•All fats belong to a group of chemical substances called esters, which are compounds of alcohols and acids. Milk fat is a mixture of different fatty acid esters called triglycerides.
•Triglycerides (triacylglycerols) account for 97-98% of the total fat in cows’ milk. The remaining lipid classes are di- and mono-acylglycerols, phospholipids, free fatty acids, and cholesterol and its esters.
•A triglyceride contains a glycerol backbone. This glycerol backbone is in red on the picture on this slide. Free fatty acids are attached to the glycerol backbone by an ester bond. Each glycerol molecule can bond three fatty acid molecules. About 437 fatty acids have been identified in the milk fat of cows.
These fatty acids all vary in chain length, number, position, and geometric isomerization of double bonds (cis or trans). If you are have not learned about fatty acids in your other classes yet, you can refer to the Wikipedia website
(http://en.wikipedia.org/wiki/Fatty_acid) or you can schedule a time to talk to me about it.
•A fatty acid molecule is composed of a hydrocarbon chain and a carboxyl group. In saturated fatty acids, the carbon atoms are linked together in a chain by single bonds. In unsaturated fatty acids, there are one or more double bonds in the hydrocarbon chain.
•Fatty acids in milk come from three main sources: from the feed, from the mobilization of reserve tissue, and from de novo (from the beginning) synthesis within the cow.
•The main source of lipid is from the cow’s food. Little synthesis of fatty acids occurs in the mammary gland. When the cow consumes food, the lipid is hydrolyzed to free fatty acids within the rumen of the cow. This means that unsaturated fatty acids are usually hydrogenated into saturated fatty acids.
Hydrogenation is the addition of hydrogen on unsaturated bonds between carbon atoms.
•Although it seems that milk fat would be mostly saturated in nature, over 30% of milk fat is still unsaturated. The unsaturated milk fat is predominately oleic acid (C18:1). Stearic acid (a saturated fatty acid) enters the mammary gland, but within the gland there is a specific C18:0 desaturase which converts stearic acid to the unsaturated oleic acid. As a result, there is a significant amount of unsaturated fat in milk. Just know that a desaturase is an enzyme which removes two hydrogen atoms from the fatty acid, creating a double bond, but you can learn more about these enzymes in upper level biochemistry classes.
•From the rumen, the fatty acids have two places they can go: 1. the bloodstream to be deposited as reserve fat or to be metabolized to produce energy, or 2. the mammary gland.
Distribution of fatty acids
•Milk fat of cows can be separated into long-chain and short-chain fatty acids fractions by distillation or chromatography. The short chain fraction contains mostly butyrate.
Milk Fat
Changes in milk fat characteristics
– dietary manipulation
– seasonal changes
•The characteristics of milk fat are highly dependent on the nature of the pre- formed fatty acids available in feedstuff. Therefore, it is possible to manipulate the composition of milk fat by dietary means.
•The physical properties of butterfat can be controlled by dietary means because the melting properties of the triglyceride are related to fatty acid composition. For example, by dietary manipulation, butter which can be spread directly from the refrigerator can be produced. On the other hand, butter that is physically stable at high temperatures can also be made.
•During colder winter months, there are more pre-formed fatty acids in the cows diet than during the summer months. During the summer, cows graze on grass and drink more water than in the winter months.
•This table lists the most important fatty acids in milk fat triglycerides.
•Milk fat is characterized by the presence of relatively large amounts of butyric and caproic acid.
•This table shows that the four most abundant fatty acids in milk are myristic, palmitic, stearic, and oleic acids. Myristic, palmitic, and stearic acids are solids at room temperature and oleic acid is a liquid at room temperature.
•The amounts of fatty acids in milk are variable. This variation affects the hardness of fat. Fat with a high content of high-melting fatty acids, such as palmitic acid, will be hard. However, fat with a high-content of low-melting oleic acid makes soft butter.
Fatty acid composition of human and bovine milk fat1
18:2+3 7.3 43 1.9 1.5
18:1 trans n/a n/a 6.4 2.5
18:1 cis n/a n/a 24.3 19.9
18:1 36.9 31.3 n/a n/a
18:0 3.2 3 15.8 11.6
17:0 n/a n/a 0.7 0.7
16:1 6.8 1.4 0.9 1.3
16:0 23.5 13.1 25.4 34.4
15:0 n/a n/a 1.1 1
14:0 9 2.3 9.8 12.7
12:0 7.9 3 2.7 3.9
10:0 n/a n/a 2.4 2.7
8:0 n/a n/a 0.9 1.1
6:0 n/a n/a 1.3 1.4
4:0 n/a n/a 3.6 3.5
Fat-free diet Corn oil diet Summer Winter
Fatty Acid2 Human Cow
1 Data are weight percent taken from studies of human and bovine milks.
2 Number of carbons in the chain colon number of double bonds. The cis and trans forms of 18:1 were only measured in the cow samples. The cis double bond was in the 9-position and the trans double bond in the 11-position.
•These data show the differences between the milk fat of humans and cows.
•The milk fat from the cow contains a set of unique short carbon chain fatty acids (4:0 – 10:0) and very little polyunsaturated fatty acids (18:2+3).
•These differences are a result of the digestive system of the cow. The cow has a rumen, which you all may already know about. It is basically a large fermentation tank in which food is broken down and modified by microorganisms before it goes into the intestines. The abundant polyunsaturated lipids in the grasses, legumes, silages, and grains consumed by the cow are hydrogenated (saturated) in the rumen, which is why cows milk has saturated fats.
•Acetic and beta-hydroxybutyric are products of fermentation in the rumen and they are absorbed directly from the rumen into circulation and are carried to the mammary gland where they are used to make short chain fatty acids.
•When the human mother was fed corn oil, which is rich in linoleic acid (18:2), she produced a level of 43% of linoleic acid in her milk fat.
