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

The objective of this study was to determine the effect of warm, chlorinate d water on the survival and subsequent growth of naturally occurring microo rganisms 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, Enterobacte riaceae, lactic acid bacteria, and yeasts and molds were determined. The vi sual appearance and development of brown discoloration were monitored. Trea tment of lettuce in warm (50 degreesC) chlorinated water delayed browning o f lettuce. Shelf life of lettuce stored at 5 degreesC, as determined by sub jective 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 2 0 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 i ncreased, regardless of treatment, as storage time at 5 degreesC and 15 deg reesC increased. The same trends were observed in populations of psychrotro phs and Enterobacteriaceae. Yeast populations increased slightly in lettuce stored at 5 degreesC but were consistently about 3 logs lower than mesophi lic 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 ini tial populations of some groups of micro-organisms naturally occurring on i ceberg lettuce but enhanced microbial growth during subsequent storage. (C) 2001 Academic Press.