Milk

Milk most often means the nutrient fluid produced by the mammary glands of female mammals. The female ability to produce milk is one of the defining characteristics of mammals and provides the primary source of nutrition for newborns before they are able to digest more diverse foods. It is also processed into dairy products such as cream, butter, yoghurt, ice-cream, gelato, cheese, casein, whey protein, lactose, condensed milk, powdered milk, and many other food-additive and industrial products.

It can also be used to mean:

• The white juice and the processed meat of the coconut in more-or-less liquid form, used especially in Thai, Indian (Kerala), and Polynesian cuisine.
• A non-animal substitute such as soy milk, rice milk, and almond milk.
• Crop milk, the regurgitated substance pigeons feed their young.

Human milk is fed to infants through breastfeeding, either directly or by the female expressing her milk to be saved and fed later. The early lactation milk is known as colostrum, and carries the mother's antibodies to the baby. It can reduce the risk of many diseases in both mother and baby.

Composition and nutrition

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.[1] For example:

• Human milk contains, on average, 1.1% protein, 4.2% fat, 7.0% lactose (a sugar), and supplies 72 kcal of energy per 100 grams.
• Cow's milk contains, on average, 3.4% protein, 3.6% fat, and 4.6% lactose, and supplies 66 kcal of energy per 100 grams.

Lactose in milk is digested with the help of the enzyme lactase. In humans, production of lactase can decline in adulthood, leading to an inability to digest milk; this is known as lactose intolerance. Some human populations (most notably Europeans) retain the ability to digest lactose into adulthood.

Cow's milk

In the Western world, cow's milk is extracted on an industrial scale for human consumption and industrial uses. It is the most commonly consumed form of milk. Dairy farming has become such a large business that in many countries the process is highly automated, with farmers using machines that attach directly to the teats of the cow's udder to speed milking, and breeds of cattle, such as Holstein, specially bred for increased milk production.

Commercial processing of milk

In North America a dairy facility processes milk and products obtained from milk (dairy products), such as cream, butter, and cheese. Most dairies are local companies, as opposed to large or nationwide companies found in the southern hemisphere.

Upon standing, fresh milk has a tendency to separate into a high-fat cream layer on top of a larger, skim-milk layer. The cream is often sold as a separate product with its own uses. In the United States, a blended mixture of half cream and half milk is often sold in smaller quantities and is called half-and-half. Half-and-half is used for creaming coffee and similar uses.

Milk produced for commercial consumption usually undergoes several processes. Pasteurization kills many harmful microorganisms 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.

A complementary process for commercial milk is homogenization. This mechanically reduces the size of the fat globules, so that they will not separate out into a cream layer.

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, followed recently by other flavors of milk and cream.

South Australia has the highest consumption of flavoured milk per person, where Farmers Union Iced Coffee outsells Coca-Cola, a success shared only by Inca Kola in Peru and Irn-Bru in Scotland.

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 indigestability, be less nutritious, and turn rancid (as opposed to sour) with age. Raw Milk Versus Pasteurized Milk

Nutritional benefits

A serving (1 cup or 250 ml) of 2%-fat milk contains 285 mg of calcium, which represents 22% to 29% of the daily recommended intake (DRI) of calcium for an adult, depending on the age, 8 grams of protein, and a number of other nutrients (either naturally or through fortification):

• Vitamins D and K - essential for bone health
• Iodine - a mineral essential for thyroid function
• Vitamin B12 and Riboflavin - necessary for cardiovascular health and energy production
• Biotin and Pantothenic Acid - B vitamins important for energy production
• Vitamin A - critical for immune function
• Potassium and Magnesium - for cardiovascular health
• Selenium - cancer-preventive trace mineral
• Thiamine - B-vitamin important for cognitive function, especially memory
• Conjugated linoleic acid - beneficial fatty acid that inhibits several types of cancer in mice, has been shown to kill human skin cancer, colorectal cancer and breast cancer cells in in vitro studies, and may help lower cholesterol and prevent atherosclerosis; only available in milk from grass-fed cows

Studies show possible links between low-fat milk consumption and reduced risk of arterial hypertension, coronary heart disease, and obesity. Overweight individuals who drink milk may benefit from decreased risk of insulin resistance and type 2 diabetes. [2]

Nutritional side-effects

Overconsumption of Cow's milk is argued to be unhealthy primarily due to its fat and cholesterol content. The following studies are used to support this position:

