Yeast is the simplest eukaryotic organism of our days. They are unicellular microorganisms classified in the kingdom Fungi. Nevertheless, yeasts were probably the first microorganism to be domesticated and since early in human history have been used on a daily basis in bread making and in alcoholic beverages. Nowadays, yeast has become a key microorganism for many types of industrial and food processing manufactures, including the production of beer, wine, cheese and bread. In particular, its use in baking industry is quite relevant due to the central role of bread as a dietary product all over the world. Moreover, yeasts are regarded with reasonably interest as nutrients and health provider sources both for humans as well as for animals. We dare to appoint yeast as the one of the world finest chefs.
Yeasts are found in diverse natural environments; colonizing from terrestrial, to aerial and aquatic environments. They can be found on decomposing fruit, on soils, as opportunistic pathogens in human beings, in the gut of the fish and free living in the sea. In general they contribute to the decay of organic material, but their successful colonization is intimately related to their capacity of physiologically adapt at diverse milieus. Hitherto, it has been described approximately 1500 species .
This chapter aims at contribute to a comprehensible analysis of the role of yeasts on the actual feed lifestyle, mainly in what regards the yeast
1.1. Yeast metabolism
Yeasts, resembling other heterotrophic organisms, have the energy and carbon metabolism operating in concert,
1.1.1. Greedy yeasts – Sugar metabolism
The yeast metabolize diverse sugars, hexoses such as glucose, fructose, galactose or mannose, some can use pentoses like xylose or arabinose, disaccharides as maltose or sucrose; yet, glucose and fructose are the preferred substrates. The metabolic routes for the dissimulation of hexoses and disaccharides share the same pathways, with the great majority of the metabolic elements arising from intermediaries of glycolysis, the tricarboxylic acid cycle (TCA) and the pentose phosphate pathway, and differ only in the initial basic steps of metabolism.
The sugar dissimilation may occur in anaerobic or in aerobic environment. In the first case is called
For the sugar utilization, yeast has primarily to sense the presence of glucose in the environment and then to transport it across the plasma membrane [4, 5]. The presence and levels of glucose sensed by the yeast can influence the enzyme levels through several processes, alteration of mRNA translation rates; mRNA stability or protein degradation, but also the concentration of intracellular metabolites (for a review see ). Yet, the major outcome is the extensive transcriptional regulation of a large number of genes leading to the adaptation to fermentative metabolism (alcoholic fermentation). These encompasses the induction of genes required for the utilization of glucose, such as genes encoding glycolytic pathway enzymes (discussed below), whereas genes required for the metabolism of alternative substrates, and those encoding proteins in the gluconeogenic and respiratory pathways are repressed by glucose (for reviews see  and ).
The gene family of hexose transporters in
The first step of the glycolytic pathway consists on the phosphorylation of glucose to glucose 6-phosphate by the action of the hexokinases (Hxkp) and the glucokinase (Glkp); which are linked to high-affinity glucose uptake. Then glucose-6-phosphate is isomerized by the phosphoglucose isomerase, encoded by
Yeast phosphofructokinase, Pfkp, is a heterooctameric enzyme subject to a complex allosteric regulation. Aldolase (fructose 1,6-bisphosphate aldolase- Fbap) in turn, catalyses the reversible cleavage of fructose 1,6-bisphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. These two compounds can be converted one into another, again in a reversible way, by the triosephosphate isomerase (Tpip). Subsequently, glyceraldehyde 3-phosphate yields the pyruvate by the action of a series of acting enzymes, whereas some of the dihydroxyacetone phosphate follows gluconeogenesis. Glyceraldehyde 3-phosphate is firstly oxidised by NAD+ (with the production of a reducing equivalent, which will take part in the latter steps of glycolysis when acetaldehyde gives ethanol (Figure 1)) and then phosphorylated, under the catalysis of the 3-phosphate dehydrogenase (Tdhp). The resulting 1,3-diphosphoglycerate, by the action of phosphoglycerate kinase (Pgkp), donates a phosphate group to an ADP molecule originating the 3 phosphoglycerate and releasing 1 molecule of energy (ATP). The next step is just a relocation of the phosphate group on position 2, done by the phosphoglycerate mutase (Pgmp); preparing this way the following reaction, the dehydration by the enolase (Enop) and from which results the phosphoenol pyruvate, a high energetic molecule. This is then phosphorylated by the pyruvate kinase (Pykp) giving the pyruvate and also releasing another molecule of ATP.
