Milk and dairy products
Milk is a complete food, physically, chemically and biologically balanced, whose components are present in ideal proportions to ensure the normal development of the young organism and to meet the needs of the child and adolescent in terms of promoting growth, bone formation, dentition and so on. Milk and dairy products have a high biological value and are particularly important in the diet of healthy and sick people, whatever their age.
In our country cow's milk is used most often, but recently goat's and buffalo milk have also come strongly onto the market, and sheep's milk is used above all in making cheese.
Milk contains minerals, enzymes, vitamins, protein substances, fat globules and so on.
The main proteins in milk are: casein, which makes up the largest share, lactalbumin and lactoglobulin. They are notable for their high biological value; they are class I proteins, whose composition contains all the essential amino acids in optimal proportions to maintain nitrogen balance and to promote the growth and development of young organisms.
Casein precipitates (coagulates) in the presence of lactic acid (which is formed from lactose under the action of lactic acid bacteria) or under the action of an enzyme called rennet (the curd that everyone knows). This process of precipitating casein is the basis for producing the various kinds of cheese.
Under the action of certain micro-organisms, milk can undergo a lactic fermentation process in which lactic acid and other substances are formed that give some dairy products their characteristic taste. At other times lactose can undergo a double fermentation, lactic and alcoholic, a phenomenon that is the basis of kefir production.
Among the mineral salts, calcium and phosphorus are more strongly represented in milk. Their ratio is above unity, which favours their absorption in the digestive tract. Sodium is also present in a relatively high proportion.
Milk is the only food of animal origin with an alkalising effect, which is particularly useful in the dietary treatment of illnesses involving raised acidity.
Both fat-soluble and water-soluble vitamins are abundantly represented in milk. The fat-soluble ones are contained mainly in its fat fraction: vitamins A, D and E, and among the water-soluble vitamins we should mention vitamins B12 and B6, pantothenic acid and riboflavin, which are present in a higher proportion.
Besides its many qualities, milk also has some drawbacks linked to its composition, namely: it is low in iron and, if eaten one-sidedly, can lead to iron-deficiency anaemia; because of its sodium content, milk is only suitable to a limited extent in a low-sodium diet — the right amount is best discussed with a health professional. It is also low in vitamin C and thiamine, so these vitamins need to be supplemented during a milk diet.
These shortcomings are minor, however, compared with its many nutritional qualities, which make it a particularly valuable food, both in the diet of the child and in the diet of the healthy or sick adult.
Soured dairy products
The dietary dairy products most commonly used in our country include: soured milk, yogurt, whole milk and kefir. They are obtained by pure lactic fermentation (carried out by the bacteria contained in the milk or introduced in the form of selected cultures) or by mixed, alcoholic and lactic fermentation. They play an important role in the rational diet of healthy people and at the same time occupy an important place in the therapeutic diet of some sick people.
Their nutritional value is similar to that of milk, but they have the advantage that the nutrients are in a form the body can easily assimilate.
Soured milk is obtained by fermenting whole or semi-skimmed milk with the help of lactic acid bacteria (lactic streptococci) that occur naturally in the milk, or by inoculating previously pasteurised milk with the help of pure cultures of lactic acid bacteria.
Yogurt is a widespread soured dairy product that originally came from Asia Minor and the Balkan peninsula. It is made from previously pasteurised cow's milk or the milk of other animal species (sheep, goats, buffalo), to which a yogurt starter is added. This consists of Streptococcus thermophilus and Lactobacillus bulgaricus, which bring about a lactic fermentation and form the characteristic aroma. The end of the fermentation can be recognised by tipping the yogurt jar. It must neither come away from the walls of the jar nor release the whey.
Kefir is a dietary soured dairy product that arises from a double fermentation, lactic and alcoholic, and comes from the Caucasus, where it is widely used. It is obtained with the help of kefir grains, which look like cauliflower florets. In fact, kefir grains are nothing other than accumulations of casein that contain a microflora specific to kefir: lactic acid bacteria (streptococci and bacilli) and yeasts. They live in symbiosis and are responsible for the two kinds of fermentation.
