The dairy foods sector of the food manufacturing industry is a traditional user of enzymes. The best-known dairy enzyme preparation is, of course, rennet, a collective name for commercial preparations containing acid proteases extracted from animal tissues. These products clot milk by removing a highly charged peptide fragment from κ-casein on the surface of micellar casein, the majority form of milk protein. Destabilized casein micelles aggregate and form the structure of the milk clot that is then acidified by lactic cultures to make cheese curd.

In addition to the use of milk-clotting enzymes to make cheese, the dairy industry also makes use of enzymes such as lipases, non-coagulant proteases, aminopeptidases, lactases, lysozyme, lactoperoxidase and transglutaminase. Some of these applications are traditional (lipase for flavour enhancement) whilst others are relatively new (lactose hydrolysis, accelerated cheese ripening, control of microbiological spoilage, modification of protein functionality).

Milk-Clotting Enzyme

Of the animal rennets, calf rennet is widely regarded as the ideal milk-clotting enzyme for cheese making. This is partly through traditional familiarity with the product through long use, but the preference also has a sound scientific basis in that calf rennet is typically 80–90% chymosin (EC 3.4.23.4). This means that most of the casein breakdown in the cheese vat is directed very specifically at κ-casein to clot the milk, and not at the other caseins. Non-specific proteolysis of α- and β-casein during curd formation can result in the loss of casein nitrogen in the whey, reducing the yield of the cheese process.

The most widely used alternative to calf rennet in the cheese industry worldwide is fermentation-produced chymosin (FPC), which currently accounts for half of the world production of enzyme-coagulated cheese. It is produced by large-scale fermentation of GM Kluyveromyces lactis or Aspergillus niger. In both cases, the microorganism has been modified using gene technology by the incorporation of the calf prochymosin gene into the host organism with a suitable promoter to ensure its efficient secretion into the growth medium. The enzyme is relatively easy to harvest and purify from the culture, unlike the earlier production system using Escherichia coli to produce chymosin in inclusion bodies.

Cheese-Ripening Enzyme

The enzymes and enzyme ‘packages’ used to modify, enhance or accelerate the maturation of cheese are generally composed of more than one class of enzyme, and for the sake of clarity they are discussed here as a technological group, rather than as individual classes. The classes used in commercial ripening technology include many hydrolases represented by proteinases, peptidases and lipases, and if current research is successful, this list may soon extend to metabolic enzymes such as acetyl-CoA synthases and amino acid-catabolizing enzymes to generate volatile esters and sulphur compounds. Considering the very extensive worldwide research effort and literature on the enzymology of cheese ripening, it is remarkable that only a few enzyme companies have successfully developed commercial enzyme packages for cheese technology, other than the ageing enzyme-modified cheese (EMC) production methods used to make flavour ingredients for processed cheese and cheese-like foods.

Lipase

Although lipases are used in cheese flavor technology, they are also used to produce modified milk fat products for other food applications. Lipolyzed milk fat (LMF) has a creamy, buttery and cheesy aroma derived from short to medium chain fatty acids and fatty acid chemical derivatives released from milk fat by lipases. The raw material substrate for manufacturing LMF is either condensed milk or butter oil emulsified to maximize the fat surface area to activate the lipase. Lipases are added and left in contact with the substrate at the optimum temperature for the enzyme(s) used, until the required flavor/aroma is achieved, or until a predetermined acid degree value (ADV) is reached, corresponding to a measurable release of fatty acids by the lipase.

Chemical interesterification, acidolysis, alcoholysis and transesterification have been used for many years to modify the physical/functional properties of milk fat, but more recently lipase technology has replaced this chemical technology to give more precise and ‘cleaner’ processing. In particular, milk fat substitutes have been prepared as a partial replacement for the milk fat in baby foods. However, fat fractionation by physical methods is the commercially preferred option for milk fat modification in dairy product applications.

Lactoperoxidase

Lactoperoxidase (LP) (EC 1.11.1.7) occurs naturally in raw milk, colostrum and saliva; it is thought to be part of the protective system for suckling animals against enteric infections. Lactoperoxidase is bactericidal to gram-negative bacteria and bacteriostatic to gram positives. It is a peroxidase that uses hydrogen peroxide to oxidize the thiocyanate ion to hypothiocyanate the active bactericidal molecule.

Lysozyme

Lysozyme is sold by the major dairy enzyme suppliers as an alternative control agent for ‘late blowing’, the textural defect of slits and irregular holes caused by the butyric fermentation in Gouda, Danbo, Grana Padano, Emmental and other important hard and semi-hard cheese varieties. Traditionally the defect, caused by Clostridium tyrobutyricum in raw milk, has been controlled by the addition of potassium nitrate to the cheese milk. However, this practice will be phased out because it is associated with the production of carcinogens, and lysozyme has become the preferred control agent.

Reference

1. Whitehurst R J , Law B A , Whitehurst R J , et al. Enzymes in food technology. [J]. Enzymes in Food Technology, 2002, 32(4):1-17.

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