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Recently, the use of minimally processed lettuce has continued to increase in salad bars and fast food. One of the major causes of quality loss is the browning of the cut pieces. The development of brown discoloration in shredded lettuce reduces consumer acceptability and is thus of significant economic importance to the primary producer and the food processing industry. Browning is common in plant tissues and is thought to be brought about through the metabolism of phenolic compounds. The phenolic pathway beings with the deamination of L-phenylalanine by the enzyme phenylalanine ammonia-lyase (PAL) to cinnamic acid 1), 2). Then cinnamic acid is sequentially hydroxylated into various phenolic compounds. In the presence of O2, the enzyme polyphenol oxidase (PPO) oxidizes these compounds to quinones, which simultaneously polymerize into brown pigment3). Increased use of minimally processed lettuce, and increased restrictions on chemical treatments to prevent browning provide an impetus to better understand the causes of browning and how they can be controlled. Therefore, the objective of this work is to investigate changes in PAL and PPO activities during the development of wound induced lettuce tissue browning at various temperatures.

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