Food Biochemistry and Food Processing (2 edition)

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448 Part 4: Milk

chylomicrons are derived from the animal’s feed or synthesised

in the liver. Butanoic acid (C4:0) is produced by the reduction of

β-hydroxybutyrate, which is synthesised from dietary roughage

by bacteria in the rumen and therefore varies substantially with

the animal’s diet. All C6:0–C14:0and 50% of C16:0are synthe-

sised in the mammary glandviathe malonylCoA pathway from

acetylCoA produced from acetate synthesised in the rumen.

Essentially 100% of C18:0,C18:1,C18:2and C18:3and 50% of

C16::0are derived from blood lipids (chylomicrons) and repre-

sent approximately 50% of total fatty acids in ruminant milk

fat. Unsaturated fatty acids in the animal’s diet are saturated by

bacteria in the rumen unless they are protected, for example by

encapsulation.

Mainly due to saturation in the rumen, ruminant milk fats are

quite saturated, approximately 65% of the fatty acids in bovine

milk fat are saturated, and are considered nutritionally undesir-

able although not all to an equal extent. However, according to

Parodi (2009), the case against saturated fatty acids as causative

factors for coronary heart disease is not proven and further re-

search is required.

When milk production is seasonal, for example Australia,

New Zealand and Ireland, very significant changes occur in the

fatty acid profile of milk fat throughout the production season

(see Fox 1995, Fox and McSweeney 1998, 2006). These varia-

tions are reflected in the hardness of butter produced from such

milk; the spreadability of butter produced in winter is much

lower than that of summer butter. Owing to the lower degree

of unsaturation, winter butter should be less susceptible to lipid

oxidation than the more unsaturated summer product but the

reverse appears to be the case, probably owing to higher levels

of pro-oxidants, for example Cu and Fe, in winter milk.

Although a ruminant’s diet, especially if grass-based, is rich

in polyunsaturated fatty acids (PUFA), these are hydrogenated

by bacteria in the rumen and, consequently, ruminant milk fat

contains very low levels of PUFAs, for example bovine milk fat

contains approximately 2.4% C18:2compared to approximately

13% in human or porcine milk fat. PUFAs are considered to be

nutritionally desirable and consequently there has been interest

in increasing the PUFA content of bovine milk fat. This can

be done by feeding encapsulated PUFA-rich lipids or crushed

PUFA-rich oil seed to the animal. Increasing the PUFA content

also reduces the melting point (MP) of the fat and makes butter

produced from it more spreadable. However, the lower MP fat

may have undesirable effects on the rheological properties of

cheese, and PUFA-rich dairy products are very susceptible to

lipid oxidation. Although the technical feasibility of increasing

the PUFA content of milk fat by feeding protected PUFA-rich

lipids to the cow has been demonstrated, it is not economical

to do so in most cases. Blending milk fat with PUFA-rich or

C18:1-rich vegetable oil appears to be much more viable and is

now widely practised commercially.

Conjugated Linoleic Acid

Linoleic acid (cis, cis9, 12-octadecadienoic acid) is the princi-

pal essential fatty acid and has been the focus of nutritional re-

search for many years. However, conjugated isomers of linoleic

acid (CLA) have attracted very considerable attention recently

(for review, see Bauman and Lock 2006). CLA is a mixture of

eight positional and geometric isomers of linoleic acid, which

have a number of health-promoting properties, including anti-

carcinogenic and anti-atherogenic activities, reduction of the

catabolic effects of immune stimulation and the ability to en-

hance growth and reduce body fat (see Parodi 1999, Yurawecz

et al. 1999). Of the eight isomers of CLA, only thecis-9,trans-

11 isomer is biologically active. This compound is effective at

very low concentrations, 0.1 g/100 g diet.

Fat-containing foods of ruminant origin, especially milk and

dairy products, are the principal sources of dietary CLA, which

is produced as an intermediate during the biohydrogenation of

linoleic acid by the rumen bacterium,Butyrivibrio fibrisolvens.

Since CLA is formed from linoleic acid, it is not surprising that

the CLA content of milk is affected by diet and season, being

highest in summer when cows are on fresh pasture rich in PUFAs

(Lock and Garnsworthy 2000, Lawless et al. 2000) and is higher

in the fat of milk from cows on mountain pasture than on lowland

pasture (Collomb et al. 2002). The concentration of CLA in milk

fat can be increased five to seven folds by increasing the level

of dietary linoleic acid, for example by duodenal infusion (Kraft

et al. 2000) or by feeding a linoleic acid-rich oil, for example

sunflower oil (Kelly et al. 1998)

A number of other lipids may have anticarcinogenic activity,

for example sphingomyelin, butanoic acid and ether lipids, but

little information is available on these to date (Parodi 1997,

1999)

Structure of Milk Triglycerides

Glycerol for milk lipid synthesis is obtained in part from hy-

drolysed blood lipids (free glycerol and monoglycerides), partly

from glucose and a little from free blood glycerol. Synthesis

of triglycerides within the cell is catalysed by enzymes located

on the endoplasmic reticulum. Esterification of fatty acids is not

random (Table 24.1). The concentrations of C4:0and C18:1appear

to be rate-limiting because of the need to keep the lipid liquid at

body temperature. Some notable features of the structure are as

follows:


 Butanoic and hexanoic acids are esterified almost entirely,

and octanoic and decanoic acids predominantly, at thesn-3

position.


 As the chain-length increases up to C16:0, an increasing

proportion is esterified at thesn-2 position; this is more

marked for human than for bovine milk fat, especially in

the case of palmitic acid (C16:0).


 Stearic acid (C18:0) is esterified mainly atsn-1.


 Unsaturated fatty acids are esterified mainly at thesn-1 and

sn-3 positions, in roughly equal proportions.

The fatty acid distribution is significant from two viewpoints:

1. It affects the MP and hardness of the fat, which can be
   reduced by randomising the fatty acid distribution. Trans-
   esterification can be performed by treatment with SnCl 2
   or enzymatically under certain conditions: increasing