Storage Temperature Affects Phenylalanine Ammonia-Lyase (PAL) Activity and Tissue Browning in Fresh-cut Lettuce

Discoloration due to enzymatic browning is one of the limiting factors on the shelf-life of lettuce. A key enzyme in tissue browning, phenylalanine ammonia-lyase (PAL) stimulates phenolics production in wounded lettuce tissues leading to enzymatic browning. In this study, effects of storage temperature and packaging materials on PAL activity and tissue browning in fresh-cut lettuce were studied. Oxygen and carbon dioxide concentrations in the packaging containers, and the phenolic content of cut lettuce were also determined. Cut lettuce was rinsed with tap water, centrifuged with a manual salad spinner and then placed in plastic containers (Poly lactic acid; PLA and Polyethylene terephthalate; PET). Samples were stored at 10 ºC and 4 ºC for up to 3 or 4 days. Tissue browning was visually evaluated using a 0-3 hedonic scale: 0, none; 1, light; 2, moderate; and 3, severe browning. The samples stored at 10 ºC for 3days in both PET and PLA, the enzyme activity reached its maximum level after 21h and then gradually decreased, while slight tissue browning was observed on the final day of the storage. However, the samples stored at 4 ºC for 3days, no tissue browning was found in any of the samples stored either in PLA or PET containers. Compared with the samples stored at 10 ºC, PAL activity increased and remained at a higher level in the samples stored at 4 ºC for 4 days. No major differences were found in the enzyme activity between the samples stored in PET and PLA containers at 4 ºC. Low temperature storage has led to the unexpectedly higher levels of the enzyme activity. However, slight browning was found in the samples stored in PET and PLA only on the final day of the storage. The oxygen concentration in PET container remained 19.5% and no major changes were found in PLA container during storage. The CO₂ concentration in PET container increased markedly (1.7%), while the CO₂ level in PLA container increased only up to 0.4% and then decreased. Total phenolics increased in both storage-samples at both temperatures, while a slight decrease in phenolics was detected on the final day of the storage at 10 ºC. The results conclude that tissue browning can be delayed when cut lettuce was stored at 4 ºC, in spite of the higher level of PAL activity and phenolic content.