Proteins and amino acids

Proteins and amino acids

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María López-Pedrouso⁎, José M. Lorenzo†, Carlos Zapata⁎, Daniel Franco† ⁎ Department of Zoology, Genetics and Physical Anthropology, University of Santiago de Compostela, Santiago de Compostela, Spain, †Meat Technology Center of Galicia, Parque Tecnológico de Galicia, Ourense, Spain

5.1 Introduction-protein quality The foodstuff industry needs to scale up production by at least 70% over the coming decades as the growth of the world’s population is expected to reach 10 billion people in 2050 (UN, 2009). Dietary proteins are an essential macronutrient for survival, health, and human reproduction. Human proteome comprises an extensive repertoire of proteins formed by 21 different amino acids in variable amounts and sequences linked by peptide bonds. Proteins carry out a plethora of fundamental functions for life, such as biochemical catalysis, structure, movement, transport, regulation of cellular processes, protection of the body and individual cells, and storage. Hydrolysis of dietary proteins in the gastrointestinal tract by proteolytic enzymes provides amino acids that are used for de novo protein synthesis and as precursors for the synthesis of other nitrogen-containing compounds, such as nucleic acids, hormones, and vitamins. Dietary requirements for protein depend on a variety of factors, including body weight and composition, physiological state (growth, pregnancy, and lactation), coexisting pathological conditions, and physical activity level. The Recommended Dietary Allowance (RDA) by the Food and Nutrition Board of the Institute of Medicine of the United States is an estimate of the average daily level of intake sufficient to meet the nutrient requirements of 97% of healthy individuals (IOM, 2005). The RDA of protein for healthy adults ≥18 years of age is 0.8 g/kg body weight per day. However, a growing body of evidence suggests that protein intakes higher than the current RDA help to achieve optimal health outcomes in adults (Campbell, Trappe, Wolfe, & Evans, 2001; Kim et al., 2015; Phillips, Chevalier, & Leidy, 2016; Wallace & Frankenfeld, 2017; Wolfe, Cifelli, Kostas, & Kim, 2017). Specifically, higher protein intakes may help prevent the loss of skeletal muscle mass in older adults (sarcopenia), prevent age-related loss of bone mineral density and hip fractures, improve appetite control, prevent and/or treat obesity, and maximize athletic performance. According to the Organization for Economic Co-operation and Development (OECD), the population older than 65 years of age reached 17.4% in 2014 with a projection of 29.5% for 2060 (De la Maisonneuve & Martins, 2015). This sociodemographic context is crucial to assess the daily level of protein intake sufficient to meet the optimal nutrient requirements in elderly populations. The nutritional quality of proteins is very heterogeneous and is dependent on ­multiple factors. The amino acid profile and digestibility of proteins are important Innovative Thermal and Non-Thermal Processing, Bioaccessibility and Bioavailability of Nutrients and Bioactive Compounds https://doi.org/10.1016/B978-0-12-814174-8.00005-6 © 2019 Elsevier Inc. All rights reserved.

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parameters for routine evaluation of their nutritive value. High-quality or complete proteins include all nine essential (EAA) or indispensable (IAA) amino acids that cannot be synthesized de novo by the organism (i.e., histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) and must be acquired from foods. A limiting amino acid is an EAA that is present in insufficient amounts to meet metabolic needs. The content of EAA is an important difference between foods of animal and plant origin (López-Pedrouso, Bernal, Franco, & Zapata, 2014; Tahergorabi & Hosseini, 2016; Young & Pellett, 1994). Meat, fish, poultry, eggs, or dairy products are the principal source of high-quality proteins. In contrast, proteins of major food crops are often deficient in one or more EAA. By way of illustration, legumes are usually deficient in methionine and rich in lysine, while, on the contrary, cereals are usually deficient in lysine but rich in methionine. The nutritional quality can be improved with diets containing a mixture of proteins with differentiated limiting EAA profiles (e.g., legumes and cereals), supplementing the diet with the limiting EAA and enhancement of EAA composition through biotechnology (Hoffer, 2016; Osuji et al., 2015; Tahergorabi & Hosseini, 2016). It is well known, however, that amino acids or small peptides (di- and tri-peptides) resulting from protein digestion can be totally absorbed into veins through enterocytes and used for new protein synthesis and other physiological functions (Wu, 2016). The digestibility of protein is defined as the proportion of food nitrogen that is absorbed after ingestion. The FAO/WHO (1991) recommended the use of the Protein Digestibility Corrected Amino Acid Score (PDCAAS) as a suitable method to evaluate protein quality in humans. The score is computed as the ratio of the amount of the first-­limiting dietary IAA in the food protein to the amount of the corresponding amino acid in preschool-aged children pattern representative of human requirements (mg of first limiting amino acid in 1 g of test protein/mg of same amino acid in 1 g of reference protein). This ratio is subsequently multiplied by the percentage of protein digestibility based on true fecal nitrogen digestibility using a rat assay as a model for the adult human. PDCAAS values higher than 1.0 are truncated to 1.0. It follows that an excess over the content of the digestible dietary EEA in the reference pattern does not translate into an additional nutritional benefit. PDCAAS is a widely used and useful assay for protein quality evaluation by its simplicity and direct relationship to human protein requirements (Schaafsma, 2005). It has, however, been subject to a number of criticisms (Millward, Layman, Tomé, & Schaafsma, 2008; Rutherfurd, Fanning, Miller, & Moughan, 2014; Schaafsma, 2005, 2012). Most of them refer to the validity of the EEA composition of the recommended reference proteins, accuracy of values based on the digestibility of nitrogen, use of fecal protein digestibility as a measure of amino acid bioavailability, impact of anti-nutritional factors in protein sources, truncation to 1.0, and suitability for predicting biological efficiency of supplemental amino acids to improve protein quality. In particular, compelling evidence suggests that ileal digestibility provides more accurate estimates for correction of the amino acid score (Schaafsma, 2005, 2012). Accordingly, the FAO (2013) recommended a new and improved scoring system for the assessment of dietary protein quality called Digestible Indispensable Amino Acid Score (DIAAS). It is calculated as DIAAS % = 100 × (mg of digestible dietary IAA in 1 g of the dietary protein/mg of the same dietary IAA in

