Effect of water activity on the inactivation kinetics of Escherichia coli O157:H7 by electron beam in ground beef, chicken breast meat, and trout fillets

Water activity (a(w)) of ground beef, chicken breast meat, and trout fillets was modified to intermediate (a(w) 0.98-0.99) and lowest (a(w) 0.94-0.96) levels. The meat samples with modified and unaltered (native, a(w) 1.00) a(w) were inoculated with Escherichia coli O157:H7 and subjected to electron beam (e-beam). Survivor curves were plotted and the D-10-values were calculated. The D-10-values ranged from 0.22 kGy for trout at native a(w) to 0.33 kGy for beef at intermediate, and chicken and trout at lowest a(w). Regardless of the species, a(w) reduction increased E. coli resistance to e-beam, suggesting that even small depletion of unbound water from food increases survival. The difference of the D-10-values between the samples at intermediate and lowest a(w) was insignificant. E-beam could be used before a(w)-reducing techniques are applied to food products. However, this would require stringent microbial control following e-beam processing. The 'tailing' of survivors was observed for some samples with reduced a(w).