Depending on the two kinds of fermentation, various kinds of kefir are obtained, namely:
- weak (one-day) kefir – the lactic fermentation predominates over the alcoholic fermentation and it resembles soured milk
- medium kefir (two days old)
- strong kefir (three days old) – the alcoholic fermentation predominates over the lactic fermentation. It has a frothy consistency and a sharp, sour taste.
Preserved milk means condensed milk, obtained by partly removing the water and then pasteurising or sterilising it, with or without added sugar, and milk powder, obtained by intensively dehydrating whole milk or cream, so that the end product has a water content of only 4-6 %.
Cream is a milk derivative obtained by letting the milk rise spontaneously or by centrifuging, a process in which the fat is separated from the remaining milk components. Depending on how the creaming was carried out, the end product can have a lipid content of between 10 and 30 %.
Butter is a dairy product obtained from milk fat (cream), which usually has a lipid content of between 65 and 85 %.
Cheese is obtained from milk (cow's, goat's, buffalo or sheep's milk, or mixtures) by variously processing the curd (coagulate) obtained by coagulating casein. Casein can be coagulated in two ways: by lactic fermentation (souring) or with the help of rennet. The cheeses obtained from curd formed by lactic souring are mostly eaten fresh and have a soft consistency. The cheeses obtained from curd produced with the help of rennet require more elaborate processing, carried out in several stages over a longer period.
Cow's cheese is made from pasteurised milk at coagulation temperature (22–28 °C), to which a starter (a selected culture of lactic streptococci), calcium chloride and sometimes rennet is added. The curd formed in this way is cut into pieces and left to rest so that the whey drains away. Sometimes light pressure (pressing) is applied. To obtain an even, as fine as possible consistency, the product prepared in this way is passed through grinding machines or rollers, which give it a butter-like consistency.
Telemea is made from cow's, sheep's, goat's or buffalo milk, or mixtures. Standardised and pasteurised milk is mixed with a starter (a culture of Streptococcus lactis and Streptococcus casei), calcium chloride, potassium nitrate and rennet. The resulting curd is pressed and then placed in brine. It is kept in the form of blocks (pieces of a particular size) in wooden barrels covered with brine, where the ripening takes place (about 20 days), and then stored in the cold.
Cheese is made from sheep's or cow's milk. The curd is made from pasteurised milk and inoculated with selected cultures. After pressing and ripening, the curd obtained is cut into slices and scalded with water at 80–90 °C. This creates a soft and elastic paste that can then be worked. It is kneaded and filled into special moulds to take up the whey and form a smooth rind. After 24 hours the cheese wheels are taken out of the moulds and put into special chambers for turning. It is left to ripen for a certain time (45-90 days – depending on the variety) and is then washed, dried and waxed.
Cheese is an excellent source of nutrients with a high biological value and has the advantage of being concentrated in a reduced volume, which makes it easier to digest. Compared with milk, cheeses have a higher content of proteins and lipids and a lower content of lactose and fat-soluble vitamins. Fat-soluble vitamins are abundantly represented in the full-fat varieties, however. The calcium content varies according to how the curd was obtained. It is lower in cheeses produced by lactic fermentation and higher in cheeses produced with the help of rennet.
Milk and its derivatives become harder to digest as we get older, because the production of lactase, which splits lactose into glucose and galactose, declines.
Combining milk with meat, coffee and tea leads to a drastic worsening of its digestibility because of the formation of proteopeptide tannates.
Fresh fermented dairy products (yoghurt, kefir, buttermilk) contain live lactic acid bacteria. These bacteria (lactobacilli, bifidobacteria) break down part of the lactose and partly split casein – which is why many people tolerate fermented dairy products well. Live yoghurt cultures improve the digestion of the lactose in the product in people who have difficulty digesting lactose.
The action of light on milk can destroy a greater part of the vitamins it contains. Vitamin C can be 80 % destroyed in 2 hours and vitamin B2 30 % destroyed in half an hour. In addition, if pasteurisation takes place under the action of light, the milk loses 12 % of its riboflavin content.