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1 g of the reference protein). DIAAS is based on true ileal amino acid digestibility of each IAA amino acid of the dietary protein and gives non-truncated scores that can be lower than or higher than 100% depending on the quality of the dietary protein (Rutherfurd et al., 2014; Wolfe, Rutherfurd, Kim, & Moughan, 2016). Other important concepts are the bioaccessibility and bioavailability of food compounds and their effects in human health. Bioavailability can be defined as the amount of a compound that is extracted or released from the food matrix into the gastrointestinal tract and thereby made accessible for intestinal absorption (Saura-Calixto, Serrano, & Goñi, 2007). The concept of bioavailability is related to complex processes involving different steps such as releasing from a food matrix, absorption, distribution, metabolism, tissue distribution, bioactivity, and elimination phases. Bioavailability includes in its definition the utilization of a nutrient and hence can be defined as the fraction that can be obtained from a food during digestion, absorbed, and then used for physiological functions (Bordoni et  al., 2014; Wood, 2005). Regarding protein issue, the protein bioavailability is the extracted fraction from the food that is turned into single amino acids or small peptides. This is important because protein supply can be increased improving the digestibility and bioavailability during food digestion (de Jongh & Broersen, 2012), and consumer preference for foods that contain ­bioactive compounds (peptides and free amino acids) is rapidly growing. The absorption rate for proteins is highly variable depending on the protein source, taking into account that animal proteins are more completely digested than plant proteins. For this reason, to define food quality based only on a high amount of protein is insufficient due to poor protein quality, protein-deficient absorption, or by undesirable organoleptic properties. Food processing can alter the structural and functional properties of proteins depending on the type and severity of the processing conditions. It is usual that foods are processed with heat treatments, oxidizing agents, organic solvents, acids, and alkalis, with final treatments of sterilization or pasteurization to improve shelf life, modifying food textures or flavors. This variety of processes may produce Maillard compounds, oxidation of sulfur amino acids, and other side reactions, resulting in lower amino acid bioavailability and reduction of protein quality (Bender, 1972). In addition, the nutritional value of the proteins is affected by their amino acid sequence and conformation. Thus unfolded or hydrolyzed proteins have different functionality and these alterations often lead to denaturalization and subsequent aggregation, reducing their bioavailability (Corzo-Martínez, Villamiel, & Javier Moreno, 2017). This issue together with the improvement of protein extraction from the food matrix and the control of protein interactions with other ingredients during food processing are relevant challenges facing the modern food industry. However, the protein functional properties (gel formation, emulsifying effect, viscosity, solubility, foam formation, and foam stabilization) related to their structural and physicochemical characteristics could be modified by thermal food processing. From a functional and nutritional point of view, knowledge of the possible changes of these properties under food processing has a great importance to improve protein features. In this chapter, however, we will examine more closely those changes affecting the nutritional protein properties when new food technologies are tested.

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5.2 Influence of conventional-thermal food processes on protein quality Nowadays, consumers are demanding healthier, more natural foods with fewer additives and longer shelf life. The balance between safety and preservation of nonshelf-stable food, during storage and commercialization, is a challenging task for food industries. For such products, thermal processing (e.g., pasteurization and sterilization) is not the best option to maintain nutritional quality. The use of innovative and non-thermal technologies (ultrasound, high hydrostatic pressure [HPP], pulsed electric fields [PEF], ohmic heating [OH], and enzymatic techniques) can assist the production of safe food and also achieve the desired nutritional, physicochemical and texture characteristics of non-shelf-stable foodstuffs. Consequently, novel and emerging technologies require further development to reach this goal in contrast to traditional methods (thermal-conventional process). Hence, in the next sections we will review the effect of thermal processing and novel technologies on food proteins.

5.2.1 Thermal-conventional treatments Boiling, cooking, baking, roasting, frying, grilling, pasteurization, and sterilization are the main treatments employed in the food industry. While the first ones are used to develop the sensory characteristics of the final products, pasteurization and sterilization are used to avoid microbiological spoilage increasing food shelf life. It is well known that the heating process produces protein modifications leading to differences in protein digestibility and bioavailability (Kaur, Maudens, Haisman, Boland, & Singh, 2014). Heat generally induces important conformational changes that depend on many other factors, such as pH and ionic strength of the food matrix. Pompei, Rossi, and Marè (1988) described the principal reactions of amino acids with other components of food during the thermal process that lead to amino acid profile modification and a decrease in nutritional food quality. They are: (1) interactions between amino acids producing bonds and decreasing the bioavailability, (2) degradation reactions of side chains, (3) alterations of amino acids with SH and SS groups, (4) denaturing and loss of native structure, (5) interactions with lipids, reducing the availability of sulfur-containing amino acids, and (6) interactions between lysine because of its double amino group and carbohydrates (Maillard reaction). From these, the Maillard reaction has been pointed out as the primary cause of diminution of the food protein’s nutritional value during thermal steps (Sohn & Ho, 1995). Therefore modifications in the amino acid profile depend on composition, structure, and heat susceptibility (Jovanović, Barać, Maćej, & Djurdjević, 2005). On the one hand, heat treatments could make proteins easily digestible due to denaturation and trypsin inhibitor activity reduction. On the other hand, other changes are related to the unfolding of secondary and tertiary structures, which may lead ­proteins to aggregate or precipitate decreasing their digestibility (Wada & Lönnerdal, 2014). Heat treatments generally have influence on protein solubility causing a higher level of aggregation and coagulation with negative consequences on the nutritional ­quality of protein (Korhonen, Pihlanto-Leppäla, Rantamäki, & Tupasela, 1998).

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Other ­adverse effects of heat treatment are related to the presence of anti-nutritional factors (trypsin inhibitors, inhibitors and hemagglutinins in legumes, tannins in legumes and cereals, phytates in cereals and oilseeds, and glucosinolates in mustard and canola protein products) that may appear during the heating process (Gilani, Xiao, & Cockell, 2012). Lately, food allergies are acquiring great importance worldwide, because in the last two decades the percentage of population affected has increased considerably (Pereira et  al., 2005). Recently, it has been determined that food allergies are related to conformational changes of proteins and thus the different food-processing methods can reduce or enhance the allergic potential of food proteins (Rahaman, Vasiljevic, & Ramchandran, 2016). One of the most known reactions that occurs due to the heating of protein-rich foods is the Maillard reaction, which is a complex reaction affected by temperature, time, water activity, presence of oxygen, and other components. This non-enzymatic chemical reaction is responsible for different tastes, colors, and aromas, such as browning in meats, roasting in coffee, darkened crust in baked bread, and caramelization. Hence it has a great impact on organoleptic properties. The heating process triggers the Maillard reaction that occurs between amino acids and reducing sugars, producing a wide range of products. The Maillard reaction can occur in any food that contains proteins and carbohydrates; it can even occur spontaneously in foods such as milk and dairy products (Van Boekel, 1998), meat and fish (Sugimura, Wakabayashi, Nakagama, & Nagao, 2004; Yang et  al., 2015), potatoes (Liska, Cook, Wang, & Szpylka, 2015), rice (Li et al., 2013), or wheat (Kocadağlı & Gökmen, 2016). Briefly, the Maillard reaction consists of the followings steps (Fig. 5.1): Step 1: sugars and amino acids are condensed followed by Amadori condensation to produce ketosamines. Lysine residues

Reducing sugar O HO

+ NH2

H

Glycosylamine

H

HO

OH

O

OH

H

OH

HO

H

HO

H

HO

H

HO

H

HO

H

HO

H

HO

HO

NH

H

H

HO

O

Amadori compound

N

H

HO

Hydroxymethylfurfural

OH

OH

OH

CH3

HO

Advanced glycation end products (AGE)

HO O

CH2

O

O

Polymeric compounds (Melanoidins)

Fig. 5.1  Main steps of the Maillard reaction during food heating.

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Step 2: dehydration and fragmentation of the sugars and degradation in the amino acids; these intermediate products can produce a slight yellow color. Step 3: some heterocyclic nitrogenous compounds called melanoidins are formed by aldol condensation, causing a brown color (Tamanna & Mahmood, 2015).

