<br><h3> Chapter One </h3> <b>The Quality of Milk for Cheese Manufacture</b> <p> T.P. Guinee and B. O'Brien <p> <p> <b>1.1 Introduction</b> <p> World production of milk in 2008 is estimated at ~576 x [10.sup.6] tonnes (ZMP, 2008), with India/Pakistan, the Americas and Europe being the major producing regions. The proportions of total milk produced by cow, water buffalo, goat, ewe, camel and other are ~84.0, 12.1, 2.0, 1.3, 0.2 and 0.2, respectively (International Dairy Federation - IDF, 2008). Cows' milk is the major milk used for cheese manufacture; however, significant quantities of cheese are also made from goat, sheep and water buffalo milks in some European Union (EU) countries, such as France, Italy and Spain. <p> Based on an estimated yield of 1 kg cheese 10 [kg.sup.-1] milk, the percentage of total milk used for cheese is ~25%, but varies widely from ~70-90% in some European countries (Italy, France, Denmark and Germany) to ~0.5% in China. While cheese-like products are produced in most parts of the world, the principal cheese-producing regions are Europe, North America and Oceania. Cheese production has increased consistently over the last two decades at an annual average rate of ~1.5%. As discussed in Chapter 8, this may be attributed to a number of factors including increases in global population and per capita income, globalisation of eating trends/habits, changing lifestyles, growth in use of cheese as an ingredient in the food service (in pizza-type dishes, cheese burgers and salad dishes) and industrial sectors (cordon bleu entrees, co-extruded products with cheese and gratins). <p> The increase in consumption has been paralleled by a greater emphasis on improved quality and consistency with respect to the levels of particular nutrients (fat, protein, calcium -[Ca.sup.2+] and sodium -[Na.sup.+]), physical properties (texture and cooking attributes), sensory characteristics and processability (size reduction attributes, such as shredability; ability to yield processed cheeses or other cheese products when subjected to secondary processing). Consequently, this has necessitated an increase in the quality and consistency of all inputs (milk composition/quality, enzyme activity/purity, starter cultures characteristics, for example, acid productivity, phage resistance, autolytic properties and flavour-imparting characteristics) and standardisation of the manufacturing process (cf. Chapter 8). In an overall context, milk quality for cheese manufacture may be defined as its suitability for conversion into cheese and deliver cheese of the desired quality and yield. The current chapter examines milk quality for cheese manufacture and the factors affecting it, together with broad-based strategies for improving quality and consistency. <p> <p> <b>1.2 Overview of milk composition</b> <p> Milk consists of protein (caseins and whey proteins), lipid, lactose, minerals (soluble and insoluble), minor components (enzymes, free amino acids, peptides) and water (Table 1.1). <p> The casein fraction coexists with the insoluble minerals as a calcium phosphatecasein complex. The water and its soluble constituents (lactose, native whey proteins, some minerals, citric acid and minor components) are referred to as serum. During cheese manufacture, the milk is subjected to a partial dehydration, involving controlled expulsion of serum and concentration of fat, caseins (and in some cases denatured, aggregated whey proteins) and some of the minerals. The methods engaged to affect the dehydration include limited destabilisation and aggregation of the calcium phosphate casein in the form of a gel network which encloses the fat and serum via specific enzymatic hydrolysis of the casein, acidification (by fermentation of milk lactose to lactic acid by added bacterial cultures), elevated temperature and various mechanical operations as discussed in Chapter 8. Amongst others, the degrees of casein aggregation and dehydration are critical parameters controlling the properties and quality of the final cheese. <p> Although manufacturing procedures for most cheese types are very defined (at least in large modern cheesemaking facilities) in terms of technology applied and the type and levels of operations imposed on the milk (cf. Chapter 8), variations in cheese quality do occur. Seasonal variation in the composition and quality of milk are considered to be crucial factors contributing to the inconsistency in quality. Consequently, an overview of milk composition in terms of its relevance to cheese manufacture is presented below. The main focus of this chapter is on cows' milk, which accounts for an estimated 95% of total milk used in cheese manufacture; the characteristics of other milks are discussed elsewhere (Anifantakis, 1986; Juarez, 1986; Remeuf & Lenoir, 1986; Muir <i>et al</i>., 1993a,b; Garcia-Ruiz <i>et al</i>., 2000; Bramanti et al., 2003; Huppertz <i>et al</i>., 2006; Kuchtik <i>et al</i>., 2008; Caravaca <i>et al</i>., 2009). <p> <p> <b>1.2.1</b> <i>Casein</i> <p> The nitrogenous fraction of cows' milk typically consists of casein, whey protein and nonprotein nitrogen (urea, proteose-peptones, peptides) at levels of ~78, 18 and 4 g 100 [g.sup.-1], respectively, of total nitrogen (Table 1.1). <p> Casein, which is typically present at a level of 2.5 g 100 [g.sup.-1] in cows' milk, is the main structural protein of both rennet- and acid-induced milk gels (Table 1.1). The casein is heterogeneous, comprising four main types: [alpha][s.sub.1], [s.sub.2], and [kappa], which represent ~38, 10, 35 and 15 g 100 [g.sup.-1] of the total casein, respectively (Fox & McSweeney, 1998; Fox, 2003; Swaisgood, 2003). Model studies in dilute dispersions indicate that the individual caseins vary in the content and distribution of phosphate (Table 1.2); the respective number of (serine) phosphate residues <i>per</i> mole of casein are ~8, 10-13, 5 and 1 for [alpha][s.sub.1-], [alpha][s.sub.2-], - and [kappa]-caseins, respectively. The serine phosphates bind calcium and calcium phosphate, and consequently, different caseins have different calcium-binding properties. Generally, [alpha][s.sub.1-], [alpha][s.sub.2-] and -caseins bind calcium strongly and precipitate at relatively low calcium concentrations (~0.005-0.1 M Ca[Cl.sub.2] solutions), inclusive of the calcium level in milk (30 mM); in contrast, [kappa]-casein is not sensitive to these calcium concentrations and can, in fact, stabilise up to 10 times its mass of the calcium-sensitive caseins. <p> Casein in milk exists in the form of spherical-shaped colloid particles (~40-300 nm diameter), known as casein micelles (Fox & Brodkorb, 2008; McMahon & Oommen, 2008). Different models have been proposed for the structure of the casein micelle on the basis of the location of individual caseins (in response to their calcium sensitivity) and the calcium phosphate. These include: <p> sub-micelle model (Schmidt, 1982), in which sub-micelles are 'cemented' together by colloidal calcium phosphate (CCP) and [kappa]-casein-rich sub-micelles are mainly concentrated at the surface of the micelle; the hydrophilic C-terminal region of the [kappa]-casein orient into the serum as a highly hydrated 'hairy layer' that is in a state of constant flux and confers stability to the micelle by steric repulsion; <p> dual bonding model (Horne, 1998), in which the interior of the micelle is composed of [[alpha].sub.s]- and -caseins which form a lattice through interactions between hydrophobic regions (hydrophobic-induced) and between hydrophilic regions containing phosphoserine clusters (that attach to CCP clusters), while -casein molecules located at the surface interact hydrophobically with the other caseins ([[alpha].sub.s]- or -) and orient their highly hydrophilic regions (hairs) into the serum; <p> tangled, cross-linked web model (Holt & Horne, 1996), comprising a 'tangled' mass of rheomorphic casein chains cross-linked by calcium phosphate nanoclusters, similar in casein composition throughout but with the chains becoming more diffuse at the micelle periphery (on moving outwards from the dense centre); and <p> interlocked lattice model (McMahon & Oomen, 2008), featuring a system of interlocking sites composed of anchoring calcium phosphate nanoclusters (several hundred per micelle), which bind the phosphoserine domains of [[alpha].sub.s]- and -caseins; the hydrophobic ends of these caseins orientate away from the calcium phosphate nanocluster and interact hydrophobically with other [alpha]- and -caseins, while [kappa]-casein is predominantly surface located because of its lack of phosphoserine domains (to bind to the calcium phosphate nanoclusters) and its highly charged C-terminal regions (which prevents strong electrostatic interactions). <p> <p> In all of the above models, the arrangement of casein within the micelle is such that the interior is mainly occupied by the calcium-sensitive caseins ([[alpha].sub.s]- and -) and [kappa]-casein is principally located at the surface, with its hydrophilic C-terminal region (caseinomacropeptide) oriented outwards toward the serum phase in the form of protruding negatively charged hairs, which create an electrokinetic potential of ~-20 mV and confer stability to the micelle by electrostatic repulsion, Brownian movement and a consequent steric repulsion (de Kruif & Holt, 2003; Horne & Banks, 2004). The [kappa]-casein C-terminal projecting from the micelle surface has been considered as an extended polyelectrolyte brush (de Kruif, 1999), a region containing 14 carboxylic acid groups and immersed in a milk serum with a high ionic strength (~0.08 M) due to the presence of various ions (e.g. potassium, sodium, chloride, phosphate, citrate). Consequently, electrostatic interactions (between the C-terminal regions) at physiological conditions are very short and highly screened (by the high ionic strength). This is conducive to a high degree of 'solvency' and extension of the -casein C-terminal hairs and to the stability of the micelle as a whole. Moreover, the C-terminal region of the [kappa]-casein is glycoslyated to varying degrees (Table 1.2; Saito & Itoh, 1992; Molle & Leonil, 1995; Fox & McSweeney, 1998; Molle <i>et al</i>., 2006), containing galactose, <i>N</i>-acetylgalactosamine (GalNAc) and/or <i>N</i>-actetylneuraminic (sialic) acid (NANA) (Dziuba & Minkiewicz, 1996). These may further enhance the ability of [kappa]-casein to increase micelle stability by steric impedance and electrostatic repulsion via their contribution to increase in water binding (to carbohydrate moieties) and to negatively charged carboxylic groups (on the NANA molecule). O'Connell & Fox (2000) found that the level of glycosylation of [kappa]-casein and protein surface hydrophobicity increased as a function of micelle size. <p> While a predominant surface location of [kappa]-casein confers stability to the casein micelle in native milk, it renders it susceptible to aggregation/flocculation by processes which reduce the solvency of (and collapse/flatten) the [kappa]-casein hairs or remove them, and thereby enable contact between the more hydrophobic micelle cores, for example cleavage of the [kappa]-casein by acid proteinases, reducing the negative charge by acidification, reducing ionic strength by microfiltration/diafiltration at native pH. However, the interactions between the micelle cores are modified by many factors, including pH, composition of the serum phase, ionic strength, protein concentration and conditions to which milk is subjected (heat, acidification, ultrafiltration/diafiltration homogenisation, shearing). <p> The casein micelles on a dry weight basis consist of ~7 g 100 [g.sup.-1] ash (mainly calcium and phosphorous), 92 g 100 [g.sup.-1] casein and 1 g 100 [g.sup.-1] minor compounds including magnesium and other salts. They are present in milk at [10.sup.14]-[10.sup.16] [mL.sup.-1], are highly hydrated (~3.7 g [H.sub.2]O [g.sup.-1] protein), are spherical and have a diameter of ~80 nm (100-500 nm), a surface area of ~8 x [10.sup.-10] [cm.sup.2] and a density of ~1.063 g [cm.sup.-3] (Fox & McSweeney, 1998). <p> <p> <b>1.2.2</b> <i>Whey protein</i> <p> Whey protein in cows' milk is typically 0.60.7 g 100 [g.sup.-1] and consists of four main types-[beta]-lactoglobulin ([alpha]-Lg), -lactalbumin ([alpha]-La), immunoglobulin(s) (Ig) and bovine serum albumin (BSA) at levels of ~54, 21, 14 and 6 g 100 [g.sup.