Here is a list of top seven products of milk:- 1. Yoghurt 2. Kefir 3. Koumiss 4. Butter Milk 5. Butter 6. Cheese 7. Rennin.
1. Yoghurt:
Yoghurt is derived from a Turkish word ‘Jugurt’ which is the most popular fermented milk in the world now-a-days. It is made from milk, skimmed milk or flavoured milk. For the preparation of yoghurt, the milk should be free from contamination. The solid content (not fat) should be between 11-15% which can be obtained by adding skim or whole milk powder in fresh milk that normally contains 8% solids.
The product can be further improved by adding small amount of modified gums which bind water and impart thickening to the product. At this stage, the size of the fat particles in the milk should be around 2µm because this improves the milk’s viscosity, product’s stability and milk appear form. The milk is then heated at 80-90°C for 30 min., starter culture is added to it.
Heating improves the milk by inactivating immunoglobulins, remove excessive oxygen to produce microaerophilic environment which supports the growth of starter culture. Besides, heating also induce the interactions between whey or serum proteins and casein which increase yoghurt viscosity.
The milk is now cooled to 40-43°C so as to allow fermentation using starter organisms such as S. salivarius sub sp. thermophilus and Lb. delbruckii sub sp. bulgaricus together at a level of 2% by volume (106-107 cfu/ ml).
It is to be carried out for about 4h during which lactose is converted into lactic acid, pH decreases to a level of 6.3-6.5 to 4.6-4.7. The flavour in yoghurt is due to acetaldehyde which should be present at 23-41 mg/kg.
Both the organisms are lacking in alcohol dehydrogenase, and produce acetaldehyde from glucose portion of lactose via pyruvate and due to the action of threonine aldolase. Diacetyl, an important flavour giving agent is sometimes present in yoghurt. Finally, after completion of fermentation, the yoghurt is cooled to 15-20°C before adding fruity flavoured agents. It is then cooled to 5°C which helps in keeping upto 3 weeks.
Microbial pathogens or contaminants generally do not occur in yoghurt due to high acidity and low pH. But sometimes, it is spoiled by acidoduric microbe such as molds and yeasts.
Yeasts, particularly lactose fermenters Kluyveromyces fragilis and Saccharomyces cerevisae are particularly important but the yeast-like fungus, Geotricum and molds such as Mucor, Rhizopus, Aspergillus, Alternaria and Penicillium also spoil yoghurt. A good yoghurt should contain not more than 10 yeast g-1 with almost complete absence of coliform and molds.
2. Kefir:
Kefir is in-fact, fermented milk, produced by a mixed lactic acid bacteria and alcoholic yeast. The microflora responsible is not spread uniformly throughout the milk but is supplemented as discrete kefir ‘grains’.
The kefiran, i.e. large layers of polysaccharide material folds upon to produce a cauliflower like florets produce kefir. The outer smooth layer contains lactobacilli while the inner, rough side contains yeast and lactic acid bacteria. The capsular homo-fermentative Lactobacillus kefiranolaciens produces kefiran.
Lactobacillus kefir contributes the required effervescence in the product. Several yeasts such as Candida kefir, Saccharomyces cerevisiae and S.exiques have been observed, the latter utilize galactose preferentially glucose.
For commercial production of kefir, milk is homogenized and heated at 85- 95°C for 3-10 minutes. It is then cooled to 22°C before addition of kefir grains at a size of 5%. The fermentation should be carried out for 8-12 h. The product contains 0.8% acidity, 0.01% to 1.0% ethanol, carbon dioxide, acetaldehyde, and diacetyl etc.
3. Koumiss:
Koumiss is produced from mare’s milk which is greyish white drink, produced mostly in East European countries. It has L. delbruckii sub sp. bulgaricus and a number of lactose fermenting yeast responsible to produce koumiss. These are spread throughout the koumiss. The lower fat content and higher carbohydrate level in the milk is suitable for koumiss production.
