Milk is an emulsion of butterfat
globules within a water-based fluid. Each fat globule is surrounded by a
membrane consisting of phospholipids and proteins; these emulsifiers keep the
individual globules from joining together into noticeable grains of butterfat
and also protect the globules from the fat-digesting activity of enzymes found
in the fluid portion of the milk. In unhomogenized cow's milk, the fat globules
average about four micrometers across. The fat-soluble vitamins A, D, E, and K
are found within the milkfat portion of the milk (McGee 18).
Schematic
of a micelle.
The largest structures in
the fluid portion of the milk are casein protein micelles: aggregates of
several thousand protein molecules, bonded with the help of nanometer-scale
particles of calcium phosphate. Each micelle is roughly spherical and about a
tenth of a micrometer across. There are four different types of casein
proteins, and collectively they make up around 80 percent of the protein in
milk, by weight. Most of the casein proteins are bound into the micelles. There
are several competing theories regarding the precise structure of the micelles,
but they share one important feature: the outermost layer consists of strands
of one type of protein, kappa-casein, reaching out from the body of the micelle
into the surrounding fluid. These Kappa-casein molecules all have a negative electrical
charge and therefore repel each other, keeping the micelles separated under
normal conditions and in a stable colloidal suspension in the water-based
surrounding fluid[3] (McGee 19–20).
Both the fat globules and
the smaller casein micelles, which are just large enough to deflect light,
contribute to the opaque white color of milk. The fat globules contain some
yellow-orange carotene, enough in some breeds — Guernsey and Jersey cows, for
instance — to impart a golden or "creamy" hue to a glass of milk. The
riboflavin in the whey portion of milk has a greenish color, which can
sometimes be discerned in skim milk or whey products (McGee 17). Fat-free skim
milk has only the casein micelles to scatter light, and they tend to scatter
shorter-wavelength blue light more than they do red, giving skim milk a bluish
tint.[4]
A
simplified representation of a lactose molecule being broken down into glucose
and galactose.
Milk contains dozens of
other types of proteins besides the caseins. They are more water-soluble than
the caseins and do not form larger structures. Because these proteins remain
suspended in the whey left behind when the caseins coagulate into curds, they
are collectively known as whey proteins. Whey proteins make up around
twenty percent of the protein in milk, by weight. Lactoglobulin is the most
common whey protein by a large margin (McGee 20–21).
The carbohydrate lactose
gives milk its sweet taste and contributes about 40% of whole cow milk's
calories. Lactose is a composite of two simple sugars, glucose and galactose. In
nature, lactose is found only in milk and a small number of plants (McGee 17). Other
components found in raw cow milk are living white blood cells, mammary-gland
cells, various bacteria, and a large number of active enzymes (McGee 16).
A milking
machine in action.
In most Western countries,
a centralised dairy facility processes milk and products obtained from milk (dairy
products), such as cream, butter, and cheese. In the United States, these
dairies are usually local companies, while in the southern hemisphere
facilities may be run by very large nationwide or trans-national corporations
(such as Fonterra).
Pasteurization kills many
harmful micro organisms by heating the milk for a short time and then cooling
it for storage and transportation. Pasteurized milk is still perishable and
must be stored cold by both suppliers and consumers. Dairies print expiration
dates on each container, after which stores will remove any unsold milk from
their shelves. In many countries it is illegal to sell milk that is not
pasteurized.
Milk may also be further
heated to extend its shelf life through ultra-high temperature treatment (UHT),
which allows it to be stored unrefrigerated, or even longer lasting sterilization.
Those preferring raw milk
argue that the pasteurization process also kills beneficial microorganisms and
important nutritional constituents. The resulting pasteurized product is said
to contribute to its own indigestibility, be less nutritious, and turn rancid
(as opposed to sour) with age. However, unpasteurized milk can harbor harmful disease-causing
bacteria such as tuberculosis, brucellosis, salmonella, diphtheria, polio, and escherichia
coli.[5] The cows must be maintained in very sanitary
conditions and a watchful eye kept as to disease testing and vaccinations for
this to be completely safe. Cheeses made with raw milk are regarded as safer as
the milk typically had to be heated to some extent anyway to make the cheese,
and this would kill many of the dangerous organisms possibly present.
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 the cream from the 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
smaller the globules, the more other molecular-level forces prevent this from
happening. In fact, the cream rises in cow milk much quicker than a simple
model would predict: rather than isolated globules, the fat in the milk tends
to form into clusters containing about a million globules, held together by a
number of minor whey proteins (McGee 19). These clusters rise faster than
individual globules can. The fat globules in milk from goats, sheep, and water
buffalo do not form clusters as readily and are smaller to begin with; cream is
very slow to separate from these milks (McGee 19).
Milk from Ireland.
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.[6] A greater
number of smaller particles possess more total surface area than a smaller
number of larger ones, and the original fat globule membranes cannot completely
cover them. Casein micelles are attracted to the newly-exposed fat surfaces;
nearly one-third of the micelles in the milk end up participating in this new
membrane structure. The casein weighs down the globules and interferes with the
clustering that accelerated separation. The exposed fat globules are briefly
vulnerable to certain enzymes present in milk, which could break down the fats
and produce rancid flavors. To prevent this, the enzymes are inactivated by
pasteurizing the milk immediately before or during homogenization. Homogenized
milk tastes blander but feels creamier in the mouth than unhomogenized; it is
whiter and more resistant to developing off flavors (McGee 23). Creamline, or
cream-top, milk is unhomogenized; it may or may not have been pasteurized. Some
have suggested that homogenized milk is harder to digest or not as suited to
some people as is unhomogenized, it was just for convenience so one didn't have
to shake the bottle, and isn't as important as is pasteurization which is done
for safety purposes.
Unhomogenized has made a
small comeback in a few areas, such as the west coast of the United States
where Srauss 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.
Milk, sold commercially in
countries where the cattle (and often the people) live indoors, commonly has vitamin
D added to it to make up for lack of exposure to UVB radiation. Milk often has flavoring
added to it for better taste or as a means of improving sales. Chocolate
flavored milk has been sold for many years and has been followed more recently
by such other flavors as strawberry and banana.
South Australia has the
highest consumption of flavoured milk per person in the world, where Farmers
Union Iced Coffee outsells Coca-Cola, a success shared only by Inca Kola in Peru
and Irn-Bru in Scotland.
Switzerland has a soft
drink based on milk that tastes and looks much like SevenUp. This popular
"milk-cola", named Rivella, is in fact the national soft drink and
comes complete in low calorie & low sugar varieties even. To taste it one
would not think at all that it is based from milk.
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