Introduction

According to the Spanish Food Code, natural milk is understood as “the whole, unaltered and unadulterated product, without colostrum, from the hygienic, regular, complete and uninterrupted milking of healthy and well-fed domestic female mammals”.

In Spanish legislation, “milk” refers solely to cow’s milk; milk from other animals must be designated by specifying the species, for example, goat’s milk, sheep’s milk…

Types of Milk

We are going to classify types of milk according to thermal treatment, nutritional content and physical form.

By thermal treatment

Pasteurised or hygienised milk is commonly marketed under the incorrect name of fresh milk. The most common treatment is heating to 71–72 ºC for 15 seconds or to 84–85 ºC for 2 seconds. This treatment eliminates pathogenic microorganisms, but not harmless ones, and it may curdle. The downside is its short shelf life of just a few days, and it must be kept refrigerated. The advantage is that it retains more vitamins than sterilised milk and contains no additives.

Sterilised milk is that which, after a technological process, ensures the destruction of germs.

• Milk packaged before sterilisation, sterilised milk, is treated for 6–20 minutes at temperatures of 115–120 ºC. It loses many vitamins.
• Milk packaged after sterilisation:
  • UHT milk, temperatures of 130 ºC for 2 seconds.
  • Superheated, 135–150 ºC for 4–5 seconds. Most nutrients are preserved.

By nutritional content

Whole milk contains all nutrients.

Semi-skimmed milk contains less fat and fewer fat-soluble vitamins A, D and E.

Skimmed milk contains no fat or fat-soluble vitamins (unless artificially added).

Lipid-modified milk: milk from which the fat has been removed and replaced with vegetable oils. This manipulation aims to improve the fat profile of the milk to enhance the lipid quality of the diet (milk with ω-3).

Enriched milk is skimmed milk to which nutrients such as vitamins A and D and calcium are added.

By physical form:

Liquid milk is the most widely known.

Evaporated milk is milk from which part of the water has been removed. It is obtained industrially through the process of pasteurisation-homogenisation-evaporation under vacuum, removing 60% of the water. It is then packaged, sealed and sterilised in an autoclave. Homogenisation breaks the fat globules to avoid cream formation.

Condensed milk is concentrated milk with sugar, deprived of part of its inherent water. It contains up to 40–50% sucrose in the final product. It is made by heating to 125ºC, adding and dissolving sucrose, vacuum evaporation, rapid cooling, seeding with microcrystals of lactose, and hygienic packaging.

Powdered milk is dehydrated milk with less than 5% water content. It is produced by evaporating milk until a 50% solids concentration is achieved, dehydrating the concentrated milk and granulating it. It is low in vitamins and amino acids.

Physico-chemical Characteristics

Milk is an opaque white liquid, more or less yellowish depending on its beta-carotene content, with a faint but species-specific smell.

It is a homogeneous mixture of a wide variety of substances such as sugars, fats, proteins, minerals and vitamins.

It has three phases: a fatty and phospholipid-containing emulsion, a colloidal suspension with caseins and calcium salts, and a solution of sugars, water-soluble vitamins and minerals.

Composition of Milk

Milk is a very complete yet complex food, consisting of three phases:

• An aqueous phase containing salts, sugars, proteins, vitamins and amino acids
• A solid colloidal phase formed by complex proteins (casein), phosphates and calcium salts
• A lipid phase composed of fats, sterols and fat-soluble vitamins, mainly A

Milk composition:

• Water (86%)
• Fat (5%)
• Carbohydrates (5%)
• Nitrogenous substances (3%)
• Minerals (1%)

Trace components include:

• Carotenoid pigments
• Enzymes
• Water- and fat-soluble vitamins
• Dissolved gases (O₂, N₂, CO₂).

Milk Sugars

The most abundant sugar in milk is lactose. Milk contains a balance between the alpha and beta forms of lactose, depending on temperature and pH. Other sugars such as glucose, galactose, and phosphorylated sugars are present in much smaller amounts.

Lactose Intolerance

Some human populations, especially people of African descent, cannot digest lactose due to the absence of the enzyme lactase, which leads to severe diarrhoea.

Milk Lipids

The fat fraction of milk is distributed in very fine droplets (microglobules) 3–10 microns in diameter.

Composition of fat globules:

• 41% proteins
• 30% phospholipids
• 2% cholesterol
• 14% glycerides
• 13% water

Milk Proteins

Milk is rich in high biological value proteins (around 90%) as they are made up of essential amino acids. It is particularly rich in glutamic acid (around 25% of total amino acids) and low in methionine, cysteine, and tryptophan according to FAO guidelines.

Proteins are present in colloidal dispersion. Milk protein consists of:

• Caseins (80%), which coagulate in acidic medium at pH 4.6, thus separating from the whey
• Whey proteins such as lactoglobulin and lactalbumin, comprising the remaining 20%. These are nutritionally important due to their high biological value.

Milk Minerals

The mineral content of milk is less than 1% (calcium, potassium, sodium, phosphorus…).

Vitamins

• Fat-soluble: A, D, E. These are lost when fat globules are removed during the production of skimmed milk.
• Water-soluble: found in the aqueous portion. Particularly rich in B₂ and B₁₂.

Nutritional Aspects of Milk

Milk is a complete food. It is only slightly deficient in iron, vitamin C and folic acid when compared with the recommended dietary allowance (RDA) for an adult consuming 2000 kcal/day.

In skimmed products, most fat-soluble vitamins (A, D and E) are lost, but the nutritional density of the remaining nutrients increases significantly.

Milk nutrients depend on the type of animal origin:

With today’s article, this common household food has been explained. The topic of the importance of milk consumption for humans will be left for a future article.