E technology also determines that it is difficult to improve efficiency, so it is particularly important to find economical and suitable solar-hydrogen conversion methods. PC is a simple and cost-effective solar-hydrogen conversion method, but its conversion efficiency is less than 1%. In addition, the potentially explosive hydrogen-oxygen mixture produced requires expensive equipment for separation to avoid reaction, which greatly increases production costs . In this case, PEC provides considerable conversion efficiency at an affordable cost [20, 21]. PEC integrates the light absorption and electrochemical processes of PV-E into a single unit. Two gases generated separately at the anode and cathode avoid further separation, which is helpful for reducing costs. If the conversion efficiency can reach 10% and the life span reaches 5 years, PEC is expected to be a replacement for traditional hydrogen production methods [22, 23, 24].
Basically, solar energy is converted into chemical energy stored in the form of hydrogen molecules by PEC devices [25, 26]. And a PEC device usually includes a metal electrode and a semiconductor photoelectrode. Ideally, semiconductors need to have a proper band gap and band structure to provide sufficient reaction potential and cover the solar spectrum as much as possible. In addition, excellent carrier transport performance and good physicochemical stability are also essential. Although a large number of semiconductor materials such as ZnO [27, 28], TiO2 [29, 30], WO3 [31, 32], and BiVO4 [33, 34] have been studied for photohydrolysis experiments, no dependent material meets all the critical conditions described above. Usually, overall water splitting consists of two half-reactions: oxidation of water and reduction of protons.
It can be seen from the equation that the minimum voltage for water splitting is 1.23 V, which requires that the energy absorbed by exciting an electron is not less than 1.23 eV. In order to meet this requirement, the photon energy absorbed by the photoelectrode must also be at least 1.23 eV. But in fact, the energy required due to the energy loss caused by the failure to reach the ideal structure is far more than 1.23 eV [35, 36].
In general, PEC water splitting includes the following processes:
Under light irradiation, carriers are generated in the semiconductor with a suitable band gap.
Photogenerated carriers separate and migrate to the surface of the semiconductor.
The number of photogenerated carriers is determined by the absorption efficiency of the semiconductor, which also reflects the utilization of sunlight. The separation and migration processes of carriers are related to how many can reach the semiconductor surface. Unfortunately, some carriers are lost resulting from recombining on their way to the surface. And the carriers that reach the surface of the semiconductor want to trigger an efficient water splitting reaction, which must meet the following requirements. First, the conduction band edge potential of the semiconductor material should be lower than H2 evolution potential, while the valence band edge potential should be higher than O2 evolution potential . This means that the band gap of the semiconductor should be greater than 1.23 eV. Semiconductor materials need to have stronger absorption in the solar spectrum to generate more photogenerated carriers. Although wide band gap semiconductor materials are likely to meet matching at the band edge positions, the absorption of sunlight is very limited [7, 9, 40]. Second, carriers need to be separated and transmitted quickly to reduce recombination, thereby improving the utilization efficiency of photogenerated carriers for PEC water splitting. Finally, materials used for PEC water splitting should be cost-effective and have good stability in the catalytic process .
Among semiconductor materials commonly used in PEC water splitting, gallium nitride (GaN) has been regarded as a promising candidate [42, 43]. GaN is likely to achieve self-driven overall water splitting because its band gap has good energy alignment with the water redox potential [43, 44]. In addition, GaN is inherently chemically inert even in a harsh environment, which guarantees the stability of the device [45, 46]. Furthermore, the band gap of GaN and its alloys can be tuned by alloying with Indium (In) to span nearly the entire solar spectrum [47, 48]. However, to achieve practical hydrogen production, GaN is still facing many challenges as an excellent photoelectrode material, including how to get a larger reaction area, how to enhance the absorption of light, and how to separate and transport photogenerated carriers more quickly and effectively [49, 50]. Correspondingly, many strategies have been proposed to address the mentioned drawbacks of GaN photoelectrode. Compared with thin-film and bulk counterpart, nanostructures have a smaller size and a larger surface area, which is helpful for shortening the transmission distance and promotes the separation of carriers. Thus the efficiency of carrier collection and utilization will be higher [51, 52, 53]. Doping is also one of the commonly used approaches to effectively improve the electrical and optical properties of GaN, which can directly tune the energy band structure and carrier transmission [54, 55]. Moreover, PEC water splitting kinetics can be promoted through the surface decoration of co-catalysts, which can enhance the transmission of carriers for water redox reaction [56, 57].
