During a study of the effect of various methods of cooking beef round on the collagen content of the meat, the method of Hartley and Hall ( 4 ) was used for the first 31 cooked samples. Since collagen is hydrolyzed to gelatin during cooking, losses in collagen were expected for all the cooking methods. Therefore it was surprising to find that 21 of these cooked samples appeared to contain more collagen nitrogen, expressed as percentage of total nitrogen, than did the matched raw samples when analyzed by this method. Similarly, when the total amounts of collagen in the slice of round before and after cooking were compared, 19 of these 31 samples appeared to have gained collagen. EXPERIMENTAL METHODSTo study the analytical method, qualitative tests (Millon, Hopkins-Cole, xanthoproteic) were made on the filtrate obtained after autoclaving the samples. Since the Hartley-Hall method assumes that gelatin is the only source of nitrogen in this filtrate, and since gelatin contains no tryptophane and only a trace of tyrosine ( 8 ) , negative tests f o r these two amino acids were expected. However, qualitative tests of the filtrates from four raw samples indicated some contamination, while tests of six cooked samples indicated considerable contamination with tyrosine and tryptophane. Parallel tests of a solution of commercial gelatin calculated to contain twice as much nitrogen as the filtrates were negative. These tests appeared to indicate that the apparent gains of collagen found on cooking some of the samples were due to decomposition of proteins other than collagen during autoclaving. This decomposition of non-collagen proteins was evidently greater in the cooked than in the raw samples.Because of these findings, the method of Lowry, Gilligan, and Katersky ( 5 ) seemed a good choice f o r studying the effect of cooking on the collagen content of meat. In this method, protein other than collagen and elastin is removed by solution in 0.1 N .sodium hydroxide before autoclaving. When the method was followed as published, the last washing with sogium hydroxide gave a positive biuret test, and qualitative tests of the filtrate obtained after autoclaving were faintly positive f o r tyrosine and tryptophane. I f washing with sodium hydroxide was continued until a negative biuret test was obtained on the last filtrate, indicating the removal of all protein other than connective tissue, tests on the filtrate obtained after autoclaving were negative for tyrosine and tryptophane. However when washing with sodium hydroxide was done according to the original directions, followed by washing with water t o a negative biuret test, qualitative tests for tyrosine and tryptophane on the filtrate obtained after autoclaving were again positive. Evidently, non-collagen protein decomposed during autoclaving when the washing with sodium hydroxide was incomplete. Because of this fact, the method was modified to include washing with sodium hydroxide until a negative biuret test was obtained on the filtrates. The volume of 0.1 N . sodium hyd...
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