There are two distinct categories of milk consumption: all infant mammals drink milk directly from their mothers’ bodies, and it is their primary source of nutrition; and humans obtain milk from other mammals for consumption by humans of all ages, as one component of a varied diet.

Normal bovine milk contains 30–35 grams of protein per liter of which about 80% is arranged in casein micelles. Total proteins in milk represent 3.2% of its composition (nutrition table).

For many years the most widely accepted theory of the structure of a micelle was that it was composed of spherical casein aggregates, called submicelles, that were held together by calcium phosphate linkages. However, there are two recent models of the casein micelle that refute the distinct micellular structures within the micelle.

The first theory, attributed to de Kruif and Holt, proposes that nanoclusters of calcium phosphate and the phosphopeptide fraction of beta-casein are the centerpiece to micellar structure. Specifically in this view unstructured proteins organize around the calcium phosphate, giving rise to their structure, and thus no specific structure is formed.

Under the second theory, proposed by Horne, the growth of calcium phosphate nanoclusters begins the process of micelle formation, but is limited by binding phosphopeptide loop regions of the caseins. Once bound, protein-protein interactions are formed and polymerization occurs, in which K-casein is used as an end cap to form micelles with trapped calcium phosphate nanoclusters.

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 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.

These compositions vary by breed, animal, and point in the lactation period.

Milk products are sold in a number of varieties based on types/degrees of:

Milk preserved by the UHT process does not need to be refrigerated before opening and has a much longer shelf life (six months) than milk in ordinary packaging. It is typically sold unrefrigerated in the UK, U.S., Europe, Latin America, and Australia.

Due to the short shelf life of normal milk, it used to be delivered to households daily in many countries; however, improved refrigeration at home, changing food shopping patterns because of supermarkets, and the higher cost of home delivery mean that daily deliveries by a milkman are no longer available in most countries.

The current milk chain flow in India is from milk producer to milk collection agent. Then it is transported to a milk chilling center and bulk transported to the processing plant, then to the sales agent and finally to the consumer.

In Pakistan, milk is supplied in jugs. Milk has been a staple food, especially among the pastoral tribes in this country.

Almost 95% of all milk in the UK is thus sold in shops today, most of it in plastic bottles of various sizes, but some also in milk cartons. Milk is hardly ever sold in glass bottles in UK shops.

In the United States, glass milk bottles have been replaced mostly with milk cartons and plastic jugs. Gallons of milk are almost always sold in jugs, while half gallons and quarts may be found in both paper cartons and plastic jugs, and smaller sizes are almost always in cartons.

A primary school child in England drinking milk out of a glass bottle with a straw
Commonly sold in 1-liter bags and cardboard boxes. The bag is then placed in a plastic jug and the corner cut off before the milk is poured.
Used to be sold in cooled 1-liter bags, just like in South Africa. Today the most common form is 1-liter aseptic cartons containing UHT skimmed, semi-skimmed or whole milk, although the plastic bags are still in use for pasteurized milk. Higher grades of pasteurized milk can be found in cartons or plastic bottles. Sizes other than 1-liter are rare.
Distributed most commonly in aseptic cartons for up to 1 liter, but smaller, snack-sized cartons are also popular. The most common flavors, besides the natural presentation, are chocolate, strawberry and vanilla.
Sweetened milk is a drink popular with students of all ages and is often sold in small plastic bags complete with straw. Adults not wishing to drink at a banquet often drink milk served from cartons or milk tea.
Sizes of 500 mL, 1 liter (the most common), 1.5 liters, 2 liters and 3 liters are commonplace.
Commonly sold in 1 L or 1.5 L cartons, in some places also in 2 dl and 5 dl cartons.
Commonly sold in 1-liter cartons. Sale in 1-liter plastic bags (common in the 1980s) is now rare.
Milk is sold in glass bottles (220 mL), cartons (236 mL and 1 L), plastic jugs (2 liters) and aseptic cartons (250 mL).
Commonly sold in 500 mL plastic bags and in bottles in some parts like in the West. It is still customary to serve the milk boiled, despite pasteurization. Milk is often buffalo milk. Flavored milk is sold in most convenience stores in waxed cardboard containers. Convenience stores also sell many varieties of milk (such as flavored and ultra-pasteurized) in various sizes, usually in aseptic cartons.
Usually sold in 1-liter cartons, but smaller, snack-sized cartons are available.
Commonly sold in 1-liter cartons or bottles and less commonly in 0.5 or 0.25-liter cartons. Whole milk, semi-skimmed milk, skimmed, lactose-free, and flavored (usually in small packages) milk is available. Milk is sold fresh or UHT. Goat's milk is also available in small amounts. UHT semi-skimmed milk is the most sold, but cafés use almost exclusively fresh whole milk.
Milk is supplied in 500 mL plastic bags and carried in jugs from rural to cities for selling
Milk is supplied in 1000 mL plastic bottles and delivered from factories to cities for selling.
UHT milk is mostly sold in aseptic cartons (500 mL, 1 L, 2 L), and non-UHT in 1 L plastic bags or plastic bottles. Milk, UHT is commonly boiled, despite being pasteurized.
Commonly sold in 1-liter bags. The bag is then placed in a plastic jug and the corner cut off before the milk is poured.
Sold in cartons (180 mL, 200 mL, 500 mL 900 mL, 1 L, 1.8 L, 2.3 L), plastic jugs (1 L and 1.8 L), aseptic cartons (180 mL and 200 mL) and plastic bags (1 L).
Commonly sold in 0.3 L, 1 L or 1.5 L cartons and sometimes as plastic or glass milk bottles.
Commonly sold in 500 mL or 1L cartons or special plastic bottles. UHT milk is more popular. Milkmen also serve in smaller towns and villages.
Pasteurized milk is commonly sold in 1-liter bags and ultra-pasteurized milk is sold in cardboard boxes called Tetra Briks. Non-pasteurized milk is forbidden. Until the 1960s no treatment was applied; milk was sold in bottles. As of 2017, plastic jugs used for pouring the bags, or "sachets", are in common use.