• Some milk is rich in saturated fat, which studies have linked to increased risk of atherosclerosis and coronary heart disease. Low-fat and non-fat forms of milk may mitigate any such risk.
• Up to 70% of humans have an incomplete ability to digest milk, lactose intolerance. For those individuals, milk may induce symptoms such as cramping, bloating, gas, and diarrhoea. Certain ethnic groups may be more susceptible to these effects.
• Critics dispute the claim that drinking large amounts of milk can reduce the risk of bone fractures, especially in the elderly. Studies have failed to associate high calcium intakes with lower risk of hip and forearm fractures in men[3] or women[4].
• Critics of milk claim that plant-based sources of calcium are preferable, on the grounds that animal proteins in milk causes leaching or excretion of calcium from bones.[5] Such critics refute the claim that milk prevents osteoporosis and make the counterclaim that milk, in fact, contributes to that disease.
• A study published in June 2005 suggests that consumption of milk by 9- to 14-year-old children is associated with weight gain, although the researchers identify that excessive calorie intake is the cause rather than dairy specific factors. Researchers were surprised by their conclusion that weight gain was associated with dietary calcium and low-fat or skim milk, but not dairy fat.[6]
• A February 2005 study found a positive association between acne and the consumption of whole milk, skim milk, and other dairy products in high-school-age women.[7].
• Two studies show a correlation between high galactose consumption, and high rates of ovarian cancer. [8][9]
• A study suggests a correlation between high calcium intake and prostate cancer.[10]. There is no evidence that any such problem is specific to milk. A review published by the World Cancer Research Fund and the American Institute for Cancer Research states that at least 11 human population studies have linked dairy product consumption and prostate cancer.

Also there has been some controversy over whether or not humans, or any other mammals, are designed to consume milk after infancy. This article, published by PETA, highlights some of the debate. http://www.notmilk.com/kradjian.html

Distribution

Because of the perishable nature of milk, expeditious distribution is desirable. In many countries milk used to be delivered to households daily, but economic pressure has made milk delivery much less popular, and in many areas daily delivery is no longer avialable. People buy it chilled at grocery or convenience stores or similar retail outlets. Prior to the widespread use of plastics, milk was often distributed to consumers in glass bottles, and before that in bulk that was ladled into the customer's container. In the UK, milk can be delivered daily by a milkman who travels his local milk round (route) using a battery-powered milk float, although this is becoming less popular as a result of supermarkets selling milk at cheaper prices. In New Zealand, milk is no longer distributed in glass bottles.

In the United States bottles were replaced with milk cartons, tall boxes with a square cross-section and a peaked top that can folded outward upon opening to form a spout. Now milk is increasingly sold in plastic bottles. First the gallon and half-gallon sizes were sold in plastic jugs while the smaller sizes were sold in milk cartons. Recently milk has been sold in smaller resealable bottles made to fit in automobile cup holders.

The half-pint milk carton is the traditional unit as a component of school lunches. In the US, pictures of missing children were printed on the larger milk cartons as a public service until it was determined that this was disturbing to children.

Milk preserved by the UHT process is sold in boxes often called a "brick" that lack the peak of the traditional milk carton. Milk preserved in this fashion does not need to be refrigerated before opening and has a longer shelf life than milk in ordinary packaging.

Glass milk containers are rare these days. Most people purchase milk in plastic jugs or bags or in waxed-paper cartons. Ultraviolet light from fluorescent lighting can destroy some of the proteins in milk so many companies that once distributed milk in transparent or highly translucent containers are now using thicker materials that block the harmful rays. Many people feel that such "UV protected" milk tastes better.

Milk comes in a variety of containers with local variants:

• United States: Commonly sold in gallon, half-gallon and quart containers (U.S. customary units) of rigid plastic or, occasionally for sizes less than a gallon, waxed cardboard. The US single-serving size is usually the half-pint (about 240 ml).
• Canada: A 1 1/3 litre plastic bag (sold as 4 litres in 3 bags) is the most common, while 2 litre, 1 litre, 500 millilitre, and 250 millilitre cartons are also available. 4 litre plastic jugs are also available.
• Parts of Europe: Sizes of 500 millilitres, 1 litre (the most common), 2 litres and 3 litres are commonplace.
• United Kingdom: Most stores stock the equivalents of old Imperial sizes: 568 ml (1 pint), 1.136 l (2 pints), 2.273 l (4 pints) or, rarely, a combination including both metric and imperial sizes).
• Australia and New Zealand: Distributed in a variety of sizes, most commonly in Tetra Paks for up to 1 litres, and plastic screw-top bottles beyond that. Most UHT-milk is packed in 1 litre paper containers with a sealed plastic spout.
• South Africa: Commonly sold in 1 litre bags. The bag is then placed in a plastic jug and the corner cut off before the milk is poured.