In this review, we summarize the recent progress of using GaN as photoelectrode for PEC water splitting and enumerate some commonly used strategies to improve the performance of photoelectrode. In the end, we also have a brief outlook of GaN for PEC water splitting.
In the introduction section, we briefly introduced the three types of solar water splitting. In this section, we will focus on the different structures of the PEC cell, which can be achieved by an n-type semiconductor as photoanode (or p-type semiconductor as photocathode) or connecting two different semiconductors.
For a semiconductor PEC cell with a half-reaction to occur on working electrode, a counter electrode is required to complete the other half-reaction circuit. Generally, a reference electrode is connected to the working electrode to characterize an externally applied voltage. If necessary, there are two compartments or ion exchange membranes between the working and counter electrodes to avoid product crossover. To overcome the thermodynamic obstacles of water splitting and the potential losses caused by the recombination process, the band gap of the working electrode is at least 1.6 eV [58, 59, 60]. However, the visible light absorption efficiency will be attenuated if the band gap is too wide. To solve this problem, that is, potential loss mechanisms that include reverse contact and overpotential caused by poor catalytic activity, the semiconductor material should be deposited on a highly conductive substrate to form a good ohmic contact, which allows most carriers to be quickly injected from the working electrode into the counter electrode [61, 62].
Obtaining enough photovoltage from a single photoelectrode to achieve solar water splitting is a challenge. It will be more favorable that combinates with dual semiconductors, because the second photoelectrode can replace the opposite electrode where the other half-reaction occurs and compensate for the lack of photovoltage . To increase the light utilization, lighting should be irradiated from a larger band gap photoelectrode (transparent substrate) to a smaller band gap photoelectrode. In addition, these two semiconductors can form wireless back-to-back ohmic contacts, sharing a transparent conductive substrate . By doing so, the potential loss in the electrolyte and the pH gradient between the two photoelectrodes can be reduced. Similarly, lighting should pass from a larger band gap material to a smaller band gap material. This series of battery structure is a relatively effective device .
Comparing onset potentials and photocurrent density (normalized to the projected surface area of the photoelectrode) at 1.23 V versus RHE (photoanode) and 0 V versus RHE (photocathode) is a well-known method to evaluate the performance of water splitting. Since the product of water splitting is hydrogen, solar-to-hydrogen (STH) is the most critical parameter of merit to evaluate the performance and the efficiency of PEC water splitting on the device. It is defined as the following equation: .
where ΦH2 is the hydrogen gas production rate, is the Gibbs free energy of hydrogen gas (237 kJ mol−1 at 25°C), and Plight is the total solar irradiation input. The light source should match the solar spectrum of air mass 1.5 global (AM1.5 G). Since the redox reaction needs to consider the current loss, the Faraday efficiency needs to be considered. So, the STH formula is expressed as:
In general, can use current density instead of under zero bias and stable-state conditions. Applied bias photon to current conversion efficiency (ABPE) is also an important parameter for PEC water splitting systems, which is often used to evaluate the performance of a single photoelectrode independently. It can be written as: .
where Vapp is the applied potential between photoelectrode and the counter electrode.
It is important to understand the efficiency of photons to convert electrons/holes at certain wavelengths of PEC water splitting. Therefore, the incident photon-to-current conversion efficiency (IPCE) or external quantum efficiency (EQE) is proposed and expressed as: .
where λ is the wavelength, Pλ is the incident light power, h is Planck’s constant, c is the speed of light, and jph is the photocurrent density. Besides, integrating the IPCE value with the standard AM1.5G solar spectrum can estimate the total photocurrent density under solar light illumination. Its formula is defined as: .
where e is the elementary electron and Φλ is photon flux of irradiation.
