Optimization of acidified warm water treatment to improve the microbiological status and sensory quality of iceberg lettuce

The optimal conditions of washing iceberg lettuce with acidified warm water to improve the hygienic and sensory product quality were determined with the aid of a central composite design. To determine the criteria of product quality, in the course of storage for 13 days at 4 °C lettuce was subjected every 2 days to sensory evaluation. The sensory values were found to correlate with the physical measurements of color (a* value) (R²=0.83) and texture (specific energy of deformation) (R²=0.77), respectively. Color, texture and overall visual quality correlated with product acceptability (R²=0.96, 0.97, 0.96, respectively). In the experiments of series I, the effect of temperature at washing (45–50 °C), pH (4–5) and treatment time (5–10 min) were investigated and the results were used for the construction of a model to describe the effects on color and texture and to define an area of sensory product acceptance. In series II the parameters were changed to ranges of temperature (45–50 °C), pH (4.6–6) and treatment time (2–5 min), and the effect on the reduction of bacterial counts within the area of product acceptance was defined. In that area reduction rates of total counts by 0.9–2.9 log cfu/g and Enterobacteriaceae by 0.8–3.0 log cfu/g were achieved. The initial germ- reducing effect was sustained until the end of storage for 7 days at 4 °C. At 50 °C, washing for 5 min at pH 4.93, the total bacteria and Enterobacteriaceae were reduced by 2.9 and 3.7 log cfu/g, respectively. The removal of the two groups of bacteria exhibited characteristic differences. Two independent experiments confirmed the reliability of the models.