Enzymic Milk Coagulation: Role of Equations Involving Coagulation Time and Curd Firmness in Describing Coagulation

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Abstract
Digitized curd firmness data were fitted to an exponential equation by a nonlinear, least squares regression computer program. Solutions were obtained for actual coagulation time, curd firming rate and theoretical maximum firmness. Actual coagulation time occurred before an increase in firmness was detectable and coincided with the inflection point of apparent absorbance changes of coagulating milk. Only the initial portion of the curve of firmness on time could be fitted to the exponential equation. Beyond this region, experimentally observed curd firmness lagged behind predicted firmness. Apparent absorbance measurements indicated this was from effects of microsyneresis the consolidation of casein micelle chains into strands and the concomitant formation of micro-droplets of milk serum between strands of the gel network. Actual coagulation time corresponds to the instant when an extended space network of casein particles is formed. For this network to be formed, aggregating micelle clusters must reach a critical volume subsequent to which any further diffusion results in immediate collision with another cluster. Dilution of milk inhibits attainment of this critical point, and a curd is not formed. Determination of coagulation time visually or rheologically is an estimate of actual time and close correspondence of coagulation time to actual depends on the method. In general, visual time is more than actual because a finite amount of rigidity must be produced before coagulation can be detected.