The final products produced by Maillard reaction will have influence on final product properties, depending on reaction time-temperature, cooking method, and ratio of amino and reducing groups, among others. In fact, frying and grilling meat could lead to a tenfold increase in the amount of Maillard reaction compared to boiling meat (Trevisan, de Almeida Lima, Sampaio, Soares, & Bastos, 2016). Within Maillard final products, advanced glycation end products have been associated with beneficial effects such as antimicrobial, antioxidant, and anti-carcinogenic properties. On the contrary, melanoidins are considered harmful to human health (mutagenic) and decrease protein digestibility (De Oliveira, Coimbra, De Oliveira, Zuñiga, & Rojas, 2016; Hellwig & Henle, 2014). Another important aspect is the Maillard reaction products such as pyrazines, alkanes, and esters, which produce a diverse range of different flavors (Guan, Wang, Yu, Xu, & Zhu, 2010). Milk and dairy products are treated with industrial heating processes to increase milk safety and quality and to increase shelf life. Ultra-high temperature (UHT) is commonly used for sterilization purposes; it consists of heating milk at 135–150°C for 2–6 s followed by aseptic packing. However, it has been reported that UHT and posterior milk storage can decrease protein solubility, and nonenzymatic post-­transcriptional changes may occur, such as the Maillard reaction products (Le, Holland, Bhandari, Alewood, & Deeth, 2013; Wada & Lönnerdal, 2014). An intermediate Amadori product, called lactulosyllysine, is produced during the milk industrial process changing the protein bioavailability and modifying its nutritional value. In addition, lactulosyllysine could be used for assessing the Maillard reaction together with carboxymethyllysine (Mehta & Deeth, 2015). Due to the Maillard reaction, skimmed milk powder has a lower biological value. The lysine is blocked by a reducing sugar preventing the protein tryptic digestion, decreasing the protein digestibility. In addition, cross-linked products such as lysinoalanine and lanthionine, which inhibit enzymatic proteolysis, are produced (Guyomarc’h, Warin, Muir, & Leaver, 2000). Wada and Lönnerdal (2014) studied the milk protein digestibility after industrial thermal treatments. Using the proteomic approach, these authors showed a decrease in digestibility for some proteins such as α-lactalbumin, β-lactoglobulin, and caseins after a thermal process. Finally, it has been demonstrated that homogenization and heat treatment also affect protein coagulation and clots formation during stomach digestion. Hence these changes in structures are relevant in the protein hydrolysis rate (Ye, Cui, Dalgleish, & Singh, 2017). Heating of meat products produces important changes in proteins, such as structural changes, aggregates formation, carbonylation, modification of aromatic residues, and Maillard reaction products. Meat muscle is composed of 15%–22% protein, which is divided into three groups based on their solubility: myofibrillar, sarcoplasmic, and connective tissue proteins. The most abundant groups are myofibrillar (50%–55%) and sarcoplasmic (30%–34%) followed by connective tissue proteins (Tornberg, 2005). In meat, thermal processing is important to ensure its preservation by e­ liminating

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s­ poilage microorganisms and to achieve desired sensorial properties. However, thermal processes may produce negative effects such as protein degradation or unpleasant flavor, texture, and color (Yu, Morton, Clerens, & Dyer, 2016). In general, heat causes structural changes in proteins losing their native conformation; myofibrillar proteins show an important denaturing between 55°C and 60°C and sarcoplasmic between 50°C and 70°C (Kemp, North, & Leath, 2009). In meat muscles, this protein denaturation and the sarcomere structure shrinkage lead to the coagulation of proteins, forming large aggregates with cross-links between unfolded polypeptide chains. It is well known that cooking procedures reduce enzymatic proteolysis, and the bioavailability of amino acids could be affected. Recent studies have simulated gastric conditions to study the effect of cooking conditions using stews and curries. These studies have demonstrated that digestive enzymes randomly attacked raw meat myofibrils, whereas in cooked meat with a compact structure enzymes could attack only myofibrillar ends. In addition, it has been proven that the mass generated from raw meat during the digestion was more resistant to pepsin action than the mass generated from cooked meat (Kaur et al., 2014; Kondjoyan, Daudin, & Santé-Lhoutellier, 2015).

5.3 Influence of emerging non-thermal food processes on protein quality 5.3.1 Ultrasound Ultrasound as a technology can play an important role in the food manufacturing industry, although it needs to be adopted commercially. Ultrasound is an alternative to conventional processes because it is an eco-friendly and economical technology that improves food safety and reduces physical hazards and the need for chemicals in a wide range of processes. Ultrasounds consist of vibrations with too high of a frequency for the human ear, generating a vibrational energy. They cause acoustic cavitations, which is a phenomenon of generation of bubbles due to pressure changes. The bubbles grow and collapse, generating hot spots that cause the thermal, mechanical, and chemical effects (Fig. 5.2). The use of high-power ultrasound generates bubbles and leads to hot spots (>5000 K) and high pressure (more than thousands of bars) affecting the structural properties of proteins. Indeed, cell structures can modify through the formation of bubbles, varying quality properties such as texture and color and affecting microbial inhibition (Alarcon-Rojo, Janacua, Rodriguez, Paniwnyk, & Mason, 2015). Effectively ultrasound treatment could be used as a non-thermal sterilization technique inactivating microorganisms or microbial spores without affecting sensorial parameters. In recent years, ultrasound techniques have been used in two different ways: (1) directly applied on food (food preservation, extraction technique, or with the aim of modifying functional properties) and (2) indirectly applied on food (cleaning and disinfection of instruments and material). In general, ultrasound applications are divided into two mains categories: low- and high-intensity ultrasounds. Both types have been demonstrated to be useful for industrial processing.

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Amplitude pressure

Low frequency wave

Time

Cell lysis Implosion

Cavitation bubble

Amplitude pressure

High frequency wave

Time

Cavitation bubble

Implosion

Fig. 5.2  Schematic graph of cavitation bubbles in high and low ultrasounds and their effect on cell lysis.

Low-power ultrasounds (high frequencies between 2 and 10 MHz; power up to 10 W) are used as non-destructive tools to monitor the industrial processes. The passage of the waves through the food matrix changes the food material properties. Generally, they are used in quality control (Chandrapala, Oliver, Kentish, & Ashokkumar, 2013; Ghaedian, Coupland, Decker, & McClements, 1998), extraction, functionality modification, or microbes deactivation (Ashokkumar, 2015). The quality control applications are based on wave speed, which depends on the moisture content or chemical composition. Several researches have studied the effect of low-power ultrasound on different foodstuffs, such as fruit and vegetables (Elvira, Durán, Urréjola, & de Espinosa, 2014; Mizrach, 2008), cakes (Gómez, Oliete, García-Álvarez, Ronda, & Salazar, 2008), cheese (Benedito, Carcel, Gonzalez, & Mulet, 2002) and meat products (Simal, Benedito, Clemente, Femenia, & Rosselló, 2003). Conversely, high-power ultrasounds (low frequencies between 20 kHz and 100 kHz; power up to hundreds of watts) are used to generate intense cavitation. In the review by Corzo-Martínez et al. (2017), structural modifications of proteins during food processing with high-intensity ultrasound are described. These structural changes of proteins were usually denaturalization and subsequent aggregation; even biological value and organoleptic properties decreased depending on the severity of processing. Structural properties of the proteins are affected by high-intensity ultrasound treatment due to formation of covalent and/or noncovalent bonds, enhancement in dissolution and solubilization, or foaming ability. Functional properties are also altered by inter and intramolecular interactions, and the changes induced by ultrasound ­depends on the type of protein (Ozuna, Paniagua-Martínez, Castaño-Tostado, Ozimek, & Amaya-Llano, 2015). Zou et al. (2017)

