Partial renneting of pasteurised bovine milk: Casein micelle size, heat and storage stability

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

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.

Rennet-Induced Casein Micelle Aggregation Models: A Review

Two phases are generally recognized in the enzymatic coagulation of milk: hydrolysis and aggregation, although nowadays more and more researchers consider the non-enzymatic phase to actually be a stage of gel formation made up of two sub-stages: micellar aggregation and hardening of the three-dimensional network of para-κ-casein. To evaluate this controversy, the main descriptive models have been reviewed. Most of them can only model micellar aggregation, without modeling the hardening stage. Some are not generalizable enough. However, more recent models have been proposed, applicable to a wide range of conditions, which could differentiate both substages. Manufacturing quality enzymatic cheeses in a cost-effective and consistent manner requires effective control of coagulation, which implies studying the non-enzymatic sub-stages of coagulation separately, as numerous studies require specific measurement methods for each of them. Some authors have recently reviewed the micellar aggregation models, but without differentiating it from hardening. Therefore, a review of the proposed models is necessary, as coagulation cannot be controlled without knowing its mechanisms and the stages that constitute it.

Effects of cold-renneted and pre-heated milk protein concentrates (MPCs) addition on the properties of alginate composite gels

This study investigated the effects of milk protein concentrate (MPC85) (untreated, cold-renneted, and pre-heated (80 °C, 30 min)) addition on the physical/mechanical properties of sodium alginate (2% w/w) composite gels in millimeter-size (bead) and centrimeter-size gel forms. The gels were characterized for the degree of syneresis, swelling behavior, hardness, stiffness, viscoelastic behavior, and surface morphology of freeze-dried gel. The results showed that the addition of untreated and treated MPCs reduced the hardness, the stiffness and the solid-like behavior of the alginate gels. Untreated MPC and pre-heated MPC caused no effect on syneresis of alginate gels. The addition of cold-renneted MPC reduced the degree of syneresis in composite gels by 13.7%. Similarly, the addition of cold-renneted MPC reduced (by 16.4%) the degree of syneresis of the alginate composite gels after incubation in simulated gastric fluid (SGF). After incubation in simulated intestinal fluid (SIF), the gel containing alginate only swelled while gels containing untreated MPC and pre-heated MPC experienced degradation and a severe mass loss. Meanwhile, the gels containing cold-renneted MPC swelled and at the same time eroded. Moreover, only cold-renneted MPC composite gels showed lower shrinkage and wrinkles on the surface of the beads during lyophilization. Therefore, based on these results, it is indicated that cold-rennet induced gelation method could increase the effectiveness of MPC as a composite material for alginate gels.