Exudation of whey from cheese during storage : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University

Abstract

Cheeses of low pH, such as Feta, Blue, Cream and Cheshire, often exude whey after manufacture. This exudation lowers the yield and reduces product acceptability. Virtually no scientific study has been undertaken on this subject. Investigations were therefore undertaken to determine the factors affecting exudation and to elucidate the underlying mechanism. Cream cheese made by the hot-pack method and recombined Feta cheese made by the traditional method, representing unripened and ripened varieties of cheese respectively, were studied. In Cream cheese the amount of exudate increased with decreased protein to fat (P /F) ratio, decreased homogenisation pressure, decreased pasteurisation temperature, decreased pH at cooking, decreased cooking temperature, increased storage temperature and increased storage time. Within the selected limits of variation of P /F ratio, fat did not affect exudation. However, an increase of moisture in non-fat substance resulted in an increased amount of exudate . The effect of homogenisation pressure appears to be due to the increase in the fat globule surface area and the increase in the coating of fat globule with casein. The partial heat-denaturation of the whey proteins in the cheesemilk was effective in reducing the rate of exudation, possibly due to the complex formation between 13-lactoglobulin and K-casein that prevented fusion of casein micelles. Residual lactose and pH did not change, and proteolysis was not detected up to 16 weeks in Cream cheese stored at 5 · C. It is concluded that exudation from Cream cheese does not occur due to any gross chemical changes during storage. Manufacture of Feta cheese involved the use of recombined cow's milk and vacuum packaging of cheese after brining. A storage study of Feta cheese up to 6 months showed steady proteolysis, slow metabolism of residual lactose and a gradual decrease of pH. The water activity of the cheese depended on the salt-in-moisture concentration. In Feta cheese the amount of exudate increased with increased P /F ratio, i ncreased pH at draining, increased residual rennet, packaging cheese without vacuum, increased storage temperature and increased storage time. Variation of priming time, with a constant curd pH at draining, did not affect exudation. Unlike Cream cheese, an increase in protein and a decrease in fat content in Feta correlated with increase in the amount of exudate. The effects of change in pH and calcium (within a range expected in normal Feta) on exudation were minor. Homogenisation was effective in reducing the rate of exudation in Feta cheese. However, a variation in the homogenisation pressure had no effect. The type of material adsorbed to the fat globule surface influences syneresis during manufacture as well as subsequent exudation during storage. The effect of a reduction in the size of fat globules on exudation appears to be less important. In Feta cheese the incorporation of heat-denatured whey proteins did not affect exudation. However, there was a substantial increase in yield. Proteolysis is the dominant factor affecting exudation. Its influence is apparently due to the disintegration of the casein network and the release of water physically held in the capillaries. Exudation is also substantially influenced by the gradient in NaCl concentration in Feta cheese following brining. Denaturation of whey proteins in Cream cheese; and homogenisation, controlled proteolysis, decreased salt gradient, use of vacuum packaging in Feta cheese appear to be the main factors available for reducing the extent of exudation. Based on the findings of the investigation a hypothesis is proposed to explain the exudation from cheeses.