Traditional cheese making depended on the natural micro-organisms present in the dairy and milk. Microbes sour that milk, producing lactic acid and lowering the pH, so that when rennet (from the stomach of young mammals) was added, the milk curdled and separated into solid curds and liquid why (which was used as a substitute for the full milk food for the calves). The curd was packed into moulds and matured in cool rooms or caves where sometimes local fungi grew on and through the cheese to make a ‘blue cheese’
Some delicious cheeses are still made in small amounts by farmers using the old skills, although most of the cheese manufactured today comes from ‘factory diaries’ using pasteurized milk really sterile conditions and cultures of lactic acid bacteria and genetically engineered microbial rennet (a common name for the enzyme rennin or renninase) The manufacture of cheddar-type cheese uses the bacteria Lactobacillus sp. and Streptococcus sp. They cause the change of lactose sugar to lactic acid. The pH changes curdles the milk and the rennet causes the formation of the solid curds, a rich mixture of protein and fat. The process of ‘cheddaring’ cuts the curd into blocks so allowing the whey to drain out. Salt is added and the curd is compressed. Flavor develops over a period of time; the more mature the cheese is the stronger the flavor. Bacteria and fungi continue the ripening process by metabolizing the protein and fat by releasing ‘flavor by-products’. Some hard cheese (like cheddar) mature for 6-9 months, although they can be eaten as mild cheese in a few weeks.
The soft cheese, such as Camembert ripen quickly and are more moist because yeasts grow on the outer surface on the outer surface and for a rind. The blue cheese such as Roquefort have an added fungus. Penicillium roqueforti and are produced with needles to allow entry of oxygen to the centre of the cheese as the fungus us aerobic. The formation of the holes in the Swiss cheese is due to another bacterium. Propionibacterium shermani, that makes propionic acid (which adds the flavor) and bubbles of carbon di oxide.