Properties of Amino Acids
Proteins are polymers of amino acids joined by peptide bonds. Twenty amino acids commonly exist and their possible combinations result in the potential for an incredibly large number of sequence and 3D structural protein variants. Amino acids consist of a carbon atom (Cα) that is covalently bonded to an amino group and a carboxylic acid group. Thus, they have an N–Cα–C ‘backbone’. In addition, the Cα is bound to a hydrogen atom and one of 20 ‘R groups’ or ‘side chains’, hence the general formula: + NH3 − CHR − COO−
Protein Nutritional Considerations
Single source plant proteins are referred to as incomplete proteins since they do not have sufficient quantities of the essential amino acids in contrast to animal proteins, which are complete. For example, cereals are deficient in Lys, while oilseeds and nuts are deficient in Lys as well as methionine. In order for plant proteins to become ‘complete’, complementary sources of proteins must be consumed, i.e. the deficiency of one source is complemented by an excess from another source, thus making the combined protein ‘complete’.
Animal Protein Structure and Proteolysis in Food Systems
Animal tissues have similar structures despite minor differences between land and aquatic (fish and shellfish) animal tissues. Post-mortem, meat structure breaks down slowly, resulting in desirable tenderization and eventual undesirable degradation/spoilage. Understanding meat structure is critical to understanding these processes, and Table 1.4 lists the location and major functions of myofibrillar proteins associated with the contractile apparatus and cytoskeletal framework of animal tissues. Individual muscle fibers are composed of myofibrils, which are the basic units of muscular contraction. The skeletal muscle of fish differs from that of mammals, in that the fibers arranged between the sheets of connective tissue are much shorter.
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Protein Modifications
A protein’s amino acid sequence is critical to its physicochemical properties, and it follows that changes made to individual amino acids may alter its functionality. In addition to the many chemical alterations that may occur to amino acids during food processing, e.g. deamination, natural, enzymatic protein modifications collectively known as post-translational modifications may also occur upon their expression in cells.
Protein Structure
Protein folding largely occurs as a means to minimize the energy of the system where hydrophobic groups are maximally shielded from aqueous environments and while the exposure of hydrophilic groups to aqueous environments is maximized. Protein structures follow a hierarchy: primary, secondary, tertiary and quaternary structures.
Oxidative Browning
Oxidative browning, also called enzymatic browning, involves the actions of a group of enzymes generally referred to as polyphenol oxidase (PPO) or phenols. PPO is normally compartmentalized in tissue such that oxygen is unavailable.
Quality Index
Trim ethylamine and its N-oxide have long been used as indices for freshness in fishery products. Degradation of trim ethylamine and its N-oxide leads to the formation of ammonia and formaldehyde with undesirable odors. The pathway on the production of formaldehyde and ammonia from trim ethylamine and its Oxide.
Last Word
Color is an intrinsic property of foods, and therefore, a change in color is often caused by a change in quality. Vision is the most important sensory perception in selecting food and appreciating its quality (Diehl 2008). Chlorophylls are the most abundant natural pigments and are responsible for the green color of plants (Marquez Ursula and Snicker 2008) and are the biomolecules responsible for capturing light energy in its transformation into chemical energy during photosynthesis.
