Effect of warm, chlorinated water on the microbial flora of shredded iceberg lettuce

An optimization technique was used to determine suitable times of exposure and temperatures for maximum destruction of microorganisms in shredded iceberg lettuce dipped in warm, chlorinated (100 mu g/ml) water in a model system. Shredded lettuce washed for 3 min at 47 degrees C and packaged in high oxygen transmission rate (6000-8000 cc m(2) 24 h) bags maintained an acceptable appearance after 7 days in storage at 1 +/- 1 degrees C. Initial microbial loads were reduced by approximately 3 log cfu g(-1) in lettuce washed in chlorinated water at 47 degrees C, and 1 log cfu g(-1) at 4 degrees C. Pilot plant scale studies confirmed observations in the model system. The microbial flora of shredded lettuce packaged in the raw state and after a 3 min wash in 100 g ml(-1) chlorine at 4 or 47 degrees C was dominated by psychrotrophic bacteria: particularly species of Pseudomonas, throughout the storage period. Numbers of these microorganisms were significantly lower than in raw lettuce or lettuce washed at 4 degrees C until the 15th day in storage. Gas composition inside the bags was not affected by the washes. (C) 1999 Crown Copyright. Published by Elsevier Science Ltd on behalf of the Canadian Institute of Food Science and Technology. All rights reserved.