Dairy Chemistry And Biochemistry

118 DAIRY CHEMISTRY AND BIOCHEMISTRY

may be improved by:

0 using fresh milk;

0 adding disodium phosphate or sodium citrate, which sequester Ca2+,

increase protein charge and dissociate casein micelles;

0 standardizing the cream with buttermilk which is a good emulsifier owing

to its high content of phospholipids.

‘Bitty cream’ is caused by the hydrolysis of phospholipids of the fat

globule membrane by phospholipases secreted by bacteria, especially Bacil-

lus cereus, but also by psychrotrophs; the partially denuded globules

coalesce when closely packed, as in cream or in the cream layer of milk,

forming aggregates rather than a solid mass of fat.

3.11.1 Free fat

‘Free fat’ may be defined as non-globular fat, i.e. fat globules from which the

membrane has been totally or partially removed. Damage to fat globules

may be determined by measuring the level of free fat present. The fat in

undamaged globules is not extractable by apolar solvents because it is

protected by the membrane, damage to which permits extraction, i.e. the

amount of fat extractable by apolar solvents is termed ‘free fat’.

Free fat may be determined by a modified Rose-Gottlieb method or by

extraction with carbon tetrachloride (CCl,). In the standard Rose-Gottlieb

method, the emulsion is destabilized by the action of ammonia and ethanol

and the fat is then extracted with ethyl/petroleum ether. The free fat in a

sample may be determined by omitting the destabilization step, i.e. by

extracting the product directly with fat solvent, and expressed as the

percentage of free fat in the sample or as a percentage of total fat.

Alternatively, the sample may be extracted with CCl,. In both methods, the

sample is shaken with the fat solvent; the duration and severity of shaking

must be carefully standardized if reproducible results are to be obtained.

Other methods used to quantify free fat include: centrifugation in

Babcock or Gerber butyrometers at 40-60°C (the free fat is read off directly

on the graduated scale); release of membrane-bound enzymes, especially

xanthine oxidase or alkaline phosphatase, or the susceptibility of milk fat to

hydrolysis by added lipase (e.g. from Geotrichum candidum).

3.12 Churning

It has been known since prehistoric times that if milk, and especially cream,
is agitated, the fat aggregates to form granules (grains) which are converted
to butter by kneading (Figure 3.26). Buttermaking has been a traditional
method for a very long time in temperate zones for conserving milk fat; in