Cow's milk is mainly used to make cheeses, less often sheep, goat, buffalo and the milk of other domestic animals, e.g.. mares etc..
Milk consists of water, protein bodies, fat, milk sugar and mineral salts. Some of these ingredients are dissolved in water, part suspended or swollen. When all the water is evaporated from the milk, we will get solid components in the form of a white precipitate, the so-called. dry weight. In cow's milk, water makes up approx 7/8, dry weight only 1/8 part of the total weight of milk. From this it follows, that the measure of the value of milk in terms of its content is dry weight.
Mixed milk from more cows contains a percentage :
water 86,0 do 89,5, on average 87,5
dry weight 10,5 do 14,0, „ 12,5
They make up the dry mass as a percentage:
fat 2,7 do 4,5, on average 3,5
protein 3,0 do 4,0, „ 3,5
milk sugar 3,6 do 5,5, „ 4,8
minerals 0,6 do 0,8, „ 0,7
In the milk of individual cows and from separate milkings, the above-mentioned components vary within even wider limits.
On the chemical composition of milk, especially for fat content, mainly influenced by the following factors: taste, individuality, period of milk production, time lapse between milkings, finally, the method of milking and handling of cattle. Fat fluctuates the most, the smallest milk sugar. The amount of fat is approximately equal to the protein content, which is also about the same ratio that the two ingredients end up in the cheese.
Thanks to the non-fat dry matter, the milk is heavier., that is, denser than water: its specific gravity at 15 ° C is from 1,028 do 1,034, on average 1,0315.
When freezing, the milk components are not distributed evenly, namely, the outer parts of the ice closest to the walls of the vessel are more watery and less solid than the original milk. The ratio is reversed in the middle section, which later freezes. It is therefore important to melt the ice and mix it thoroughly before taking a sample.
Fat is, next to casein, the most valuable component of milk: we make butter from it; in cheeses it is a spread that significantly increases their value. In milk it is distributed in the form of very fine, only under the microscope visible spheres of unequal size. From this heterogeneous mixture, that is, suspensions, a significant proportion of especially larger fat globules, as a species lighter than other milk components, it rises to the top and forms a layer of cream (patrz podstoje). The fragmentation of the fat globules is still preserved in both cheese and butter, which increases their dietary value.
The fat in milk has a specific gravity 0,93, melting point from 31 do 36°C, and clotting from 21 do 26°C. In fresh milk, fat is in a liquid state and retains this property even at a temperature close to 0 ° C. This supercooling phenomenon is closely related to surface tension. The fat atomizes even more subtly under the influence of vibrations at the temperature above its melting point, on the other hand, when shaken at a temperature below the freezing point, it concentrates and emits in the form of butter lumps. Hence the harmfulness of careless transportation, too vigorous mixing of milk in heaters, pumping, etc..
Milk fat is a mixture of at least nine different fats, being a compound of glycerin with corresponding fatty acids. Most of them are common fats: stearyna, palmityna i oleina, which are a compound of glycerin with non-volatile stearic acids, palmitic and oleic, and only about 5% goes to butyrin, which is a combination of glycerin with volatile butyric acid. It is the main feature of milk fat, because it is not found in other animal fats. The fats become rancid under the influence of some microorganisms, i.e.. that they are split into free fatty acid and glycerin. Volatile acids, in particular, have a sharp taste and smell. The sun's rays, when exposed to air, oxidize the fat, as a result it takes on a sebum-like taste. Therefore, you should protect the milk from sunlight. Fat in milk, especially warm milk, is absorbed greedily and retains foreign odors for a long time, e.g.. fertilizer, musty, smoke, tracks, carbolu itp. Therefore, you should protect dairy products from any fumes.
Green feed colors the fats and gives the milk and milk products a more or less yellowish appearance.
Protein bodies. We find them in the body of every animal, and similarly to plants, carbohydrates predominate, yes, protein in animals. Protein bodies contain the following elements: coal, hydrogen, azote, oxygen, sulfur, and sometimes some phosphorus as well.
There are several different protein bodies in milk, the most important of which are:
casein, that is, cheesecake - on average 2,6%, albumin and globulin - average 0,7%.
Of these, casein is of the greatest importance in cheese making, that is, cheesecake, because it is used to make cheese. Chemically, free casein behaves like a weak acid. It is found in connection with calcium in fresh sweet milk as calcium caseinate. It is an acid salt that is insoluble in water, which forms a colloid in milk.
Casein differs from other protein bodies in milk mainly in its phosphorus content and its behavior under the influence of rennet. By acting on calcium caseinate with a weak acid, we will release casein from its combination with calcium and we will obtain free casein, that is cottage cheese. The same thing happens as a result of the spontaneous souring of milk.
Unlike acids, rennet acts on calcium caseinate. According to Hammarsten, it splits casein into two completely different bodies, namely: 1. and paracaseinę, more specifically calcium paracaseinate and 2. for whey protein, i.e. whey protein in the presence of calcium salt, with the most paracasein being formed, and little whey protein.
Calcium paracaseinate is insoluble in water and therefore separates as a dense clot, which more or less whey-free is the raw rennet cheese.
