Dairy products

Milk can be fermented to produce a variety of products, including butter, yoghurt and cheese (Figure 17.3). In each case, acid produced by the fermentation process causes coagulation or curdling of the milk proteins.

In cheese-making, this coagulation is effected by the addition of the protease rennin, or by the action of lactic acid bacteria (especially Streptococcus lactis and S. cremoris). Coagulation allows the separation of the semisolid curd from the liquid whey. The subsequent steps in the cheese-making process depend on the specific type of cheese (Table 17.2). Following separation, the curd of most cheeses is pressed and shaped, removing excess liquid and firming the texture. During the ripening process, salt is often added, and flavour develops due to continuing microbial action on the protein and fat components of the cheese. In some cases, a fresh inoculation of microorganisms is made at this point, such as the addition of Penicillium spores to Camembert and Brie. The length of the ripening period varies from a month to more than a year according to type, with the harder cheeses requiring the longer periods.

Rennin is traditionally derived from the stomachs of calves, but nowadays is more frequently the product of genetically engineered bacteria, allowing its consumption even by strict vegetarians.

Soured cream Buttermilk Cheeses

Soured cream Buttermilk Cheeses

Yoghurt Butter

Acidophilus milk

Yoghurt Butter

Acidophilus milk

Figure 17.3 Fermented dairy products. Fermentation is initiated by the inoculation of a starter culture of lactic acid bacteria to convert lactose to lactic acid. Heterolactic fermenters such as Leuconostoc are added when aromatic flavouring compounds such as diacetyl are required

Yoghurt is another milk derivative. Thickened milk is exposed to the action of two bacteria, Streptococcus thermophilus and Lactobacillus bulgaricus, both of which ferment lactose present in milk into lactic acid. In addition, L. bulgaricus contributes aromatics responsible for imparting flavour to the yoghurt.

Other dairy products, such as soured cream and buttermilk, are also produced by means of the fermentative properties of species of streptococci and lactobacilli.

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