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reported that ultrasounds improved functional properties such as solubility, foamability, and gelation, with respect to a traditional treatment. Protein agglomerates can be disrupted, decreasing their size in aqueous solution. In addition, the ultrasonic energy can also break disulphide bonds destabilizing the secondary structure and maintaining the denatured structure (O’Sullivan, Park, Beevers, Greenwood, & Norton, 2017). Furthermore ultrasounds produce changes in the post-translational modifications of proteins varying their level of oxidation, glycosylation, hydroxylation, phosphorylation, methylation, and acylation. Moreover they can trigger chemical reactions in live organisms changing edibility and nutritional quality of proteins, such as seed germination stimulation. For example, Yang, Gao, Yang, and Chen (2015) reported improvements in the nutritional value of soybean sprouts. Stefanović et al. (2014) studied the effect of ultrasound treatment on enzymatic hydrolysis of egg protein with several proteases. They found that optimized conditions of ultrasound in the enzymatic process improved the degree of hydrolysis compared with a thermal treatment. Milk and whey have been extensively studied with power ultrasounds to achieve different aims focused on microbiological and functional properties (Cameron, McMaster, & Britz, 2009; Muthukumaran, Kentish, Ashokkumar, & Stevens, 2005; Yanjun et al., 2014). In addition, the effects of ultrasounds on milk homogenization have been previously studied (Villamiel & de Jong, 2000). These authors have tested that the ultrasonic process can influence fat separation in raw milk due to reduction in fat globule size. In the same way, Juliano et al. (2011) used ultrasound to move fat particles and generate coalescence; these larger particles have low density floating on the milk surface. When whey was subjected to ultrasounds treatment, some evidence of denaturation was indicated by Villamiel and de Jong (2000) at temperatures higher than 60°C. However, when lower temperatures (<25°C) were employed, no changes in protein profiles were observed (Bermúdez-Aguirre & Barbosa-Cánovas, 2008; Muthukumaran et al., 2005). Seed proteins are typically of lower quality due to poor digestibility caused by exogenous and endogenous factors, such as enzyme inhibitors or anti-nutrients. In this sense, ultrasound treatment increased the hydrolysis degree of sorghum kafirin and improved its antioxidant properties. The explanation was due to phenols being more easily degraded in comparison with traditional heating treatment, leading to protein aggregates with lower digestibility (Sullivan, Pangloli, & Dia, 2017). In meat products high-power ultrasound has a great impact on tenderization due to proteolytic degradation of sarcoplasmic and myofibrillar fractions and the connective tissue. During the process, the proteins are intensely degraded weakening the muscle fibers and causing tenderization (Jayasooriya, Bhandari, Torley, & D'Arcy, 2004). In addition, the high-power ultrasounds can disrupt membranes and release specific enzymes such as cathepsins. The cathepsins activate the calpains altering calcium concentration affecting meat tenderization. In the same vein, disrupting the walls and membranes of cells by sonicating would increase the calcium concentration from sarcoplasmic reticulum, achieving more meat tenderness (Roncales, Ceña, Beltran, & Jaime, 1993). Ultrasound has also been described in applications to modify the

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diffusivity of sodium chloride and water in dry-cured products, improving textural properties during meat brining. During the salting stage of meat products the water moves through meat to brine and the salt penetrates into the meat. Ultrasound accelerates the brining process changing the mass transfer driving forces between meat and brine (Alarcon-Rojo et al., 2015; Ozuna, Puig, García-Pérez, Mulet, & Cárcel, 2013).

5.3.2 High pressure A high-pressure process (HPP) is an alternative to a thermal process that often produces changes in flavor, color, and protein functionality. HPP is used as a sterilization technique, increasing the shelf life of foods with high value (Fig. 5.3). HPP is considered as a cold pasteurization technique by which products are sealed in a flexible container or plastic bag and introduced into a vessel and subjected to high levels of isostatic pressure transmitted by water. Generally it is a process in which the food matrix is under a high pressure (100–1000 MPa) at room temperature. Foods and beverages can be sterilized by HPP as an alternative to thermal-conventional methods. Combining high temperatures in short times with pressures can eliminate pathogens and spores in fluids. However, temperature can be modified through the process to influence protein denaturation. The unfolding of proteins, which affects the gelation of the final product, can be modulated by process parameters like pressure level, temperature, and cycles, or even food-intrinsic parameters. HPP can affect secondary, tertiary, and quaternary structure of proteins leading to denaturation as well as inactive microorganisms, and avoids the Maillard reaction (Korhonen et al., 1998). An important advantage of HPP is reducing the Maillard reaction rate of furosine, carboxymethyl-l-lysine, and carboxyethyl-l-lysine. Avila Ruiz et al. (2016) concluded that 700 MPa at high temperature reduced Maillard products and undesirable browning in whey protein-sugar solutions. The effect of HPP on protein structure has been exposed extensively by Balasubramaniam, Barbosa-Cánovas, and Leieveld (2016). The native protein structure can be disturbed by factors like pH,

Fig. 5.3  Positive aspects related to the renewed interest of use of HPP in the food industry.

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temperature, or pressure. The sequence of amino acids joined by peptide bonds is not mainly affected by high pressure, but the hydrogen bonds could be shortened or even ruptured. HPP mainly influences on tertiary and quaternary structure causing denaturation by changes in hydrophobic and ionic interactions (Balasubramaniam et al., 2016). Protein solubility depends on conformational changes and new establishment of interactions between proteins, leading to aggregation. Functional properties such as solubility, gelation, emulsification, foaming, and coagulation can change with pressure and it has either positive or negative effects on organoleptic and nutritional quality of food. The influence of HPP on protein functionality has been discussed by Messens, Van Camp, and Huyghebaert (1997). Indeed, HPP produces protein unfolding and afterward aggregation with different proteins in the food. Both phenomena have consequences on the textural properties of foods. Effectively, small compounds related to aroma molecules and vitamins are not mainly affected by HPP. Conversely, proteins and starch can change their structure (Barba, Terefe, Buckow, Knorr, & Orlien, 2015). There are only a few reports on the effects on nutritional features of HPP-treated foods. Yin, Tang, Wen, Yang, and Li (2008) studied the effect of HPP on in vitro digestibility of bean protein isolate. These authors assessed that protein isolate digestibility could change at different pressures. In fact they confirmed that bean protein isolate after a high pressure treatment >200 MPa had a negative effect on digestibility due to protein aggregation and also induced aggregation decreasing protein solubility. On the contrary, reducing the effect of anti-nutritional factors through HPP can increase the protein digestibility. These constrains are important issues in legumes, peas, and beans. However, HPP was not enough to complete the cooking of legumes (Linsberger-Martin, Weiglhofer, Thi Phuong, & Berghofer, 2013). Thereby Butz et al. (2002) studied the effect of HPP on the functional properties of carrots, tomatoes, and broccoli. In the majority of cases HPP did not produce loss of healthy compounds and only changes in the vegetable matrices were induced, altering physicochemical properties, related to water-holding capacity and extractability. As noted, one of the challenges for the food industry is to remove or minimize the allergen effects of foods. Accordingly the digestibility of β-lactoglobulin, the major food allergen in bovine milk, was studied. High-pressure treatment (600 and 800 MPa) increased significantly in  vitro pepsin digestion of β-lactoglobulin under simulated gastric conditions, enhancing its nutritional value (Zeece, Huppertz, & Kelly, 2008). Regarding milk quality, it has been reported that HPP treatment has affected casein micelles and whey proteins. The whey proteins (α-lactalbumin and β-lactoglobulin) are denatured to different conditions and the micelles change physicochemical features such as lightness and turbidity. Depending on pressure, temperature, time, and pH, the casein micelles and whey proteins can be affected (Huppertz, Fox, & Kelly, 2004). HPP is increasingly becoming an important new tool in the meat industry to extend the shelf life and improve food safety. Most of meat proteins are affected by high pressure, which has an important effect on meat quality. Chapleau, Mangavel, Compoint, and de Lamballerie-Anton (2003) have reported that HPP (from 50 to 600 MPa) modified the secondary structure of myofibrillar protein with significant increase of the level of aggregation when pressure was >300 MPa. In agreement with this study,

