Inhibitory Mechanism of Onion juice on Browning of Shredded Lettuce

Abstract

The Inhibitory mechanism of onion juice on the browning of shredded lettuce during storage was investigated. Browning and phenylalanine ammonia-lyase (PAL) activity of the control sample dipped in distilled water stored at 5°C increased immediately after cutting. When shredded lettuce dipped in onion juice for 5 minutes, browning and PAL activity of shredded lettuce was suppressed for 48 hours. Polyphenol oxidase (PPO) activity of the shredded lettuce was slightly lower than the control sample, but the rise time of PPO activity was familiar to the control sample. Direct inhibition of onion juice on the PAL activity was not observed at the level of concentration that could inhibit browning almost completely, if treated immediately after cutting. On the other hand, the PPO activity was suppressed about 60% of the control upon the addition of ten percent onion juice in the reaction mixture. However, browning was suppressed negligibly, if the shredded lettuce was treated with onion juice of the concentration immediately after cutting. When the sample turned to brown already after cutting was dipped in onion juice, the Hue angle rised rapidly. After the treatment, browning of shredded lettuce was almost suppressed. The results indicate that onion juice act on the delay in the increase of PAL activity of shredded lettuce, the partial direct inhibition of PPO activity and elimination of color formed by browning reaction. Especially, the inhibition of browning in shredded lettuce during storage mainly depended on the suppression of the increase in PAL activity, if the shredded was treated with onion juice immediately after cutting.