Influence of a commercial warm chlorinated water treatment and packaging on the shelf-life of ready-to-use lettuce

Ready-to-use (RTU) lettuce washed in 100 ppm chlorine at 4 °C and packaged in oxygen-permeable film has a limited shelf-life due to continued respiration and enzymatic activity within the lettuce tissue. Preliminary laboratory work has revealed that washing in 100 ppm chlorine at 47 °C has the potential to extend the shelf-life of RTU lettuce; however, this has not been demonstrated in a commercial processing setting. A flume was constructed which allowed the continuous treatment of chopped lettuce at 48 °C for 30 s, with rapid cooling to below 20 °C. Lettuce processed by either warm or cold water treatment was packaged at 340, 680 or 1134 g in 28 × 30 cm bags made from two different films with oxygen transmission rate of 981 and 550 mL m −2 h −1, and stored for up to 18 days at 4 °C. Loss of quality was assessed by a sensory panel, and residual O 2 and CO 2 concentrations in the bags were determined at various intervals. Quality of cold washed lettuce was retained over 18 days in packages containing 1134 g of lettuce. The warm water treatment had a significantly beneficial effect on the visual quality of lettuce packaged in 340 or 680 g amounts between days 8 and 14 of storage at 4 °C. CO 2 levels were lower in packages with 340 or 680 g of lettuce; however, the O 2 levels reached 0.1% with all treatments after 4 days of storage. Treatments in which the CO 2 reached an equilibrium concentration of 15–20% gave the least deterioration of quality. Equilibrium gas concentrations predicted from respiration experiments were higher (>0.5% O 2) or lower (