Classification of Milk: 3 Types | Milk Microbiology

The following points highlight the three types of milk. The types are: 1. Fresh Milk 2. Pasteurized Milk 3. Fermented Milk.

Type # 1. Fresh Milk:

This milk is endowed with all the naturally occurring constituents provided the milking is done perfectly asceptically and the milking cattle are healthy. This milk has great nutritional value.

In Ayurveda this milk has been referred to as Dharoshna milk meaning thereby that the freshly drawn milk has the same temperature as the body of the milking cattle. In this milk no constituent is lost and destroyed. But, use of this milk has its own limitations.

Type # 2. Pasteurized Milk:

The milk is treated at a definite temperature for a definite time period so as to get it free from the microorganisms without losing any constituent. This treatment is called milk-pasteurization which results in the destruction of most of the diseases and spoilage causing microorganisms in milk with heat.

Actually, the term ‘milk-pasteurization’ refers to the process of heating every particle of milk to at least. 145°F and holding it continuously at or above this temperature for at least 30 minutes, or to at least 161°F or above this temperature for at least 15 seconds.

However, the account of milk-pasteurization is the following:

Home Pasteurization of Milk:

Home pasteurization of milk is the simplest and one of the most satisfactory methods. The milk is usually pasteurized in regular milk bottles. In this process, cover from one bottle is removed, a little of the milk is poured out, a hole is punched in the cover, the cover is replaced and a thermometer is inserted there.

Then all the bottles of milk are set on a milk rack and heated until the thermometer registers 145°F. The pail is removed from the heat and the bottles are left in the hot water for 30 minutes; reheating is done to keep the temperature at 145°F, if necessary. After the 30 minutes period, the hot water is gradually replaced with cold water until the milk is cooled, preferably using ice in the last water.

Commercial Pasteurization of Milk:

Two methods of milk pasteurization are used commercially:

(i) Low temperature holding (LTH) method and

(ii) High temperature short-time (HTST) method.

(i) Low Temperature Holding (LTH) Method:

In low temperature holding method, or ‘vat pasteurization’, the milk is heated to 145°F (62.8°C) for 30 minutes. The milk is heated in closed vats by steam coils, hot water jackets or continuous spraying of heated water along the sides of the container. It is then cooled and bottled.

It has been found in practice that LTH method is the more efficient as it destroys a larger percentage of the bacteria. Usually over 99% of the pathogenic bacteria present in milk are destroyed by proper pasteurization by this method.

(ii) High Temperature Short-time (HTST) Method:

In this method milk is heated to a temperature of 161°F (71.7°C) for 15-30 seconds. The heating is done by means of electricity or hot water. The heated milk is then cooled and maintained at a low temperature until distributed.

Test of Pasteurization (Phosphatase Test):

Phosphatase is an enzyme present in raw milk. This enzyme is destroyed by adequate pasteurization of the milk. If the milk is under-pasteurized it gives positive reaction to phosphatase test.

This test can be conducted as follows:

5.0 ml of buffer substrate is added to 0.5 ml of milk sample to be tested, shaken for a short period and incubated in a water bath at 37°C for 10 minutes. To this is then added 6 drops of BQC (2-6 dibromo-quinone- chloroimide) solution and allowed to stand for 5 minutes.

The colour developed is compared with opaque standard. Grey or brown colour indicates proper pasteurization. Blue colour is the sign of improper pasteurization. The intensity of the colour being proportional of the extent of improper pasteurization.

Type # 3. Fermented Milk:

Fermentation is used to a very large extent with dairy area. Here the purpose is souring and developing flavour substances. Fermentation is done commonly of the pasteurized milk. The “souring of sweet milk” was practiced long before the microorganisms had been discovered.

This was done to preserve milk and to provide a new beverage with a distinctive and desirable flavour. Consumption of fermented milk is widespread due to its therapeutic value.

The most important organisms used in the preparation of fermented milk are species of Streptococcus, Leuconostoc, and Lactobacillus. The starter culture consists of either pure strain of a species or combination of species best suited for the production of desired product.

Therapeutic Value of Fermented Milk:

It goes to the credit of the work of the Russian bacteriologist, Metchnikoff, who first explored the therapeutic value of fermented milk. He observed that people in the Balkan countries lived to an exceptionally old age. He related this to the regular consumption of fermented milk by them.

He believed that drinking fermented milk containing lactobacilli caused these lactic acid bacteria to become ‘implanted’ in the intestinal tract replacing the putrefying micro-flora.

The metabolic products of these putrefying microorganisms were considered responsible for various human ailments that ultimately lead to premature death. This theory of longevity was widely expounded resulting in the development of considerable interest of people in fermented milk.

Kinds of Fermented Milk:

(i) Cultured Buttermilk:

It is produced by inoculating pasteurized skim milk with a selected starter culture and allowing the fermentation until desired aroma, acidity and flavour have developed. The starter culture consists of Streptococcus lactis or S. cremoris together with Leuconostoc citrivorum or L. dextranicum.

The streptococci develop the acidity and Leuconostoc spp. are responsible for volatile acids and neutral products (acetyl acid, acetylmethylcarbinol and diacetyl) which give the characteristic aroma and flavour.

