This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Temperature. When collagen is heated with water, gelatin is formed. Bogue states that this occurs at 80° to 90°C. or at higher temperatures. The reaction occurs more rapidly at higher temperatures, but at these temperatures more of the gelatin is hydrolyzed to proteoses, peptones, or amino acids. When meat is cooked in hot water the collagen of the connective tissue is converted to gelatin. The gelatin is soluble in the hot water, and as it is dissolved the muscle fibers are separated and fall apart. At temperatures below the boiling point of water a longer time is required to hydrolyze the collagen to gelatin.
Acids. The addition of a dilute acid hastens the hydrolysis of collagen, but this also increases the hydrolysis of the gelatin to peptones and amino acids. The latter products do not form gels, so that, in the manufacture of gelatin, it is not desirable for the gelatin to contain very much of these products. Softening of connective tissue in cooking meat is brought about in a shorter time by the addition of a small amount of acid. Tomatoes added to a stew will accomplish this purpose. Sometimes vinegar is used.