Food Biochemistry and Food Processing (2 edition)

BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come

452 Part 4: Milk

functions as a pro-oxidant, apparently as a complex with

Cu. Sulphydryl groups of proteins are also effective antiox-

idants; cream for butter making is usually heated to a high

temperature to denature proteins and expose and activate

sulphydryl groups.

Fat-Soluble Vitamins

Since the fat-soluble vitamins (A, D, E and K) in milk are derived

from the animal’s diet, large seasonal variations can be expected

in their concentration in milk. The breed of cow also has a

significant effect on the concentration of fat-soluble vitamins in

milk; high-fat milk (Jersey and Guernsey) has a higher content

of these vitamins than Friesian or Holstein milk. Variations in

the concentrations of fat-soluble vitamins in milk have a number

of consequences:


 Nutritionally, milk contributes a substantial portion of the

RDA (recommended daily allowance) for these vitamins

to Western diets; it is common practice to fortify milk and

butter with vitamins A and D.


 The yellow-orange colour of high-fat dairy products de-

pends on the concentrations of carotenoids and vitamin A

present, and hence on the diet of the animal. New Zealand

butter is much more highly coloured than Irish butter,

which in turn is much more yellow than American or Ger-

man products. The differences are due in part to the greater

dependence of milk production in New Zealand and Ireland

on pasture and to the higher proportion of carotenoid-rich

clover in New Zealand pasture and the higher proportion of

Jersey cows in New Zealand herds.


 Goats, sheep and buffalo do not transfer carotenoids to

their milk, which is, consequently, whiter than bovine milk.

Products produced from these milks are whiter than cor-

responding products made from bovine milk. The darker

colour of the latter may be unattractive to consumers accus-

tomed to caprine or ovine milk products. If necessary, the

carotenoids in bovine milk may be bleached (by benzoyl

peroxide) or masked (by chlorophyll or TiO 2 ).


 Vitamin E (tocopherols) is a potent antioxidant and con-

tributes to the oxidative stability of dairy products. The to-

copherol content of milk and meat can be readily increased

by supplementing the animal’s diet with tocopherols, which

is sometimes practised.

MILK PROTEINS

Introduction

Technologically, the proteins of milk are its most important

constituents. They play important, even essential, roles in all

dairy products except butter and anhydrous milk fat. The roles

played by milk proteins include the following:


 Nutritional: All milk proteins.


 Physiological: Igs, lactoferrin (Lf), lactoperoxidase,

vitamin-binding proteins, protein-derived biologically-

active peptides.

 Physico-chemical:


 Gelation. Enzymatically, acid- or thermally induced

gelation in all cheeses, fermented milks, whey protein

concentrates and isolates.


 Heat stability: All thermally processed dairy products,


 Surface activity: Caseinates, whey protein concentrates

and isolates.


 Rheological: All protein-containing dairy products.


 Water sorption: Most dairy products, comminuted meat

products.

Milk proteins have been studied extensively and are very well

characterised at molecular and functional levels (for reviews, see

Fox and McSweeney 2003).

Heterogeneity of Milk Proteins

It has been known since 1830 that milk contains two types of

protein which can be separated by acidification, to what we now

know is pH 4.6. The proteins insoluble at pH 4.6 are called

caseins and represent approximately 78% of the total nitrogen

in bovine milk; the soluble proteins are called whey or serum

proteins. As early as 1885, it was shown that there are two types

of whey protein, globulins and albumins, which were thought

to be transferred directly from the blood (the proteins of blood

and whey have generally similar physico-chemical properties

and are classified as albumins and globulins). Initially, the term

casein was not restricted to the acid-insoluble proteins in milk

but was used to describe all acid-insoluble proteins; however,

it was recognised at an early stage that the caseins are unique

milk-specific proteins.

The casein fraction of milk protein was considered initially

to be homogeneous but from 1918 onwards, evidence began to

accumulate that it is heterogeneous. Through the application of

free boundary electrophoresis (FBE) in the 1930s and especially

zone electrophoresis in starch or polyacrylamide gels (SGE,

PAGE) containing urea and a reducing agent in the 1960s, it has

been shown that casein is in fact very heterogeneous. Bovine

casein consists of four families of caseins:αs1-,αs2-β- andκ-,

which represent about 38%, 10%, 36% and 12%, respectively,

of whole casein. Urea-PAGE showed that each of the casein

families exhibits micro-heterogeneity due to:


 genetic polymorphism, usually involving substitution of

one or two amino acids;


 variations in the degree of phosphorylation;


 variations in the degree of glycosylation ofκ-casein;


 inter-molecular disulphide bond formation inαs2- and

κ-caseins;


 limited proteolysis, especially ofβ- andαs2-caseins, by

plasmin; the resulting peptides include theγ- andλ-caseins

and proteose peptones.

In the 1930s, FBE showed that both the globulin and albumin

fractions of whey protein are heterogeneous and, in the 1950s,

the principal constituents were isolated and characterised. It is

now known that the whey protein fraction of bovine milk com-

prises four main proteins:β-lactoglobulin (β-Lg),α-La, Igs and