At this point, pyruvate can follow distinguished metabolic routes (Figure 2) depending on the environmental conditions, which in turn regulate the enzymes involved as well as their kinetics properties, but also of the yeast species . Conversely, the carbon flux gets to a branching point in which may be divided among the respiratory and the fermentative pathways.
In alcoholic fermentation, pyruvate is decarboxylated to give acetaldehyde and CO2, by the pyruvate decarboxylase (Pdc1p). In the final reaction, catalysed by the alcohol dehydrogenase (Adhp), acetaldehyde is reduced yielding the ethanol and promoting the re-oxidation of NADH to NAD+. At the same time, and in addition to the 2 molecules of CO2 and of ethanol, formed per molecule of glucose, the sugar is incorporated into other by-products such as yeast biomass, acids (pyruvic, acetaldehyde, ketoglutaric, lactic) and also importantly glycerol. This is generated from dihydroxyacetone-phosphate and is, to a certain extent, very desired by the wine producers in order to get fuller bodied wines (discussed below). Furthermore, alcoholic fermentation is a redox-neutral process; given that the NADH produced during the oxidation of glyceraldehyde 3-phosphate is afterwards reoxidized in the reduction of acetaldehyde to ethanol . Yet, one must keep in mind that with fermentation is associated culture growth and, biomass composition is more oxidized than glucose, consequently an excess of reducing equivalents may be attained. The way yeast circumvent this problem, under anaerobic conditions, consists on the production of glycerol by reduction of the glycolytic intermediate dihydroxyacetone phosphate to glycerol 3-phosphate catalysed by NAD+-dependent glycerol 3-phosphate dehydrogenase (encoded by the two isogenes
Although fermentation usually happens in the absence of oxygen, this is not a strict rule. Even in the presence of high levels of oxygen, if the sugars are fully accessible to be metabolized, yeasts choose to ferment instead of respire. This phenomenon is called the Crabtree effect , defined as the inhibition of aerobic metabolism when glucose is available, which occurs both in the presence or absence of oxygen. For instance,
S. cerevisiae, the party starter
Beverages with an alcoholic content are largely consumed by mankind since ancient times. Such beverages made from fermentation of sugar-rich goods, namely cereals and fruits, are present in oldest records . Beer, made from germinated barley, and wine, produced from grapes, are among the most popular and their worldwide consumption is second only to non-alcoholic drinks as water, tea and coffee .
Wine and beer history is hand to hand with human civilization history, as most likely only the agriculture advent and the establishment of permanent settlements provided the conditions for its production. Nevertheless, wine and beer are most probably result of an “accident”, as some harvested grapes were not consumed rapidly enough or some cereal wet pulp was left aside, and
In fact, our relationship with
Wine was also very popular in the ancient cultures, with references to this beverage in religious ceremonies of Egypt and Phoenicia. Pharaohs tombs were frequently adorned with vintage scenes and jars filled with wine accompanied the Kings afterlife [11, 23]. Wine consumption spread with the rise of the Greek and Roman Empires. Under the Greek and Roman influence, wine earned the status of “Civilized” drink, becoming very popular with the Empire upper classes, and beer was labelled as a “Barbarian” drink. Wine production and vine cultivation spread across Europe and replaced beer as the main drink in many countries. Some of these are still nowadays associated with wine production like Portugal, Spain and France. The production and consumption of beer continued mainly in northern borders of the Roman Empire, where Germanic tribes ruled and Roman influence was weaker [26, 28].
