How Microorganisms Help Make Cheese
Cheese-making usually comprises three stages: coagulation or curdling, draining, and ripening. Microorganisms are an indispensable part of cheese-making: lactic bacteria are often used in curdling, and ripening usually requires the use of bacteria, molds, and sometimes even yeast.
Cheese-Making, Step By Step
1) Curdling (or coagulation)
The first step in making any cheese is curdling, the process of destablizing the network of milk proteins known as caseins. The disruption of the casein network results in the production of a curd that eventually becomes the cheese, and of a watery whey which is discarded. Curdling is achieved by adding a coagulating enzyme to the milk, acidifying the milk, or both. The properties of the cheese are determined in large part by the amount and activity of added enzymes and by the extent to which the milk is acidified.
The details of the draining process are determined by the type of curd involved. Acid curds are drained by simply separating the curds from the whey, sometimes with the help of stirring, heating, or centrifugation. Rennet curds, such as Oka cheese, automatically expulse the whey from the curd as the enzyme causes the milk’s casein network to contract. This type of curd readily lends itself to mechanical processing such as cutting, stirring and pressing. The draining operation for mixed curds depends on the amount of coagulating agents used and the time at which they were added.
Ripening gives the curd a specific appearance, texture, aroma and flavour. The microorganisms used in ripening vary from cheese to cheese. Fresh-milk cheeses such as cheddar and mozzarella are not ripened.
Surface-ripened cheeses become coated with molds, yeasts, or even bacteria. Oka cheese, for example, is covered by red bacteria known as Brevibacterium linens. These bacteria degrade certain amino acids in cheese and give Oka its characteristic aroma.
The molds Penicillium camemberti and Penicillium roqueforti contribute to the ripening of downy-rinded cheeses such as Camembert and the blue cheeses. These molds cause physical changes in the cheese and modify its flavour and aroma. It should be noted that the microbial population continues to multiply throughout the entire ripening stage. Factors such as temperature, atmospheric composition and the humidity of the ripening chamber, which greatly influence the microbiological and enzymatic activity in progress, are therefore strictly controlled.
The holes found in some cheeses are formed during ripening. The characteristic holes and flavour of Emmental and Gruyère cheeses are the result of the carbon dioxide and propionic acid produced by propionic bacteria.