Extent of κ-casein hydrolysis during renneting of bovine milk: A critical assessment of the analytical and estimation approaches

Abstract

Renneting is an enzymatic process that turns milk into curd which is then transformed into cheese. Rennet-induced coagulation of caseins (CNs) is the critical step during this process and the key is the primary hydrolysis of κ-CN's Phe₁₀₅-Met₁₀₆ bond by chymosin. This article comprehensively reviews the existing data on the extent/degree of κ-CN hydrolysis during renneting of bovine milk and critically evaluates its determination methods. The data show that under normal cheese-making conditions, milk gelation occurs at a degree of κ-CN hydrolysis <80%, which varies due to several factors including analytical and estimation approaches. The common approach involves isolating the macropeptides released, by precipitating whey proteins and residual CN in 1%-12% trichloroacetic acid (TCA), then assuming that the maximum amount obtained is 100% κ-CN hydrolysis. The drawback is that the estimated degree of κ-CN hydrolysis may be higher than the actual value as TCA partially precipitates the macropeptide fractions. Moreover, macropeptide isolation seems unnecessary based on current advances in chromatographic and electrophoretic techniques. The present work proposes a simple mass balance-based approach that will provide accurate estimates in future studies. The accuracy of measuring the degree of κ-CN hydrolysis has implications on the precision of the data in relation to its partitioning (% distribution between the curd and whey) which is essential for improving whey quality.

Keywords: bovine milk; caseinoglycomacropeptide; caseinomacropeptide; cheese; rennet; κ‐CN hydrolysis.

FIGURE 1

(a): Major stages of the general aged cheese‐making process. (b): Primary enzymatic phase of κ‐CN hydrolysis by chymosin at Phe105‐Met106 bond which releases the caseinomacropeptide [soluble C‐terminal fraction (f106–169)] into whey allowing para‐κ‐CN [hydrophobic fraction (f1–105)] together with other micellar components (collectively referred to as para‐casein micelles) to aggregate. Further details of the reaction mechanism are available (Palmer et al., ; Yegin & Dekker, .

FIGURE 2

Perspectives on factors affecting the degree of κ‐CN hydrolysis estimated at gelation point.