Food Biochemistry and Food Processing (2 edition)

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25 Biochemistry of Milk Processing 471

multi-stage, with integrated or external fluidised bed drying

steps, choice of atomiser, air inlet and outlet temperatures) de-

pends on the final product characteristics required. Because of

a wide range of applications of milk powders, from simple re-

constitution to use in cheese manufacture or incorporation as an

ingredient into complex food products, the precise functionality

required for a powder can vary widely.

Skim milk powders (SMP) are usually classified on the basis

of heat treatment (mostly meaning the intensity of preheating

during manufacture), which influences their flavour and solubil-

ity (Car ́ıc and Kalab 1987, Kyle 1993, Pellegrino et al. 1995).

Instant powders (readily dispersible in cold or warm water) are

desirable for applications requiring reconstitution in the home,

and are produced by careful production of agglomerated pow-

ders containing an extensive network of air spaces that can fill

rapidly with water on contact (Pisecky 1997). A number of ́

techniques are used to produce agglomerated powders, includ-

ing feeding fines (small powder particles) back to the atomisa-

tion zone during drying (in straight-through agglomeration pro-

cesses) or wetting dry powder in chambers that promote aggre-

gation of moistened particles that, when subsequently dried, pro-

duce porous agglomerated powder particles (rewet processes).

Milk powder contains occluded air (i.e., contained in vacuoles

within individual powder particles) and interstitial air (i.e., en-

trapped between neighbouring powder particles). The amount of

occluded air depends on the heat treatment applied to the feed

(whey protein denaturation affects foaming properties), method

of atomisation and outlet air temperature, while the interstitial

air content of a powder depends on the size, shape and surface

geometry of individual powder particles. The drying process can

be manipulated (e.g., by using multiple stages or by returning

fines to the atomisation zone) to increase the levels of interstitial

and occluded air and instantise the powder (Kelly et al. 2003).

In the case of whole milk powders (WMP), a key characteristic

is the state of the milk fat, whether fully emulsified or partially

in free form (the latter is desirable for WMP used in choco-

late manufacture). The production of instant WMP requires the

use of an amphiphilic additive such as lecithin, as well as ag-

glomeration, to overcome the intrinsic hydrophobicity of milk

fat (Jensen 1975, Kim et al. 2009). Lecithin may be added ei-

ther during multi-stage drying of milk (between the main drying

chamber and the fluidised bed dryer) or separately in a rewet

process.

During spray-drying of milk, there is relatively little change

in milk proteins, apart from that caused by preheating and evap-

oration. The constituent most affected by the drying stageper se

is probably lactose that, on the rapid removal of moisture from

the matrix, assumes an amorphous glassy state, which is hygro-

scopic and can readily absorb moisture if the powder is exposed

to a high relative humidity. This can result in the crystallisation

of lactose, with a concomitant uptake of water, which can cause

caking and plasticisation of the powder (Kelly et al. 2003).

Amorphous lactose is the principal constituent of both SMP

and WMP and forms the continuous matrix in which proteins, fat

globules and air vacuoles are dispersed. In products containing a

very high level of lactose (e.g., whey powders, SMP), the lactose

may be pre-crystallised to avoid the formation of an amorphous

glass. Usually, small lactose crystals are added to milk or whey

concentrate prior to drying to promote crystallisation under rel-

atively mild conditions (i.e., in the liquid rather than the powder

form); the added crystals act as nuclei for crystallisation.

Freezing of Milk

When milk is cooled to temperatures below approximately

–0.54◦C, pure ice crystals form, and the remainder of the milk,

thus, becomes progressively more concentrated at lower tem-

peratures. This results in changes in milk that are analogous

to those occurring on concentration by other means, including

decrease in pH and increase in Ca^2 +activity. At temperatures

below approximately –23◦C, the non-frozen portion of milk is

in a glassy state, and changes cease, due to diffusion coefficients

being close to zero. On thawing of frozen milk, aggregation of

casein micelles due to salting out may occur, as may clumping of

fat globules, due to mechanical damage to globule membranes

by ice crystals.

CHEESE AND FERMENTED MILKS

Introduction

About 35% of total world milk production is used to produce

cheese (∼ 16 × 106 tonnes per annum), mainly in Europe, North

and South America, Australia and New Zealand. Cheese man-

ufacture essentially involves coagulating the casein micelles to

form a gel that entraps the fat globules, if present; when the

gel is cut or broken the casein network contracts (synereses),

expelling whey. The resulting curds may be consumed fresh as

mild-flavoured products, or ripened for a period ranging from

2 weeks, for example for Mozzarella, to>2 years, for example

for Parmigiano-Reggiano.

Cheese is undoubtedly the most biochemically complex dairy

product. However, since the action of rennet on milk and dairy

fermentations are described in Chapters 33 and 27, respectively,

only a brief summary is given here. For more detailed discus-

sion of the science and technology of cheesemaking, see also

Robinson and Wilbey (1998), Law (1999), Eck and Gillis (2000)

and Fox et al. (2004), McSweeney (2007) and a number of arti-

cles in the Encyclopedia of Dairy Science (2011).

The coagulation of milk for cheese is achieved by one of three

methods:

1. Rennet-induced coagulation, which is used for most
   ripened cheeses, and accounts for approximately 75% of
   total cheese production,


2. Acidification to pH approximately 4.6 at 30–36◦Cbyin
   situproduction of acid by fermentation of lactose to lac-
   tic acid by lactic acid bacteria (LAB;Lactococcus,Lac-
   tobacillusorStreptococcus) or direct acidification with
   acid or acidogen (e.g., gluconic acid-δ-lactone). Most
   acid-coagulated cheeses are consumed fresh and repre-
   sent approximately 25% of total cheese production. Major
   examples of acid-coagulated cheeses are Cottage, Quarg
   and Cream cheeses,