Survival of acid-adapted Escherichia coli O157:H7 and not-adapted E. coli on beef treated with 2% or 5% lactic acid

Abstract

Decontamination of beef by spraying with solutions of lactic acid is common practice in North America and is to be permitted in the EU. Validation of each such treatment is necessary for HACCP purposes. The utility of validation by reference to reduction in numbers of Escherichia coli naturally present on beef is questioned. This is because reductions of E. coli generally might be greater than reductions of the main target organism, E. coli O157:H7, which can be more acid tolerant. To investigate the effects of lactic acid sprays on the two types of E. coli on beef surfaces, slices of beef with cut muscle, fat or membrane surfaces were prepared. The surfaces were inoculated with a five strain cocktail of acid-adapted E. coli O157:H7 or with a not-acid-adapted strain of E. coli at number of 5, 1 or −1 log cfu/cm2. Inoculated slices were not treated or were sprayed with water or 2% or 5% lactic acid at 0.5 ml/cm2. For each slice of lactic acid-treated meat the numbers of E. coli or E. coli O157:H7 recovered on agars that allowed resuscitation of injured cells or did not allow resuscitation were similar. The differences in the log mean numbers (log A) and in the log total numbers (N) of bacteria recovered from slices that were not treated or treated with acid were calculated. Differences in log A and N for bacteria recovered from slices treated with water or acid were calculated also. Means of the differences indicated that each acid treatment gave similar reductions in the numbers of E. coli or E. coli O157:H7. However, reductions were somewhat greater with 5% than with 2% lactic acid. The findings suggest that the E. coli or E. coli O157:H7 that survived acid treatments of meat surfaces were protected from exposure to injurious concentrations of undissociated acid. Consequently, strains of E. coli that are or are not relatively tolerant of acid conditions will be inactivated to similar extents by solutions of lactic acid applied to beef surfaces.