Effect of temperature and chlorination of pre-washing water on shelf-life and physiological properties of ready-to-use iceberg lettuce

The effects of cold and warm, chlorinated water as well as warm water without chlorination for pre-washing trimmed, cored iceberg lettuce heads was assessed regarding the shelf-life and physiological properties of the resulting ready-to-use (RTU) produce. On a pilot-plant scale, lettuce heads were shredded with or without pre-washing (50 degrees C, no chlorine; 4 and 50 degrees C 200 mg/l free chlorine; 60 s). After shredding, lettuce was washed for 90 s (tap water, 4 degrees C, spin-dried, packaged in 200 g consumer-sized bags, and stored at 4 degrees C for <= 9 days. Samples were analyzed for their headspace 0, and CO, levels, sensory properties, microbiological status, and phenylalanine ammonia-lyase (PAL) activity at regular intervals throughout processing and storage. Parallelly, instrumental color and texture measurements were performed. All pre-washing procedures of lettuce heads lowered the initial counts of total aerobic bacteria, pseudomonades, and Enterobacteriaceae by >= 1.63, >= 1.55, and >= 1.85 log(10) cfu/g, respectively, while the single-wash of shredded lettuce in tap water without any pre-washing resulted in significantly smaller reduction rates (0.23, 0.11, and 0.71 log(10) cfu/g, respectively). During storage, all pre-washing procedures had a persistent effect on bacterial counts of the RTU lettuce. Furthermore, all pre-washing procedures delayed vascular tissue browning at cut edges and retarded the decline in the overall visual quality of the samples. Whereas wound-induced PAL synthesis was reduced by the use of warm water (+/- chlorine), O-2 depletion and CO, evolution in the corresponding bags were slightly enhanced, thus proving that processing kept the food as a living tissue. (c) 2005 Elsevier Ltd. All rights reserved.