Milk is a secretion produced by mammals, which a mother uses to feed her young. It is a balanced blend of nutrients that supplies a growing organism with energy, proteins, fats, carbohydrates, minerals and vitamins, and protects it against infections. It is this balance that makes milk such a valuable part of the human diet.
The composition of cow’s milk and human milk is surprisingly similar, which is why cow’s milk fits so well into our everyday diet. Between species, however, milk varies enormously: the fat content of whale and dolphin milk exceeds 40%, and it is this whale fat that gives the young the energy they need in cold water. Elephant milk contains on average 20% fat and reindeer milk over 15%, while cows and humans both come in at roughly 4%.
What milk is and why its composition matters
People milk cows, buffalo, goats and sheep and use the milk as an important part of the diet. Alongside the macronutrients, milk also contains micro-components that support the endocrine system and the transport of substances in the body.
Cows produce considerably more milk than their calves need. Breeding programs have noticeably improved both the quantity and the quality of output. Cow’s milk contains on average 87.3% water and 12.7% dry matter. The dry matter in turn consists on average of 3.9% fat, 3.2% protein, 4.7% milk sugar, or lactose, and 0.9% minerals and vitamins. The components occur in milk in different forms: lactose is dissolved, proteins are colloidal and lipids are emulsified in water.
Milk fat and the products made from it — butter, cheese, yogurt, curd cheese and cream — are rich in nutrients, but also delicate. It is precisely this delicacy that places strict demands on how milk is handled.
Milk fat: composition and measurement
The lipids in milk are collectively called milk fat. The fat occurs as fat globules coated with a protein membrane, with a diameter ranging from 0.1 to 20 mm. Because milk fat is lighter than the other components, it rises to the surface on standing and forms a cream layer.
This has to be taken into account when sampling milk, because the sample result is the basis for payment for the milk. After just a few hours of standing, the difference in fat content between the bottom and top layers of the milk can exceed one percent. When a sample is taken in the morning from unmixed evening milk, it is common to find 0.5–1.0% fat in the bottom layer and 6.0–6.5% in the top. Bacteria and somatic cells also stick to the fat globules and rise to the surface with them.
Milk fat has the most varied composition of all natural fats, containing several hundred fatty acids. Some of them inhibit the growth of disease-causing microbes. Among them are the essential fatty acids arachidonic acid and linolenic acid, which are needed to build nerve cells. Biologically, milk fat is an energy-rich nutrient and at the same time the most digestible fat — milk fat that is already in emulsified form to begin with reaches a digestibility of up to 99%.
In the dairy industry, fat content has commercial and technological importance. The reference method is the Rose-Gottlieb method: alcohol and ammonia are added to the milk, the alcohol precipitates the proteins, the ammonia dissolves them and the fat is released, then extracted and weighed. Extraction, however, is time-consuming and costly, so faster methods have been developed.
The first quick and economical methods were butyrometric. In the Gerber method, a set amount of milk is pipetted into a butyrometer, and sulfuric acid and amyl alcohol are added. When concentrated sulfuric acid mixes with the water in the milk, heat is released, which breaks down the protein; the amyl alcohol helps to release the fat. After centrifugation under standard conditions and warming in a water bath, the fat content is read from the butyrometer scale.
Two historical periods can be clearly distinguished in fat analysis: the development of gravimetric methods and the introduction of routine butyrometric methods. The latter was used in milk quality control from the early 1900s until the end of the 1960s. Today dairies work with analyzers, but the standards needed to calibrate them are still obtained using the Rose-Gottlieb method.
Proteins in milk: casein and whey proteins
From the standpoint of human nutrition, proteins are the most valuable nutrient in milk. The nitrogen compounds in milk fall into three main groups: caseins, whey proteins and non-protein nitrogen (NPN). Studies show that about 76% of the nitrogen compounds are casein, 18% whey proteins and 6% NPN.
Casein, which is used in cheesemaking, occurs in milk as a calcium caseinate–calcium phosphate complex that binds water well and supplies the growing organism with the amino acids needed to build body proteins. The whey proteins, which have a high biological value, end up in the whey during cheesemaking, but they can be separated by ultrafiltration and put to further use. NPN consists of low-molecular-weight nitrogen, about half of which is urea, with the rest made up of creatine and free amino acids.
Rapid instrumental methods have made protein content one of the main quality indicators on which farmers are paid for their milk. Unlike fat, protein stays evenly distributed in milk and does not rise to the surface.
Proteins are complex organic compounds containing carbon, hydrogen, oxygen and nitrogen, and they may also contain sulfur, phosphorus and other elements. Protein molecules are made up of amino acids, and the list of different proteins is very long. All proteins, however, have a relatively constant nitrogen content, on the basis of which the Danish chemist Kjeldahl developed the classic reference method in the 19th century. In this method, milk is digested with concentrated sulfuric acid and potassium sulfate, with copper sulfate acting as a catalyst; the organic matter burns off and the nitrogen is converted into ammonium sulfate.
