Milk is a dispersed system made of water and what is known as the total dry matter, which includes lactose, fats, nitrogenous substances including proteins, minerals, vitamins and micro-organisms. A delicate raw material, if ever there was one, milk must be treated with great care, under controlled sanitary conditions in order to preserve its qualities as well as possible. The cold chain is a major factor in this preservation

Depending on the length and holding temperatures, it will undergo what is called a biological maturing, which corresponds to an acidification of the medium. The lactic bacteria then transform the lactose into lactic acid and this acidification will have an impact on the milk's mineral configuration, and protein aggregates. The milk's behaviour during the cheese-making process will depend strongly on this, which will in turn affect a variety of the future cheese's properties. As they develop, the micro-organisms will also release enzymes which will trigger a "biochemical breakdown" of the milk's components. Cheese transformation companies - which have standards that must be met - often make a selection of specific flora by applying a heat treatment to the milk, and by reintroducing selected cultures, whether live or not. Milk preparation includes the adjustment of the future cheese's fat content as well as the standardisation of major parameters such as the pH and the quantity and quality of the proteins.

The second means of coagulation also leads to the destabilisation of the milk but this time thanks to the action of an enzyme compound called rennet which will break down part of the casein (one of milk's major proteins) and thus cause the aggregation of protein clusters (micelles). The rennet may be of animal (gastric enzyme) or microbial origin. Both of these processes are used in combination in cheese making. The fact of favouring one or the other will lead to different cheeses. For example, to obtain spreadable fresh cheeses we will favour the acidification method but, on the contrary, for boiled cheeses we will favour the enzymatic method. The curdling of milk then corresponds to a protein network starting to become established, in which the minerals are the cement, and in which the physical-chemistry bonds will gradually solidify the structure. The globules of fatty matter and the micro-organisms will, in particular, be held back in the network's meshes. Milk curdling in a cheese-making plant is controlled by means of the following: quality and quantity of the proteins and minerals, temperature, coagulant dosage, the milk's pH at the time of renneting.

The speed at which the serum is expelled is one of the major parameters in a cheese recipe. Because, although the transformation of the lactose into lactic acid by the lactic bacteria is carried out inside the curd grain, it will have a demineralising effect whose consequences on the finished product will be real. On the contrary, when the serum leaves the grain, it carries part of the lactose away with it thus preserving the cheese from excessive demineralisation. Depending on the type of cheese being made, the combined kinetics of the acidification and of the expulsion of the serum are decisive. The serum will be completed in the vat by mechanical actions: stirring. The purpose is to prevent too fast an agglomeration of the curd grains as that would hinder the outflow of serum. It is a question of putting or maintaining the curds in suspension. The draining into vats can be improved by increasing the temperature (eg: hard pressed cheeses). The temperature, favourable for the construction of chemical bonds, increases the gel's powers of contraction, which will cause more serum to be expelled. Other processes are applied to promote the extraction of the serum, such as centrifugation of the curds (spreadable fresh cheeses).

Indeed, when the lactic acid accumulates inside the grains, it makes the curds more permeable. And this promotes the extraction of the water. Draining, which depends on the time/temperature pair, continues as required through to complete acidification of the product. The cheese can then be salted, by brining or surface salting. This will further complete the draining as the salt helps to expel the water.

The temperature, the ambient humidity and the renewal of the air in the plant are all parameters that help to control the ripening conditions We also use the term 'cellar care' when we turn the cheeses over or rub them. The goal being to make the surface of the products uniform so that the chosen microbes can develop correctly. The overturning operation also helps to distribute the humidity. The products' packaging is specific to each type of cheese. But in every case it interacts with the cheese to complete or delay its ripening.
Anyway, the cheese, a living substance, will always evolve through until its consumption limit date. The seat of complex biochemical reactions, its evolution will be controlled to obtain the best trade off between taste and nourishment.