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Milk Composition & Synthesis
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Milk Composition
Milk Fat

Milk Fat :
  • is primarily triglycerides.
    - 3 fatty acids bound to 1 glycerol
  • is initially used by neonate for deposition of body adipose
  • is an energy source for the neonate.
  • is secreted as a milk fat globule.
    - bounded by a lipid-bylayer membrane
  • is extremely variable in concentration between species and within a species.
  • is dispersed when milk is homogenized.

Milk fat is composed of a complex mixture of lipids. Triglycerides are the major type of lipid in milk fat. Triglycerides are composed of three fatty acids covalently bound to a glycerol molecule by ester bonds. Milk fat is the major source of lipid used by the neonate mammal for accumulating body adipose in the initial days after birth. Most mammalian neonates are born with little body adipose for insulation or as a source of stored energy. A few days after birth most neonates begin to be able to metabolize milk fat as an energy source.

Adipocytes of the mammary fat pad. These fat cells are similar to the body adipose cells. Note the broad sheaths of dense connective tissue (stained red in this image) that course through the lobules of adipocytes. Histological section of mammary fat pad.

Milk fat is the most variable component of milk. Marine mammals have the highest fat contents, while some of the ungulates have the lowest milk fat contents.

Milk fat is secreted from mammary epithelial cells as fat globules which are primarily composed of a globule of triglyceride surrounded by a lipid bilayer membrane similar to the apical membrane of the epithelial cells (discussed in more detail in the Lesson on Milk Fat Synthesis). This fat globule membrane helps to stabilize the fat globules in an emulsion within the aqueous environment of milk (remember that cow milk is about 87% water). Lipid has a lower buoyant density than water, so when raw milk is centrifuged the fat rises to the top resulting in the cream layer (see image at right).

There are so many fat globules that they also carry some of the milk protein to the top, so cream also contains a small amount of protein in addition to the milk fat component; this protein component contributes to the whipping characteristics of cream. The stability of the fat emulsion in milk can be overcome by allowing raw milk to stand for a period of time, which results in the cream rising to the top. This is especially true for high fat milks such as those from the Jersey and Guernsey breeds.

Milk fat is the primary component of cream. Below milk was centrifuged to separate the cream and the skim milk (also called the plasma phase of milk).

Homogenization is the process by which fat globules in fluid milk are broken into sizes small enough that they will not rise in the milk to form cream under normal milk storage conditions. This is important for processing, storage and consumption of cow milk.

In past times dietary energy was at a premium for humans. Milk fat is a ready source of dietary energy and milk producers were paid a premium for the milk fat content of the milk they produced. In today's society in the US, dietary fat, especially saturated fats, are considered to have a negative impact on health. Whereas fifty years ago cows producing greater than 4 % milk fat were considered important, the average fat content of fluid milk consumed in the US today is less than 2%. More emphasis has been placed upon lower fat and higher protein content of cow milk.

Milk Fat
Milk Composition & Synthesis