Rennet would therefore be a problem for the kosher consumer. Rennin is the name given to the enzyme fraction of rennet. Until recently the only source of rennin was calves. Science has created a system for the enzyme chymosin that does not require calves. Using genetic engineering, the gene for chymosin was cut from a calf cell and they were able to produce an exact microbe copy of the calf chymosin.
Microbes replicate and grow rapidly, thus a supply of rennin is available.
With starter cultures you can control the flavor, aroma, inhibit undesirable organisms etc. There are single strain cultures and mixed or compound cultures. In the compound cultures, each strain will provide its own specific characteristics. The commercial manufactures of starter cultures will make them in different forms. There are a number of steps necessary for preparing starter culture ready for cheese production.