(ii) Bulgarian Milk:

This fermented milk differs from buttermilk in having higher acidity and lacking aroma because of incubation of starter culture inoculated milk at 37°C. The starter culture consists of Lactobacillus bulgaricus. The bulgarian milk is also prepared from skim milk which is mostly pasteurized.

(iii) Kefir:

This is the result of a mixed lactic acid and alcoholic fermentation. The starter culture is the small granules of organisms called ‘kefir grains’ which consists of Streptococcus lactis, Lactobacillius bulgaricus and lactose fermenting yeasts.

The bacteria produce acids (0.6-1.0% lactic acid) and yeasts produce alcohol (0.5-1.0% ethanol). Kefir is usually produced from the milk of cow; goat or sheep. Among the lactobacilli used, the addition of Bacillus caucosi is of particular significance because it grows in nugets and is responsible for a special type of kefir.

(iv) Yogurt and Yogurt-like Products:

Yogurt and Yogurt-like products (laban and dahi) are the traditional form of sour milk in India, Romania, Greece, Hungary, etc. The microorganisms responsible for the fermentation are Streptococcus thermophilus and Lactobacillus bulgaricus.

The fermentation takes place at 42- 46°C. Yogurt has a characteristic flavour and texture. The Indian curd (Dahi) is traditionally prepared from concentrated whole milk and is used either as such or in other forms such as mattha, chhach or takra.

(v) Kumiss (Kumys):

A similar product fermented in the pattern of kefir is known by a different name, the ‘kumiss’. It is prepared by tradition in certain parts of Russia. This fermented milk product is mostly used in Asia minor.

It is prepared by fermenting milk with yeasts and lactobacilli. It contains foam and some carbon dioxide and ethanol. These constituents render this product specific type of characteristics. The most important type of kumiss is prepared from mare’s milk.

(vi) Sweet Acidophilus Milk:

The sweet acidophilus milk has recently been introduced. It consists of large number of Lactobacillus acidophilus. To prepare it, a concentrated cell suspension of L. acidophilus is added to the cold pasteurized milk containing low fat, mixed well, packaged and then held at a temperature below 40°F. The product has the same flavour as pasteurized milk.

(vii) Taette:

This fermented milk product is commonly used in Scandinavia. It is much different from Kefir and Kumiss. For preparing taette, milk is fermented with yeast and rope forming strains of Streptococcus lactis. It is highly appreciated for its special texture and taste.

(viii) Cheese:

A large part of the milk produced is used in the manufacture of cheese. Cheese is the partially degraded concentrate of milk fat and casein, with a small amount of milk fluid. The manufacture of all cheeses depends upon the activity of microorganisms. Several hundreds of varieties of cheese are manufactured, and the amazing fact is that most of them can be made from the same batch of milk.

This is possible by providing conditions that favour the development of selected type of microorganisms. Stated in another way, the quality and characteristic of cheese are determined by the biochemical activities of specific microorganisms.

(ix) Butter:

Butter is made by churning of sweet or sour cream of a specified butterfat content; the butterfat globules coalesce into granules. The liquid portion, butter milk, is drained off and the granules are further processed.

The bacteria normally present in the cream usually produce the acid and other products desired, and dependence is placed upon these in the manufacture of butter in home. In the commercial manufacture of butter in the creamery, the cream is pasteurized and then inoculated with a culture of lactic add bacteria. The inoculant is termed starter.

The changes brought about in satisfactory ripening of cream are not the result simply of the growth of the starter culture, the Streptococcus lactis.

Other organisms closely related to it must grow with it. Cream which has been ripened by means of a pure culture of the ordinary Streptococcus lactis does not produce butter with as satisfactory a flavour and aroma as that produced by cream which has been inoculated with a mixture of these organisms.

The ripening produced by a starter is, therefore, an example of an associative action. The associative organisms are Leuconostoc citrivorum or L. dextranicum.

The most important of the substances contributing to a desirable butter aroma is diacetyl (CH₃ CO.CO.CH3). In the butter culture, the citric acid normally present in it is attacked with the formation of acetylmethylacarbinol or acetoin. This compound is readily absorbed onto the butterfat. It is gradually oxidized to diacetyl which is volatile and when present in the right amount gives the desirable butter aroma.

(x) Cream:

Cream is prepared by stirring or centrifugation of whole milk which results in the separation mostly of the fat content of the milk.

Following two are the important types of cream:

(a) Sweet Cream:

Sweet cream is prepared by inoculating milk with the strains of Leuconostoc citrivorum so that only butter aroma is produced and not the lactic acid. In this type of cream the amount of casein is less as compared to sour cream. This is because of failure of the milk curding which is possible only on the production of lactic acid.

(b) Sour Cream:

Streptococcus lactis and Leuconostoc citrivorum are used in producing sour cream and the ratio of these starter organisms is important. When L. citrivorum is employed, it produces diacetyl (CH₃ CO.CO CH₃) which has a butter flavour. Because casein is least soluble, its isoelectric point is 4.6 and the acid pH affects the curdling with milk.