Changes in Phenylalanine Ammonia-lyase and Polyphenol Oxidase Activities with Occurrence of Browning in Shredded Lettuce during Storage

Degree of browning, phenolic content, phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) activities were measured during storage in air at 5°C, 10°C, 15°C and 20°C to examine the undesirable enzymatic browning in shredded lettuce tissue. The browning of tissues was significantly dependent on storage temperature. High temperature markedly promoted the development of tissue browning. There was no significant change in activity of PPO at all temperatures during the storage. On the other hand, wounding induced marked increase in PAL activity. The initial increase of PAL activity was greater at higher temperatures. These results suggest that the increase in PAL activity due to wounding is probably one of the major reasons for the development of shredded lettuce tissue browning.