MONITORING CHEDDAR CHEESE RIPENING BY CHEMICAL INDEXES OF PROTEOLYSIS .1. DETERMINATION OF FREE GLUTAMIC-ACID, SOLUBLE NITROGEN, AND LIBERATED AMINO-GROUPS

Free L-glutamic acid (Glu), soluble nitrogen (N), and liberated amino groups (amino-N) were evaluated as indicators of proteolysis and maturation in a study of Cheddar cheese ripening. The analytical methods involved standardized extraction and fractionation procedures, enzymatic determination of Glu, the measurement of N by Kjeldahl and amino-N by a rapid spectrophotometric assay with o-phthaldialdehyde. Cheddar cheeses from two different production plants were stored at both 4-degrees-C and 10-degrees-C for nine months. All cheeses were periodically analyzed for Glu in an aqueous extract and for N and amino-N in citrate extracts soluble at pH 4.6 and in 12% trichloroacetic acid. All indices correlated well with each other and with the age of the cheese at a highly significant level. It was demonstrated that these parameters are well suited to determine the extent of proteolysis as an indication of the degree of maturity of ripening cheese, thus providing an objective measurement for age classification and raw material control. The convenient assays for Glu and o-phthaldialdehyde-reactive amino-N were proved suitable for routine applications, enabling a fast and sensitive check on the degree of proteolysis in cheese without sophisticated equipment.