Factors affecting the efficacy of washing procedures used in the production of prepared salads

Abstract

External leaves of whole lettuce were found to have counts approximately 1 log cycle higher than subsequent inner leaf layers. A standard washing in tap water resulted in the removal of an average of 92.4% of the lettuce leaf microflora. Inclusion of 100 mg l−1 (pH c. 9) available free chlorine reduced the count by 97.8%. Adjusting the pH of hypochlorite solutions from c. 9 to 4.5–5.0 with inorganic or organic acids produced a 1.5–4.0 fold increase in the microbiocidal effect. Increasing the washing time in hypochlorite from 5 to 30 min did not decrease microbial numbers further whereas extended washing in tap water produced a reduction comparable to hypochlorite. Addition of a surfactant, Tween 80, to hypochlorite reduced microbial numbers by 99.6% but resulted in organoleptic differences. Failure of conventional water and hypochlorite washing to remove more of the microflora is ascribed to the survival of bacteria in protective hydrophobic pockets or folds in the leaf surface and some supportive electron microscopy evidence is presented.