CHEESE TECHNOLOGY

Module 4. Pretreatments of milk for cheesemaking

Lesson 10
HEAT TREATMENT AND HOMOGENIZATION OF CHEESE MILK

10.1 Introduction

Most cheese-making today involves the treatment of milk by one or more processing steps prior to addition of coagulant and starter culture. Perhaps the simplest and earliest technological intervention, driven by safety concerns, was the pasteurization of milk. Pasteurisation inactivates some enzymes, reverses shifts in the mineral balance of milk and influences the microflora of non-starter lactic acid bacteria (NSLAB) in the final cheese. Pasteurization unacceptably impairs the cheese flavor as a result of its influence on NSLAB. On the other hand, more severe heat treatments than pasteurization result in significant denaturation of whey proteins and their resulting incorporation into cheese curd, with significant effects on cheese yield and composition.

10.2 Heat Treatments

Milk for cheese manufacture is heated to eliminate pathogenic bacteria, to minimise damage to caseins by proteolytic bacteria on storage or to incorporate heat-denatured whey proteins in curd, thereby improving cheese yield. Furthermore, more severe heat treatment of milk may be applied to inactivate spores from Clostridium tyrobutyricum. Heat treatment of milk at conditions severe than those used for conventional pasteurisation results in denaturation of whey proteins, interactions between whey proteins and casein micelles and transfer of soluble calcium, magnesium and phosphate to the insoluble colloidal state. Casein micelles are very stable at high temperatures, although changes in zeta potential, size, hydration of micelles and some association-dissociation reactions do occur under severe heat treatments. Denaturation of whey proteins exposes side chain groups originally buried in the native structure, particularly reactive thiol groups, and the unfolded proteins may self-aggregate or interact with casein micelles, through interactions with κ-casein. The extent of association of denatured whey protein with casein micelles is dependent on the pH of the milk prior to heating, levels of soluble calcium and phosphate, milk solids concentration and mode of heating (direct or indirect). For cheese-makers, the principal interest has been in increasing yield by exploiting this heat-induced association of caseins with whey proteins, while attempting to minimise undesirable changes in cheese quality.

In the cheese vat, high heat treatment of milk prolongs rennet coagulation times and reduces the strength of rennet gels leading to impaired syneresis. The adverse effects on coagulation are attributed to the inhibition of hydrolysis of κ-casein by chymosin due to the β-lactoglobulin/κ-casein complex at the micelle surface impairing the accessibility of κ-casein to the coagulant, to reduced reactivity of renneted micelles with attached denatured whey proteins to aggregation, or to a reduction in the concentration of micellar calcium.

10.3 Homogenization of Cheese-Milk

The primary aim of homogenization of milk is to reduce the size of the fat globules, thereby delaying their creaming rate. In raw milk, fat globule size commonly ranges from 0.2–15 μm, and homogenization generally aims to reduce the maximum to < 2 μm. For this purpose, two-stage valve homogenizers are used, which operate at pressure of 20 MPa. Recently, novel homogenization devices, e.g. high-pressure homogenizers and microfluidisers, which can operate at pressures of several hundred MPa and achieve greater reductions in fat globule size, have been developed. In cheesemaking, homogenization of cheese milk can be of interest for preventing creaming of fat globules, reducing fat losses in the whey or controlling development of free fat in the cheese. Due to the reduction in fat globule size on homogenization, the total surface area of the fat globules increases and the amount of original fat globule membrane material is by far insufficient to fully cover the newly-formed surface. As a result, other surface-active components of milk, primarily caseins and, to a lesser extent, whey proteins, become adsorbed onto the surface of the newly formed globules. Thus fat globules in homogenized milk almost resemble casein covered emulsion droplets. The adsorption of caseins onto the fat globules has the following implications for cheese-making characteristics of milk:

(1) Casein surface area in milk is increased, but the amount of micellar casein is reduced;

(2) Two types of particles with a casein micelle surface layer exist: native casein micelles and casein-covered fat globules;

(3) When adsorbed, casein micelles tend to spread over the surface of the fat globule and hence increase in effective surface area but with reduced surface density of κ-casein.

The rennet coagulation time (RCT) of unhomogenised milk is generally lower than that of homogenized milk. This is probably related to the larger casein surface area in homogenized milk, as well as the lower surface density of κ-casein. The former increases the probability of interactions between particles, whereas the latter reduces the amount of κ-casein that needs to be hydrolyzed before micellar flocculation is induced. Negative aspects of homogenization occur in the subsequent stages of cheesemaking, i.e., the syneresis of the paracasein matrix and the fusion of the paracasein micelles into a strong and cohesive network. Cheese curd from homogenized milk shows poor syneresis and, as a result, has high moisture content. Furthermore, cheese curd prepared from homogenized milk is also often characterized by a coarse and brittle structure.

10.4 High-Pressure Treatment of Cheese milk

Cheese prepared from raw milk is better than the cheese manufactured from pasteurized milk but for safety reasons, pasteurization of milk before cheesemaking is essential. High pressure treatment (HPT) of milk can be used as an alternative to pasteurization so that raw milk quality cheese can be produced without compromising safety aspects. High pressure treatment is a non-thermal process wherein a high pressure in the range of 200 to 1000 MPa for different time periods is used for destruction of microorganisms.

HPT of milk causes several protein modifications such as whey protein denaturation and micelle fragmentation and alters mineral equilibrium. These changes improve rennet coagulation of cheese milk and yield of cheese. HPT of milk results in smaller casein micelles due to which RCT decreases. Cheese yield increases owing to denaturation of whey proteins by HPT (resulting in their incorporation in cheese curd), which also leads to increased moisture retention.