Dairy Chemistry And Biochemistry

MILK LIPIDS 77

dairy products, they impart strong flavours which are undesirable in milk

or butter (they cause hydrolytic rancidity) but they contribute to the

desirable flavour of some cheeses, e.g. Blue, Romano, Parmesan.

2. Ruminant milk fats contain low levels of polyunsaturated fatty acids
   (PUFAs) in comparison with monogastric milk fats. This is because a
   high proportion of the fatty acids in monogastric milk fats are derived
   from dietary lipids (following digestion and absorption) via blood.
   Unsaturated fatty acids in the diet of ruminants (grass contains con-
   siderable levels of PUFAs) are hydrogenated by rumen micro-organisms
   unless protected by encapsulation (section 3.16.1). The low level of
   PUFAs in bovine milk fat is considered to be nutritionally undesir-
   able.


3. The milk fats from marine mammals contain high levels of long-chain,
   highly unsaturated fatty acids, presumably reflecting the requirement that
   the lipids of these species remain liquid at the low temperatures of their
   environments.


4. Ruminant milk fats are also rich in medium-chain fatty acids. These are
   synthesized in the mammary gland via the usual malonyl CoA pathway
   (section 3.5) and are released from the synthesizing enzyme complex by
   thioacylases; presumably, the higher levels of medium chain acids in
   ruminant milk fats compared with those of monogastric animals reflect
   higher thioacylase activity in the mammary tissue of the former.


5. The fatty acid profile of bovine milk fat shows a marked seasonal pattern,
   especially when cows are fed on pasture in summer. Data for Irish milk
   fat are shown in Figure 3.5; the changes are particularly marked for C,:,,
   C,,:, and c18:I. These changes affect the Reichert Meissl, Polenski and
   iodine (a measure of unsaturation) (Figure 3.6) numbers and the melting
   point and hardness (spreadability) of butter made from these milks:
   winter butter, with low levels of C4:o and c18:] and a high level of C,,:, is
   much harder than summer butter (Figure 3.7).


6. Unsaturated fatty acids may occur as cis or trans isomers; trans isomers,
   which have higher melting points than the corresponding cis isomers, are
   considered to be nutritionally undesirable. Bovine milk fat contains a low
   level (5%) of trans fatty acids in comparison with chemically hydro-
   genated (hardened) vegetable oils, in which the value may be 50% due to
   non-stereospecific hydrogenation.

Bovine milk fat contains low concentrations of keto and hydroxy acids
(each at c. 0.3% of total fatty acids). The keto acids may have the carbonyl
group (C=O) at various positions. The 3-keto acids give rise to methyl

ketones (R-C-CH,) on heating (high concentrations of methyl ketones
are produced in blue cheeses through the oxidative activity of Penicilliurn
roqueforti). The position of the hydroxy group on the hydroxy acids also

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