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Kaur et al. (2016) have showed that both myofibrillar proteins and in vitro digestion were affected by HPP. The structure of myofibrils was changed by denaturation, extraction, and solubilization of proteins and it can be observed through microscopy after treatment at 600 MPa. HPP caused a change of visual appearance and texture similar that caused by the heating process, but HPP improved digestibility. In addition, sarcoplasmic proteins of muscle significantly modified their solubility influencing the processed meat quality. Furthermore the effects on sarcoplasmic proteins are correlated with quality parameters such as solubility, color, and water-holding capacity (Marcos & Mullen, 2014). For the food industry, the reduction of salt levels is a major strategic goal (WHO, 2011), because consumption of excessive amounts of salt may be harmful to health mainly related to hypertension. Thus the meat industry is involved in this objective and HPP could be an alternative technology to reduce sodium chloride and polyphosphates in processed meat products. Polyphosphates combined with sodium chloride have been used to improve binding properties, although this interaction is not fully understood. High pressure is used as a non-thermal preservation technique with low/reduced sodium contents. Technical modifications of high pressure lead to changes in functional properties of proteins due to denaturation with consequences on water-holding capacity becoming more soluble and emulsifying (Rodrigues, Rosenthal, & Tiburski, 2016; Villamonte, Simonin, Duranton, Chéret, & De Lamballerie, 2013). As a concomitant effect HPP helps to control risks associated with Salmonella and Listeria monocytogenes in raw and marinated meats (Hugas, Garriga, & Monfort, 2002).

5.3.3 Pulsed electric field Pulsed electric field (PEF) is a non-thermal method that consists of electrical pulses of short time with high voltage. PEF treatment can kill or inactivate microorganisms with small changes in sensorial and nutritional properties of fresh products. The main effect is based on the damage of the cell plasma membrane, resulting in increased permeability of the membrane and electrical conductivity of the cellular material. Thus electroporation is the resulting damage in the cell membrane, both reversible and irreversible, which increases its permeability. Depending on the PEF conditions, the electroporation (Fig. 5.4) can cause the death or survival of the microorganisms. If the PEF Cell lysis

Electroporation

Microorganisms inactivation

Sterilization

Fig. 5.4  Main effects of PEF on microorganisms and proteins.

Protein unfolding

Damage of enzymes

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intensity is too high, the bacteria will not be capable of repairing itself and it will start releasing small molecules or even cellular lysis (Amiali, Ngadi, Smith, & Raghavan, 2006). For this reason, there has been an increasing interest in using PEF with other technologies to extract different intracellular components in high selectivity, such as proteins (Barba et  al., 2015). Indeed, electroextraction of cytoplasmic proteins was carried out from microalgae with less damage to protein stability compared with standard methods. In addition the protein was slightly fragmented allowing its removal by centrifugation or filtration (Coustets et al., 2015). Commercial applications of PEF to inactivate microorganisms and enzymes in aqueous solutions include water purification, fruit juices pasteurization, and milk processing. In addition, previous reports have demonstrated that small and volatile compounds are slightly affected by electric field and thus keep their natural aroma (Chen et al., 2015). As these types of liquid foods contain heat-labile and volatile components, it would be interesting to use PEF combined with low temperatures to kill all microbes without impairing sensory quality. The efficiency of inactivation depends on strength of electric fields, pulse duration, temperature, pH, ionic strength, and the conductivity of the medium (Saulis, 2010). As noted previously, proteins play an important role in quality of food. In this sense, there are few papers about the effects of PEF on food proteins. Under the action of a strong PEF, some chemical groups in the polypeptide chain could be ionized, disturbing electrostatic interactions between proteins and changing protein structure, affecting biological activity (Zhao, Tang, Lu, Chen, & Li, 2013). PEF mainly affects secondary structure, causing inactivation of enzymes and influencing protein digestibility, but less so than a thermal pasteurization does. It is worth highlighting that changes at the molecular level can lead to enhanced protein digestibility. Indeed, Vanga et al. (2015) reported that PEF treatment of peanut protein provoked changed in the α-helix and β-sheets secondary structure, increasing the amount of random coils and aggregated strands. This effect of PEF on secondary structure enhanced its in vitro digestibility and even its allergenicity. It has also been reported that PEF cause a pH decrease or even oxidation of amino acid residues, which produced the inactivation of native enzymes (Buckow, Ng, & Toepfl, 2013). During the pulses, temperature can increase in small, localized areas allowing an increased movement of charged groups. The combined effects of heat and PEF act on protein electrostatic interactions and their stability. In tomatoes, pectin methyl-esterase has been studied under different conditions of PEF and temperature (Samaranayake & Sastry, 2016). Vagadia, Vanga, Singh, and Raghavan (2016) also studied the effect of temperature and PEF on anti-nutritional factors of soybean using a molecular modeling study. Rearrangements of trypsin inhibitor occur especially in turns and coils with temperatures and oscillating electric field. Studies have also been carried out in bovine milk under the influence of PEF as alternative treatment to thermal pasteurization. Singh, Orsat, and Raghavan (2013) indicated that PEF altered the casein micelle causing a reduction in the micelle size, and this change was due to unfolding or orientation of proteins in the direction of the applied electric field, although this change could be reversible. Other important proteins present in milk are whey proteins with a high nutritional value. In this sense,

