Upon standing for 12 to 24 hours, fresh milk has a tendency to separate into a high-fat cream layer on top of a larger, low-fat milk layer. The cream is often sold as a separate product with its own uses; today the separation of cream from milk is usually accomplished rapidly in centrifugal cream separators. The fat globules rise to the top of a container of milk because fat is less dense than water.
The fat globules in milk from goats, sheep, and water buffalo do not form clusters so readily and are smaller to begin with, so the cream is very slow to separate from these milks.
October 2007 Grand Champion Senior Doe, Finesse in Raleigh, NC
Homogenization and Pasteurization
Milk is often homogenized, a treatment which prevents a cream layer from separating out of the milk. The milk is pumped at high pressures through very narrow tubes, breaking up the fat globules through turbulence and cavitation. Homogenized milk tastes blander but feels creamier in the mouth than unhomogenized; it is whiter and more resistant to developing off flavors. Creamline, or cream-top milk is unhomogenized, and it may or may not have been pasteurized. Unlike pasteurization, homogenization confers no health or safety benefits to the milk, only the convenience of not needing to shake the bottle oneself.
Pasteurization is not intended to kill all of the pathogenic micro-organisms in the food or liquid, but to achieve a reduction in the number of viable organisms, reducing their number so that they are unlikely to cause disease (assuming the pasteurised product is refrigerated and consumed before its expiration date). Pasteurisation typically uses temperatures below boiling since at temperatures above the boiling point for milk will make it “curdle”.
Unhomogenized milk has made a small comeback in a few areas, such as the west coast of the United States where Straus Family Creameries, based originally out of Sonoma, sells one line of organic milk with the cream still on top in old-fashioned glass bottles. They still however pasteurize it to prevent harmful microorganisms.
Nutrition and health
The composition of milk differs widely between species. Factors such as the type of protein; the proportion of protein, fat, and sugar; the levels of various vitamins and minerals; and the size of the butterfat globules and the strength of the curd are among those than can vary.
Studies show possible links between low-fat milk consumption and reduced risk of arterial hypertension, coronary heart disease, colorectal cancer and obesity. Overweight individuals who drink milk may benefit from decreased risk of insulin resistance and type 2 diabetes.
Conjugated linoleic acid is a beneficial fatty acid that inhibits several types of cancer in mice, it has been shown to kill human skin cancer, colorectal cancer and breast cancer cells in vitro studies, and may help lower cholesterol and prevent atherosclerosis; only available in milk from grass-fed cows.
Interestingly, a study has shown that for women desiring to have a child, those who consume full fat dairy products may actually slightly increase their fertility, while those consuming low fat dairy products may slightly reduce their fertility due to interference with ovulation. However, studies in this area are still inconsistent.
Cow milk (whole)
Nutritional value per 100 g (3.5 oz)
Source: USDA Nutrient database
Percentages are relative to US recommendations for adults.
Energy 60 kcal 250 kJ
Carbohydrates 5.2 g
- Sugars 5.2 g
– Lactose 5.2 g
Fat 3.25 g
- saturated 1.9 g
- monounsaturated 0.8 g
- polyunsaturated 0.2 g
Protein 3.2 g
Water 88 g
Vitamin A equiv. 28 µg 3%
Thiamin (Vit. B1) 0.04 mg 3%
Riboflavin (Vit. B2) 0.18 mg 12%
Vitamin B12 0.44 µg 18%
Vitamin D 40 IU 20%
Calcium 113 mg 11%
Magnesium 10 mg 3%
Potassium 143 mg 3%
Vitamins D and K are essential for bone health.
Iodine is a mineral essential for thyroid function.
Vitamin B12 and riboflavin are necessary for cardiovascular health and energy production, and B12 is difficult to get outside of animal products or else as supplemental pills.
Biotin and pantothenic acid are B vitamins important for energy production.
Vitamin A is critical for immune function.
Potassium and magnesium are for cardiovascular health.
Selenium is a cancer-preventive trace mineral.
Thiamine is a B-vitamin important for cognitive function, especially memory.