Reactions in Food Processing: Amino Acids, Peptides, and Proteins

1. Amino acids, peptides and proteins 15/1 /2019

2. Reactions Involved in Food Processing • The nature and extent of the chemical changes induced in proteins by food processing depend on a number of parameters, for example, composition of the food and processing conditions, such as temperature, pH or the presence of oxygen. • As a consequence of these reactions, the biological value of proteins may be decreased: -

3. • Destruction of essential amino acids • • Conversion of essential amino acids into derivatives which are not metabolizable • • Decrease in the digestibility of protein as a result of intra- or inter chain cross-linking. • • Formation of toxic degradation products is also possible. • The nutritional/physiological and toxicological assessment of changes induced by processing of food is a subject of some controversy and opposing opinions.

4. The Maillard reaction of the ε-amino group of lysine prevails in the presence of reducing sugars. • A nonproducing sugar (e.g. sucrose) can also cause a loss of lysine when conditions for sugar hydrolysis are favorable

5. Enzyme-Catalyzed Reactions • A great number and variety of enzyme-catalyzed reactions are known with protein as a substrate these include: • hydrolytic reactions (cleavage of peptide bonds or other linkages, e.g., the ester linkage in a phosphoprotein) • Transfer reactions (phosphorylation, incorporation of acyl residues, sugar residues and methyl groups) • Redox reactions (thiol oxidation, disulfide reduction, amino group oxidation or incorporation of hydroxyl groups). • Only enzymes that are involved in hydrolysis of peptide bonds (proteolytic enzymes, peptidases) .

6. And requirements of food processing operations is a perennial endeavor . • Food production is similar to a standard industrial fabrication process: on the one hand is the food commodity with all its required properties, on the other hand are the components of the product, each of which supplies a distinct part of the required properties. • Such considerations have prompted investigations into the relationships in food between macroscopic physical and chemical properties and the structure and reactions at the molecular level.

7. Modification of the properties of proteins is possible by changing the amino acid composition or the size of the molecule, or by removing or inserting hetero constituents. Such changes can be accomplished by chemical and/or enzymatic reactions. From a food processing point of view,theaimsof modification of proteins are:

8. • Blocking the reactions involved in deterioration of food (e.g., the Maillard reaction) • Improving some physical properties of proteins (e.g., texture, foam stability, whippability, solubility) • Improving thenutritional value(increasingthe extent of digestibility, inactivation of toxic or other undesirable constituents, introducing essential ingredients such as some amino acids).

9. Chemical Modification • Acylation • succinylated wheat gluten is quite soluble at pH 5 . • This effect is related to disaggregation of high molecular weight gluten fractions . • In the case of succinylated casein it is obvious that the modification shifts the isoelectric point of the protein (and thereby the solubility minimum) to a lower pH . • Succinylation of leaf proteins improves the solubility as well as the flavor and emulsifying properties.

10. Alkylation Modification of protein by reductive methylation of amino groups with formaldehyde/NaBH4 retards Maillard reactions. The resultant methyl derivative, depending on the degree of substitution, is less accessible to proteolysis . Hence, its value from a nutritional/physiological point of view is under investigation.

11. Enzymes 30/4/2019

12. Enzymes • Enzymes are proteins with powerful catalytic activity. • They are synthesized by biological cells and in all organisms, they are involved in chemical reactions related to metabolism. Specificity • In addition to an enzyme’s ability to substantially increase reaction rates, there is a unique enzyme property related to its high specificity for both the compound to be converted (substrate specificity) and for the type of reaction to be catalyzed (reaction specificity).

13. Structure • Enzymes are globular proteins with greatly differing particle sizes , the protein structure is determined by its amino acid sequences and by its conformation, both secondary and tertiary, derived from this sequence. Larger enzyme molecules often consist of two or more peptide chains arranged into a specified quaternary structure . • Reaction Specificity: The substrate is specifically activated by the enzyme so that, among the several thermodynamically permissible reactions, only one occurs. This is illustrated by the following example: L(+)-lactic acid is recognized as a substrate by four enzymes

14. Isolation and Purification Most of the enzyme properties are clearly and reliably revealed only with purified enzymes. As noted under enzyme isolation, prerequisites for the isolation of Apure enzyme are selected protein chemical separation methods carried out at 0– 4◦C since enzymes are often not stable at higher temperatures. Nomenclature The Nomenclature Commitee of the “International Union of Biochemistry and Molecular Biology” (IUBMB) adopted rules last amended in 1992 for the systematic classification and designation of enzymes based on reaction specificity.

15. All enzymes are classified into six major classes according to the nature of the chemical reaction catalyzed: 1.Oxidoreductases. 2. Transferases. 3. Hydrolases. 4. Lyases (cleave C−C, C−O, C−N, and other groups by elimination, leaving double bonds, or conversely adding groups to double bonds). 5. Isomerases (involved in the catalysis of isomerizations within one molecule). 6. Ligases (involved in the biosynthesis of a compound with the simultaneous hydrolysis of a pyrophosphate bond in ATP or a similar triphosphate).

16. Multiple Forms of Enzymes Chromatographic or electrophoretic separations of an enzyme can occasionally result in separation of the enzyme into “isoenzymes”, i.e. forms of the enzyme which catalyze the same reaction although they differ in their protein structure. The occurrence of multiple enzyme forms can be the result of the following: a) Different compartments of the cell produce genetically independent enzymes with the same substrate and reaction specificity, but which differ in their primary structure. .

17. An example is glutamate-oxalacetate transaminase occurring in mitochondria and also in muscle tissue sarcoplasm. This is the indicator enzyme used to differentiate fresh from frozen meat • b) Protomers associate to form polymers of differing size. An example is the glutamate dehydrogenase occurring in tissue as an equilibrium mixture of molecular weights Mr = 2.5·105−106. c) Different protomers combine in various amounts to form the enzyme. For example, lactate dehydrogenas • (a protomer is the structural unit of an oligomericprotein)

18. Technical Enzyme Preparations Production (H.W) Immobilized Enzymes ------------------------------------ Immobilized Enzymes Enzymes in solution are usually used only once. The repeated use of enzymes fixed to a carrier is more economical. The use of enzymes in a continous process, for example, immobilized enzymes.

19. Individual Enzymes 1.Oxidoreductases: a. Glucose Oxidase. b. Catalase. c.Lipoxygenase. d. Aldehyde Dehydrogenase. 2.Hydrolases: a.Peptidases b. α- andβ-Amylases 3. Isomerases example(H.W) 4. Transferases example(H.W)

21. Aroma Compound When food is consumed, the interaction of taste, odor and textural feeling provides an overall sensation which is best defined by the English word “flavor”. Flavor results from compounds that are divided into two broad classes: Those responsible for taste and those responsible for odors, the latter often designated as aroma substances. However, there are compounds which provide both sensations. Compounds responsible for taste are generally nonvolatile at room temperature. Therefore, they interact only with taste receptors located in the taste buds of the tongue. The four important basic taste perceptions are provided by: sour, sweet, bitter and salty compounds

22. Examples of key odorants

23. Impact Compounds of Natural Aromas The amount of volatile substances present infood is extremely low (ca. 10–15mg/kg). In general, however, they comprise a large number of components. Threshold Value The lowest concentration of a compound that is just enough for the recognition of its odor is called the odor threshold (recognition threshold). The detection threshold islower, i.e., the concentration at which the compound is detectable but the aroma quality still cannot be unambiguously established