Changes in appearance and natural microflora on iceberg lettuce treated in warm, chlorinated water and then stored at refrigeration temperature

The objective of this study was to determine the effect of warm, chlorinated water on the survival and subsequent growth of naturally occurring microorganisms and visual quality of fresh-cut iceberg lettuce. After dipping cut lettuce leaves in water containing 20 mg l(-1) free chlorine for 90s at 50 degreesC, samples were stored at 5 or 15 degreesC for up to 18 or 7 days, respectively. Populations of aerobic mesophiles, psychrotrophs, Enterobacteriaceae, lactic acid bacteria, and yeasts and molds were determined. The visual appearance and development of brown discoloration were monitored. Treatment of lettuce in warm (50 degreesC) chlorinated water delayed browning of lettuce. Shelf life of lettuce stored at 5 degreesC, as determined by subjective evaluation of color and general appearance, was about 5 days longer than that of lettuce stored at 15 degreesC. Treatment in warm (50 degrees) wafer, with or without 20 mg l(-1) chlorine, and in chlorinated water at 20 degrees C significantly (alpha = 0.05) reduced the initial population of mesophilic aerobic microflora by 1.73-1.96 log(10) cfu g(-1). Populations increased, regardless of treatment, as storage time at 5 degreesC and 15 degreesC increased. The same trends were observed in populations of psychrotrophs and Enterobacteriaceae. Yeast populations increased slightly in lettuce stored at 5 degreesC but were consistently about 3 logs lower than mesophilic aerobes. Populations of molds and lactic acid bacteria were less than 2 log(10) cfu g(-1) throughout storage at 5 or 15 degreesC. Results suggest that hear (50 degreesC) treatment may have delayed browning and reduced initial populations of some groups of micro-organisms naturally occurring on iceberg lettuce but enhanced microbial growth during subsequent storage. (C) 2001 Academic Press.