Caseins proteolytic modification

Proteolytic modification has special importance for the improvement of solubility of proteins. This effect becomes significant even after very limited proteolysis. Hydrolysis of casein to DH of 2 and 6.7% with Staphylococcus aureus V8 protease increased the isoelectric solubility to 25 and 50%, respectively (Chobert et al., 1988a). However, it should be noted that the solubility profiles were not identical, due to a shift of the isoelectric point of the modified proteins. Solubility of a protein hydrolysate depends on the enzyme used (Adler-Nissen, 1986a). Protamex (a Bacillus proteinase complex) hydrolysates of sodium caseinate (DH 9 and 15%) displayed 85-90% solubility between pH 4 and 5 (Slattery and FitzGerald, 1998). 

The properties of many dairy products, in fact their very existence, depend on the properties of milk proteins, although the fat, lactose and especially the salts, exert very significant modifying influences. Casein products are almost exclusively milk protein while the production of most cheese varieties is initiated through the specific modification of proteins by proteolytic enzymes or isoelectric precipitation. The high heat treatments to which many milk products are subjected are possible only because of the exceptionally high heat stability of the principal milk proteins, the caseins. 

There have been a limited number of studies on the effects of enzymic modification of protein concentrates on functional properties other than solubility. Studies on functional properties, as modified by enzymic treatments, emphasize foam formation and emulsifying characteristics of the hydrolysates. Treatment of chicken egg albumen alters the functional properties of the egg proteins in terms of foam volume and stability and the behavior of the proteins in angel food cakes (25). Various proteolytic enzymes were used to degrade the egg albumen partially. However, proteolytic enzyme inhibitors indigenous to the egg proteins repressed hydrolysis of the egg proteins compared with casein. 

Proteolytic activity was assayed as described by Kembhavi et al. [9], with modification. The reaction mixture was made up of 0.4 mL of casein (Sigma) 0.5% (w/v) in distilled water and 0.4 mL 0.2 M acetate buffer, pH 5.0, to whieh 0.2 mL of the crude enzyme solution was added. The reaction was carried out at 60°C and stopped after 30 min with 1 mL of 10% trichloroacetic acid (TCA). Test tubes were centrifuged at 5,000 rpm/5 min, and the absorbance of the supernatant was measured at 280 nm. An appropriate control was prepared in which the TCA was added before the enzymatic solution. One unit of enzyme activity (U) was arbitrarily defined as the amount of enzyme required to cause an increase of 0.01 in absorbance at 280 nm under the assay conditions. 

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