Enzymes and the Production of Cheese

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There are hundreds of different types and brands of cheese, and their flavors, ways of production, and ingredients generally differ to create the plethora of cheeses we know and love. But despite their differences, all types of cheese have one similarity in the way they are produced: they all use a natural complex of enzymes, referred to as rennet, to coagulate (solidify) milk, separating it into the solid curd and liquid whey.

At the start of the cheese making process, milk, the basis of all cheese, is prepared and treated. Different types of milk are selected to produce a diverse range of flavors, including cow, goat, sheep, buffalo, or camel. The milk is often pasteurized, or heated in order to kill off harmful bacteria, and the starter culture is added to begin the acidification process of milk curdling. After acidification, the milk begins to coagulate, and the casein proteins present in milk change, leading to the formation of solidified milk fat (aka. the curd).

The process of coagulation can occur naturally, however rennet is usually added to speed up the process. Rather than being one specific enzyme, rennet is a complex of enzymes, including protease, and pepsin, which break down protein to form peptides and amino acids, and lipase, which converts fats into glycerol and fatty acids. The most important of these enzymes are rennin, which curdle milk, and chymosin, which react with the substrate K-casein, causing the proteins in milk to clump together to form curds, and separate from the whey known as “glyco macro peptide”.

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adding dissolved rennet to milk

external image 08curds-whey.jpg curd seperating from whey
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Afterwards, the whey is drained, leaving the creamy milk curds, which are still in a semi-liquid state at this point. They are then flavored, then pressed in a cheese press to remove excess whey, and in this process the cheese finally becomes solidified. Finally, the cheese is left to age for up to several years, and then packaged and sold to consumers like us.

History and Production of Rennet
Rennet was first indirectly discovered thousands of years ago, when people found out that milk stored in a dried stomach, specifically in the fourth stomach of a young calf, would remain hard and edible for long periods of time. Nowadays, a milk-fed calf’s fourth stomach is removed for rennet production in conjunction with the production of veal. The stomachs are dried and cleaned, then soaked in a solution of saltwater and vinegar to lower pH levels. After a few days, the solution produced is filtered, and can then be used for the coagulation of milk. Generally, 1 gram of rennet can coagulate 2000 to 4000 grams of milk.

There are other methods of extracting rennet, however because of the scarcity of milk-fed calf stomachs, the use of animal rennet is generally only used by very experienced and traditional cheese makers, such as those in Switzerland, France, and other areas of central Europe. Nowadays, only about 35% of global cheese production uses animal rennet.

Other alternatives for animal rennet include:
- Vegetable Rennet: for example, rennet removed from fig tree bark, nettles, thistles, and the mold mucor miehei.
- Microbial Rennet: some molds, such as mucor miehei, can produce proteolytic enzymes which help to digest proteins, much like protease and pepsin in animal rennet. These molds are produced in a fermenter, then added texternal image cheese.jpgo milk in the same fashion as rennet. However, microbial rennets can alter the flavor of cheese, usually giving it a slightly bitter taste.
- Genetic Rennet: Because of the imperfections of microbial rennet (ex. the need for purification in order to avoid contamination), genetic rennet is often used as a replacement. Developments in genetic engineering allow calf-genes to be used to modify certain types of bacteria, fungi or yeast to make them produce the chymosin found in animal rennet. Genetically engineered rennet is very similar to naturally produced calf rennet, with only slight differences in the type of chymosin, making it a popular substitute for animal rennet. In fact, “in 1999, about 60% of US hard cheese was made with genetically engineered Chymosin” (wikipedia).
- Acid Coagulation: sometimes, cheaper brands of cheese are produced by using acid (ex, citric acid), or even bacteria, to coagulate milk.

Chi-Chi Chuang, SL Biology E block


For a detailed account with visuals of the process of cheese making and the history of cheese: http://shannak.myweb.uga.edu/Process.html
Another visual step-by-step guide to home cheese making: