Differentiation of Food Spoilage Bacteria on the Basis of Their Ability to Utilize Different Proteins

Methods designed to evaluate qualitatively the proteolytic potential of spoilage bacteria must be sufficiently sensitive to differentiate among the broad spectrum of bacterial proteases. By using four unfractionated proteins (egg albumen, gelatin, casein, and water soluble proteins of beef muscle) and three casein fractions (α s ‐, β‐, and κ‐), the proteolytic activity of 122 bacterial isolates from chicken were studied. Three distinctive groups of proteolytic bacteria were found. One group completely hydrolyzed all of the test proteins (elaborated proteases with low substrate specificity). Another group, which elaborated proteases with high substrate specificity, selectively hydrolyzed one or more, but not all of the test proteins and fractions of casein and/or the water soluble sarcoplasmic proteins. The third group demonstrated an inability to hydrolyze test proteins to obtain nutrients for growth. However, the latter group did grow when cultured in the presence of an added protease. Further, it was observed that a significant number of isolates unable to hydrolyze one protein, hydrolyzed other test proteins, thus, indicating the need in an assay method of being able to detect these differences. Comparisons between undenatured and denatured proteins, as test substrates, revealed no significant difference between protease preference by spoilage isolates for either type of protein. However, the use of undenatured protein substrates seemed to be a more logical approach to simulate actual conditions in the food industry. The use of a single protein substrate to evaluate proteolytic activity, as current methods employ, increases the possibility of making an incorrect appraisal of the true proteolytic capacity of a microorganism.