After cooling, the contents of the flask are made alkaline with sodium hydroxide, and the released ammonia is distilled and titrated. This gives the total nitrogen in the sample, from which total protein is calculated. Most countries base quality premium payments on the total amount of protein.
Milk sugar, minerals and vitamins
After water, lactose is the main component of milk — 4.7% on average. Milk, cow’s milk included, is the main source of lactose, one of the most common natural disaccharides. In milk, lactose is present in dissolved form and provides about 30% of the energy you get from milk. Together with minerals, lactose creates the osmotic pressure of milk, which is tied to the osmotic pressure of blood — which is why the lactose content of a healthy cow’s milk cannot change significantly. Lactose also promotes calcium absorption in the body. Milk also contains small amounts of the monosaccharides glucose and galactose.
Carbohydrates are also food for the bacteria that sour milk and for starter culture bacteria, whose activity converts lactose into lactic acid — the very basis of yogurt and cheese making. In cheese production, a large share of the milk sugar goes unused, and because lactose dissolves in water, it ends up in the whey. In recent years, whey proteins have begun to be separated by ultrafiltration; lactose is hydrolyzed into glucose and galactose, which are used as a sweetener in the confectionery industry, and the pharmaceutical industry also uses lactose to coat medicines.
Today, the content of lactose, fat and protein is mostly measured with infrared analyzers. Since this is not a standard method, its accuracy depends on calibration carried out against the standard method. The reference methods for lactose are the polarimetric and gravimetric methods.
Cow’s milk contains on average 0.7% minerals, including most of the elements the body needs. Milk is a good source of calcium, phosphorus, potassium, zinc, iodine and selenium. All the calcium in milk is absorbable by humans. Iron, magnesium, copper and fluorine are present in milk in relatively small amounts. Minerals regulate acid-base balance, maintain the structure of tissues, maintain osmotic pressure and mediate the transport of nutrients; they are also components of many enzymes and vitamins.
The toxic compounds lead, cadmium and mercury get into milk mainly from feed and the surrounding environment, although the vast majority is deposited in the cow’s body and does not reach the milk. Radioactive isotopes of strontium and iodine are found in milk in very small amounts.
Milk contains all the known vitamins, above all vitamins A, B and D. Among the enzymes, lipase deserves special attention: it breaks down milk fat and can cause a bitter taste and smell. Cooling temperatures close to the freezing point and frequent temperature changes increase the effect of lipase. So do overly powerful centrifugal pumps when milk is received and pumped from one tank to another — they damage the protein membrane surrounding the fat globules.
Physical properties of milk
Water is the largest component of milk by quantity. Its job is to keep the other substances dissolved, emulsified or suspended, which ensures the body absorbs them well. At the same time, the high water content is also the reason milk keeps for only a short time and spoils easily.
The specific gravity (relative density) of milk depends on the proportions of the substances in it. A high fat content lowers density, while a high content of protein, milk sugar and minerals raises it. The average density of cow’s milk at 20 °C is between 1028 and 1035 kg/m3. A liter of milk is heavier than a liter of water and weighs 1.033 kg on average. When water is added to milk, its density drops noticeably — which is the simplest way to detect substantial dilution.
Adulteration can be proven even more precisely by measuring the freezing point. Thanks to the dissolved components, above all milk sugar and minerals, the freezing point of milk is lower than that of water — in individual cows roughly -0.512 to -0.545 °C. When water is added, the freezing point rises and approaches 0 °C.
Producing high-quality milk
Milk drawn from a clean udder does not contain significant amounts of bacteria. Some contamination of the milking environment and equipment is unavoidable, but in chilled milk that meets good hygiene requirements, the total bacterial count should stay below 10,000 bacteria/ml. When the bacterial count rises, fats, proteins and milk sugar begin to break down and off-flavors develop.
FAQ about milk
Why is cow’s milk so well suited to humans?
The composition of cow’s milk and human milk is similar: the fat content of both is about 4%, and the ratio of macro- and micronutrients matches human needs quite well. It is precisely this similarity that makes cow’s milk a valuable everyday food.
Has water been added to the milk?
The simplest clue is density: if it falls below 1028 kg/m3, the cow’s milk has probably been watered down. Measuring the freezing point is even more precise — when water is added, it rises toward 0 °C.
Why does milk separate when it stands?
Milk fat is lighter than the other components and rises to the surface as the milk stands, forming a layer of cream. Within a few hours, the difference in fat content between the lower and upper layers can exceed one percent, which is why milk must always be mixed carefully when taking samples.
Author: Marek Morozov
Source: WHO – healthy eating.
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