Up to now, considerable efforts have been investigated on surface decoration to enhance PEC water splitting performance . In this regard, various co-catalysts were studied by depositing on the surface of GaN to improve the efficiency of PEC water splitting. For instance, the quantum efficiency of the solid solution of GaN and ZnO for overall water splitting in the visible light region achieves the highest value of 2–3% after modified with a mixed oxide of Rh and Cr nanoparticles . A Co-Pi catalyst photoelectron deposited on GaN thin-film photoelectrodes eliminated the anomalous two-plateau behavior and current spikes, which revealed that the Co-Pi catalyst is helpful for suppressing surface recombination and increases the photocurrent . A similar but deeper achievement was carried out by Tricoli et al. for hybridizing highly transparent Co3O4 nano-island catalysts on GaN nanowire to enhance the water oxidation activity. The result shows that the per-metal turnover frequencies in 1 M NaOH aqueous solution are 0.34–0.65 s−1 at an overpotential of 400 mV, which is the best result of Co-based electrocatalysts until this report. This was attributed to Co3O4 that can play a role as hole scavenger, collecting photogenerated holes rapidly and suppressing carrier recombination . Additionally, a size-controlled effect of poly-protected Rh nanoparticles on the photocatalytic activity of (Ga1 − xZnx)(N1 − xOx) was studied by Teranishi et al. for the first time. Their results show that the activity of smaller Rh cores is higher than the larger ones, which benefits from its increased surface area and improves charge separation efficiency . This study was inconsistent with the previous report by Kamat et al. The greater the shift in the Fermi level observed in smaller gold nanoparticles, which is reflected in the higher photocatalytic reduction efficiency, the stronger the photocurrent .
Apart from nanoparticles, core-shell heterostructure is another important approach for surface decoration. GaN-InGaN core-shell rod arrays as photoanode for visible light-driven water splitting were studied by Waag et al. The core-shell structure extends the use of sunlight to the visible light region, thereby greatly improving the efficiency of water splitting. The photocurrent density of (0.3 mA/cm2 at 1.35 V) GaN-InGaN was 10-fold higher than that of GaN (0.03 mA/cm2 at 1.35 V), as shown in Figure 3 . Mi et al. employed GaN-InGaN core-shell nanowire for PEC water splitting, and the high incident photon-to-current conversion efficiency of up to ∼27% is obtained . It is expected to achieve higher PEC activity by surface treatment of GaN. And as far as the current development is concerned, it is foreseeable that surface modification is still a good strategy to achieve efficient water splitting.
As an important part of the PEC water splitting system, the morphology of semiconductor materials is very important. Different morphologies have a great influence on the efficiency of PEC water splitting. Many different morphologies of GaN for PEC water splitting have been proposed. Xi and co-workers used metal–organic chemical vapor deposition (MOCVD) to fabricate GaN nanowires, and it has obtained high photocurrent density value at an applied bias voltage from −1 to 1 V . Its morphology was shown in Figure 4a. It can be found from Figure 4b that compared to the planar structure and other diameters, 300 nm has a stronger current density due to a larger body-to-surface ratio, thereby increasing the efficiency of PEC water splitting. GaN microwires still have problems such as low crystal quality and light absorption. To further improve the efficiency of PEC water splitting, Park et al. used the plasma-assisted molecular beam epitaxy (PAMBE) technique to grow InGaN/GaN multiple quantum wells (MQWs) grown on hollow n-GaN nanowires (Figure 4c) . The hollow and InGaN/GaN multiple quantum well structures of the nanowires allow the incident light to be refracted multiple times, increasing the absorption of light. Figure 4d shows the incident photon-to-current conversion efficiency value of the device, which can be found that the highest IPCE value of the device is as high as 33.3% and 415 μmol of hydrogen gas was generated within 1 hour.
