Effect of mild heat-shock treatments on pink discoloration and physiological parameters in fresh-cut iceberg lettuce

Pink discoloration is one of the major quality issues occurring in fresh-cut lettuce. Low oxygen atmosphere in modified atmosphere packaging prevents pinking but may lead to other major quality losses such as fermentation, growth of lactic acid bacteria and associated production of off-odours. The application of mild heat-shock treatment on fresh-cut lettuce is known to reduce the activity of Phenylalanine Ammonia Lyase (PAL), an enzyme involved in pink discoloration. Heat-shock treatment, however, may also affect the vitality of the plant tissue, leading to early senescence symptoms. In the present study several heat-shock treatments were tested for their effect on pink discoloration and on lettuce tissue vitality. The optimal heat-shock treatment was found within a narrow range of temperatures (between 45 and 47.5 °C) and application times (between 30 and 180 s). Heat-shock treatment reduced significantly the PAL activity by a factor 3 during the first 5 d of storage but increased the respiration rate of the produce by a factor 2 on day 5. We recommend to optimize the packaging to suit the heat-shock associated higher respiration rate of the treated product in order to increase the shelf life of the fresh-cut lettuce.