Milk is a source of various products which brings smile on our face like cheese, butter, ghee or milk itself. The huge transforming capacity of milk to curds has been utilized to produce yoghurt, butter ghee and cheese. Cheese is a product of curdled milk having its own history and legacy. It is said by Archaeologists that. cheese was discovered as far back as 6000 BC and had been made from cow's and goat's milk. It is said to be discovered by the nomadic tribes of Central Asia who used to carry milk in animal skin bags milk on animal backs when on the move and later found that there was a separation seen after a period of time as milk coagulates and a transparent liquids formed due to the high temperature and continuous motion. Both the products were tasty and edible!!! Fermentation of the milk sugars has caused the milk to curdle and the swaying motion would have broke up the curd to provide a refreshing whey drink which is also the basic principle involved in natural cheese making.
Acidification is the key step in cheese making. Acidification is nothing but the lowering of pH or increasing acid content of the milk, making it more acidic which is achieved by addition of suitable bacteria after the milk has been pasteurised. Starter culture is an important component in cheese making. Starter culture is that boost given up in the process due to the presence of known specific strains of microorganisms. Manufacturers maintain their own specialized starter culture which not only lowers the pH level initially but later also provides flavor to the cheese. Acid is an essential component in the production of good cheese, but excess amount of acid in the milk will make cheese crumbly and whose absence or low concentration will make curd pasty. Acidification is followed by coagulation of the milk to convert it into curds and whey by addition of the enzyme Rennin which is a Component of Rennet.
Rennin also known as Chymosin is a proteolytic enzyme synthesized by chief cells in the stomach. Its role in digestion is to curdle or coagulate milk in the stomach to facilitate digestion by holding on so that it would block the rapid flow of milk from the stomach. It is synthesized in the young mammals and earlier stomach lining of young calves was utilized to extract this enzyme to produce cheese. Now it is possible to produce it through genetic engineering in Fermentors which has not only avoided killing of the young calves but also has helped to meet the increased demand of consumption all over the world. Animal source of Rennin was utilized for several years ,it was in 1990, the recombinant version of calf chymosin, usually denoted as fermentation produced chymo-sin was utlised in production of cheese.
The chymosin gene was successfully isolated from the calf stomach cells and is inserted into nontoxicogenic and nonpathogenic strains of bacterium, yeast or filamentous fungus to produce Chymosin which has been registered by the U.S.Food and Drug Administration. The iso-lation of mRNA from the host's abomasum cells is followed by reverse transcription to produce the complimentary DNA or cDNA. After incorporation into a vector DNA it can be transferred into suitable micro-organisms namely E.coli, Kluyveromyces lactis, Aspergillus niger and specific strains of yeasts Chymosin is by far the most prominent genetically engineered clotting enzyme. After cloning preprochymosin or pro-chymosin cDna into Bacteria, yeasts or filamentous fungi they were allowed to multiply in submerged fermentors as they are more economical and produced the required recombinant enzyme. It depends on the micro-organism whether prochymosin is directly released into the medium(e.g. yeasts and fungi) or enclosed in inclusion bodies (e.g E.coli) so that cells have to be lysed before further processing. Prochyomysin is then usually activated at pH 2.similar to the activation schedule in natural calf rennet production.Thus the active rennin is added to set the curd. Once the curd has been set firmly it is followed by cutting and heating the cheese in order allow the whey to ooze out from the curd and tenders the curd hard. After which salting is one of the very old procedure followed which not only adds flavor to the curd but also inhibits the growth of certain microorganism and aids in aging process of the cheese. Aging of cheese vary from days to months to several years. As Clifton Paul Fadiman states Cheese-milk, leap towards immortality, the art of cheese making utilizes the labor of millions of microorganisms.
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