Cheese is concentrated milk five- to tenfold by the removal of water. The basic character of the milk defines the basic character of the cheese. Milk from cows is more neutral than others. Sheep and buffalo milk have relatively high fat and protein contents therefore make richer cheeses. Goat's milk has less casein protein, so it produces a more crumbly curd.
The cow breed also produces distinct flavors. Today most dairy comes from black and white Holstein or Friesian cows. These cows have been bred to maximize milk yields on a standarized feed. Traditional breeds, such as the Brown Swiss and others, produce a lower volume of milk but one that is richer in protein, fat, and other favorable cheese constituents.
The animal's diet also affects flavor. Today most cows are fed on a standarized diet year round. The diet is composed of silage and hay from few fodder corps such as alfalfa or maize. This feed produces neutral milk that can be made into very good cheese. However, herds allowed to graze on pastures give milk with aromatic complexities that make extraordinary cheese. Pasture cheese has traces of local climate and seasonal flavors. It is also deeper yellow color due to greater carotenoid pigments in fresh vegetation. Beware of bright orange cheeses as these have been artificially dyed.
Flavor is also affected by whether the milk used is raw or pasteurized. Pasteurization, to eliminate disease and spoilage bacteria, has been a practical necessity in industrial cheese making, which requires milk to be pooled and stored. Since 1940s, the FDA requires that any cheese made from unpasteurized milk be aged at least 60 days at a temperature above 35 degrees F/2 degrees C. In the 1950s the US also banned imports of raw-milk cheeses aged less than 60 days. This essentially means that soft cheeses made with raw milk are contraband. The French, Swiss, and Italian regulations actually forbid the use of pasteurized milk for the traditional production of cheeses such as Brie, Camembert, Comté, Emmental, Gruyère, and Parmesan. Pasteurization kills useful milk bacteria, and inactivates enzymes in the milk that work on flavor development. However, pasteurization is no guarantee of safety as milk and cheese can be contaminated in later processing. Most outbreaks in recent years have involved pasteurized products.
Rennet
At least 2,500 years ago, shepherds began using pieces of the first stomach of a young calf, lamb, or goat to curdle milk for cheese. Later, people made a brine extract from the stomach. This was, conceivably, the first semipurified enzyme. Modern methods allow for the production of that same enzyme, chymosin, to be produced in a bacterium, a mold, and a yeast. Most cheese today uses this engineered "vegetable rennets." In Europe, rennet from a calf stomach is required for traditional cheese making.
Traditional rennet is made from the fourth stomach or abomasum of a milk-fed calf less than 30 days old, before chymosin is replaced by other protein-digesting enzymes. Chymosin is unlike other enzymes because it attacks only one milk protein at just one point. It targets the negatively charged kappa-casein that repels individual casein particles from each other. Thus, chymosin allows the casein particles to bond and form a continuous solid gel which is better known as the curd.
Acidity alone reduces the zeta potential and causes milk to curdle, so why rennet? Acid disperses casein proteins and their calcium glue before it allows the proteins to come together. Some casein and most of the calcium are lost in the whey. In addition, the acidity required to curdle milk is so high that some of the flavor-producing enzymes work very slowly or not at all. The curd produced is weak and brittle. By contrast, rennet leaves the casein micelle proteins intact and causes them to bond into a firm, elastic curd.
Microbes
A handful of modern cheese is made with purified cultures, but mostly it is made using a portion of the previous batch's starter.
Starter bacteria consists of lactic acid bacteria which initially acidify the milk, persist in the drained curd, and generate much of the flavor during the ripening process of semihard and hard cheeses such as Cheddar, Gouda, and Parmesan. The numbers of starter bacteria drop dramatically during cheesemaking, but their enzymes continue to work for months. There are two broad groups of starters: Lactococci (mesophilic) and Lactobacilli and Streptococci (thermophilic). Most cheeses are acidified by the mesophilic group, while the few that undergo a cooking step, such as mozzarella and the Italian hard cheeses, are acidified by the thermophilic group.
In addition, there are other microbes that give some cheeses their characteristic looks and flavors:
- The Propionibacteria- Propionibacter shermanii is the hole maker, important in Swiss starters. It produces carbon dioxide gas that makes up the holes in cheese.
- The Smear Bacteria-Brevibacterium linens gives some strong cheeses their characteristic stink, such as Münster, Epoisses, Limburger.
- Molds, especially Penicillium, require oxygen to grow, can tolerate drier conditions than bacteria, and produce powerful protein- and fat-digesting enzymes that improve the texture and flavor of certain cheeses.
- Blue molds include Penicillium roqueforti, which gives Roquefort cheese its veins of blue; and P. glaucum, which colors the interior of Stilton and Gorgonzola.
- White molds include P. camemberti, which contributes to the creamy texture of Camembert, Brie, and Neufchâtel.
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