In Middle Ages, wine and beer production gained a new impetus with the shift from the familiar production to a more centralized production in monasteries . Such happened because, at the time, water was frequently polluted, so alcoholic beverages were safer than water for the monks’ consumption. Additionally, during the long fasting periods that the monks subjected themselves, the drinking of these highly nutritious beverages became common to satisfy hunger. This happened because wine and beer were considered similar to water and didn’t constitute a breach of fast. In fact, in some monasteries monks were allowed to drink up to 5 litres of beer per day . Southern monasteries produced mainly wine, as the weather was warmer and suitable for vines, but in the north the colder weather was more fitting barley and wheat growth and therefore Northern monasteries were more devoted to beer production. Each monastery developed its own methodologies for wine and beer making, leading to new wines and new brews and to a great technical improvement. Later these products become a source of income for the monasteries.
From the sixteenth century, with the discovery of the New World by Portuguese and the Spanish explorers, wine and beer spread to new territories. Vines were introduced in Brazil by the Portuguese around 1500 , and in Africa by the Dutch around 1650. In the Australian continent and North America this happened later, around 1800.
In the nineteenth century, wine and beer making suffered probably the major scientific advances. Around 1860 Louis Pasteur, a name forever associated with wine and beer production, developed studies on the conservation of wine through a heating-cooling process later known as “pasteurization”, showing that wine could be stored for longer periods after such treatment. Moreover, in 1870 Pasteur made known to the world the role of
The intensive study of this amazing microorganism and its role in fermentation showed the specificities of each yeast species and strains. The necessity of consistent properties and quality in different fermentations, both in brewing and winemaking, paved the way for the selection of the right yeast for the job. The quest for stable and improved yeast began.
2.1. The “Right” yeast for the job
The final product, either wine or beer, is greatly influenced by the sugar-rich fermentable broth, grape juice or malted cereals, with different composition in fermentable sugars and nitrogen sources. The progression of the fermentation is another very important aspect of winemaking and brewing,
2.1.1. Yeast physiology
Beer is the denomination commonly attributed to a carbonated alcoholic beverage produced by fermentation of malted barley, while wine is made of the fermented juice of any of several types of grapes. However, there are as many different wines and beers as there are different producers, all with their unique character and flavour influenced by the selected ingredients, kind of fermentation and yeast selected.
As said, the choice of the ingredients greatly impacts the fermentation final product; usually beer is the product of malt, hops, water and yeast. Malt is the result of germinating and drying (kilning) barley, yet other cereals besides barley can be used to produce beer, as wheat and rye. Malt extract will provide the entirety of the carbohydrates and nitrogen to the fermentation process and as such, it will influence the final ethanol concentration as well as colour and flavour development. Conversely, another important aspect is the intervention of hop, the female flower cluster of
Brewer’s yeast can be distinguished in top fermenting and bottom fermenting yeasts, based in the position at which the fermentation occurs. This division accounts with the yeast flocculation behaviour, and it is such an important element of brewing that defines the two main classes:
These brewer’s yeasts present several differences in their genomes.
In winemaking, most wine yeasts belong to
The utilization of dried yeast as a starter culture is very common in the wine industry. Cells are dehydrated through a cycle of filtrations and centrifugation to remove external water and then submitted to streams of dehumidified hot air. Such procedure can reduce yeast cells’ content in water to as low as 6%. However, even though yeast cells can survive such treatment, it causes cellular damage. Damages to cell wall and plasma membranes caused by the changes on cell size and shape, as well as damages to proteins produced by free radicals were reported .
One of beer brewing specificities is the utilization of a freshly grown starter culture. In one hand, it ensures a healthy population fully adapted to growth medium. Cells are usually collected at the late exponential phase, preventing a large percentage of aged cells, and at the same time ensuring metabolic fitness. On the other hand, it meets the requirement for flavour consistency of the final product, even though it is more expensive than the alternatives. The pattern of metabolic products of yeast is highly dependent on its growth conditions, and cells fully adapted to
During fermentation, yeast is constantly facing new pressures. The high osmotic stress due to the sugar high content of
Nitrogen assimilation is especially important in flavour development. The main sources of nitrogen are free amino acids and ammonium ions, which are used by the cell for protein formation [35, 40]. Such amino acids are also relevant for the production of alcohols and esters, important in these beverages flavour. During fermentation, amino acids are always used following a certain order, independent from the fermentation conditions. Group A, including arginine, asparagine, aspartate, glutamate, glutamine, lysine, serine and threonine, are used first. Group B amino acids are utilized slowly and include histidine, isoleucine, leucine, methionine and valine. Group C is composed by alanine, glycine, phenylalanine, tyrosine, tryptophan, and are only absorbed after the complete exhaustion of group A. Group D is composed of proline, which require an aerobic metabolism for its uptake and it is poorly used during fermentations .