Nanopores, nanocones, and honeycombs are other nanostructures of GaN. Figure 5a shows the GaN nanopore structure , nanopore structure used electrochemical lateral etching and ICP etching to prepare laterally porous, vertically holes well-ordered GaN. This structure reduces the UV reflectivity. The ordered vertical holes not only help open the embedded channels to the electrolyte on both sides and reduce the migration distance of bubbles in the water splitting reaction but also help to modulate the light field. Incident light can be modulated and captured into the nanopore to enhance the absorption of light, so the saturation photocurrent was 4.5 times that of the planar structure, as shown in Figure 5d. Moreover, GaN with aligned nanopore structure had been fabricated by combining MOCVD using a lateral anodic etching, as shown in Figure 5b . Laterally porous 3D hierarchical nanostructures not only provided a large contact area between the electrode and the electrolyte but also increased the absorption of light and provided a channel for the transmission of light and electrons. The device also achieved high values of photocurrent of 0.32 mA/cm2 by using etching voltages at 10 V (Figure 5e). Kim et al. had prepared GaN truncated nanocones , which was shown in Figure 5c. GaN truncated nanocones have concentrated incident light inside the nanostructure and enhanced the light trapping with reduced light losses from surface reflection. The relationship between current density and potential was shown in Figure 5f, which indicated that the photocurrent of GaN truncated nanocones was three times higher than the planar structure.
The above structures are expected, and GaN can also have nanorods , nanocolumns , nano-pyramids , and so on. It can be known from the above results that changing the morphology of GaN influences the efficiency of PEC water splitting, which mainly affects the light absorption efficiency of GaN and reduces light reflection and loss. Therefore, it is very important to choose the appropriate semiconductor morphology for PEC water splitting system.
Doping is a commonly used and effective method to improve the performance of materials. It mainly adjusts the energy band of the material, so that the photogenerated electrons and holes are better transported and high efficiency of PEC water splitting is obtained. Zhou and co-workers doped ZnO-GaN (GZNO) solid solution with La, as shown in Figure 6a . La-dopant incorporation is optimized to adjust the bending of the band gap, which increases the thickness of the space charge region, thereby improving the separation of photogenerated carriers. Figure 6c shows the photocatalytic performance of GZNO and 3% La GZNO. It can be clearly seen that the photocatalyst doped with La produces more hydrogen and oxygen under the same conditions, which indicates that the performance of the photocatalyst is significantly improved after doping. Figure 6b shows the schematic of Ni-doped AlN and two-dimensional GaN monolayers . By controlling the doping content of Ni, it can adjust the band bending of GaN. Figure 6d displays the binding strength of GaN and AlN composites with different transition metals doped. It can be found that Ni doping is the best for OER because they have small OER overpotentials.
GaN doped with Mn , Mg , or CrO are also reported . Doping is also a good method to improve the efficiency of PEC water splitting. It mainly adjusts the energy band of GaN through doping, thereby promoting the separation of photogenerated electrons and holes and effectively preventing the recombination of carriers. However, excessive doping will deteriorate the crystal quality of the material. So, it is important to choose the doping material and control dopant incorporation.
The solid solution is a wurtzite structure composite material composed of GaN and ZnO mixed in a certain proportion. It adjusts the doping content of ZnO to change the band gap of the solid solution and realizes PEC of water splitting under the visible light. This concept was first proposed by Maeda and co-workers . And then, Ohno et al. used Rh2 − yCryO3 nanoparticles to modify the solid solution, and the device shows outstanding stability; it has been working continuously for half a year under light irradiation, as shown in Figure 7a . The co-catalyst is beneficial to suppress the oxidative decomposition of the solid solution, thereby making the device more stable. NiCoFeP and flux-assisted method can also be used to modify the solid solution to improve the efficiency of PEC water splitting [87, 88]. The conversion efficiency of solar energy using NiCoFeP-modified solid solution exceeds 1% at 1.23 V vs. RHE. To further improve the efficiency of PEC water splitting, solid solution nanosheets modified with Rh nanoparticles have been proposed, as shown in Figure 7d . This shows 0.7 μmol h−1 g−1 of hydrogen production in an aqueous H2SO4 solution. The nitridation process was used to change the morphology from hexagonal 2D ZnGa2O4 nanosheets to 2D (GaN)1 − x(ZnO)x nanosheets, reducing the path of carrier transportation and decreasing the recombination of electrons and holes. So, the composition of a solid solution or multiple-metal incorporation can expand the light absorption range of the device, improving the absorption of light and increasing the efficiency of PEC water splitting.