-1] of total (Table 1.2). The properties of the individual whey proteins have been extensively reviewed (Table 1.2; Mulvihill & Donovan, 1987; Brew, 2003; Fox, 2003; Hurley, 2003; Sawyer, 2003). In milk, they exist as soluble globular proteins and are characterised by a relatively high level of intramolecular disulphide bonding, and -Lg and BSA each contain one cysteine residue per mole. On heat-induced denaturation, the whey proteins can interact via thioldisulphide bonds with other whey proteins and with [kappa]-casein. The latter results in the formation of [kappa]-casein/-Lg aggregates either at the surface of the casein micelle or in the serum phase or both (cf. Chapter 8). The size and location (serum/micelle surface) of these aggregates are affected by severity of heat treatment of milk, pH at heating, ionic strength, calcium level and casein-to-whey protein ratio. The degree of interaction and size/location of aggregates have a profound effect on the structure and physical properties of rennet- and acid-induced milk gels, and hence on cheeses (see Chapter 8). For example, a high level of case in whey protein interaction, induced by high heat treatment of the milk (e.g. 95C for [is greater than or equal to] 1-2 min, ~[is greater than or equal to] 40% denaturation of total whey protein; Guinee et al., 1995), is highly favoured in the manufacture of yoghurt and smooth-textured cheeses with a high moisture-to-protein ratio, such as cream cheese and ultrafiltration-produced Quark. In these products it increases protein recovery and moisture binding (reduce syneresis), contributes smoothness and enhances yield (Guinee <i>et al</i>., 1993). In contrast, high heat treatment of milk is unsuitable for acid-curd cheeses with a granular structure (Cottage cheese) or for Quark manufactured using a mechanical separator, as it impedes whey expulsion during separation and makes it difficult to achieve the desired dry matter and texture characteristics. High heat treatment of milk is generally undesirable for rennet-curd cheeses as denatured protein at levels of [is greater than or equal to] 25% of total (at heat treatments of 82C for 26 s, or greater) impedes the ability of the milk to gel on rennet addition, causes marked deterioration in melt properties of the cheese (Rynne <i>et al</i>., 2004) and reduces the recovery of fat from milk to cheese (see Chapter 8). However, a higher-than-normal heat treatment that gives a moderate degree of whey protein denaturation may be desirable as a means of modulating the texture of reduced fat cheese, e.g. reduce firmness (Guinee, 2003; Rynne <i>et al</i>., 2004). <p> <p> <b>1.2.3</b> <i>Minerals</i> <p> Cows' milk contains ~0.75 g 100 [g.sup.-1] ash, which comprises [K.sup.+], [Ca.sup.2+], [Cl.sup.-], [P.sup.5+], [Na.sup.+] and [Mg.sup.2+] at concentrations (mg 100 [g.sup.-1]) of ~140, 120, 105, 95, 58 and 12, respectively (Table 1.2; White & Davies, 1958a; Chapman &Burnett, 1972; Keogh <i>et al</i>., 1982; Grandison <i>et al</i>., 1984; O'Brien <i>et al</i>., 1999c). These minerals are partitioned to varying degrees between the serum (soluble) and the casein (colloidal or insoluble) in native milk (pH ~6.6-6.7) at room temperature. Serum concentrations as a percentage of the total concentration for each of the minerals are ~100, 100, 100, 66, 43 and 34 for [Na.sup.+], [K.sup.+], [Cl.sup.-], [Mg.sup.2+], [P.sup.2+] and [Ca.sup.2+], respectively. The partition concentrations of [Ca.sup.2+] and [P.sup.2+] between the colloidal and soluble states in native milk is controlled mainly by the degree of ionisation of the casein (micelle), which in milk may be considered as a very large dominant anion that regulates the degree of binding of the counterion calcium, to an extent affected by the concentration of calcium <i>per se</i> and those of citric acid and phosphate. A major difference between the calcium salts of citrate (tricalcium citrate - [Ca.sub.3][([C.sub.6][H.sub.5][O.sub.7]).sub.2]) and phosphate (tricalcium phosphate - [Ca.sub.3][(P[O.sub.4]).sub.2]) is their solubility, with the solubility product of the latter being very low (2.07 x [10.sup.-33] mol [L.sup.-1] at 25C) compared to the former (3.23 x [10.sup.-3] mol L-1 at 25C). <p> <i>(Continues...)</i> <p> <p> <!-- copyright notice --> <br></pre> <blockquote><hr noshade size='1'><font size='-2'> Excerpted from <b>Technology of Cheesemaking</b> by <b>Barry A. Law Adnan Tamime</b> Copyright © 2001 by John Wiley & Sons, Ltd. Excerpted by permission.<br> All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.<br>Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.