In winemaking, fermentations are usually developed under anaerobic conditions, but it is common in brewing to oxygenate the
The inorganic ions are necessary, but at nanomolar concentrations. These trace elements, as calcium, zinc or copper, are mainly required as cofactors of enzymes or in the flocculation process. For instance, the response to oxidative stress is dependent on enzymes such as the different superoxide dismutase isoforms that require manganese, zinc or copper . On the other hand, calcium is vital for the flocculation advance . Conversely, insufficient amounts of such elements can lead to cellular damage and stress, and consequent stuck fermentations.
The use of antimicrobials in vineyards is common to control fungi that spoil grapes. But, when grapes are macerated these compounds are incorporated into the juice. Even though they may help to prevent the wine oxidation and microbial spoilage, a concentration to high may lead to off-flavours and in worst case, yeast death. So antimicrobials, especially sulphur dioxide, are an important stress to yeast during fermentation. Commercially available yeast also has to deal with toxins produced by wild yeasts derived from the vineyards. These toxins are produced to give those wild yeast advantages over others species in accessing to the nutrients. Isolation of strains resistant to both antimicrobials and natural toxins is an important research field [11, 43].
Certain compounds are extremely important for brewing and wine making not as substrates but as by-products. Such metabolites greatly influence the final product’s colour and flavour, as well as its stability. In fact, the importance of these compounds is such that lager beers are usually stored from several days to weeks, lagering, solely to remove diacetyl, an off-flavour causing metabolite. This time consuming maturation phase consists in a second fermentation at low temperature to eliminate the butter-like flavour caused by this vicinal diketone. Studies are being conducted in order to minimize this metabolite formation and reduce the maturation time . Sulphur containing compounds are other family of by-products receiving great attention. Such group comprises sulphite, sulphide and dimethyl sulphide, and while sulphite is a beneficial and flavour stabilizing metabolite, the remaining compounds are responsible for off-flavours. The equilibrium of such compounds formation could lead to better wine and beer and shorter fermentations .
Ethanol is one of the most important by-products of beer fermentation. Nevertheless, it represents an important stressor for yeast cells due to its high toxicity. Ethanol concentration can reach 10% in higher gravity fermentations, and acts especially upon biological membranes . Reports showed ethanol effects in growth inhibition , lipid modification and loss of proton motive force across the membrane and increased membrane permeability/fluidity . Yet, cells exposed to oxygen, with high levels of sterols in membranes, and adequate levels of nutrients, amino acids and trace elements in the fermentation broth are able to respond efficiently to such effects .
Nutritional stress occurs at the end of fermentation and cells enter stationary phase. This occurs because fermentable carbon sources tend to be depleted, and cells have to change their metabolism from fermentative to respiratory (explained in section 1), entering in a quiescent state . Such phenomenon induces flocculation, a cell-cell interaction process dependent of lectins and calcium that promotes sedimentation. Flocculation in turn is influenced by several other factors besides nutrient depletion. Reports showed the influence of ethanol content, calcium concentration, pH changes, oxygen concentration and temperature . The onset of flocculation is an important area of interest in brewing. If flocculation occurs to soon, stuck fermentation may occur, which results in a high sugar and low ethanol content. If, on the other hand, happens in a later stage, it has a high impact in beer filtration as most cells tend to be kept in suspension.
Fermentation is the most yeast-dependent phase of these alcoholic beverages production, but yeast also interferes with others proceedings. The metabolic fitness of the starter culture, the storage and maintenance of both dried and fresh yeasts, and the storage of the products submit yeast to different conditions to which they have to respond/adapt. To obtain detailed information on such processes please see reviews  and .
2.2. Old beverages, new solutions
Wine and beer industrial production led to a demand for better and more efficient yeast. Yeasts with improved utilization of substrates, carbohydrates and nitrogen, and consistent flocculent behaviour, as well as high fermentative capacity and high ethanol production are the industry goal. Enhancing beer and wine flavour through modification of by-products formation is another field of intensive research [30, 39]. As it is the improvement of the fermentation process, through encapsulation/immobilization of yeast .