Condensed milk is distributed in metal cans, 250 and 125 ml paper containers and 100 and 200 ml squeeze tubes, and powdered milk (skim and whole) is distributed in boxes or bags.

Varieties and brands

Cow's milk is generally available in several varieties. In some countries these are:

• Full cream (or "whole" in North America, about 3.25% fat)
• Semi-skimmed ("reduced fat" or "low fat", about 1.5-1.8% fat)
• Skimmed (about 0.1% fat)

Milk in the U.S. and Canada is sold as:

• Whole varieties
• 2 % (reduced fat)
• 1 % (low fat)
• 1/2 % (very low fat)
• Skim (no fat)

Note: In Canada "whole" milk refers to unhomogenized milk. "Homogenized" milk (or "Homo milk" in short) refers to milk which is 3.25% butterfat. Generally all store-bought milk in Canada has been homogenized, yet the term is also used as a name to describe butterfat content for a specific variety of milk. Modern commercial dairy processing techniques involve first removing all of the butterfat, and then adding back the appropriate amount depending on which product is being produced on that particular line.

In Britain, it is possible to get Channel Island milk, which is 5.5% fat.

In the United States, skim milk is also known as "fat free" milk, due to USDA regulations stating that any food with less than 1/2 gram of fat per serving can be labeled "fat free".

Full cream, or whole milk, has the full milk fat content (about 3-4% if Friesian- or Holstein-breed are the source). For skimmed or semi-skimmed milk, all of the fat content is removed and then some (in the case of semi-skimmed milk) is returned. The best-selling variety of milk is semi-skimmed; in some countries full-cream (whole) milk is generally seen as less healthy and skimmed milk is often thought to lack taste. Whole milk is recommended to provide sufficient fat for developing toddlers who have graduated from breast milk or infant formula.

Other milk animals

In addition to cows, the following animals provide milk for dairy products:

• Sheep
• Goats
• Horses
• Donkeys
• Camels (including the South American camelids)
• Yaks
• Water buffalo
• Reindeer

In Russia and Sweden, small moose dairies also exist. Donkey and horse milk have the lowest fat content, while the milk of seals contains more than 50% fat. [11]

Curdling

When raw milk is left standing for a while, it turns sour. This is the result of fermentation: lactic acid bacteria turning the milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products such as cheese and yogurt. There are four noted periods of milk decay:

• Rancid (also called "on the turn". Milk is still consumable at this stage)
• Curdling (separation of curd and whey will occur but may still be consumable)
• Coagulation (beyond use. A period of aromatic decay sets in accompanied by mould)
• Dry (beyond use. The milk has dehydrated and become hard and chalky)

Pasteurized cow's milk, on the other hand, spoils in a way that makes it unsuitable for consumption, causing it to assume an unpleasant odor and pose a high danger of food poisoning if ingested. The naturally-occurring lactic acid bacteria in raw milk, under suitable conditions, quickly produce large amounts of lactic acid. The ensuing acidity in turn prevents other germs from growing, or slows their growth significantly. Through pasteurization, however, these lactic acid bacteria are mostly destroyed, which means that other germs can grow unfettered and thus cause decomposition.

In order to prevent spoilage, milk can be kept refrigerated and stored between 1 and 4 degrees Celsius. Most milk is pasteurized by heating briefly and then refrigerated to allow transport from factory farms to local markets. The spoilage of milk can be forestalled by using ultra-high temperature (UHT) treatment; milk so treated can be stored unrefrigerated for several months until opened. Sterilized milk, which is heated for a much longer period of time, will last even longer, but also lose more nutrients and assume a still different taste. Condensed milk, made by removing most of the water, can be stored for many months, unrefrigerated. The most durable form of milk is milk powder which is produced from milk by removing almost all water.

Why milk expands on heating

Milk does not actually reach boiling point faster than water. The reason behind the quick expansion of milk on heating is due to its chemical composition. Proteins have the ability of coating bubbles and stabilizing foams; milk is an emulsion of very small fat droplets coated by casein.

At and below room temperature, droplets are arranged in a way that protects them from coalescing. On heating, this subtle architecture is destroyed: milk reaches a temperature (below boiling point of water) in which protein molecules are irreversibly changed in their spatial arrangement (denatured). Casein, together with other components, thereby forms a tough film which surrounds the water vapour bubbles of boiling milk, preventing them from breaking. This has the effect that the milk is transformed into a relatively stable foam, which occupies a much larger volume than the original liquid.

References

[12] Introduction to Dairy Science,Guelph