On enzymatic clotting processes. II. The colloidal instability of chymosin-treated casein micelles. |
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Authors: | T A Payens |
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Institution: | Netherlands Institute for Dairy Research, Ede, The Netherlands |
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Abstract: | The enzymatic clotting of casein micelles dispersed in 0.01 M CaCl2 was monitored by turbidimetry and electrophoresis. The relation between the duration of the lag phase and the enzyme concentration, (e), can be represented by t = K(e)?γ, where K is a constant and the exponent γ is found to vary between 0.92 and 1.00. This result is interpreted in terms of a flocculation rate constant increasing with the concentration of the enzyme. It is shown that the colloidal instability of chymosin-treated casein micelles cannot be explained on the basis of the well-known theory of the stability of lyophobic colloids, but that clotting is achieved through short-range interactions. The short-range effects that most probably account for the clotting are: hydrophobic bond formation, Ca-bridgas and electrostatic interactions. Under typica'. experimental conditions (33°C; maximum rate of enzymatic product formation about 1.8 × 10?10 mol ml?1 s?1) the flocculation rate constant of clotting micelles was found to be 5 × 105 mlmol?1s?1. Various factors, which could be responsible for this low value, are discussed. In the initial stages of the clotting process the turbidity of the system passes through a shallow minimum, which is ascribed to the cleavage of a macropeptide from K-casein by the clotting enzyme. The condition for the minimum has been derived. |
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