The Liberation of Free Amino Acids in Raw and Pasteurized Milk Cheddar Cheese during Ripening

Abstract

Observations reported in the preceding paper (6) on the distribution of free amino acids in commercial cheddar cheese showed the usefulness of paper par- tition chromatography when applied to analyses of water-solubh nitrogenous com- pounds liberated during cheese ripening. It seemed timely and opportune to study other important issues related to cheese ripening using this procedure. Two topics which need investigation include the rate of production of free amino acids during cheese ripening and the possible differences existing in the character of amino acid and amine make-up of raw- and pasteurized-milk cheese. Recently Reihard and Gary (7), using microbiological methods, followed the pro- duction of a number of amino acids during ripening, but as yet no broad picture has been evolved. Certainly, little is known concerning differences in specific water-soluble nitrogenous compounds of raw- and pasteurized-milk cheese, al- though it is known that the water-soluble nitrogen generally is higher in the former. Dahlberg and Kosikowsky (2) showed that more tyrosine and less tyramine can be expected in pasteurized-milk cheese than in the raw-milk product. This problem was studied and the results of this investigation are the basis for this paper. METHODS Paper partition chromatography as applied to the commercial cheeses in the preceding paper (6) was used. In addition, a rough quantitative procedure sug- gested by Block (1) was adapted, colored areas on the test paper being compared to colored chalk standard areas ranging from 0 to 5 in value. The colored areas then were cut out of the large filter paper and weighed. The weight multiplied by the intensity value gave an index, and these indices were compared to standard curves whose index values were obtained in the same manner but using highly purified known amino acids. In this study, four cheddar cheeses were made in each lot. Three lots of cheese were made over a period of 6 too., thus providing triplicate series of results. The four cheeses in each lot varied as to whether cheese was made from raw or pasturized milk and as to whether the starter was a regular commercial Strep- tococcus lactis starter or a combination commercial lactis-DK (Streptococcus faecalis) starter. The amount of commercial starter used was 1.0 per cent for the pasteurized milk and 0.75 per cent for the raw milk. Combination lactis-DK was used in a 0.75 per cent + 0.75 per cent quantity for the other raw and pasteur- ized milk. Milk from which the twelve cheeses were made averaged about 3.5 per cent fat and was of average commercial grade with an initial bacterial plate count of