Composition, physico-chemistry and microbiology of milk

Overview

In view physicochemical, milk is a very complex product. A thorough knowledge of its structure is essential to understanding the changes taking place in him and its derivatives in the various industrial processes. The purpose of this chapter is to examine this structure.

Composition of milk

Milk is a colloidal system consisting of an aqueous solution of lactose, raw salt and several other elements to the dissolved state, in which proteins are in state of suspension and fat in the state of 'emulsion. The total dry extract of milk is on average 13.1% dry matter and fat (not fat) is 9.2%.

The overall composition of cow's milk is presented in Table 1, whose data are quantitative estimates, which vary depending on many factors: animal, food and health status of the animal, lactation period, and during trafficking. However, the exact composition of a sample of milk can only be achieved by analysis.

Physicochemical properties of milk

Knowledge of the physicochemical properties of milk is of undoubted importance because it allows to better evaluate the quality of raw material and to provide treatments tailored technology and operations.

Acidity of milk

The pH (active acidity) of a normal milk ranges from 6.2 to 6.8, but the majority of milk has a pH between 6.4 and 6.6. Colostrum is more acidic than normal milk, while milk and late lactation cows that patients usually have a higher pH, which approximates the pH of the blood.

All components can be combined with basic ions contribute to the acidity of milk. It is the balance between the basic constituents (sodium, potassium, magnesium, calcium and hydrogen) and acidic (phosphates, citrates, chlorides, carbonates, hydroxyl and protein) in milk which determines acidity. These two groups of constituents may exist in all combinations. We must also recognize that these combinations vary in degree of ionization in dissociation constant and solubility product. It should also be noted that the degree of dissociation increases with the neutralization or pH and calcium salts that are less differentiated than the salts of sodium or potassium. It is for this reason that in the milk, especially in acidic media, there is a predominance of calcium salts which tend to combine with proteins.

The acidity of the milk expressed as a percentage of lactic acid can vary from 0.10 to 0.30%. Most milk has an acidity of 0.14 to 0.17%. The natural constituents of milk that contribute to acid phosphates are (0.09%), caseins (0.05 - 0.08%), other proteins (0.01%), citrates (0.01 %) and carbon dioxide (0.01%). The acidity of the milk can also be expressed as "degree Dornic. Fresh milk as the acidity may have between 16 and 18 ° Dornic (with 1 ° D = 0.1 gd'acide lactic per liter).

Dairy Technology, focuses on changes in acidity during treatment. Indeed, these changes may affect the stability of the milk.

The heating of milk causes the loss of carbon dioxide, can break down the lactose into various organic acids or cause blockage of amino groups of proteins and then causes an increase in acidity. Similarly, high temperature, the tricalcium phosphate may precipitate and cause an increase in acidity triggered by dissociation of the phosphate radicals.

The development of lactic acid bacteria in milk transforms lactose into lactic acid in particular. This means that new acidity developed by acidity and leads to the destabilization of proteins. According to the use of milk, it can develop its acidity.

Freezing Point

The freezing point is the temperature of transition from liquid to solid state. This is one of the constants of the most stable milk. This consistency results from the fact that the osmotic pressure of the milk is kept in balance with that of blood. The lowering of the freezing point is directly related to the concentration of solutes in a solution. It is therefore a measure of the number of molecules or ions dissolved in the aqueous phase of milk.

The freezing point of milk can vary from -0.52 to -0.56 ° C, any change greater than -0.52 ° C is an index of anchorage. It allows the detection of anchor milk from 3%. The lowering of the freezing point can also be caused by the subdivision of lactose into several smaller molecules. It can also be used to assess the degree of hydration of proteins.

Boiling Point

At normal atmospheric pressure, the boiling point of water is 100 ° C and the milk is 100.5 ° C. As for the freezing point, it depends on the number of particles in solution and therefore increases with the concentration of milk and decreases with pressure. This phenomenon is applied in the processes of concentration of milk.

Density of milk

The weight of a substance per unit volume is the density, while the density is the ratio of density with that of water. Since the density of any substance varies with temperature, it is important to specify that in reporting the results. In practice, the density of water at 4 ° C is 1000 g / l and therefore, at this temperature, density and the density of water are identical.

The density of milk at 15 ° C is on average 1032 (1028-1035). It is the result of the density of each constituent of milk. For whole milk, it is necessary to measure the density at 30 ° C so that the fat is in the liquid state, otherwise in solid form, fat has a higher density and variable. Consider also that if there is air in the milk, the density will be lower. The density of milk constituents at 30 ° C is as follows:

• Fat (Fat): 0.913;
• Dry defatted: 1,592;
• Lactose (L): 1.63;
• Protein (P): 1.35;
• Ash (A): 5.5.

So the density of milk at 30 ° C will be calculated by the following formula:

  Density milk to 30 ° C = [(% fat x 0,913) + (% L x 1.63) + (% P x 1.35) + (% A x 5.5) + (% Water x 1)] 

Dairy Microbiology

Microbiology is intimately linked to the dairy industry: it applies to all sectors. Its principles, in fact, justify the method of hygienic milk production, treatment and control several processes at its processing plant, and are the basis of methods of dairy products. The quality of milk and milk products depends in large part, although if we take into account the microbiological standards in its formal assessment.
The application of general principles of hygiene can achieve three goals:

• The first, prevent and stop the transmission of pathogenic bacteria in milk and dairy products and thereby protect the health of consumers;
• The second, prevent and reduce microbial growth in milk and dairy products and thus prevent their deterioration and the appearance of defects;
• Third, encourage and guide the development of bacteria in some dairy products such as fermented products (yogurt, lben, ...).

Classification of microorganisms associated with milk

It is possible to establish an identification key to identify the species encountered most frequently in a product. The method of identification used is mainly based on tests for Gram, of catalase to oxidase, fermentation of sugars and sporulation, including the methodology to be followed in the case of the first four, is found in any textbook of General Microbiology . For its part, the sporulation can be recognized by the survival of an organism subjected to a heating of 85 ° C for 10 minutes.

Table 2 presents the classification of some species of bacteria associated with milk, obtained with these tests.