Food Biochemistry and Food Processing

19 Chemistry and Biochemistry of Milk Constituents 437

Lipid oxidation can be prevented or controlled by

• Avoiding metal contamination at all stages of
  processing through the use of stainless steel
  equipment.


• Avoiding exposure to UV light by using opaque
  packing (foil or paper).


• Packaging under an inert atmosphere, usually N 2.


• Use of O 2 or free radical scavengers, for
  example, glucose oxidase and superoxide
  dismutase (indigenous enzymes in milk),
  respectively.


• Use of antioxidants that break the free-radical
  chain reaction; synthetic antioxidants are not
  permitted in dairy products, but the level of
  natural antioxidants, for example, tocopherols
  (vitamin E), in milk may be increased by
  supplementing the animal’s feed. Polyphenols
  are very effective antioxidants; their direct
  addition to dairy products is not permitted, but it
  may be possible to increase their concentration in
  milk by supplementing the animal’s feed.
  Antioxidants are compounds that readily supply
  a H•to fatty acid and peroxy radicals, leaving a
  stable oxidized radical. Many antioxidants are
  polyphenols, which give up a H•and are
  converted to a quinone; examples are tocopherols
  (vitamin E) and catechins. At low concentrations,
  ascorbic acid is a good antioxidant, but at high
  concentations it functions as a prooxidant,
  apparently as a complex with Cu. Sulphydryl
  groups of proteins are also effective antioxidants;
  cream for butter making is usually heated to a
  high temperature to denature proteins and
  expose/activate sulphydryl groups.

FAT-SOLUBLEVITAMINS

Since the fat-soluble vitamins (A, D, E, and K) in
milk are derived from the animal’s diet, large season-
al variations in their concentration in milk can be
expected. 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 high-
er 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 for these vitamins to Western

diets; it is common practice to fortify milk and

butter with vitamins A and D.

• The yellow-orange color of high-fat dairy
  products depends on the concentrations of caro-
  tenoids and vitamin A present, and hence on the
  diet of the animal. New Zealand butter is much
  more highly colored than Irish butter, which in
  turn is much more yellow than American or
  German 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 corresponding
  products made from bovine milk. The darker
  color of the bovine milk may be unattractive to
  consumers accustomed 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 contributes to the oxidative stability of dairy
  products. The tocopherol contents of milk and
  meat can be readily increased by supplementing
  the animal’s diet with tocopherols, which is
  sometimes practiced.

MILK PROTEINS

INTRODUCTION

Technologically, the proteins of milk are its most im-

portant constituents. They play important, even es-

sential, roles in all dairy products except butter and

anhydrous milk fat. The roles played by milk proteins

are nutritional, physiological, and physicochemical:

• Nutritional:All milk proteins.


• Physiological:Immunoglobulins, lactoferrin,
  lactoperoxidase, vitamin-binding proteins,
  protein-derived biologically active peptides.


• Physicochemical:
  
  
  • Gelation.Enzymatically, acid or thermally
    induced gelation in all cheeses, fermented
    milks, whey protein concentrates and isolates.
  
  
  • Heat stability.All thermally processed dairy
    products.