Milk is a liquid food produced by all female mammals. Breast milk or cow milk is the only food given to humans in the first months of their lives. But milk is a fragile product. It turns sour easily because lactose, the sugar in milk, ferments under the action of naturally occurring lactic bacteria, which cause the protein to coagulate. The ease with which milk curdles gave rise to cheese. It is also the reason that milk, whether from a cow, goat, sheep, or other mammals, is rarely consumed raw, but treated with heat so that it can keep for a longer time. It can also have part of its fat content removed through the process of skimming.

The term “milk” refers to cow milk. The source of other types of milk must be indicated, e.g., goat milk, sheep milk, etc. The stringent hygiene standards enforced in the European Union mean that milk is always of an almost impeccable microbial quality.

The choice between whole (full-fat), low-fat (semi-skimmed), or skim milk is made on the basis of intended use, and sometimes nutritional needs. Whether it is pasteurized or sterilized, milk can be packaged in opaque bottles or cartons. Condensed milk is canned, while instant milk powder is usually sold in a bag. Heat treatment removes all the fat from the milk, and then cream is added back to bring the milk up to the desired fat content. Milk in France always has color-coded packaging, regardless of format: red: whole milk, 3.6% fat blue: low-fat milk, 1.5 to 1.8% fat green: skim milk, less than 0.3% fat yellow: raw milk, 37 to 45% fat

Milk is used extensively in both cooking and baking. Its constituent ingredients (water, proteins, lipids, and carbohydrates) mean that it can be used for thinning sauces, creating emulsions, setting, fixing flavor, and softening. In cooking, it is an important ingredient in purees, certain sauces (such as béchamel), various soups, gratins, soufflés, etc. It is essential in pastry making; without it, there would be no crème caramel, crème anglaise or pastry cream, other desserts (rice pudding, for instance), or ice cream. Milk goes well with all products, including cereals, vegetables, fruit, eggs, meat, and fish.

Pasteurized milk and microfiltered milk must be stored in a refrigerator between 1 and 5°C. Sterilized and concentrated milk can be stored at room temperature but must be refrigerated after opening. Instant milk can also be stored at room temperature. Once the package is opened, it should be consumed within ten days.

The main ingredient in milk is water, followed by proteins, which are well balanced in essential amino acids, some carbohydrates, and minerals, including a balanced proportion of calcium and phosphorus, which are essential to the development and protection of the skeletal system. For these reasons, milk is a liquid source of nutrition that is essential throughout a person’s life. It also contains fat and vitamin A (varying proportions for whole, low-fat, and skim milk). It also contains vitamin B2. This does not mean that milk is a complete food, because it contains no vitamin C and is low in iron.

It can be indigestible, because of the presence of casein, a protein specific to milk, which coagulates into large flakes in the stomach. These are smaller if bread is eaten at the same time. In any case, it is easier to digest when taken warm or cool, but not iced. Milk is not tolerated by people with an intolerance to lactose (its carbohydrate), which is why lactose-free milk is now available.

Raw milk does not undergo any treatment, and so still contains all its natural lactobacilli. Boiling prevents it from turning sour too quickly (5 minutes in winter, 10 minutes in summer). It can be stored in the refrigerator, but not for more than 24 hours. In France, it is bottled directly at the farm. The bottle has a yellow cap. It is only found in the fresh food aisle of stores. Pasteurized milk sold as fresh milk is heated to 72°C for 20 seconds. This kills any harmful microbes but preserves the other microorganisms. Either whole or low-fat milk can be produced this way, and it is this product that tastes the most like raw milk, especially the whole version. It must be stored at 4°C and its use-by or expiration date is within seven days of production.

Microfiltered milk is processed to separate the cream. It is then filtered through porous membranes and recombined with the necessary volume of cream. This milk tastes even more like raw milk. It can be stored longer: 14 days at 4°C. Sterilized milks are classified into: Sterilized—milk is heated to 115°C for 15 to 20 minutes in its packaging. This removes all of its flavor. It can be stored for 150 days at room temperature. UHT (Ultra High Temperature) treatment—the temperature of the milk is raised to 140/150°C for just a few seconds. It is then cooled in a sterile environment and packaged in bottles or cartons. It can be stored for 90 days at room temperature. Its flavor is slightly less affected than with the other sterilization method. Both must be kept in the refrigerator once opened. This category includes milk with a guaranteed vitamin content (where synthetic vitamins are added to the milk to compensate for loss during sterilization), iron-fortified milk, lactose-free milk, and a variety of flavored milks.

Evaporated milk has 50 to 55 percent of its water content removed, before packaging and sterilization. It can be stored for several months at room temperature and must be refrigerated once opened. This milk can be reconstituted by adding water. Its flavor is nothing like that of fresh milk. Condensed milk has 75 percent of its water content removed, before the addition of 40 to 42 percent sucrose syrup. This is a high-calorie product, often eaten as a sweet treat. Instant or dried milk powder is almost entirely dehydrated; the milk may not contain more than 4 percent water. It can be stored unopened for several months in a dry place at room temperature away from moisture or heat. It can be reconstituted by adding water.

Milk sold in the form of powdered infant formula is subject to special regulations. In France, organic milk, whether UHT treated or pasteurized, whole, low-fat, or skim, must come from cows raised and fed according to the rules of organic agriculture. Goat milk is sold in whole and low-fat form and is always UHT treated. Sheep milk is quite rare, but can be found in areas where a large volume of cheese is produced using this milk.