Ripening of Ossau-Iraty cheese: determination of free amino acids by RP-HPLC and of total free amino acids by the TNBS method

The free amino acids and sulphosalicylic acid-soluble N fraction in the French ewes’-milk cheese, Ossau-Iraty, were determined to evaluate the degree of proteolysis during ripening of the cheese. RP-HPLC was used to assess the free amino acids, the trinitrobenzenesulphonic acid method to assess the sulphosalicylic acid-soluble N fraction. The total free amino acids content as determined by RP-HPLC ranged from 200 mg/100 g dry matter on the first day of ripening to 2200 mg/100 g dry matter after 120 d of ripening. The major amino acids were glutamic acid, asparagine, glutamine, valine, leucine, phenylalanine and lysine, which accounted for between 50% (on day 1) and 70% (on day 120) of the total free amino acids. Correlations between all the main free amino acids and ripening time were good. Regression of the sulphosalicylic acid-soluble N fraction as determined by the trinitrobenzenesulphonic acid method on the total free amino acids as determined by RP-HPLC yielded a correlation coefficient value of 0.985