MILK  FATS

Milk fats of mammals generally differ radically from the fat depots. The greatest differences are seen at the level of the short and medium chain fatty acids (from 4 to 12 carbon atoms) and about 3% butyric acid (C4:0).
If the fat content of milk is not very high (1.3% in horse, about 4% in cow, goat, camel or human, 7.5% in sheep), fats account for approximately one third of the dry matter.
In total, roughly 20 million tons of milk fats are produced per year, 85% being from cows leading to about 7 million tons of butter (2007-08).

Milk fats are the most complex dietary fats. It was reported that dairy fat contains about 40 major fatty acids but a total of about 400 minor ones have been detected (Jensen R G, J Dairy Sci 2002, 85, 295). A more recent study using several technical steps reported the detection of 430 fatty acid methyl esters, including cyclohexyl, methyl-branched, hydroxylated, and conjugated fatty acids (Schroder M et al., JAOCS 2013, 90, 771).
Despite that complexity, the study of the fatty acid composition of milks enabled the determination of the area of their origin (Gaspardo B et al., J Dairy Sci 2010, 93, 3417).
A unique feature of dairy fat is the occurrence of trans fatty acids (vaccenic and rumenic acids). The archaeological presence of milk in preserved sites may be asserted in determining the stable carbon isotope (d¹³C) compositions of individual fatty acids since milk and adipose fat from animals raised on similar pastures have distinct isotopic signatures (Evershed RP et al., Acc Chem Res 2002, 35, 660).

The average fatty acid compositions (weight percent) of six important milk fats are given in the table below:

Data from Devie H et al., Eur J Lipid Sci Technol 2012, 114, 1036 
and Zou X et al., J Agric Food Chem 2013, 61, 7070

It must be noticed that milk from ruminant animals (cow, buffalo, goat, ewe, camel…) contains butyric acid (4:0), in contrast to others (Human, horse, donkey) which have low amounts of this fatty acid produced by the bacterial hydrogenation in stomach.
The fatty acid composition of milk fats is under the influence of the diet but the positional distribution of fatty acids is not. The location of the short chain acids appears restricted to the position 3 and only one short chain can occur per molecule. 
In human milk fat the most abundant triacylglycerol species are : PPO (20.6%), OPO (19%), POL (16,7%), and POLa (10.9%) (Zou XQ et al., J Agric Food Chem 2013, 61, 167). A review on the structure of triacylglycerols contained in milk lipids and its role in infant nutrition has been reviewed (Innis SM, Adv Nutr 2011, 2, 275). 
Human milk fat substitute has been developed from vegetable oils (Betapol from Loders Croklaan). The product has a structure similar to human milk fat, matching its fatty acid composition and fatty acid distribution (about 70% of palmitic acid at the sn-2 position). Betapol production involves position-targeted reactions catalyzed by 1,3-specific lipases.

Specific distribution and association of the fatty acids can be deduced from the table given below:

(Data for ewe, goat and donkey are  from Blasi F et al., J Food Comp Anal 2008, 21, 1)