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Barsotti, Dumay, Mu, Diaz, and Cheftel (2001) observed that PEF caused a slight unfolding and aggregation of β-lactoglobulin and ovalbumin, which are heat-sensitive proteins. In contrast, when milk was exposed to long-duration pulses or high-intensity electric fields of 45–55 kV/cm (Floury et al., 2005), the milk protein structure was apparently modified. Sharma, Oey, and Everett (2014) confirmed that thermal treatment caused more denaturation in whey proteins than PEF processing. In addition, they comported that whey protein’s stability depends on pH that changed with the process. Published studies about egg and milk proteins indicated that PEF processing did not induce denaturation of egg-white protein or permanent conformation modifications of ovalbumin (Fernandez-Diaz, Barsotti, Dumay, & Cheftel, 2000; Jeantet, Baron, Nau, Roignant, & Brule, 1999). On the contrary, Perez and Pilosof (2004) showed that the structures of egg-white protein and ß-lactoglobulin were partially modified when subjected to long pulses with high-strength PEF. The meat industry is currently searching new technologies to enhance quality of meat, including its color, tenderness, and juiciness. PEF can induce structural changes in muscle fibers due to electroporation and chemical and biochemical reactions, depending on meat muscle fibers’ orientation, electric field strength, and pulse duration (Toepfl & Heinz, 2007). Indeed, as PEF is a directional phenomenon, when the current is perpendicular to the muscle fibers, the electroporation is greater than in parallel, causing fragmentation of the myofibrils when they are observed in transmission electron microscopy (O'Dowd, Arimi, Noci, Cronin, & Lyng, 2013). These facts will facilitate meat tenderization due to fragmentation of protein myofibrils, such as myosin heavy chain, troponin T, and desmin, which are considered good candidates as biomarkers for proteolysis and meat tenderization (Suwandy, Carne, van de Ven, Bekhit, & Hopkins, 2015). In addition, and as a concomitant effect in meat products, electroporation increases cellular permeability releasing calcium ions that activate μ-calpain, improving proteolytic enzyme activity.

5.3.4 Enzymatic reaction The enzymatic process has been used in food applications such as baked goods, dairy, brewing, and meat processing for centuries. The most frequently used enzymes are hydrolases (lipases, amylases, proteases, and hemicellulases) and oxidoreductases (­lipoxygenases and glucooxidases), which are extracted from plants and animal tissues (Table 5.1). Despite the fact that the use of enzymes is increasing, the synergistic effect on complex foods is often quite intricate and variable in function of specific conditions during the process (pH, temperature, and moisture). Genetically modified microorganisms have served as sources of many enzymes related to metabolic catalyzing. It should be mentioned that safety evaluation of enzymatic preparations is essential for consumers even when the enzyme is not part of the final product. Proteases, also known as peptidases or proteolytic enzymes, break polypeptide chains in different localizations (aminopeptidase, carboxypeptidase, chymotrypsin, papain, pepsin, trypsin). Proteases can be divided into two groups, exoproteases and endoproteases, depending on where protein breakdown is produced (Pandey & Teixeira, 2016). In addition, the combination of novel technologies is possible; indeed

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Table 5.1  Main enzymes and enzymatic activities used in the food industry. Lipases

Catalyzes the hydrolysis of lipids to give rise to diglycerides, monoglycerides, or glycerol

Bakery, oil industry, dairy, fish, meat, and beverages Amylases

Catalyzes the hydrolysis of starch into sugars like glucose, fructose, maltodextrin derivatives, starch hydrolysates, and cyclodextrin

Bakery and beverages Proteases

Hydrolyzes the peptide bond classified as endopeptidases or endopeptidases

Bakery and dairy Hemicellulases

Catalyzes the hydrolysis of hemicellulose into polysaccharides and oligosaccharides

Bakery and beverages Oxidoreductase

Catalyzes the exchange of electrons between donor and acceptor molecules

bakery, dairy, and beverage

ultrasound-assisted enzymatic treatment could reduce the effect of allergenic proteins through different mechanisms in peanuts (Li, Yu, Ahmedna, & Goktepe, 2013). In the baking industry, proteases reduce flour protein strength increasing the extensibility and softness of the dough (Ahmed et al., 2015). In addition, proteases have a great potential to remove gluten from food, hence a combination of sourdough lactic acid bacteria fermentation and fungal proteases was used to make gluten-free pasta (Curiel et al., 2014). One of the most important meat quality parameters is tenderness, which is caused by degradation of structural proteins. This fragmentation is governed by endogenous enzymes (endoproteases), hence the complex μ-calpain-calpastatin is one of the major enzymatic complexes that contributes to the degradation of muscle proteins (Marqués, Maróstica, & Pastore, 2010). Other factors such as sarcomere shortening during rigor mortis lead to decreased meat tenderization in low temperatures during the post-­ mortem period. Low temperature is key for minimizing product contamination, inhibiting or delaying the pathogenic microorganism’s growth, but has a negative impact on the tenderization process. Nowadays, exogenous proteases from plants, microbes, and animals are used to improve tenderness (Bekhit, Hopkins, Geesink, Bekhit, & Franks, 2014). The most widely used proteases are papain, bromelain, and ficin, characterized by poor selectivity. The main disadvantages of the exogenous proteases are associated with poor sensorial characteristics, mushy textures, bitterness, or even unpleasant flavors. However, commercial preparations such as Flavourzyme™ have been developed to decrease bitterness in the final product (Benjakul, Binsan, Visessanguan, Osako, & Tanaka, 2009; Nilsang, Lertsiri, Suphantharika, & Assavanig, 2005).

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Meat tenderness is also strongly influenced by the connective tissue integrity, which has a high content of collagen. Zhao et al. (2012) demonstrated that MCP-01, an extracellular protease, showed a strong selectivity for degrading collagen at low temperatures. Furthermore the bacterial protease G was very specific toward myofibrillar and collagen protein degradation (Ha, Bekhit, Carne, & Hopkins, 2013). Enzymatic processing is employed in by-products from the fish industry. Fishery processing generates a large volume of fish waste, which could be used as fish protein hydrolysate and other food products, as a source of proteins and amino acids for humans and animals. The enzymatic process of protein hydrolysates includes vegetal enzymes (papain, bromelain, and keratinases) and animal (trypsin, chymotrypsin, pepsin, and rennin) or microbial proteases. The main drawback of enzymatic hydrolysis is the elimination of tryptophan and cysteine and partial elimination of tyrosine, serine, and threonine (Ghaly, Ramakrishnan, Brooks, Budge, & Dave, 2013). Regarding the dairy industry, whey protein products as by-products of other dairy foods (yogurt, buttermilk, cheeses, cream, etc.) have a high nutritional value and special functional properties. Whey protein is used as a dietary supplement, even in infant formulas, and other applications (Høst et al., 1999). Main applications of proteases in the dairy industry are focused on improving hydrolysis of milk proteins to increase their digestibility and to reduce the allergic properties of casein (Patel, Singhania, & Pandey, 2016). One of the methods used to assess the degree of protein hydrolysis is measuring the level of free amino terminals by mid-infrared spectroscopy (Poulsen et al., 2016). Nowadays allergens are a priority for the dairy industry; the most effective method to reduce the amount of allergens of cow’s milk is filtration, although it is a very expensive process. Other cost-effective alternatives to decrease antigenicity is the proteolysis of allergenic components as Svenning, Brynhildsvold, Molland, Langsrud, and Vegarud (2000) reported. Furthermore enzymatic hydrolysis leads to enhance functional properties (solubility, aggregation, or emulsifying properties) for specific applications (De Castro, Bagagli, & Sato, 2015). The proteolytic activity of enzymes is of great importance in milk-based products and it should be carefully monitored and controlled for improving flavor, texture, and nutritional value. Indeed, the rennet of the cheese contains chymosin and pepsin, which act on protein coagulation producing their typical characteristics (Barbé et al., 2014).

