Influence of Protein and Fat Contents of Ultrafiltered Milk on Rheological Properties of Gels Formed by Chymosin

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Abstract
Enzymatically induced gelation of [cow] milk and milk concentrated by ultrafiltration was studied with a curd firmness tester which measured transmission of oscillatory deformations through the product. Scott Blair''s relationship between pseudo modulus of rigidity (G) and time after clotting (t), G = G.infin. e-.lambda./t, described the gelation of untrafiltered retentates satisfactorily. Maximum gel firmness G.infin., and .lambda., which is the time to reach a firmness of G.infin./e, were affected substantially by protein content. The effect of protein content on rates of firming of gels were evaluated at different stages of gel formation. At the onset of gelation, rates increased up to protein content 90 g.cntdot.kg-1, but decrease at higher protein concentrations. At a later stage, this disappeared and rates correlated with protein content. Those observations are discussed relative to effects of protein content on proteolytic and aggregation phases of milk clotting. Times from clotting to point of cutting curd were affected significantly by slight increases in protein above that in unconcentrated milk, but clotting times were not affected. Variations in fat content had inconsequential effects on test measures.