Enumeration of total heterotrophic and psychrotrophic bacteria using different types of agar to evaluate the microbial quality of blue mussels ( Mytilus edulis) and sea scallops ( Placopecten magellanicus)

Abstract

Microbial quality of Fortune Harbor, NL, cultured blue mussels stored at three temperatures (−12, 2 and 9 °C) for 10 days was evaluated using aerobic plate count (APC) and psychrotrophic plate count (PPC) on plate count agar (PCA) and marine agar (MA). The relationship between bacterial counts in Fortune Harbor mussels on PCA and MA was established using linear regression analysis. The accuracy of selected linear models to predict bacterial count on MA using bacterial counts on PCA of wild and cultured mussels and scallops, stored at 2 °C, was examined. The shelf life of stored mussels and scallops was estimated based on bacterial counts, agar type and storage temperature. Results showed that bacterial counts (APC and PPC) in Fortune Harbor mussels on MA were significantly ( p < 0.05) higher than their corresponding counts on PCA agar at all storage temperatures. A strong correlation ( r > 0.7, p < 0.01) was observed between bacterial counts in mussels stored at 2 and 9 °C on PCA and MA. The accuracy of the linear models to predict bacterial counts of bivalves on MA using the counts on PCA ranged from 60% to 93%. Both temperature and agar type influenced microbial shelf life estimation while the type of bacteria (APC or PPC) had a lesser effect. Results of this study strongly suggest the use of MA to evaluate the general microbial quality of bivalves instead of PCA or PCA + 1% NaCl.