Changes in collagen structure during maturation were studied with collagenous residues obtained from the loose connective-tissue network within bovine biceps femoris muscle from animals of different ages. A new technique for studying thermal shrinkage and collagen structure of powdered collagenous residues was described. Samples from four age groups were studied: Group I, three calves, 40–49 days old; Group II, three steers, 403–495 days old; Group III, three cows, 4 years, 8 months to 5 years, 5 months old; Group IV, two cows, 10 years, 2 months and 10 years, 5 months old.
The release of soluble protein, ninhydrin-positive material, and hydroxy-proline from collagenous residues into a phosphate-buffered medium (pH 7.0) upon incubation at gradually increasing increments of temperature from 25 to 70°C was measured. Differences between various age groups were marked, and at 60°C and above, the groups ranked I, II, III, and IV, from highest to lowest, in amounts of soluble material released. At the final temperature of 70°C, Group I samples had released 42% of their hydroxyproline in a soluble form, compared to only 2% from Group IV. The thermal shrinkage temperature, which was taken as that temperature at which a sudden release of soluble hydroxyproline occurred, increased with advancing age, from near 55°C for Group I to 70°C or above for Group IV, while the average molecular weight of the soluble protein released was greater for the younger animals. The results were discussed in relation to experiments involving the rate of collagenase digestion of the same samples. The findings indicate stronger or more extensive cross-linkages in the collagen from older animals.