Dairy Chemistry And Biochemistry

MILK LIPIDS 137

point decreases as the number of double bonds in the molecule increases

(Figure 3.36) and cis isomers have lower melting points than the corre-

sponding trans isomers (Figure 3.37). The melting point of both cis and trans

isomers increases as the double bond moves from the carboxyl group

towards the o-carbon.

Symmetrical triglycerides have a higher melting point than asymmetrical

molecules containing the same fatty acids (Table 3.16).

As discussed in section 3.6, the fatty acids in milk fat are not distributed

randomly and the melting point may be modified by randomizing the fatty

acid distribution by transesterification using a lipase or chemical catalysts.

3.16.2 Process parameters

Temperature treatment of cream. The melting point of lipids is strongly

influenced by the crystalline form, ct, fl, fl', which is influenced by the

structure of the triglycerides and by the thermal history of the product. The

hardness of butter can be reduced by subjecting the cream to one of a

variety of temperature programmes, which may be automated. The classical

example of this is the Alnarp process, a typical example of which involves

cooling pasteurized cream to c. 8"C, holding for c. 2 h, warming to 20°C,

holding for c. 2 h and then cooling to c. 10°C for churning. More compli-

cated schedules may be justified in certain cases.

All these treatments exert their effect by controlled crystal growth, e.g.

larger, fewer crystals adsorb less liquid fat and there is less formation of

mixed (liquid-solid) crystals due to reduced supercooling.

Work softening (microfixing). The liquid fat in butter crystallizes during
cold storage after manufacture, forming an interlocking crystal network and
resulting in increased hardness. Firmness can be reduced by 50-55% by
disrupting this network, e.g. by passing the product through a small orifice
(Figure 3.38) (the hardness of margarine can be reduced by 70-75% by a
similar process; the greater impact of disrupting the crystal network on the
hardness of margarine makes margarine appear to be more spreadable than
butter even when both contain the same proportion of solid fat). Microfix-
ing is relatively more effective when a strong crystal network has formed, i.e.
when setting is at an advanced stage (e.g. after storage at 5°C for 7days).
The effect of microfixing is reversed on storage or by warming/cooling, i.e.
is essentially a reversible phenomenon (Figure 3.38).

Fractionation. The melting and spreading characteristics of butter can be
altered by fractional crystallization, i.e. controlled crystallization of molten
fat or crystallization from a solution of fat in an organic solvent (e.g. ethanol
or acetone). Cleaner, sharper fractionation is obtained in the latter but
solvents may not be acceptable for use with foods. The crystals formed may