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DAIRY PRODUCTS

(i)                 Yoghurt: Prepared by inoculation (addition) of concentrated milk either with a sample of – yoghurt (traditional) or commercially available culture of Lactobacillus bulgaricus and/or Streptococcus thermophilus (modern practice). Fermentation of lactose to lactic acid at 42-45°C for 4-5 hours drops the pH to 4 that leads to the precipitation of protein casein and thus, production of the yoghurt (dahi).

(ii)               Cheese: Many kinds of cheese are produced which involves three steps:

a)      Curdling of the milk by inoculating it with starter culture of Streptococcus lactis or S. thermophilus and/or Lactobacillus lactis or L. bulgaricus. The culture ferments the lactose sugar of milk to lactic acid in about 6 hours, which leads to the precipitation of casein to form curd. Curd may also be prepared by adding rennin (rennet), which contains the enzyme chymosin (a protease), to milk. Rennet is traditionally obtained from the pancreas of calves but, now, can also be obtained from some naturally occurring microbes, like the fungus Mucor miehei or the genetically engineered microbes (e.g., Escherichia coli) containing the calf chymosin (rennin) gene. Combined actions of proteolysis (by rennet) and acidification (by culture lactic acid) leads to the coagulation of casein.

b)      Separation of the protein casein from curd by heating_it at 37°C (the liquid separated is known as whey). This heating process leads to the inactivation of rennin also. The protein is then molded into characteristic shapes and applied with salt by floating it in salt solution. This checks the contamination by undesirable microbes.

c)      Ripening of the cheese is allowed for 2-12 months at 2-10°C by microorganisms that are already present in cheese or by adding the desirable commercial microbes (e.g., Penicillium sp., Lactobacillus sp., Streptococcus sp., depending upon the kind of cheese to be prepared). This gives t1avour and aroma and improves the physical properties of the cheese due to degradation of lactose (into lactic acid), proteins and fats (into butyric acid). Sometimes, proteases from Penicillium or Bacillus amyloliquefaciens and lipases from Aspergillus or Mucor are also added. The byproduct whey can be used to produce ethanol by the yeast Kluyveromyces sp.

(iii)             Butter: It is obtained from the churning of pasteurized sweet or sour milk, which separates the fat globules from the liquid. The cream thus obtained, after the pasteurization at 71oC for 30 min., to kill the microbes, is either soured naturally or by adding the starter culture of Streptococcus lactis or Leuconostoc. This increases the butter yield and also improves the aroma and flavour. Salt is added to inhibit the contaminating microbial growth.

(iv)              Low Lactose Milk: The milk and whey contain about 5% lactose. In many parts of world, people are not tolerant to lactose as they lack the enzyme lactase which breaks down the lactose sugar into glucose and galactose. Such persons suffer from diarrhoea or stomach ache after taking the milk. Lactase obtained from the yeast Kluveromycesfragilis or the fungus Aspergillus niger is used to treat the milk conversion of lactose to glucose and galactose) so that they can be safely consumed by such persons.