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Why do humans drink so much milk?

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And given that all mammals lactate,

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why do we favor certain types of milk[br]over others?

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Milk is the first thing we drink,

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and thanks to developments in the [br]production and variety of dairy products,

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it can take on countless forms for our[br]dietary and sensory well-being.

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Milk's primary function is as a complete[br]source of nutrition for newborns.

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In fact, since it has all of the vital[br]nutrients for development and growth,

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proteins,

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carbohydrates,

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fats,

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vitamins and minerals,

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and water,

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milk is the only thing a baby [br]even needs to ingest

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for the first six months of life.

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The unique makeup of milk can vary[br]depending on factors like species,

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diet,

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and location.

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Reindeer of the Arctic Circle, [br]for example,

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make energy-dense milk [br]that's about 20% fat,

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roughly five times more than human[br]or cow's milk,

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to help their young survive the harsh,[br]freezing climate.

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So how is milk made?

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In the uniquely mammalian process[br]of lactation,

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a special class of milk-secreting cells[br]known as mammocytes

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line up in a single layer around[br]pear-shaped alveoli.

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Those cells absorb all of the building[br]blocks of milk,

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then synthesize tiny droplets of fat

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on structures called [br]smooth endoplasmic reticula.

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The droplets combine with each other[br]and other molecules

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and are then expelled and stored[br]in spaces between cells.

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Mammary glands eventually secrete the milk[br]through the breasts, udders,

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or, in the rare case of the platypus, [br]through ducts in the abdomen.

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Although this process is typically[br]reserved for females,

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in some species, like dayak fruit bats,

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goats,

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and even cats,

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males can also lactate.

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Milk drinkers worldwide consume[br]dairy from buffalo,

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goats,

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sheeps,

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camels,

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yaks,

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horses,

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and cows.

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Almost all of these species are ruminants,

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a type of mammal with [br]four-chambered stomachs

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that yield large quantities of milk.

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Of these, cows were the most [br]easily domesticated

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and produce a milk that is both[br]easily separated into cream and liquid

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and has a similar fat content[br]to human milk.

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In their natural environment,

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mammals secrete milk on call [br]for immediate consumption by their young.

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But with the demands of thirsty consumers,

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the dairy industry has enlisted methods[br]to step up production,

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enhance shelf life,

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and provide a variety of milk products.

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In the dairy, centrifugation machines[br]spin milk at high speeds,

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forcing less dense fats to separate[br]from the liquid and float up.

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After being skimmed off,

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this fat, known as butterfat,[br]can be used in dairy products

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like butter,

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cream,

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and cheese.

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Or it can be later added back to [br]the liquid in varying proportions

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to yield different fat content milks.

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Full fat milk, sometimes referred to[br]as whole milk, has 3.25% butterfat added

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compared to 1-2% for low[br]and reduced fat milk,

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and less than half a percent [br]for skim milk.

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To stop reseparation of the fat[br]from the water, or creaming,

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the mixture undergoes the high-energy[br]pressurized process of homogenization.

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Before milk hits the shelves, it's also[br]typically heat treated

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to reduce its level of microbes,

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a government-sanctioned process

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that raw milk enthusiasts argue [br]may reduce milk's nutritional worth.

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Milk spoilage is started by microbes,

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which consume and break down [br]the nutrients in milk.

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That process causes butterfat [br]to clump together,

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leading to a visually unpleasant product.

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And the byproducts of [br]the microbes' consumption

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are compounds that taste [br]and smell nasty.

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But there's a bigger problem.

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Raw milk can carry microbes that are[br]the sources of deadly diseases,

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so in order to kill as many of those[br]microbes as possible,

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and keep milk fresh longer,

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we use a technique called pasteurization.

0:04:16.669,0:04:19.495
One version of this process involves[br]exposing milk

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to about 30 seconds of high heat.

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Another version, [br]called ultra-high temperature processing,

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or ultra pasteurization,

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blasts the milk with considerably higher [br]temperatures over just a few seconds.

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UHT milk boasts a much longer shelf life,

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up to twelve months unrefrigerated,

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compared to pasteurized milk's[br]two weeks in the fridge.

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That's because the higher temperatures[br]of UHT processing

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inactivate far more microbes.

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Yet the higher processing temperatures

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may adversely affect the nutritional[br]and sensory properties of the milk.

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Ultimately, that choice lies[br]in the consumer's taste

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and need for convenience.

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Fortunately, there are many [br]choices available

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in an industry that produces in excess [br]of 840 million tons of products each year.