4. Butter Milk:
It is a liquid substance which is separated from cream during churning process. A mixture of starting cultures such as Lactobacillus lactis sub sp. diacetylactis and Leuconostoc mesenteroides sub sp. cremoris produce butter milk.
Since citrate metabolism can provide the pyruvate, lead to the accumulation of diacetyl, hence fresh milk should contain citrate for the production of butter milk. If not, the milk is supplemented with 0.1-0.2% sodium citrate to ensure good flavour development. For the production, milk is pasteurized, homogenized and fermented at 22°C for 12-16 h. The final product contains 0.7-0.9% lactic acid.
5. Butter:
Lactic acid producing bacteria are responsible for subsequent separation of butter fat in the churning process. These organisms (Uuconostoc citrovorum, Streptococcus cremoris or S. lactis) produce a small amount of acetoin which is spontaneously oxidized to diacetyl.
This substance gives butter and similar products with their buttery flavour and aroma. When the pH reaches about 4.3, it ceases growth, but its enzymes attack the citrates in the milk and produce diacetyl. But neither S. ceremoris nor Leuconostoc alone can produce the desired result in commercial practice.
6. Cheese:
There are about 2000 varieties of cheese made from mammalian milk. Cheese is thought to have originated in south western Asia some 8000 years ago. The Romans encouraged technical improvements and stimulated the development of new varieties during their invasions in Europe between 60 B.C. and A.D. 300. The cheese name is derived from Latin name caseus.
There are two groups of cheese, fresh cheese and ripened cheese. The fresh cheese are made up of milk coagulated by acid or high heat e.g. cottage cheese, while ripened cheese are made through lactic acid bacterial fermentation and coagulated by an enzyme preparation. The curd is removed and salted and whey is separated. The salted curd is held in controlled environment.
During this process, various physical and chemical changes occur to give a characteristic flavour and texture. Just as the variety of grape influences the flavour and bouquet of wine, so the mammalian origin of milk influences the flavour and aroma of a natural ripened cheese.
There are mainly three categories of cheeses:
(a) Soft cheese,
(b) Hard cheese, and
(c) Semisoft cheese.
The soft cheeses are ripened by the enzymes from yeast and other fungi that grow on the surface, hard cheese ripened by lactic acid bacteria which grows throughout the cheese, die, autolyze and release hydrolytic enzymes. The semisoft cheeses are ripened by proteolytic and lipolytic organisms which soften the curd and give it flavour.
7. Rennin (Milk-coagulating Enzymes):
The milk coagulation enzyme, rennet, has its importance in cheese production. Certain commonly used cultures which have an ability to convert lactose to lactic acid and lower the pH, are Streptococcus lactis, S. cremoris, S. thermophilus, Lactobacillus Helveticas and L. bulgaricus. These organisms bring the desired curd structure and flavour with minimum gas production.
Rennet is an extract from the fourth stomach of 3 to 4 week old calves which have been fed on milk. The enzyme so purified, called rennin, chymase or chymosin.
Rennin production of microbial origin is now widely recommended due to good coagulation of casein without hydrolysis, good odour and structure of cheese, nontoxic, low protein denaturation in order to prevent the development of bitter taste during ripening process and low lipase activity, to check the rancidity development.
There are several genera of fungi reported to produce rennin. These are species of Aspergillus, Candida, Coriolus, Rhizopus, Mucor, Penicillium, Torulopsis etc. Mucor pusillus, M. miehei and Endothecia parasiticus are widely used.The enzyme is an acid protease, which is stable at pH 4.0- 5.5 with a molecular weight of 34000-37500 Dalton at 50°C temperature.
Fermentation:
A culture consists of soy meal (3%), glucose (1%), skim milk (1%), NaNO3 (0.3%), K2HPO4, (0.05%), MgSO4.7H2O (0.025%) at pH 6 litre-1 is mixed, autoclaved and fermentation takes 48 hours at 28°C. The extracellular enzyme is concentrated after separating the mycelium. Recently cDNA from calf protein has been cloned in E. coli resulting into the microbial production of calf renin enzyme.