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.

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

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, yaks, etc.
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.

Whale's milk — not generally used for human consumption — is one of the highest-fat milks. It contains, on average, 10.9% protein, 42.3% fat, and 2.0% lactose, and supplies 443 kcal of energy per 100 grams. Human milk is not produced or distributed industrially or commercially; however, milk banks exist that allow for the collection of donated human milk and its redistribution to infants who may benefit from human milk for various reasons (premature infants, infants with allergies, metabolic diseases, etc.).

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. 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.