Microbiological quality and safety of quahog clams, Mercenaria mercenaria, during refrigeration and at elevated storage temperatures

Abstract

The effects of storage temperatures and times on the microbiological quality and safety of hard-shelled quahog clams (Mercenaria mercenaria) were examined. Samples were stored at four different incubation temperatures (3.3, 7.2, 10.0, and 12.8°C) for a period of 3 weeks, following their harvest from summer growing waters (≥27°C) and winter waters (≤4°C). Clams were analyzed for two naturally occurring pathogens, Vibrio parahaemolyticus and Vibrio vulnificus. During the summer, V. parahaemolyticus was isolated from 56% of the stored samples, with the highest concentration, 6,100/g, occurring on day 12 at 12.8°C. Also, during the summer, V. vulnificus was isolated from 11% of the stored samples, with the highest concentration of 1,500/g occurring on day 15 at 7.2°C. No Vibrio spp. were detected during the winter. During summer storage, aerobic mesophilic counts on plate count agar (PCA) containing 2% NaCl ranged from 104 to 108 CFU/g, and during storage of the winter samples, aerobic mesophilic PCA (with added NaCl) counts ranged from <100 to 104 CFU/g. Comparatively, summer storage mesophilic counts on PCA containing no added NaCl ranged from <100 to 105 CFU/g, and for the winter samples the range was <100 to 102 CFU/g. Coliform and fecal coliform counts ranged from <0.3 to 61.1/g and <0.3 to 24.4/g, respectively. There was no statistical correlation between the length of storage or the temperature of incubation and the presence of V. parahaemolyticus, V. vulnificus, coliforms, or fecal coliforms. However, storage time and incubation temperature affected the PCA counts (P ≤ 0.05) in quahog clams.