Relationships between storage quality attributes, such as russet spotting and browning intensity, and physiological attributes, such as soluble phenolic content and polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) activities, of minimally processed crisphead lettuce (Lactuca sativa L.) were studied. The processed lettuce tissue was kept in air or air plus ethylene at 2 or 5 μl·liter-1 at 2.5 or 5C for 1 to 4 days and then transferred to air at 2.5, 5, or 20C for 1 to several days. None of the above physiological attributes of the initial samples from eight lettuce cultivars (Calmar, El Toro, Sea Green, Pacific, Monterey, Salinas 88, 86-13, and Nerone) and three maturity stages (immature, mature, and overmature) correlated with their storage quality. However, ethylene-induced PPO and PAL activities and browning intensity measured 3 to 4 days after harvest consistently and significantly correlated with the final visual quality of the ethylene-treated, minimally processed lettuce after 6 to 10 days of storage. Among these three attributes, ethylene induced a 2.5- to 5.3-fold increase in PAL activity, while the relative changes in PPO activity and browning intensity were only 23% to 68%. Ethylene-induced PAL activity possibly may be used as an index to predict the storage life of minimally processed lettuce.

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