The production of volatile sulphur compounds in cheddar cheese during accelerated ripening

Abstract

The primary objective of this research programme was to select and examine chemical compounds implicated in Cheddar cheese flavour to determine their suitabilities as indicators of the state of maturity of the cheese. A literature review of current knowledge on cheese ripening indicated that methanethiol was strongly associated with Cheddar flavour. The free amino acids were also considered to be important but the nature of their contribution was uncertain. On the basis of these findings, a method for the determination of the volatile sulphur compounds, hydrogen sulphide, methanethiol and dimethyl sulphide, was developed. This method utilized gas chromatography with flame photometric detection. A fourth volatile sulphur compound, carbonyl sulphide, previousiy unreported in Cheddar cheese headspace, was detected and identified during this work. A large number of normal Cheddar cheeses were mariuf actured for these trials using standard cheesemaking procedures. The cheeses were analysed at 1, 3, 6 and 9 months of age for: (1) volatile sulphur compounds, (2) free amino acids (PTA-soluble N) and (3) free amino acids and TCAsoluble peptides (TCA-soluble tyrosine). At each age, the cheeses were also assessed for flavour development by a taste panel. The levels of dimethyl sulphide did not vary systematically with the age of the cheese. Hydrogen sulphide and methanethiol levels increased until approximately 6 months of age ~nd then decreased. Levels of carbonyl sulphide, PTA-soluble N and TCA-soluble tyrosine and the taste panel assessments all increased regularly over the ripening period. Correlation and regression analyses were carried out between the levels of compounds measured and the flavour assessments. Only PTA-soluble N and TCA-soluble tyrosine correlated well enough to be considered useful as ripening indicators. To determine the effect of accelerated ripening treatments on the levels of volatile sulphur compounds and proteolysis products, cheeses were manu~actured using (1) mutant starter, (2) higher initial ripening temperature and (c) a combination of (1) and (2). The relationships between the levels of compounds measured and the age of the cheeses were similar to those obtained with the normal cheeses except that higher levels were reached in the treated cheeses. Correlation and regression analyses between the levels of compounds measured and flavour assessments indicated that PTA-soluble N levels were the best indicators of flavour development in the cheeses. An investigation into the possible sources of carbonyl sulphide was carried out. It was detected in the headspaces above certain packaging and sealing materials. Evidence that it was produced within the chees~s was given by two sets of findings: (1) experimental cheeses with added cysteine had consistently higher levels of carbonyl sulphide and (2) it was produced chemically from mixtures of cysteine and methionine in the presence of riboflavin and ascorbic acid in laboratory trials