Large collections of yeast were assembled, as the Centraalbureau voor Schimmelcultures (CBS) collection, in The Netherlands, and the Carlsberg collection, in Denmark. Manipulation of these strains to improve wine and beer properties has been performed in several ways, from spores manoeuvring and natural mutants’ survey to genetic engineering (GE). A rather recent and extensive review in strategies for the improvement of
The presence of glucose, even in small amounts, represses the simultaneous uptake and consumption of several sugars, namely maltose and galactose. Maltose (50%), maltotriose (15%) and sucrose (5%) are the main sugars in
Conversely, in sucrose metabolism, glucose repression addresses the sucrose conversion in fructose and glucose under the action of Suc2p. Studies showed that the disruption of
Maltose metabolism is more complex than sucrose, as it responds to both glucose repression and maltose induction. Maltose induction is under the influence of the locus
Finally, maltotriose, a glucose tri-saccharide, is the second most abundant sugar in
In winemaking and brewing, where flavour has such importance, amino acids metabolism has a notorious place. As said, amino acids are involved in formation of higher alcohols and esters that significantly contribute to beer and wine flavour. Since yeast cannot hydrolyse
Flocculation is a phenotype of industrial interest. It facilitates the filtration process in the end of fermentation, saving both time and money. In the case of brewing, it also serves the cropping (recover of part of the yeast population of the fermentation to pitch the next). Flocculation is a reversible aggregation of cells, where lectins recognize sugar residues in neighbour cells. Two industrially relevant flocculation phenotypes are well-known, Flo1 and NewFlo. Both are under the control of
The control of by-products production in order to improve wine and beer organoleptic properties is an expanding research area. The production of glycerol, to improve wine and beer’s fullness, as well as sulphite, to improve stability, and the reduction in diacetyl and sulphide content are the main targets. Glycerol, as the second fermentation metabolite, is rather important to wine and beer; the increase of its concentration to improve these beverages sensory character is an active field. Overexpression of
The presence of sulphite, an antioxidant and flavour stabilizer, and reduction of off-flavour producing sulphide is another important problem addressed by the industry. Both these goals can be achieved at the same time with the directed mutagenesis of NADPH-dependent sulphite reductase, an important enzyme in sulphur–containing amino acids synthesis. This strategy lowered this enzyme activity and increased the amount of sulphite in wine while reducing the sulphide presence in wine .
The reduction of diacetyl has special importance, as the maturation time (lagering) is directly dependent on this compound concentration. The expression of the bacterial enzyme acetolactate decarboxylase (ALDC) in yeast is the main approach to reduce the amounts of this compound. ALDC catalyses the reaction of α-acetolactate to acetoin, preventing the formation of diacetyl. However, after heterologous expression of ALDC, the yeast became auxotrophic for some amino acids and the growth rate was very low in
Improvement of yeast to render fermentations faster and cheaper is an industry goal, but the enhancing of the fermentation process itself is another alternative. The fed-batch technology has already proved its benefits , and improvements of such process with yeast immobilization/encapsulation are now under the spotlight. This results in much faster fermentation rates as compared to the existing free cell fermentations. However, it has some disadvantages, such as: i) complexity of production process including the choice of the suitable carrier materials, ii) bioreactors design, iii) fine-tuning of the flavour formation during fermentation processes, and iv) cost constraints .
3. Baker’s yeast – Magic on bread making
The process of bread making relies on the fermentation carried out by a mixture of yeast and bacteria. Even when all this was unknown and the flour leavening seen as “magic”, bread was already produced and extensively consumed. On those ancient times, the leavening resulted presumably due to the action (fermentation) of the natural microbial contaminants of flour or dough ingredients. This was obviously not a controlled process, yet with the practice of maintaining a fresh inoculum from one preparation to the next, promoted the selection of yeast and bacteria biodiversity. Nowadays, some types of bread are still prepared in this fashion, sourdoughs are one example (for a review see ), but the baking industry moved for the use of commercially baker’s yeast, typically the strain
3.1. Commercial baker’s yeast production – The break of spell
Commercial baker’s yeast is produced in several forms in order to meet specific requirements of climate, technology, methodology, transportation, storage and final product. As with all biotechnology processes, this is in constant development/undergoing research not only to optimize the process technology and its components, but as to produce faster growing strains with the characteristics to deliver better quality end products.
