In iceberg lettuce leaf tissue, phenylalanine ammonia-lyase (PAL) the first committed enzyme in the phenylpropanoid pathway, increased 6-fold in activity during the first 24 h at 10 8°C after wounding (Toma´s-Barbera´n et al., 1997). The phenolic compounds chlorogenic and isochlorogenic acid, and dicaffeoyl tartaric acid increased thereafter and accumulated to levels 3-, 6- and 4-fold higher, respectively, than in unwounded tissue after 72 h. Upon oxidation and/or polymerization,these phenolic compounds cause tissue browning that reduces quality and shortens shelf-life (Bolin and Huxsoll, 1991). A heat shock treatment that reduces browning in fresh cut lettuce may work by redirecting protein synthesis away from the production of wound-induced enzymes of phenolic metabolism, and toward the production of innocuous
heat shock proteins. Administering the heat shock up to 24 h before, or8 h after wounding significantly reduced (Saltveit, 2000).