5.3.5 Ohmic heating Ohmic heating (OH), also called Joule heating is an innovative thermal processing technique. This method is defined as a process where heat is internally generated due to food electrical resistance, when electrical current (usually alternating) is passed through food that contains enough water and electrolytes (Halden, De Alwis, & Fryer, 1990; Vicente, de Castro, & Teixeira, 2006). The heating rate is mainly proportional to the electric field strength, electrical conductivity, and the temperature of food being heated. In OH, the electrodes are in contact with food, which acts as a resistance; hence the heating occurs in the form of internal energy transformation from electric to thermal in a very short time (Samaranayake, Sastry, & Zhang, 2005). OH is a high-temperature, short-time method and this fact allows for uniform heating,

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i­ncreasing the final product’s quality, especially for thermal-sensitive products like vitamins (Vicente et al., 2006). However, OH is a technology with a large number of applications, including blanching, dehydration, fermentation, extraction, pasteurization, sterilization, and food heating. Inactivation of microorganisms and/or enzymes with an increased extraction rate has been improved employing the OH technique. Indeed, the most common application is pasteurization or sterilization of liquid and particulate foods, especially ready-to-eat meals, fruit, vegetables, poultry, fish, and meats. There are many factors that affect OH, which depends on foodstuff (electrical conductivity, particle size, shape, ionic concentration, etc.) and process (temperature, electric field strength, frequency of the electric current, time, etc.) (Fig. 5.5). Among them, electrical conductivity and its temperature influence are the most significant parameters in order to evaluate the food heating rate (Ruan et al., 2002), since electrical conductivity of liquid or particulate foods increases linearly with the temperature (Wang & Sastry, 1997). Regarding the issues discussed in this chapter, there are few reports on the relationship between OH and changes in properties of protein. In addition, the majority of these studies did not directly address the protein nutritional quality; they are focused on physical modification that occurs during OH. However, it could be inferred that the loss of protein quality is likely due to protein structural changes and protein aggregation. In addition, it has been widely reported that a traditional thermal process with slow heating can activate endogenous proteases capable of degrading myofibrillar proteins before the protease becomes degraded by temperature. This situation is different with OH as it could be verified in studies on gel-functionality properties from surimi, where OH treatment can rapidly inactivate proteases avoiding enzymatic degradation of myofibrillar proteins (Yongsawatdigul, Park, & Kolbe, 1997; Yongsawatdigul, Park, Kolbe, Dagga, & Morrissey, 1995). These studies confirmed that OH increased the surimi viscosity compared to traditional heating procedures. This was due to the degradation of myosin heavy chain and actin was minimized by ohmic treatment, increasing the continuous network structure of the gels. Recently Mesías, Wagner, George, and Morales (2016) studied the effect of OH on the amino acid profile of vegetable baby foods. They reported that total protein

Fig. 5.5  Schematic diagram of ohmic heating and relevant parameters of the process.

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content and amino acids were not affected by OH, with the exception of arginine. In addition, OH maintains a similar content of essential and non-essential amino acids. These findings suggest that sterilization with ohmic technique versus sterilization under conventional treatment could be an adequate procedure to maintain the total amino acid profile in these types of products. In the dairy industry several parameters related to protein quality and thermal process indicators, such as FAST index (fluorescence of advanced Maillard products and soluble tryptophan), carboxymetylisine, protein denaturation and aggregation (betalactoglobulin), and glycoxidaction products among others, have been used to determine the influence of OH. Indeed, Sun et al. (2008) studied the effect of OH versus conventional processes on milk protein denaturation using the FAST index at different temperatures inside a range of 40°C to 80°C for 60 min. In this study there was no significant differences between the FAST index for both process, showing that there was no additional thermal influence on protein denaturation. The effect of OH in infant milks using FAST index, color variations, and other parameters has been analyzed at the laboratory (Roux, Courel, Ait-Ameur, Birlouez-Aragon, & Pain, 2009) and pilot scales (Roux et al., 2016). These studies confirmed that few differences were found between treatments, showing that OH could be a promising technology for the conservation of some nutrients in dairy foods. Pereira et al. (2016) realized a characterization of the initial steps of protein aggregation/denaturation, measuring the percentage of free thiol group and loss in solubility in whey protein isolates after OH treatment and conventional procedures. They reported that OH produced less aggregation and increased protein solubility in the first stages, providing major protein retention and a better quality product. Finally, as we described in other sections, ohmic technology has a great impact on the allergenic issue. In this sense milk and whey have great allergenic potential, since casein, beta-lactoglobulin, and alpha-lactoalbumin are the most common allergens in bovine milk (Miciński et al., 2013). Indeed, in milk products heat application could promote allergenic reduction due to protein denaturation by loss of the secondary or tertiary structure and aggregation phenomenon (Shandilya, Kapila, Haq, Kapila, & Kansal, 2013). On the contrary, heat may increase allergenicity due to the formation of neoallergenic compounds formed during the Maillard reaction (Jaeger et al., 2016). According to Cappato et  al. (2017), there is no literature measuring the allergenic potential of the milk proteins treated by OH. As electricity can modify casein micelles and protein conformational structures, different results with respect to conventional thermal techniques could be achieved (Jaeger et al., 2016). For this reason the assessment of ohmic effect on the allergenicity decrease from dairy food could represent one of the greatest challenges in the application of this innovative technology in milk and dairy products.

5.3.6 Irradiation Radiation processing of food is widely used for food preservation without altering textural, sensory, and nutritional properties. Irradiation is a physical process in which food undergoes a dose of ionizing and/or non-ionizing radiation to eliminate

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­ icroorganisms, bacteria, viruses, parasites, or even insects that might be present in m the food (Farkas & Mohácsi-Farkas, 2011). For this reason, it can damage the DNA of living cells such as microorganisms or insects without the need for chemicals (Kuan, Bhat, Patras, & Karim, 2013). This radiation is ionizing when it has enough energy to produce ions in the food matrix and it comes in contact with high-frequency radiation. Indeed, food irradiation types developed by the food industry are ionizing (gamma rays, X-rays, electron beams) and non-ionizing radiation (visible light, infrared light, and microwaves). The latter are not able to produce ions, but can produce heat under moist conditions; hence they are used for industrial purposes such as cooking or reheating food (Ortega-Rivas, 2012). Ionizing radiations are produced from radioisotopes 60Co and 137Cs (gamma rays), high-energy electrons (electron beams), or X-ray machines. The most common ionizing radiation is gamma rays from 60Co, which is measured in Grays (Gy). Irradiation facilities are increasingly developed in food processing together with operational considerations (Ehlermann, 2016; Mittendorfer, 2016). Nowadays, irradiation is employed in 40 different countries and on >60 different foods. The volume of the irradiated foodstuffs is increasing annually in the world (Arvanitoyannis, 2010). The main effects of irradiation include: sterilization and control of pests, shelf-life improvement by modification of ripening delay and sprout inhibition of fruits and vegetables, and prevention of foodborne disease by the inactivation of pathogens. In addition, irradiation techniques have showed improvement to the rehydration process as well as reduction of anti-nutritional components of plants (Bhat, Ameran, Voon, Karim, & Tze, 2011). However, indirect effects of irradiation, such as lipid oxidation, vitamin destruction, and protein denaturation, limit its application in food processing (Al-Kahtani et al., 1998; Rahman, 2007). This is a controversial issue; according to the Food and Drug Administration (FDA), irradiation has effects on food nutritional value that is similar to those of conventional food-processing techniques. In this regard, lipids, carbohydrates, proteins, minerals, and most vitamins remain almost unaffected by irradiation (Stewart, 2001). On the contrary, high-intensity irradiation may cause the degradation of some micronutrients, such as vitamins A, B1, C, and E (Smith & Pillai, 2004) and protein (Bhattacharjee & Singhal, 2010). The intensity of the effect for each type of radiation is correlated with accumulated radiation dose and its penetrability, changing the biological effect (Jaczynski & Park, 2003, 2004). Experts reviewed the data and intervals and thus concluded that food irradiation has no risk. An international committee reported that a safety interval between 1 and 10 kGy for irradiated food presents no toxicological hazard (JECFI, 1981). Safety issues are being reviewed by international agencies in order to establish a safe process for irradiated food (EFSA, 2011; JECFI, 1981; WHO, 1994). Concerning proteins, it should be noted that radiation of proteins has been studied for >30 years (Houée-Levin & Sicard-Roselli, 2001). Irradiated food absorbs energy, which also breaks down the chemical bonds producing free radicals. Free radicals are unstable molecules that are highly reactive, and consequently new bonds are formed almost instantly. In a radiation process, proteins are affected by direct and indirect effects of ionizing, which result in conformational and structural changes (Kuan et al., 2013). The most commonly occurring changes of proteins are polymerization and

