Introductory Chapter: Novel Thermal and Non-Thermal Technologies for Food Processing

1. Introduction

Food Processing is the set of techniques and methods used to transform raw materials into final products. Quality of foods is a great concern when processing methods are used for food preservation. Thus, food preservation whilst ensuring its quality and safety has been a prime goal of food processors which is mainly attributed to the preferences of the consumer which increased the demand for high quality foods in terms of nutritional physiology and technological quality. Since prehistoric times, processing of food materials including sun drying, salting, fermenting and smoking have been used to preserve foodstuffs in order to make them edible [1]. Indeed, food processing methods have been developed to inactivate pathogenic bacteria, toxins as well as detrimental constituents so that the obtained processed foods can meet safety and shelf-stability requirements to the consumer. Nowadays, more emphasis was attributed on informatively labeled, high-quality and value-added foods which are convenient to use beyond the traditional requirements [2]. In the recent years, the awareness of foods that are beneficial to consumer health has increased, especially about the effects of processing on the functional components of the various food products. Consequently, a number of health-conscious consumers try to more eat raw foods. However, many foods are available only when they are processed, furthermore, they need to be processed to make them safe for consumption [1]. The purpose of this introductive chapter is to provide a general perspective of the novel thermal and non-thermal processing technologies currently available in connection with their efficiency and their impact once implemented by the food industry. Food processing are commonly divided into two main types: thermal and non-thermal processing as shown in Figure 1 [1, 3, 4]. Thermal processing is the most common and traditional technique because of its ability and efficiency to inactivate microorganisms as well as spoilage enzymes [5]. However, severe heat treatments may induce chemical and physical changes to food products. These techniques can even induce the formation of toxic compounds and can reduce the bioavailability of some importants nutrients. Furthermore, an adverse effect on the sensory properties of foods caused by thermal processing has been reported [1]. As a consequence, softer processing techniques including novel thermal and non-thermal processing has become the new trend. Indeed, these processing techniques are claimed to combine the high quality of industrial foods and the improved functionality.

Figure 1.

Thermal and non-thermal food processing methods.

They were also reported to be more cost efficient and environmentally friendly compared to classic thermal processing techniques. Therefore, the aim of this introductory chapter is to provide a critical review of novel food preservation processes including both thermal and non-thermal technologies.

2. Thermal food processing methods

Overall, conventional food preservation processes consist of exposing food to a very high temperature, leading to the reduction of the contamination or microbial load from food.

Thermal processing techniques persists as the most widely used techniques for prolonging the shelf life of foods and ensuring their microbiological safety [1]. However, these processes result some undesirable changes in food including the loss of nutritional components which are temperature-sensitive, the change in the texture of food and in the organoleptic characteristics of treated foods [6]. The thermal treatment used for preservation result also in the formation of chemical toxicants in food which are carcinogenic and harmful to the human body. However, the amount and the type these toxicants formed depend on the treated food and the type of thermal method used for cooking food [7, 8].

Furthermore, the functional benefits of thermally processed foods are still doubtful as thermal treatment causes considerable changes in the nutritional attributes and the quality of foods [9]. For instance, vitamins are among the most sensitive components in foods to be affected by thermal treatments. Heat-sensitive vitamins include both of fat-soluble vitamins such as vitamins A (in the presence of oxygen), D, E and β-carotene, and water-soluble vitamins such as vitamin C (ascorbic acid), B1 (thiamine), B2 (riboflavin) in an acid environment, nicotinic acid, biotin C and pantothenic acid [10]. Novel thermal processing techniques are characterized by novel heating alternatives that can offer quicker heating rates leading to minimization of nutrient degradation and adverse reactions. These techniques include ohmic, radio frequency and microwave heating.

2.1 Ohmic heating

Ohmic heating is generally included to the group of novel processing technologies and especially to the novel alternatives to conventional thermal processing. This technology has been known since the nineteenth century as it was applied to pasteurize milk [11, 12]. Ohmic heating is direct electro-heating where electrical current is applied directly to the food while microwave processing and radio frequency heating are indirect electro-heating where the electrical energy is firstly converted to electromagnetic radiation that subsequently generates heat within a product [13]. Various ohmic heating processes that use electrical current for heating food have been used and developed. This technology has been also widely studied and reviewed in many scientific publications [11, 14, 15, 16].

Generally, most of scientific publications that studied the effect of ohmic heating processes on foods were carried out at either 20 kHz or 50 kHz [17, 18, 19]. For instance, ohmic heating frequency at 10 or 60 kHz on the inactivation kinetics of Geobacillus stearothermophilus spores showed an improved antimicrobial efficiency at higher temperatures as reported by Somavat et al. [20, 21]. Most commonly studied food matrices were meat, processed meat products and liquid foods including fruit, vegetable juices, milk and milk-analogues. Indeed, the main drawback for this technology is that ohmic treatments can alter the textural properties of foods despite a correct optimization of the ohmic process conditions [12, 16].

