Food Chemistry

Module 2. Food proteins

Lesson 5
REACTIONS INVOLVED IN PROCESSING AND REACTIONS WITH ALKALI

5.1 Introduction

A number of chemical changes involving proteins may occur during processing and storage of foods. These changes can be desirable or undesirable. The various treatments involved in processing of foods are heating, cooling, drying, fermentation, use of chemicals, irradiation, etc. Among these, heating is most common processing treatment. Heating is mainly done to kill pathogens, inactivate enzymes that cause oxidative and hydrolytic changes in foods during storage.

As a result of these chemical changes, nutritive value of proteins may be decreased.

• Formation of toxic compounds
• Destruction/ loss of amino acids
• Conversion of essential amino acids into derivatives which are not metabolizable
• Decrease in digestibility of proteins due to cross linking

The nature and extent of chemical changes induced in proteins by food processing depends on a number of parameters like composition of food and processing conditions like temperature, pH or presence of oxygen. As a consequence of these reactions, the biological value of proteins may be decreased.

5.2 Some common changes are described below

5.2.1 Denaturation

Denaturation is a phenomenon that involves transformation of a well-defined folded structure of protein to an unfolded state, without any change in the primary structure. Most food proteins are denatured when exposed to moderate heat treatments (60^oC-90^oC/1 h or less).

Denaturation is generally reversible when the peptide chain is stabilized in its unfolded state by the denaturing agents and the native conformation can be restabilized after the removal of the agent. Irreversible denaturation occurs when the unfolded peptide chain is stabilized by interactions with other chains.
The predenatured transition state involves minor conformational changes that occur prior to denaturation. As the reaction proceeds, changes due to denaturation occur. Following these changes, the protein may react either with themselves and/or with other food constituents resulting in the formation of higher molecular weight aggregates. These post-denaturation reactions are virtually irreversible.
Changes resulting from these mild heat treatments are usually beneficial from a nutritional standpoint, e.g.

• Digestibility is often improved. In general denatured proteins are more readily attacked by proteolytic enzymes.
• Several enzymes like proteases, lipoxygenases, polyphenol oxidases, etc. are inactivated. This limits the undesirables changes like development of off-flavours, acidity, textural changes and discoloration of foods during storage.
• Proteinaceous anti-nutritional factors present in seeds and legumes are denatured and inactivated by mild heat treatments. These inhibitors impair efficient digestion of proteins and thus reduce their bioavailability.
• Certain proteinaceous toxins, e.g. botulism toxin and enterotoxins are inactivated.

However, extensive denaturation affects certain functional properties like solubility and other related properties.

5.2.2 Desulfuration

Thermal treatments of proteins or proteinaceous foods at high temperature and in the absence of any added substances can lead to several chemical changes. Most of these chemical changes are irreversible and some of these reactions result in the formation of amino acid types that are potentially toxic. One of the first noticeable changes in proteins on heating at around 100^oC is loss of heat-labile amino acids such as cysteine, cystine & lysine and the formation of gases like hydrogen disulphide (H₂S). Thermal treatments like sterilization at temperature above 115^oC bring about the partial destruction of cysteine and cystine residues and formation of H₂S, dimethyl sulfide and cysteic acid; H₂S and other volatile compounds produced contribute to the flavor of these heat treated foods.

5.2.3 Deamidation

This reaction takes place during heating of proteins at temperatures above 100^oC. The ammonia released comes mainly from the amide groups of glutamine and asparagine, and these reactions do not impair the nutritive value of the proteins. However, due to the unmasking of the carboxyl groups, the isoelectric points get affected and therefore the functional properties of proteins are modified. Deamidation may be followed by establishment of new covalent bonds between amino-acid residues.

5.2.4 Racemization

Severe heat treatment at temperatures above 200^oC as well as heat treatment at alkaline pH (e.g. in texturized foods) invariably leads to partial racemization of L-amino acid residues to D-amino acid residues. Some racemization is also observed during acid hydrolysis of proteins and roasting of proteins or protein containing foods above 200^oC.

Since D-amino acids have no nutritional value, racemization of an essential amino acid reduces its nutritional value by 50%. Racemization of amino acid residues causes a reduction in digestibility because peptide bonds involving D-amino acid residues are less efficiently hydrolyzed by gastric and pancreatic proteases. This leads to loss of essential amino acids that have racemized and impairs the nutritional value of the protein. D-amino acids are also less efficiently absorbed through intestinal mucosal cells and even if absorbed they can’t be utilized in in-vivo protein synthesis.

