Browning susceptibility of minimally processed Baby and Romaine lettuces

This study was conducted to determine the effects of cultivar, tissue susce ptibility and storage temperature on the keeping quality of minimally proce ssed Baby and Romaine lettuces. Midrib and photosynthetic tissues were Ligh tly processed and stored for up to 7 days at 5 degrees C and 13 degrees C. Changes in L* and a* values and absorbance at 430 nm were compared. The rel ationship between polyphenols, phenylalanine ammonia lyase (PAL) and polyph enol oxidase (PPO) activities and browning was examined. Measurements of L* and a* values on midribs showed that browning discoloration was the same f or Baby as for Romaine lettuces and there was no relationship for temperatu re. In addition, L* and a* values in photosynthetic tissue indicated a decr ease in green pigmentation during storage, especially at 13 degrees C. For both kinds of tissue at the end of cold storage an increase in soluble brow n polymers was detected. In midribs, total phenolics increased significantl y throughout the storage period because of the tissue-wounding response. Th e photosynthetic tissue had a higher phenolic content than the midrib one. For both kinds of tissue an increase in PPO activity occurred throughout co ld storage. On the other hand, PAL activity in midrib tissue only increased initially, followed by a slow decline to reach normal levels. Browning pot ential estimated by L* values correlated significantly (P>95%) with PPO act ivity and with absorbance at 430 nm for Romaine lettuce. Based on colour an d browning potential no differences between Romaine and Baby lettuce cultiv ars were observed. However, photosynthetic tissue was the most suitable tis sue for the preparation of minimally processed salad mixes because of its h igh phenolic content.