2.2 Radio frequency heating

Radio frequency heating process involves the direct transfer of electromagnetic energy into food product, thus, it induced volumetric heating due to frictional interaction between different molecules [22]. The allowed frequencies for the applications of this technique are 13.56, 27.12, and 40.68 MHz [22]. The greater wavelength at radio frequencies compared to those of microwave heating justifies the significant advantages of radio frequencies over microwaves, especially in the case of food processing applications [23]. Radio frequencies heating presents similar advantages to ohmic and microwave processes when compared with conventional heat-processing technologies. Some specific advantages of radio frequencies heating over those alternative volumetric technologies are noted. However, radio frequencies processing presents some disadvantages which are: the higher equipment and operating costs when compared to conventional heating systems, the reduced power density when compared to microwave heating, and the limited research efforts regarding the determination of food treated by radio frequencies dielectric properties [23, 24].

2.3 Microwave processing

Microwave technique is considered as a novel thermal treatment whose used have increased over the last years either in food industry or for domestic use. Domestic microwaves generally operate at a frequency of 2.45GHz while industrial microwave systems operate at frequencies that range between 915 MHz and 2.45GHz.The microwave are distinguished by generating heat instantly which significantly reduces the processing time and operational cost when compared with the conventional dry-heating methods [3, 25].

Overall, microwave heating is used in both of pasteurization and sterilization. Indeed, pasteurization is a process in which only pathogenic microorganisms in the vegetative form are destroyed by thermal treatment in order to enhance food safety and shelf life. For microwave processing, the destruction of microbes at sub-lethal temperatures was attributed to the selective heating, cell membrane rupture, electroporation, and magnetic field coupling [26].

The main advantages of microwaves processing are the less time of the process and the fast and efficient heating. Indeed, product quality and food nutritional and sensory qualities are improved with reduced drying time [3]. However, microwaves also show disadvantages, such as degradation of treated food products by dry heating and food dehydration. However, one of the main disadvantages recognized in microwave heating is the non-uniform temperature distribution resulting in hot and cold spots in microwave-heated products [12].

3. Non-thermal food processing methods

Recent developments in food preservation processes operations involve novel technologies that minimize the deleterious effects of heat on the nutritional and sensory properties of foods. Promising methods include non-thermal processing which can be conducted at ambient or slightly above-ambient temperature and causes no change in the nutirional composition of food and the texture which remaines intact [1, 4].

Since the last few decades, various non-thermal technology for food processing treatments came into light including pulsed electric field, ultrasonication, cold plasma etc. These non-thermal treatments result in the reduction of the microbial load in the treated food with an increase in shelf-life and with good sensory and textural characteristics as they unmask food to treatment conditions for a fraction of seconds [27]. Furthermore, the preservation effects of non-thermal technologies are more than those of thermal technologies because there is no risk for the formation of any toxic or undesirable products in foods since it is not exposed to high temperatures [4].

3.1 Ultrasonication processing

Ultrasonication is an emerging non-thermal technology in food industry, whereas it is used in other processing sectors [28]. It refers to sound waves at a higher frequency than that of human hearing (between 20 kHz and 10 MHz). The major effects of ultrasound on liquid systems is contributed by cavitation phenomena, which is the physical processes that create and implode micro-bubbles of gases dissolved in a liquid by the compression and decompression of the different molecules that constitute the liquid medium [1]. Ultrasonication is generally used with different frequencies including low-frequency, medium-frequency, and highfrequency ultrasonication, with frequencies that range of 20 kHz–100 kHz, 100 kHz − 1 MHz, and 1 MHz–100 MHz, respectively [29]. Ultrasonication is useful for the process of degassing in carbonated beverages. It is also a good replacement for the processes of preservation including pasteurization and sterilization in order to reduce the microbial load in both of foods and food products [30]. Ultrasound has successfully proven its usefulness in the food sector in various areas such as food preservation, extraction, intensified synthesis, and improvement of the physical and chemical properties of food [4]. However, this treatment must be studied on bulk food in order to understand its effect so that it can be implemented at industrial scale.

3.2 High hydrostatic pressure processing

Application of High hydrostatic pressure mainly deals with pressure as a preservation method with great potential to produce microbiologically safer food products. During this process, the pressure ranging between 100 and 600 MPa is transmitted uniformly and instantly, with a little variation in temperature upon increasing pressure regardless of the size of the food (the rate of temperature increase is about 3°C/100 MPa) [31].