5.2.5 Effect of heat treatment at alkaline pH

Alkali treatment causes many reactions (undesirable reactions). The more common ones are hydrolysis, elimination reactions involving side chains of certain amino acids, racemization of amino acid residues, addition of compound to the proteins, scission of the peptide chain, modification or elimination of non protein constituents (prosthetic groups etc.), and the interaction of the protein with alkali-derived products from the environment. All of these reactions are affected by the pH, the temperature, ionic strength, presence of specific ions, and by the nature of the protein itself. . Heating of proteins at alkaline pH or heating above 200^oC at neutral pH can result in β-elimination reaction. The first stage of this reaction involves abstraction of proton from α-carbon atom resulting in formation of carbanion. The carbanion derivative of cysteine, cystine and phosphoserine undergoes second stage of β-elimination reaction leading to formation of dehydroalanine . The resulting dehydroalanine residues are very reactive and react with nucleophillic groups such as ε-amino group of lysine, thiol group of cysteine and delta-amino group of ornithine (degradation product of arginine).These reactions results in formation of lysinoalanine, lanthionine and ornithoalanine cross-links respectively in proteins. . Of these lysino-alanine is the major cross-link commonly found in alkali treated proteins because of the abundance of readily accessible lysyl residues.Formation of protein-protein cross-links in alkali treated proteins decreases their digestibility and biological value. Decrease in digestibility is related to the inability of trypsin to cleave the peptide bond in lysinoalanine. Cross-links also impose stearic constraints that prevent the hydrolysis of other peptide bonds in the neighbourhood of such cross links.

5.2.6 Interaction between proteins and carbohydrates/aldehydes (Maillard reaction)

Maillard reaction (nonenzymic browning) refers to a complex set of reactions initiated by reaction between amines and carbonyl compounds, which, at elevated temperatures, decompose and eventually condense into insoluble brown products known as melanoidins. This reaction occurs not only in foods during processing but can also occur in biological systems. In either case, proteins and amino acids generally provide an amino component while reducing sugars, ascorbic acid and carbonyl compounds generated from lipid oxidation provide the carbonyl component. (Fig. 5.5 Sugar Amine Condensation), (Fig. 5.6 Amadori Rearrangement), (Fig. 5.7 Degradation of Amadori Compounds), (Fig. 5.8 Dehydration and Fragmentation),

5.2.7 Significance of the Maillard Reaction

Maillard [Sugar – amino] type browning is most prevalent – because it requires relatively low energy of activation and is autocatalytic. Direct caramelization requires high energy of activation. Therefore occurs to a limited extent in food. Significance of Maillard reaction in food processing is given below.

1. Production of colour

• Desirable as in coffee, chocolate bread crust, toast etc.
• Undesirable, as in milk & milk products (khoa, condensed milk, milk powder etc) and in many intermediate moisture products.

2. Production of flavour and off-flavour

• Flavour (odour) are due to formation of volatile products e.g. fission products and strecker aldehydes
• Substances tasting sweet & bitter may be involved

3. Antioxidant properties

• Maillard reaction products are reported by have antioxidant properties.
• This is thought to be due to formation of reductones, chelating of heavy metals,which may otherwise act as a proxidant.

4. Toxicity

• Through possible formation of imidazoles N-nitroso derivatives.
• Some of the compounds are known to be carcinogenic in laboratory animals.
• Intrinsic toxicity is due to nutritional properties of Maillard products and intermediates.

5. Nutritional implications

• One of the important reasons for interest of food industry in Maillard browning is its relation to nutrition.
• Considerations in this regards are reduction in nutritive value.
• Loss of essential amino acids - especially lysine.
• Loss of some vitamins.
• Increase excretion of Zn in urine due formation of metal chelating compounds.
• Reduced digestibility due to development of cross-links between lactose and protein.
• Inhibition of trypsin, carboxypeptidases (A and B) and amino peptidase by Maillard reaction production – metabolic inhibitors.
• Inhibition of intestinal amino acid transport – disturbed amino acid utilization.
• Lowered consumption of food due to prior palatability appearance and physical properties of the brown products.

5.2.8 Oxidation of amino acids

Methionine is oxidized to methionine sulfoxide by various peroxides. Under strong oxidizing conditions, methionine sulfoxide is further oxidized to methionine sulfone, and in some cases to homocysteic acid.