Effects of Processing on the Nutritional Value of Proteins

Proteins as food materials are exposed to physical or chemical treatments during blending and application. These treatments affect the functional and nutritional properties of proteins. Besides, purposive modifications have been applied to improve the existing properties of protein or create new functions.

8.3.1. Heat Treatment

Heat treatment can cause protein structure change, peptide bond hydrolysis, amino acid side chain modification and condensation with other molecules. The later two reactions are harmful to the nutritive value of proteins. Structural changes and limited peptide bond hydrolysis caused by mild heat treatment do not affect the nutritive quality of proteins, but

significantly affect the functional properties of proteins. The degree and result (conformational change and aggregation) of thermal denaturation depend largely on the nature of proteins and environmental conditions. Mammalian collagens start to unfold, dissociate, and dissolve when heated to 65°C in the presence of large quantity of water. In contrast, actins undergo contraction, aggregation and water holding capacity reduction under same conditions.

Caseinate monomers are very resistant to heat treatment. Vigorous heat treatment dose not affect the properties of caseinate. Mild heat treatment dose not lead to the destruction or generation of covalent bonds or serious changes of higher structures in proteins. Hence, from the nutritional point of view, the changes induced by mild heat treatment are beneficial.

Heat treatment can eliminate the toxic effects of lectins in legumes, because the lectin is a thermal unstable protein which can bind polysaccharides. It can reduce the nutritional value of natural plant proteins in the diet. The presence of lectins can form compounds by combining intestinal brush border membrane polysaccharide, which can diminish the abilities of shifting and digestion in amino acid, produce toxicity to human and animal; heat treatment can eliminate their harmful effects.

Protease inhibitors in foods, including trypsin inhibitors and chymotrypsin inhibitors, are inactivated during heat treatment. High temperature treatments, such as high-pressure sterilization, puffing, sterilization, cooking and baking, can destroy all antinutrients factors (Figure 5-14). Therefore, moderate heat treatment can markedly improve the nutritional value of vegetable proteins.

Many proteins, such as soybean globulin, collagen and egg albumin, are more digestible after moderate heat treatment.

Figure 5-14. Effect of steaming on trypsin inhibitor activity and protein efficiency ratio of soybean meal feed [20]

After heat treatment, the proteins are more susceptible to enzymatic hydrolyze due to unfolding and amino acid residues exposure. Besides, heat treatment can remove off-odor compounds in soybean protein and avoid the undesired flavor caused by lipid oxidase.

8.3.2. Dehydration

When the water in a protein solution is removed, aggregation occurs to the proteins. This is the case especially when water is removed by evaporation at high temperatures, because the

solubility and surface activity decline sharply. Hence, the effects of drying on the functional properties of protein should not be ignored. Drying changes the size and internal and surface porosity of particles and consequently alters the wettability, absorption, diapersion and dissolution of food proteins. When water is quickly removed as steam, particles shrink in the minimum level and salts and carbohydrates migrate to the dried surface, yielding porous powder particles. This change can be observed in freeze-drying and spray-drying.

Introduction of bubbles to protein solution before drying or the implementation of controlled aggregation during drying can be applied to increase the porosity of particles.

8.3.3. Irradiation

Irritation causes inter- and intra-molecular cross-linking of proteins or peptide bond breakdown in foods with low moisture content. In the presence of catalase, Tyr is oxidized and condensed to dityrosine.

8.3.4. Oxidation

Oxidants are important additives in the food industry. For example, hydrogen peroxide is sterilization and bleaching agent which can be used as a low-temperature fungicide in package. It can also been used to improve the color of fish protein concentrates, flours and oil seed protein isolates. Benzoyl peroxide can oxidize cysteine in flour to cystine so as to enhance gluten strength and bleach flour. In some cases, benzoyl peroxide can also be used in whey powder bleaching. Peroxides are also found in foods without artificially added oxidants and these peroxides are often the reason for the degradation of proteins present in the foods.

Light oxidation, irradiation, trace metals, oxygen, hot air drying and aeration during fermentation can all induce oxidative denaturation of amino acids in proteins.

