The following points highlight the three main steps involved in the production of cheese. The steps are: 1. Coagulum Formation 2. Separation of Curd from Whey 3. Ripening of Cheese.
Step # 1. Coagulum Formation:
Milk coagulation occurs due to two distinct activities (Fig. 38.1, Table 38.1).
(i) Inoculation with bacterial cultures, e.g., Streptococcus lactis or S. cremoris for incubation at 31°C, or S. thermophilic combined- with Lactobacillus lactis. L. bulguricus or L. helveticus (for incubation at 50°C), results in lactose degradation to produce lactic acid, which lowers the pH to about 4.6.
(ii) Incubation with rennet cleaves K-casein into para-K-casein and caseino macropeptide (Fig. 38.2). This cleavage occurs at a specific peptide bond between phenylalanine at position 105 and methionine at position 106 (-phe 105-met 106-), and leads to coagulation of α- and β-caseins and the K-casein hydrolysis products.
Traditionally, rennet obtained from the fourth stomach of unweaned calves has been used. But at present, rennet from microbial sources is used extensively, and is responsible for about 70% of US and 30% of the worldwide cheese productions. But the rennet obtained from Mucor miehei is relatively more thermostable and hence remains active during ripening, which often produces bitter off-flavours.
Therefore, it is treated with oxidising agents like H2O2, peracids, etc., which converts the methionine residues to their sulphoxides. This reduces the temperature tolerance of the enzyme by 10°C and makes M. miehei rennet more comparable to calf rennet. Attempts to clone calf chymosin gene in E. coli and Saccharomyces cerevisiae have been successful, but active renin is secreted only by the yeast cells.
Step # 2. Separation of Curd:
The coagulum is heated to 37°C and cooled. This eliminates the remaining rennet activity and separates, to some extent the watery fluid called whey. The curd is separated from whey, salted, and mixed with proteases and/or lipases; alternatively, bricks of cheese may be inoculated with specific strains if fungi, e.g., Penicillium roquefortii, P. camembertii, etc. The bricks are pressed to remove excess moisture to enable proper ripening.
Step # 3. Ripening:
Ripening procedures will vary with the type of cheese to be produced. The cheese bricks are inoculated with specific strains of fungi for the development of appropriate flavours through protease and lipase activities. Alternatively, proteases and lipases may be used for this purpose. Proteases from Bacillus amyloliquefaciens are used to enhance flavour in cheddar cheese.
Proteases hydrolyse proteins to produce peptides of variable sizes. Peptides having terminal acidic amino acid residues produce meaty, appetising flavours. But hydrophobic amino acid residues located non-terminally produce bitter flavours: the, flavours are the strongest in medium-sized peptides, absent in longer peptides, and decrease with a decrease in the peptide size.
Therefore, the kind and the degree of flavour in cheese can be controlled by regulating protein hydrolysis. The stronger flavours of Italian cheeses are produced by a modest lipid hydrolysis, which increases the amount of-free butyric acid. Lipolysis is brought about by lipase from M. miehei or Aspergillus niger; the. lipase is added to the milk at 30 U/l before addition of rennet.
Thus cheese production presents examples of the following:
(i) Use of microorganisms for food processing to enhance flavour, texture, etc.,
(ii) Use of enzymes in food processing,
(iii) Enzyme (rennet) modification (by chemical reaction) to suit specific needs, and
(iv) Use of recombinant DNA technology to produce enzymes for food processing (cloning of chymosin gene).