Molasses (beet and cane molasses), the common carbon and energy source used in the production of baker’s yeast, is a by-product of sugar refining industries, therefore cheaper than the formerly used cereals grain. Furthermore the sugars present on those molasses (around 50%), consisting on a mixture of sucrose, fructose and glucose, are ready to be fermented by the yeast. In order to obtain the proper broth for the optimum yeast biomass yield; the mixture of molasses has to be supplemented with nitrogen sources, minerals, salts and vitamins [60, 61].
After the preparation and sterilization of the broth, the production of baker yeast can take place. It begins by the inoculation of a small closed test flask containing the prepared sterilized broth with a pure yeast culture. The growth is allowed and careful screened, and when the culture reaches an elevated density, it is transferred to larger vessels and supplied with more broth, fed-batch reactors. This scale-up process continues until a desirable biomass quantity is attained, the so-called commercial starter, able to inoculate industrial fermenters/reactors, which production ranges from 40,000 to 200,000L .
The entire fermentation process of baker’s yeast has to be directed towards maximum biomass production; by-products such as ethanol are not desired. As we saw in section 1.1.1, in anaerobic dissimilation of sugars (alcoholic fermentation) the ATP yield is quite low comparing with respiratory dissimilation, affecting drastically the biomass yield. The way to avoid anaerobic ethanol production is the use of the mentioned fed-batch reactors, in which is possible to control the specific growth rate and sugar concentration by controlling the fed of reactors with fresh broth .
Nowadays, during the industrial large reactors the addition of nutrients and regulation of pH, temperature and airflow are carefully monitored and controlled by computer systems during the entire production process. In this way, the tones of baker’s yeast obtained in the end of the fermentation have the same quality/characteristics/properties as the original pure yeast culture that started the process. These tones of yeast are suspended in a large amount of water, resulting in a creamy suspension of active yeast, being necessary the so-called
3.2. Idol baker’s yeast
Yeast has a significant role on bread making, greatly influencing the final product properties. The most important contribution is in the leavening phase; after the dough has been kneaded and the gluten network start to develop, yeast starts to consume the available fermentable sugars and to produce ethanol and CO2, as mentioned in section 1. As fermentation occurs, the dough is gradually depleted of O2 present in the air bubbles trapped in the dough, leaving small bubble
As mentioned, the common procedure of bread making today, at least in developed countries, consists of using this commercial baker’s yeast. Its quality/individuality depends on storage stability, osmotolerance and freeze-thaw resistance. Considerable efforts have been made to obtain the Idol Yeast, including evolutionary engineering, genetic engineering (mainly to provide yeast with high capacity to tolerate freeze-thaw treatments). Yet, there is still considerable space for improvement. Those several strategies to achieve the Idol Yeast has been thoroughly revised and discussed in a previous work from the beginning of this year  as well as on [3, 68].
4. Yeast à la Carte
4.1. Yeast treats for animals
The pioneering research conducted almost a century ago by Max Delbrück and his colleagues was the first to highlight the value of surplus brewer’s yeast as a feeding supplement for animals . Yeasts have been fed to animals for more than a hundred years, either in the form of yeast fermented mash produced on the farm, yeast by-products from breweries or distilleries, or commercial yeast products specifically produced for animal feeding. In animals, including pets, this practice is used to compensate for the amino acid and vitamin deficiencies of cereals [73, 74], and in fish as a substitute for other ingredients .
Brewer’s yeast biomass, as described above, which results from the cultivation of
Those yeast used for monogastrics food or feeding rations is generally inactivated because feeding of live yeast might cause avitaminosis due to the depletion of B-vitamins in the intestine . They can also cause adverse fermentation in the digestive tract of swine leading to diarrhoea and bloating . Yeasts can be killed through application of heat or using chemicals. High temperature destroys the yeast membrane, but does not necessarily inactivate all yeast enzymes, unless quite elevated temperatures are applied. Alternatively, there are the chemical treatments with propionic acid or formic acid, which also act as a preservatives for yeast, and contribute to the feed value of the yeast .
