Rennet coagulation of milk in the presence of sucrose

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.

Understanding the differences in cheese-making properties between reverse osmosis and ultrafiltration concentrates

Concentrating milk by reverse osmosis (RO) has the potential to increase cheese yield but is known to impair cheese-making properties. The main compositional differences between ultrafiltration (UF) and RO concentrates are the high lactose and mineral contents of the latter. The objective of this work was to determine the distinct effects of high lactose and high minerals on the cheese-making properties of RO concentrate, by supplementing UF concentrate with lactose. The soluble colloidal equilibria of concentrates were studied as well as several other properties: rennet gelation behavior, cheese mass balance, composition, and microstructure. Rennet coagulation time was longer and gel firming rate was lower for RO concentrate than for UF concentrate. Lactose was mainly responsible for these differences. Lactose in RO concentrate was also responsible for the 7% increase of moisture-adjusted cheese yield, relative to UF concentrate. Compared with cheese made from UF concentrate, cheese made from RO concentrate showed higher moisture content, which could not be attributed to lactose but to the high mineral concentration. This study showed the potential of using RO instead of UF concentrate to maximize cheese yield. The approach is, however, limited to applications where post-acidification can be controlled, and will require appropriate strategies to reduce the negative effects of high mineral content in RO concentrate.

Effect of pH adjustment on the composition and rennet-gelation properties of milk concentrates made from ultrafiltration and reverse osmosis

The objective of this work was to investigate the effect of pH adjustment (initial pH vs. pH 6.50) on the rennet-gelation properties of concentrates made by ultrafiltration (UF) and reverse osmosis (RO). Rennet-gelation kinetics were followed by dynamic rheology and κ-casein hydrolysis by reverse-phase HPLC. At initial pH, RO concentrates had better rennet-coagulation behavior than UF concentrates and skim milk, whereas adjusting the pH to 6.50 produced the opposite results. The kinetics of κ-casein hydrolysis were similar in skim milk, and both concentrates and were not affected by pH adjustment. Differences in rennet coagulation were then related to the extent of hydrolysis required to trigger casein micelle aggregation. Small pH adjustments (<0.2 pH unit) enabled the use of RO concentrate with similar rennet-gelation behavior to UF concentrate, despite major compositional differences. This study shows that pH adjustment of RO concentrates can be a simple approach to improve their coagulation properties; however, the mechanisms behind these improvements remain to be elucidated.

Micellar casein gelation at high sucrose content

This article investigates the effect of sucrose addition on the formation of casein gels by acidification and/or renneting of pure micellar casein. Gelation kinetics and gel properties were followed by rheological methods, and microscopy and syneresis measurements were used to obtain a more complete characterization of the structures formed. Sucrose content has been identified as a key parameter for controlling the kinetics of aggregation and the strength of the final gels. Results have shown that the effect of sucrose on gelation can vary such that effects can be completely reversed depending on the gelation route used. During acid gelation, addition of up to 30% (wt/wt) sucrose causes gels to form more rapidly and at higher pH values, and to have higher viscoelastic moduli and a more homogeneous microstructure than those without sucrose. By contrast, gels formed by renneting in the presence of sucrose are weaker and have longer gelation times. It is proposed that sucrose reduces solvent quality and causes the collapse of the "hairy" kappa-casein brush on the surface of the casein micelles. This may explain why sucrose increases the possibility of gel formation during acidification and reduces the degree of kappa-casein hydrolysis during renneting.