Food Biochemistry and Food Processing (2 edition)

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466 Part 4: Milk

particles known as casein micelles, with an average diameter of

approximately 150 nm (range 50–600 nm) and containing, on

average, approximately 5000 protein molecules.

Owing to its physical state, milk is an unstable system. The

fat globules, which are less dense than the aqueous phase, rise

to the surface where they form a cream layer. The fat-rich cream

can be skimmed off and used for the manufacture of butter,

butter oil, ghee or other fat-rich products. Alternatively, the fat-

depleted lower layer (skimmed milk) may be run off, for example

through a valve at the bottom of a separating vat. The skimmed

milk may be consumed directly as a beverage or used for the

manufacture of a wide range of products. Gravity creaming and

butter manufacture have been used since prehistoric times, but

gravity creaming has been largely replaced by centrifugal sep-

aration of the fat since the development of the cream separator

by Gustav de Laval in 1878. Gravity separation is still used to

prepare reduced-fat milk for the manufacture of Parmigiano-

Reggiano cheese. The physics of fat separation by gravity or

centrifugal separation and the process of demulsifying milk fat

to butter or butter oil will be described briefly later in this chapter.

Creaming (fat separation) is undesirable in many dairy prod-

ucts (e.g., liquid/beverage milk, concentrated milks) and is pre-

vented by a process known as homogenisation (the commonly

used valve homogeniser was developed by Auguste Gaulin in

1899). Homogenisation prevents creaming by reducing the size

of the fat globules and preventing their agglomeration (cluster-

ing), by denaturing a particular minor protein known as cryo-

globulin (type M immunoglobulin).

The casein micelles are physicochemically stable but can be

destabilised by a number of processes/treatments, which are ex-

ploited for the production of new dairy products. The most im-

portant of these are limited proteolysis and acidification, which

are used in the production of cheese, fermented milks and func-

tional milk proteins. The mechanism and consequences of pro-

teolysis and acidification will be described briefly. Thus, the

three main constituents/phases of milk, that is fat, casein and the

aqueous solution (whey), can be separated easily.

Milk is remarkably heat stable; it can be sterilised by heat

in standard or concentrated form, or dried to produce a wide

range of dairy products. The principal processes and the re-

sulting product families are summarised in Table 25.1. All

these families of dairy products contain many products, for

example approximately 1400 varieties of cheese are produced

worldwide.

In addition to the above considerations, milk is a rich medium

for the growth of a wide range of microorganisms. While this

is exploited in the production of a range of fermented dairy

products, it also means that milk can harbour microorganisms

that may cause spoilage of products, or present health risks to

the consumer. Largely for the latter reason, very little raw milk is

now consumed, the vast majority being heat treated sufficiently

severe to kill all pathogenic and food-poisoning bacteria.

Table 25.1.Diversity of Dairy Products Produced by Different Processes

Process Primary (italic) and Secondary Products

Heat treatment (generally with

homogenisation)

Market milk

Centrifugal separation Standardised milk

Cream

Butter, butter oil, ghee

Creams of various fat content (pasteurised or UHT treated); whipping, coffee,

dessert creams

Skim milk

Skim milk powder, casein, cheese, protein concentrates

Concentration (e.g., evaporation or

membrane separation)

In-container or UHT-sterilised concentrated milks, sweetened condensed milk

Concentration and drying Whole milk powders, infant formulae

Enzymatic coagulation Cheese

Numerous varieties, processed cheese, cheese sauces and dips

Rennet casein; cheese analogues

Whey

Whey powders; demineralised whey powders; whey protein concentrates;

fractionated whey proteins; whey protein hydrolysates; lactose and lactose

derivatives

Acid coagulation Cheese (fresh)

Acid casein; functional proteins

Whey(as rennet whey)

Fermentation Various fermented milk products, e.g., yoghurt, buttermilk, acidophilus milk,

bioyoghurt

Freezing, aeration and whipping Ice cream

Source: Adapted From Fox and McSweeney 1998.

UHT, ultra-high temperature.