Application of bioluminescence to rapid determination of microbial levels in ground beef

The relationship between microbial ATP measurements and aerobic plate counts (APC's at 35, 20 and 7°C) was investigated for 75 ground beef samples. Samples (n = 27) were obtained from several local retail markets in one experiment, and ground beef samples obtained from a single, processing facility were sampled throughout 15 d of storage at 1°C for a total of 48 samples in another experiment. Bioluminescent assay time for a given sample was less than 1 h. The correlation coefficient (r) between log microbial ATP and log APC (20°C) per g was 0.86 and 0.99 for retail and single source samples, respectively. Differences between actual log APC (20°C)/g and corresponding values predicted by linear regression equations were ≤log 0.5 for 25 of 27 retail samples and 48 of 48 single source samples. Variation was noted in values of ATP per bacterial cell and relative bioluminescent quenching (ATP per relative light unit, RLU) for most retail samples and for single source samples having low APC (20°C) levels (≤log 7.0).