This method causes microbial cell injury and does not alter low-energy covalent bonds. As the covalent bonds have low compressibility and would not break within the ranges of high pressure used in food processing, the primary structure of molecules in food such as proteins or fats remains intact [32]. High hydrostatic pressure can bring about a significant decimal decrease in the population of pathogenic Gram-positive bacteria and Gram-negative bacteria, furthermore, it helps in food preservation for a longer duration. However, the reduction in microbial load depends greatly on the pressure, temperature during treatment and type of food processed [4]. The quality of High hydrostatic pressure-processed food in terms of nutritional components, sensory, and texture is excellent due to the short period of food exposure to treatment conditions [33].

3.3 Radiation processing

Radiation processing of food is classified as a physical and non-thermal mode of food preservation, hence, it is called cold pasteurization [1]. Food produces can be exposed to either ionizing radiation or non-ionizing radiation in order to destroy pathigenic microorganisms or insects in the food. Ionizing radiation is generated by either electron beams, X-rays or gamma rays leading to the inactivation of microorganisms by damaging their DNA, while non-ionizing radiation is generated from ultraviolet rays, visible light, microwaves or infrared. Applications radiation processing are mostly employed in the food processing sector especially for the preservation of food products. This technique is effective against pathogenic microbes including E. coli, Salmonella and Staphylococcus [34].

However, the use of radiation processing result in some undesirable changes in food if treated at high irradiation doses. For instance, the color of meat as well its lipids have been slightly changed a which may lead to rejection by consumers. Therefore, irradiation is usually done with a low dose with the combination of the use of antimicrobial agents in order to achieve the desired inactivation in food with no change in the food composition and processed food products [35].

3.4 Pulsed electric field processing

Pulsed electric field technology is classified as a non-thermal technology for food processing which is capable to inactivate microorganisms and enzymes and to retain health-related compounds concurrently [1]. The Pulsed electric field process is commonly applied to liquid foods by the application of a series of short and high-voltage pulses (25–80 kV/cm) to a liquid food [1]. Since food is exposed to pulsed electric field for a very short duration of time ranging between few milliseconds to nanosecond, there is no chance of heating and hence, undesirable changes in food due to high temperature are eliminated [36].

The mechanism of pulsed electric field causing microorganism inactivation is known as electroporation of cells. Indeed, pulsed electric field causes damage to the cell membrane of microbes through tension in the cell membrane attributable to electromechanical compression which facilitates the formation of pores in the membrane [37].

The efficiency of this process in reducing microbial load depends on the intensity of field applied, temperature, the total exposure time, and energy [4]. Previous studies have shown that pulsed electric field process was effective against E. coli in flowable food like pineapple and orange juice and coconut water with an intensity of 5.6 W/cm² [38]. Similar results reported that pulsed electric field intensity are also effective for microbial inactivation in fruit juices [39]. Apart from microbial inactivation, pulsed electric field process is also effective in the deactivation of food spoilage enzymes [4]. For instance, the most common discussed enzymes are polyphenoloxidase and peroxidase which catalyze oxidation of phenolic compounds leading to enzymatic browning of vegetables [1].

3.5 Ozone

Ozone is generally employed as an effective antibacterial agent against many bacteria in food in gas form or in ozonated water as it can be mixed with water to form [4]. There are many mechanisms by which ozone causes microbial cell death. For instance, ozone causes damages of the microbial cell membranes as it alters the permeability of cells. Furthermore, ozone is known to damage the structure of proteins that leads to the malfunctioning of microbial enzymes, which results in microbial cell death [40]. Ozone showed its effectiveness of ozone against Listeria monocytogenes present in meat by a treatment of 280 mg O₃/m³ for 5 h with pulse of ozone passed after 10 min for 30 min duration. Furthermore, ozone was efficient for the inactivation of Salmonella and spoilage microorganisms [4, 41, 42]. Ozone treatment is also effective in the inactivation of toxins present in food [40]. However, this reactive molecule reacts with many components in food which could induce undesirable changes. It also induces oxidation in food lipids. Further studies are needed in order to reduce these undesirable changes in food and to improve its acceptability.

3.6 Cold plasma technology

In the food industry, cold plasma can be used for the reduction of the microbial load in food products or on the surface of food which enhances the physical and chemical properties of different food constituents including lipids and proteins. Thus, it is used for the sterilization of food processing equipment, treatment of food packaging material, inactivation of food spoilage enzymes, and treatment of wastewater [43]. There are no risk of thermal damage to heat-sensitive food material as the used temperature is ambient. Cold nitrogen plasma shows significant inhibitory action on Salmonella enterica after a treatment of 600 W for 2 min [44]. Furthermore, a reduction of 97.9% and 99.3% in the growth of fungal species such as Aspergillus parasiticus and Aspergillus flavus, respectively, after a treatment at 60 W plasma power on the ground nut surface [45].