Proteolysis-induced changes in meat collagen during conditioning
Changes in intramuscular connective tissue brought about by conditioning were investigated in bovine muscles of different quality. Perimysial and endomysial collagens were solubilized to a small extent during conditioning and residual insoluble collagens in both connective tissue domains were damaged by proteolytic processing. Yields of soluble perimysial material from unconditioned muscles were significantly lower (p = 0.096) than from conditioned muscles. Solubilized perimysial collagen from unconditioned muscles was significantly lower (p = 0.015) than from conditioned muscles with 1±0.8 % of original collagen solubilized for unconditioned muscles and 3.4 ± 3.3 % for conditioned muscles. 87.5 % of the muscles examined showed an increase in percentage solubilized collagen due to conditioning. The main peptide components observed on analysis of insoluble perimysial fractions after CNBr digestion were derived from types I and III collagen. No changes were observed in the major peptide bands due to conditioning. Yields of soluble endomysial fractions representedo, n average,9 4.5 % of total extracted endomysial material for unconditioned muscles compared with 97.5 % for conditioned muscles. Soluble endomysial fractions contained, on average, 0.13 % collagen from unconditioned muscles and 0.22 % collagen from conditioned muscles. The main peptide components observed on analysis of insoluble endomysial fractions after CNBr-digestion were derived from types I and III collagen. Changes observed on the peptide maps, evident as the appearanceo f a number of new bandsf rom conditioned samples,a ppearedt o be muscle specific. % Type III collagen decreased on conditioning, indicating that endomysial type III collagen was preferentially destroyed during - conditioning. In model systems, insoluble perimysium treated with pepsin over 24 h resulted in little damage to the insoluble collagenous residue remaining. Insoluble perimysium treated with cathepsin resulted in changes to the major peptide bands on one-dimensional SDSpolyacrylamide gel electrophoresisw hich were evident after 24 h treatment. Two-dimensional peptide maps obtained from conditioned insoluble perimysium and from insoluble perimysium treated with cathepsin for 24 h were altered relative to the unconditioned insoluble perimysium, indicating proteolytic damage to high molecular weight fractions. The in vitro case was extreme, but was comparable with conditioned insoluble perimysium. In addition, new peptide material in conditioned perimysium and endomysium in the molecular weight range 40 000 to 50 000 was observed, while perimysial samples revealed loss of peptide material, due to conditioning. Percentage solubilized collagen was higher (p < 0.05) from three muscles of varying quality when pre-injected with 0.1 M lactic acid and conditioned from 1 to 14 days than from untreated muscles. Analysis of the high molecular weight collagen peptides from lactic acid treated muscles by two-dimensional SDS-polyacrylamide gel electrophoresis revealed increased incidence of degradation in this region compared with untreated controls. Sensory profiling using quality descriptive analysis (QDA) was carried out on three muscles of varying quality, pre-injected with 0.1 M lactic acid and results compared with untreated muscles. The results obtained failed to correlate the observed biochemical changes due to lactic acid treatment with perceived textural changes in these muscles. However, variability of the taste panel scores contributed significantly to the results obtained.