Yeasts have been used in diets of numerous species with varying levels of success. Yeast for swines is sold for feed applications as wet slurry, as dried brewer’s yeast, or in mixtures with other brewery by-products . It is ideal for their feed as a good protein source, it contains most of the essential amino acids in adequate quantities, and numerous vitamins, selenium, copper, and phosphorus. Selenium concentrations are much higher in yeast than in soybean meal, and deficiency of this compound in the swine’s alimentation has been the cause of higher swine mortality [78, 79]. Additionally, dried brewer’s yeast contains mannan oligosaccharides, which have been reported to increase the growth performance and intestinal health of pigs . Benefits have been described as well for nursing and weanling pigs .
Either live or inactivated brewer’s yeast have been used as well in ruminants diets, consequently it was observed an increase in productivity of animal meat or milk [73, 82], but also live yeast cultures have been used. These are prepared by inoculating wet cereal grains or grain by-products with live yeast, partially fermenting the mash, and then drying the entire medium without killing yeast or destroying vitamins and enzymes . Live yeast is reported to stimulate fermentation in the rumen through its ability to stimulate the development of anaerobic, cellulolytic and acid lactic bacteria fermentations. In addition, the ingestion of yeast offers continuous supply of vitamins, dicarboxylic acids, removal of oxygen, buffering effect, and reduction in the number of protozoa. As a result, there is an improvement of the digestion of the fibrous and cellulolytic portion of the diet, which leads to a greater intake of food and better performance [83, 84].
Dried yeast was used traditionally in poultry diets in the past as a source of aminoacids and micronutrients, and though the broiler growth was improving this practice was largely discontinued for economic reasons . On the other hand, brewer’s yeast appears to be especially beneficial for breeder turkeys and laying hens . Reproductive improvements are attributed to the high level of dietary biotin and selenium in yeast, which is more beneficial than inorganic selenium added to poultry diets  and it contributes for the prevention of biotin deficiency in poultry diets, which may result in reduced feed conversion, low egg production, and poor hatchability . Brewer’s yeast is also very rich in folic acid, an important vitamin for turkeys .
Brewer’s yeast has been recognized to have potential as well as a substitute for live food in the production of certain fish or as a potential replacement for fishmeal [88-90]. In addition, it has low content in phosphorous, meaning less water and environmental contamination than common fish meal and other plant-based alternate protein sources . Multiple studies have demonstrated the immunostimulant properties of yeasts, such as their ability to enhance non-specific immune activity . That reaction can be related to β-glucans, nucleic acids as well as mannan oligosaccharides . Brewer’s yeast may serve as an excellent health promoter for fish culture as even when administered for relatively long periods is able to enhance immune responses as well as growth of various fish species, without causing immunosuppression [94, 95]. Furthermore, the relative high levels of nucleic acid nitrogen present (mostly in the form of RNA) that in humans and most monogastric animals can became toxic if taken in excess, as the capacity of excretion of the uric acid formed is limited , in fish does not happen due to their very active liver uricase .
4.2. Human little treats, big benefits
In a world of rapidly increasing population and low agricultural production, yeasts are relatively cheap and easily produced on an industrial scale representing a sustainable alternate protein source to cover the population nutritional demands. The first time that yeast was cultivated in large scale for human nutritional use was in Germany during both World Wars . The yeasts
The benefits from ingesting yeasts do not stop here, many other have been reported as: selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon ,
4.3. What the future holds
Authors would like to acknowledge Hugh S. Johnson for the several critical readings of the manuscript regarding proper English usage and Maria Manuel Azevedo for reviewing the manuscript and for valuable suggestions. Fábio Faria-Oliveira is supported by a PhD grant from FCT-SFRH/BD/45368/2008. This work was financed by FEDER through COMPETE Programme (Programa Operacional Factores de Competitividade) and national funds from FCT (Fundação para a Ciência e a Tecnologia) project PEst-C/BIA/UI4050/2011.