Trp is the most sensitive amino acids to oxidation, followed by tyrosine and histidine in sequence. The oxidation of containing amino acids occurs mainly in the sulfur-containing side chains and that of aromatic-sulfur-containing amino acids mainly occurs in the side chains containing aromatic rings.

8.3.5. Maillard Reaction

Foods containing proteins and reducing carbohydrates or carbonyl compounds (such as aldehydes and ketones produced by lipid oxidation) are subject to Maillard reactions during processing and storage. These reactions yield brown or black melanoidins, which impart breads and bakery products with the brown color. The formation of melanoidins is accompanied by the cross-linking of a small amount of proteins. The cross-linking significantly reduces the digestibility of these proteins. Studies on some protein-carbohydrate model systems reveal that melanoidins are mutagenic and the capacity depends on the degree of Maillard reactions. Melanoidins are insoluble in water and are only weakly absorbed on intestinal wall. Hence, melanoidins are only slightly physiologically toxic to human.

However, the low molecular weight precursors of melanoidin are absorbed easily.

8.3.6. Enzyme Treatment

Enzymatic hydrolysis can improve the quality of food proteins. For example, papain can tenderize meat and chymotrypsin can coagulate casein. Papain, bromelain, pepsin and protease produced by microorganisms can partially hydrolyze thermal denatured or solvent denatured proteins and improve their solubility. The hydrolytes can be used in acidic or

hot-processed drinks. The hydrolytes can also be used as alternative protein sources for patients allergic to milk and gluten or those with digestive tract diseases.

Partial hydrolysis can improve the emulsifying and foaming abilities of thermally denatured proteins, because the increased solubility of proteins facilitates their diffusion to the oil/water and gas/water interfaces. However, when the degree of hydrolysis exceeds 3%~5%, the viscosity and thickness of protein layer absorbed on the interface are insufficient to stabilize emulsion and foam. Hydrolysis reduces the volume of proteins, which is unfavorable for the gelling ability and viscoelasticity of proteins, but limited hydrolysis of gluten can enhance the expansion degree of dough in bread making and improve the flakiness of biscuits.

In addition, proteases can connect low molecular weight peptides through peptide transferring to produce new larger peptides, namely proteinoids.

8.3.7. Functional Peptides from Protein Hydrolysates

Proteins often undergo hydrolysis during processing. Hydrolysis not only improves the nutritional value of foods, but also yields functional peptides, such as those with antihypertensive, immunomodulatory, neuroactive, antimicrobial and antioxidative activities.

Hydrolysis using proteases has been preferred as the main method for producing functional peptides from food proteins. Fish and other seafood proteins have gained particular interest as potential functional peptide sources among various food protein resources due to their abundance as byproducts. For example, antioxidative peptide could be prepared from Alaska pollack (Theragra chalcogramma) and yellowfin sole (Limanda aspera) frame proteins by using a crude enzyme mixture from mackerel intestine [23, 24]. The pepsin hydrolysate, which exhibits a strong DPPH scavenging activity at a concentration of 6.80μg/mL, could be obtained from a marine fish half-fin anchovy [25] and its antibacterial activity is higher than the hydrolysates produced by other proteases [26].

Functional peptides can be generated by protease hydrolysis and/or fermention. Use of exogenous enzymes is preferred in most cases over autolysis. Various food-grade proteases, such as alcalase, flavourzyme and protamex from microorganisms, papain from plant, and pepsin and trypsin from animal sources, have been widely used in producing functional peptides in large scale.

Some functional peptides can be generated during fermentation or food processing. In fermented foods, antioxidative peptides can be produced by the action of microbes and endogenous proteolytic enzymes. For example, the free radical scavenging peptides could be produced from fermented mussel sauce [27]. Milk whey fermented by lactic acid bacteria has been reported to contain antioxidative peptides [28]. The peptide cyclo (His-Pro) is an endogenous cyclic dipeptide with antioxidant activity. It has been isolated from processed foods such as dried shrimp and fish sauce [29].

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