Whey protein (mainly albumin) remains in the whey as a soluble substance. In practice, we precipitate it with weak acids and heating to a temperature from 75 do 90°C. Precipitated forms the so-called. that is. It is inelastic and somewhat greasy, easily digestible and nutritious. It differs from casein primarily by the lack of phosphorus. It is used for the production of inferior types of cheese, the so-called. "Short circuit”.
Globulin is not found in cheese making. practical because of the small amount. It remains dissolved in the whey. Excessive amount of globulin in milk protects the calcium caseinate from the effects of rennet. This applies, for example, to. colostrum globulin, or in milk from cows affected by udder diseases. From this the harmful effects of even a small addition of colostrum or altered milk. Huge protein molecules are made up of simpler ingredients, namely with approx 20 types of amino acids. The quantitative ratio of the various amino acids to each other within each protein molecule is variable, hence the possibility of the formation of many different combinations and changes in physical and chemical terms, conditioning numerous differences in the properties of protein molecules. Taking into account the affinity of protein for water, salt and other chemical agents, the wealth of differences in protein properties is easy to understand. Of course, in its structure, casein will remain casein, but not without significance will be the quantitative ratio of other accompanying proteins to it, like albumin and globulin when it is renneted (buffering) and an effect on the subsequent shrinkage of the cheese slurry.
This subtle field of chemistry still needs a lot of explanation. The cheese maker raises questions, what effect on the building material of proteins, their quantitative relations is influenced by such factors, like an animal species, taste, period of milk production, health, way of nutrition, soil, etc.. The cheese maker copes with this by observing the ability of milk to coagulate under the influence of rennet, assesses by touch the changes taking place in the cheese thicket, in order to apply the method and course of treatments during processing.
• Differences in the structure of the protein in the feed have a variety of effects on the nutritional effect. The animal organism only then creates a specific protein, when the feed proteins contain all the building blocks and when all the necessary amino acids are present in sufficient quantity. Hence the desirability of mixing different feeds. In cheese countries, this is also considered, in order not to overdo it in milk records, which would adversely affect the cheese suitability of the milk.
The complex is not without significance here: calcium caseinate + minerals, especially calcium phosphates, which will be discussed later.
Milk sugar is only found in the milk of animals, but it is absent elsewhere in nature. His presence makes, that the milk is sweet. Lactic acid bacteria convert it into lactic acid in an aqueous solution, and this, by combining with the calcium bound to casein, precipitates it: the milk is curdled, that is, it clots. Some yeasts convert milk sugar into alcohol and carbonic anhydride. The amount of milk sugar and the acid produced from it in cheeses has a significant impact on the life of microorganisms and the direction in which the cheeses mature.. Without lactic acid, all the cheeses would rot.
Minerals. The content of mineral salts in healthy milk varies only within tight limits, namely from 0,6 — 0,8%, a is on average 0,7%. The properties of milk depend to a large extent on the quantitative and qualitative ratio of salt to proteins, especially its taste and the ability to solidify under the influence of rennet. Bitter-salty milk, alkaline, having a ratio of minerals different from the normal ratio is not affected by the shearing effect of rennet. There is more potassium in a healthy milk than sodium. The number of alkalinity in milk (Nottbohm) is expressed by the ratio K20 : Na20.
The following mineral salts are found in milk: sodium and potassium chloride, monopotassium phosphate, double-potassium, two-calcium, tricalcium and di-magnesium, potassium citrate, calcium and magnesium, plus calcium oxide bound to casein, and some iron. The mentioned salts are usually dissolved in milk, except some calcium phosphate. The presence of acidic and alkaline salts next to each other is the cause of the litmus reaction of healthy and fresh milk to the duality.
Other ingredients. In addition to those mentioned, there are still small amounts of other ingredients in milk. Lecithin is one of the most important here, citric acid, enzymes and vitamins.
Lecithin is found in milk in very small amounts. It accompanies fat, hence there is more of it in cream and buttermilk than in skim milk. It contains phosphorus and is important in the nutrition of the nerves and brain. Non-ripening cream cheeses are rich in lecithin. When heated to high temperatures, lecithin breaks down.
Vitamins. Depending on the type of feed, milk contains more or less vitamins, these essential ingredients for life and health. They also partly pass into cheeses. We find vitamins here:
A - growth factor, against the degeneration of the epithelium of several organs, and, inter alia, the conjunctiva of the eye (xerophtalmia); withstands heating up to 100 ° C (and a bit more), but without air access.
B1 (thiamine) - anti-neuritic factor, against beriberi; easily oxidizes, when hot in the presence of air it decomposes gradually.
B2 (lactoflawina) - growth factor, against skin diseases (pellagra); it is the yellow ingredient of whey; tolerates heating well. C (ascorbic acid) - anti-scurvy factor (anti-rot) and anti-infective; when hot, it is easily oxidized when exposed to air.
D - antirachitic factor (anti-rickets), withstands high temperatures also in the presence of oxygen. It is not yet fully understood.
The effect of temperatures on the loss of vitamins used in cheese brewing and the oxidation that occurs during long mixing is not yet precisely defined.
Enzymes, that is, leavens give milk a kind of life force. They are complex, chemically very active substances, which are able to make chemical transformations of many times larger amounts of other substances. The milk enzymes either come from the animal's organism, or they are products of microorganisms. We find here e.g.. catalog, diastase, oxidase and peroxidase. High temperature destroys them. We also use an enzyme measure to evaluate milk.