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f­ ragmentation because of the presence of free radicals. Another chemical reaction that occurs is the protein oxidation that occurs by the action of OH free radicals. The oxidative reactions occurring in muscle foods can result in the generation of carbonyls (aldheydes and ketones), protein polymers, and peptide scissions via deamination. Protein oxidation can occur by strong interaction between a protein and an oxidized lipid, and the oxidation can be easily transferred from lipids to proteins. Primary and secondary lipid oxidation products such as aldheydes and ketones can react with proteins, inducing protein oxidation. Different chemical reactions such as deamination, decarboxylation, reduction of disulphide linkages, oxidation of sulfhydryl groups, modification of amino acid moieties, peptide-chain cleavage, and aggregation can produce permanent changes (Kuan et al., 2013). The result of these chemical reactions can vary greatly depending on protein state (e.g., fibrous or globular, native or denatured, protein composition), the presence of other molecules (presence of oxidizable lipids, heme pigments, transition metal ions, water concentration and oxidative enzymes), and the irradiation conditions (e.g., dose, dose rate, temperature, and oxygen concentration) (Audette-Stuart, Houée-Levin, & Potier, 2005; Houée-Levin & Sicard-Roselli, 2001). The splitting and aggregation of proteins due to irradiation phenomenon are related to alterations of the secondary and tertiary protein structures. The protein damages in muscle food might lead to changes in protein functionality (e.g., gel-forming ability, meat-binding ability, emulsifying capacity, viscosity, solubility, and water-holding capacity with a significant impact on food quality). However, irradiation of low and medium energy has no significant effect on protein (Kuan et al., 2013). The study of the irradiation application on protein nutritional changes is rather scarce. Jaczynski and Park (2004) investigated the effects of electron-beam irradiation on surimi seafood, showing that degradation of myosin heavy chain was dose-­ dependent (6–8 kGy), while the actin integrity was only slight affected. Alternatively, Xiao, Zhang, Lee, Ma, & Ahn, 2011 showed that irradiation at 3 KGy significantly increased protein oxidation in chicken thighs during refrigerated storage of 7 days. These authors suggested that irradiation could produce hydroxyls radicals by splitting water molecules that react with peptides or amino acids prone to irradiation (e.g., cysteine, methionine, tyrosine, phenylalanine, histidine, tryptophan, and lysine). In addition, they postulated that irradiation can break the protein structures by splitting hydrogen and sulfur bonds, because secondary and tertiary protein structures can be unfolded due to SS bond reduction or oxidation of SH group. Food processing with ionizing irradiation can modify food antigenicity, reducing allergens by two different ways: firstly, by interaction with target protein or secondly, by formation of major products from water radiolysis (Kempner, 2001). A study conducted by Seo et al. (2007) about the effect of gamma irradiation (0–10 kGy) on ovalbumin confirmed that irradiation could be useful to inhibit and reduce the food allergy produced by hen’s egg albumin. They demonstrated that a band of SDS-PAGE associated with egg albumin disappears after a treatment either gamma and electronbeam irradiation. This result agrees with the findings of other studies realized on lectins, generally recognized as an important food anti-nutrient, in which high doses of gamma radiation suppress allergic effect induced by them (Vaz et al., 2013). These authors confirmed the absence of structural integrity in irradiated antigens at high

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doses (25 kGy). At this level, lectins do not maintain tertiary or secondary structure, indicating the presences of precipitation and insoluble aggregates, without trace of native structure. On the contrary, irradiation at low dose (1 kGy) does not mitigate the allergenic response; it can even be increased and cause severe allergenic inflammatory response, as the same research group demonstrated (Vaz et al., 2013). Recently, Sujatha, Hymavathi, Uma-Devi, Roberts, and Kumar (2017) reported that irradiation significantly improved in vitro protein digestibility of selected millet grains and the effect was more pronounced in dehulled grain (2.58%) than in whole grain (2.13%). Regarding UV radiation, it has been reported that UV radiation induces cross-linking in food proteins under certain conditions. Indeed, Kato, Uchida, and Kawakishi (1992) reported collagen degradation and fragmentation caused by UV irradiation, using its model peptides. Authors explained these results due to proline oxidation for fragmentation, whereas degradation was caused because collagen is a proline-rich protein. Even UV radiation has been used to produce rheological changes in order to improve some of the quality features of fish and meat gelatin. Indeed, Ishizaki, Hamada, Iso, and Taguchi (1993) studied the effect of UV in pork and sardine pastes, improving gel strength. The modifications were attributed to actomyosin denaturation in the samples, because UV irradiation produced a significant increase of the surface hydrophobicity and decrease in the total SH in samples. Ishizaki, Hamada, Tanaka, & Taguchi, 1993, in a posterior study, indicated that the myosin solubility decreased when irradiation time and intensity increased (Ishizaki, Ogasawara, Tanaka, & Taguchi, 1994).

5.4 Conclusions and future remarks This chapter describes how proteins are affected by food processing. Food quality is a global challenge in order to achieve food safety and a healthy and balanced life for our society. Dietary protein, probably more than any other food component, is indispensable to human nutrition. Therefore it should be necessary to emphasize the need to control food processing techniques in order to maintain nutrition and improve the texture and sensory qualities of food through protein composition and its structural organization. Traditional industrial-thermal processes have negative impacts on food proteins, decreasing their nutritional value. Novel and emerging technologies (ultrasound, high-pressure processing, pulsed electric field, ohmic heating, and irradiation) could improve food safety and sensorial qualities as well as protein quality. So far, however, there has been little research about the effects of these emerging technologies on nutritional value of proteins. Therefore still more researches are need to completely understand all effects produced by emerging technologies. It is expected that in the future more studies on bioaccessibility and bioavailability of food proteins will be conducted to assess these processes.

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