Protein in cheese and cheese products: Structure-function relationships

Abstract

Cheese manufacture involves the controlled destabilization of the casein micelle in milk by enzymatic hydrolysis of the surface ê-casein layer, acidifi cation to the isoelectric pH of the casein, or a combination of pH reduction to ~5.6 and high temperature (~90 °C) in rennet-curd, acid-curd and acid heat-curd cheeses, respectively. Under suitable conditions, the destabilized micelles undergo limited aggregation to form a gel, which is dehydrated to a curd with the desired moisture content by a series of unit operations including cutting the gel into pieces (curd particles), in situ acidifi cation, heating and stirring the curd particle/whey mixture, removal of the expressed whey, pressing and/or salting of the curd. Microstructurally, rennet-curd cheese is a matrix comprised of a hydrated calcium phosphate para -casein network that occludes the fat phase which occurs as discrete and coalesced globules or pools. The microstructure is infl uenced by the concentration of para -casein and the degree to which the component para - casein micelles are aggregated and fused, as affected by manufacturing operations. Macrostructurally, rennet-curd cheese is an assembly of curd particles or pieces (e.g., chips) that fuse to varying degrees according to their microstructure, which affects their potential to deform, and curd handling treatments (e.g., pressing) which effect the level of stress applied to the amalgam of curd particles/pieces. The matrix of acid- or acid heat-curd cheese is similar to that of rennet-curd cheese, but the network is formed from casein (with little, or no, bound calcium) or casein complexed with whey protein, denatured by high heat treatment of the milk prior to acidifi cation and gelation. Most acid-curd and acid heat-curd cheeses have a very uniform texture and lack a macrostructure as the curd particles, low in calcium and high in moisture, coalesce easily to form a structuralcontinuum. Heating of rennet-curd cheese to 90-100 °C in culinary applications leads to contraction and shrinkage of the para -casein network and liquefaction and coalescence of fat. These microstructural changes are the basis of the melt properties, including softening, fl ow and stretchability. Owing to their low pH, acid-curd cheeses generally tend to be unstable during heating, as refl ected by protein precipitation and the release of excess free moisture. The micro- and macro-structure of cheese has a major infl uence on various aspects of quality including composition, rheology, texture, cooking properties, opacity/translucence, and behaviour during curd processing operations such as portioning, shredding, slicing, and processed cheese manufacture