Meat and Fish Proteins

The characteristics of proteins that affect their properties in comminuted meat products are not well defined (52). The ability of proteins to bind water and fat, as well as to retain these two components during heating and storage, is critical in the manufacture of processed meat products. Binding properties affect not only cook yield, but also final appearance and texture. Evaluation of individual muscle proteins suggests that the salt-soluble proteins (ie, myofibrillar proteins) are the major contributors in emulsifi-cation. Meat proteins generally display improved emulsi-fication properties in the presence of increasing salt concentration, especially at pH values near or below their isoelectric points (pi). This apparent salt-induced shift in pi serves to increase or maintain protein solubility, an essential requirement for emulsification (53).

The emulsification properties of meat and fish protein are affected by changes in solubility, as induced by frozen storage, heating, and pH. However, solubility is not a good predictor of emulsification or other functional properties. One study reported that the amount of soluble protein from fresh meat sources was highly correlated with emulsification properties, irrespective of original meat source (54). For frozen or cooked meat, however, not only did the soluble protein content decrease, but the remaining non-coagulated soluble protein had lower emulsification prop erties compared to fresh meat sources. This observation was attributed to denaturation of soluble proteins that could have been caused by shearing during emulsification. Undenatured proteins are required for good emulsification properties since they possess greater conformational potential. Shear-induced protein denaturation may be followed by aggregation, this being intensified with increasing protein concentration. Dissociation of muscle protein complexes and subsequent protein denaturation without accompanying aggregation results in increased hydropho-bicity of the salt-extractable proteins, with little change in their solubility or total sulfhydryl group content. Protein denaturation may be enhanced by hydrophobic association of the polypeptide chains at oil/water interfaces, resulting in a much larger available protein volume/surface area and increasing emulsifying capacity (1). Under moderate heating conditions (ie, <50°C), emulsification and functional properties of salt-extracted meat and fish proteins are often improved. At higher temperatures (ie, 50-70°C), functional properties are impaired, ie, proteins aggregate as reflected in decreased solubility and sulfhydryl group content. In addition, at temperatures above 70°C, proteins display decreased hydrophobicity (55).

The setting of a meat emulsion in comminuted products, in addition to binding of meat pieces in restructured or reformed products, is believed to be based on the establishment of a stable protein gel. Gelation arises from protein denaturation and subsequent association to form a three-dimensional matrix and is generally heat initiated, because raw meat pieces do not significantly bind to each other (56). An exception is the low-temperature (about 4°C) setting of sols from certain fish species that can occur on storage. During gel formation, exposure of sulfhydryl groups has been observed (55), in addition to shifts in pi caused by exposure of previously masked charged groups (57).

Perishability and compositional variations affect the use of fish muscle protein as a raw food material (eg, in the manufacture of kamoboko and surimi) (52). Aside from microorganisms and oxidative rancidity, the most conspicuous determinants of the quality of fish muscle proteins are alterations in functional properties, a direct reflection of protein denaturation (58). Formaldehyde is often produced during the storage of fish, especially gadoids, and has been implicated in the toughening of fish muscle during frozen storage. Through interaction with the side chain groups of fish muscle proteins, formaldehyde can increase the rate of protein denaturation, leading to aggregation of proteins and subsequent toughening (59). Denaturation of fish muscle proteins, especially on frozen storage, may also result from oxidation of free fatty acids and lipid peroxides or lipid-protein complex formation. When fish muscle proteins are thermally or surface denatured, the degree of denaturation is intensified in the presence of lipids. Yet, in other instances, intact lipids may function to stabilize and protect these proteins (53).

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