Milk

The composition of milk is summarised in Table 10.2. For the most part, the milk employed in the production of cheese is from the cow, but essentially any milk can be converted into cheese. The key criteria are the content of protein and of fat.

The proteins, especially the caseins, form the main structural 'architecture' for the cheese. The fat, which comprises spherical globules in the milk, becomes trapped within the protein matrix in the cheese. Carbohydrate, of which lactose is the most important, is for the most part expelled with the whey, the remainder being fermented to lactic acid. The fourth major component is calcium phosphate, much of it in a micellar form, which makes a key contribution to the physical properties of cheese.

Lactic acid bacteria

Milk

| Pasteurisation

(± additions such as colour) j Pre-curing

Rennet Coagulation

Curds and whey

Ripening starter culture—».J Cooking, washing, milling, salting

Separation Whey

More microbes, additions

Compression and shaping of curd Ripening, ageing

Cheese

Fig. 10.1 Making cheese.

Table 10.2 Composition of cow's milk.

Component Percentage

Water 87.3

Lactose 4.8

Fat 3.7

Caseins 2.8

Whey protein 0.6

Ash 0.7

The main constituents of the protein fraction are caseins and the whey proteins, the latter being water soluble and therefore expelled with the whey. The caseins are phosphoproteins that precipitate at 20° C from raw milk at pH 4.6. There are three major casein fractions: a, j and k and they tend to associate via electrostatic and hydrophobic interactions to afford micelles, rendering a colloidal suspension in the milk, one which is impacted by calcium phosphate (Fig. 10.2).

Ninety-six per cent of the lipid is in the form of globules in colloidal suspension. They are coated by emulsion-stabilising membranes in a lipid bilayer with protein at interfaces. This ensures integrity of the globules which, if degraded, release free fats that give an oily mouthful and an undesirable appearance.

Short-chain fatty acids (principally C4: 0 and C6: 0) contribute to the flavour in certain cheeses. The complexity of flavour in goat and sheep cheese is dependent on these and other fatty acids.

Cow's milk comprises 4.8% lactose. This is either fermented as is or after hydrolysis to glucose and galactose. If it is not efficiently eliminated with the

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Fig. 10.2 Micelles in cheese curd (modified from http://www.foodsci.uoguelph.ca/deicon/ casein.html).

whey, then it will lead to the risk of colour pick-up in the Maillard reaction and to the growth of spoilage organisms.

Milk also contains some enzymes. Advantage is taken of its heat-sensitive alkaline phosphatase to test for the efficiency of pasteurisation: if the enzyme is destroyed, then this is indicative of sufficient heat having been applied.

The milk may be pretreated in various ways depending on the cheese that is being made. Such treatments may include

(1) heating (pasteurisation) to destroy pathogens and lower the levels of spoilage bacteria and enzymes. Such treatment may typically be a regime of 72°C for 15 s;

(2) reduction of fat by centrifugation or by adding non-fat solids such as concentrated skimmed milk or non-fat dry milk. However, this may be problematic if lactose levels are too high;

(3) concentration, which may be by applying vacuum (for high throughput cheeses) or ultrafiltration (for soft cheeses);

(4) clarification, either by high-speed centrifugation or microfiltration. This procedure optimises the number of foci that lead to 'eyes' in the finished cheese. Very high-speed centrifugation will additionally lower the level of undesirable micro-organisms;

(5) homogenisation. This involves the application of high-pressure shear to disrupt fat globules, rendering smaller globules that are coated with protein. This is important for rendering consistent texture in blue-veined cheeses and for cream cheese. It also has significance for the levels of free fatty acids and therefore of the flavour-active oxidation products that are made from them;

(6) addition of calcium chloride, which promotes clotting;

(7) addition of enzymes to enhance flavour or to accelerate maturation. For example, lipases may be employed in the manufacture of blue-veined cheeses;

(8) addition of micro-organisms. These microbes may include Propionibacter for Emmental and Swiss cheese, Penicillium roqueforti for blue cheeses and P. camamberti for camembert and brie.

Table 10.3 Lactic acid bacteria used in cheese production.

Cheese type

Organisms

Italian grana and pasta types, Swiss Thermophilics

Lactobacillus delbrueckii ssp. bulgaricus Lactobacillus helveticus Streptococcus thermophilus

Blue, Cheddar, cottage, cream, Gouda, Limburger

Blue, cottage, cream, Gouda

Homofermentative

Lactoccus lactis ssp. cremoris Lactococcus lactis ssp. lactis Lactococcus lactis ssp. lactis biovar diacetylactisa

Heterofermentative

Leuconostoc mesenteroides ssp. cremoris aThis organism has a plasmid coding for enzymes that allow the metabolism of citrate. Based on Olsen (1995).

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