Phenolic metabolism and ethanolic fermentation of intact and cut lettuce exposed to CO2-enriched atmospheres

Abstract Intact heads and cut leaf midrib tissue of crisphead lettuce ( Lactuca sativa , L., cv. ‘Salinas’ and ‘Vanguard’) were kept in air or air +5%, 10%, or 20% CO 2 for 10 or 20 days at 2.5°C and then transferred to air at 20°C for 12 hrs to study CO 2 effects on phenolic metabolism and fermentative products. Exposure of cut midribs to 20% CO 2 increased extractable activity of phenylalanine ammonia-lyase (PAL) when assayed at its optimum pH (8.5). However, under the 20% CO 2 treatment PAL activity was reduced as a result of a decrease in cytoplasmic pH. Total phenolic content of cut midribs was reduced under 20% CO 2 -enriched atmosphere probably due to this decrease in PAL activity. The phenolic content rapidly increased after the tissue was transferred from 20% CO 2 at 2.5°C to air at 20°C when cytoplasmic pH returned to its normal value and the PAL activity was restored. Such effects of CO 2 on PAL and phenolics were of lower magnitude in intact heads, probably due to their lower sensitivity to phenolic metabolism. Off-flavor developed in the intact heads exposed to 20% CO 2 , which was associated with increased concentrations of ethanol and acetaldehyde. When exposed to 20% CO 2 , levels of ethanol and acetaldehyde in cut midribs were only about half of those in intact heads.