The method of forming multiple-metal incorporation is similar to that of a solid solution. Different In content incorporation can change the band gap of GaN to widen the absorption spectrum range. Many different multiple-metal incorporations have been proposed [90, 91, 92, 93, 94]. AlOtaibi et al. grown InGaN/GaN core-shell nanowire arrays on Si substrate by catalyst-free MBE, as shown in Figure 8a . It has a photoelectric conversion efficiency of up to 27% under ultraviolet and visible light irradiation. The photoelectrode continued to work for 10 hours, and the hydrogen production was consistent with the theoretical value (Figure 8d), which indicates that the photoelectrode has good stability and hydrogen production ability. And the quadruple-band InGaN nanowire arrays were integrated on a nonpolar substrate, which includes In0.35Ga0.65N, In0.27Ga0.73N, In0.20Ga0.80N, and GaN and exhibits a solar-to-hydrogen efficiency of ∼5.2% in a relatively stable state (Figure 8b) . Multiband nanowire arrays enhance light absorption to improve the performance of PEC water splitting. Moreover, the multiband nanowire array photoelectrode has good stability and high photocatalyst efficiency for overall water splitting, as shown in Figure 8e. To improve the efficiency of the photolysis of water, InGaN heterostructures have been proposed. Kibria and co-workers have fabricated InGaN/GaN nanowire heterostructures, in which the internal quantum efficiency is about 13% . The nanowire heterostructure is shown in Figure 8c. The combination of GaN and InGaN expands the light absorption range of GaN from ultraviolet light to visible light, which greatly improves the light absorption range and improves the efficiency of photolysis. The InGaN/GaN nanowire heterostructure photoelectrode also exhibits extremely high stability and high hydrogen production capabilities, as shown in Figure 8f. Moreover, nanowire arrays , tunnel junction nanowire , have also been reported.
In summary, the multiple-metal incorporation can greatly improve the efficiency of PEC water splitting of GaN. The structures and In content will greatly affect the efficiency of PEC water splitting. So, it is important to choose a suitable structure and the In content while preparing the GaN-based photoelectrode.
This review mainly introduces the application of GaN in the PEC water splitting system and summarizes the methods to improve the efficiency of PEC water splitting. The methods to enhance efficiency are mainly carried out in the following four aspects, such as morphology, doping, surface modification, and composition of solid solution or multiple-metal incorporation. Up to now, GaN has made great progress in the application of PEC water splitting; the solar-to-hydrogen efficiency of 12.6% has already been obtained without any external bias , better than CoP catalyst electrodes (6.7%) reported recently , but it still not as excellent as TiO2 (18.5%) . And its properties need to be further optimized to improve the absorption efficiency of visible light, increase the carrier migration speed, and facilitate carrier transport. The follow-up works are suggested from the following aspects:
At present, most water splitting processes are carried out in alkaline or acidic solutions. It should be considered how to ensure the stability and catalytic activity of metal nitrides for a long time.
Although the theory of water splitting is simple, the reaction process is still not clear, and in-depth study of the mechanism is helpful for the design of the catalyst.
Reasonable design of the composition and structure of the catalyst to adjust its electronic structure, band gap, band edge potential, and microstructure help to improve the catalytic performance. We believe that with the deepening of research, the efficiency of GaN for water splitting can be further promoted.
The authors declare no conflict of interest.