The effect of pH shift on early cheese maturation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University
During the first few weeks of maturation, the texture of Cheddar-like cheeses changes substantially from an elastic material, often containing strongly oriented protein fibres from which some moisture can be expelled readily, to a uniform smooth-bodied cheese. In the past, proteolysis, particularly the cleavage of αs1-casein, has been invoked as an important factor in the early changes in cheese texture. However, some of the textural changes that occur early in cheese maturation may be related to the redistribution of water within the cheese matrix. To examine this, a model cheese curd system was devised and explored. Initially, cheese curd was prepared using starter and chymosin and the curd pH was controlled by varying the draining and salting pH values. The changes in water distribution, as measured by the quantity of centrifugal serum, seemed to be influenced by the cheese pH, but this could not be confirmed because of the continuing changes in cheese pH. Substitution of starter by dilute lactic acid to alter the pH value at setting provided a means of controlling the cheese pH during the 2-week periods of study. In some trials, glucono-δ-lactone (GDL) was used to reduce the pH of the cheese to the desired level after curd manufacture. This simulated the time-dependent pH change in a normal cheese such that the effects on the water-holding capacity, microstructure and rheological properties of the cheese could be studied. This model system of cheesemaking proved to be very effective in adjusting the pH of the cheese to the desired level. In the protocol finally adopted, milk was acidified with lactic acid and coagulated with Rennilase 46L®. After cheddaring, salting and light pressing, the curd was finely diced and mixed with GDL to give curd samples with comparable moisture contents and similar minimal casein proteolysis rates but different pH values. The quantity of centrifugal serum decreased with a decrease in the set pH of cheese curds between pH 6.30 and pH 5.30. The maximum quantity was obtained from cheese curd set at pH 6.30 whereas no serum could be centrifuged from cheese curds set at pH 5.70 or lower. The quantity of centrifugal serum was essentially constant with time for cheese curds of all set pH values. Lowering of the pH of cheese curds from the set pH by the addition of GDL also affected the quantity of centrifugal serum which decreased with a decrease in the adjusted pH value of the samples. The quantity of serum also decreased with time for all samples with adjusted pH values, the decrease being more rapid for samples with lower pH values. The set pH was also found to influence the rheological characteristics of the cheese curds. Both the maximum force, from the large strain method developed for this project (Instron), and G'(stiffness), from the small strain method (Bohlin), showed a maximum for cheese curd with a pH of 5.90. The values for maximum force of cheese curds adjusted to lower pH values using GDL were in the same range as those of cheese curds of a similar set pH value. This suggested that the samples tended to attain a new rheological equilibrium with time that was consistent with the conditions of the lower pH value. The microstructure of the cheese curds seemed to be determined by the pH at setting as changes in structure were apparent when cheese curds were made at different set pH values but not when the pH was altered from the set pH value using GDL. The changes resulting from the alteration in pH may have been too subtle for detection by the confocal microscopy technique used. Micrographs of cheese curds from which a serum phase could be centrifuged appeared to be less compact and had open spaces in the structure where water may have been present. Such areas were absent from the micrographs of cheese curd samples from which no centrifugal serum was obtained. Overall, the present study showed that changes with time in the quantity of centrifugal serum, microstructure and some of the rheological properties in cheese curds took place even when proteolysis was at a minimum and when the pH was essentially constant. This suggests that physical changes in the cheese curd, such as redistribution of water within the cheese curd, are also important during the initial stages of ripening and probably contribute to the differences in rheological properties observed in young cheese.