Postharvest Treatments to Alleviate Butt Discoloration in Iceberg Lettuce
Date Issued
2014
Date
2014
Author(s)
Hung, Ying-Hua
Abstract
Iceberg lettuce (Lactuca sativa L. var. capitata) is an important cool-season crop for export in Taiwan. Harvesting lettuce causes wounding at the stem end which turned from white to reddish-brown during storage and is called butt discoloration. It is well documented that wounding of plant tissues usually will induce phenylalanine ammonia-lyase (PAL) activity and the accumulation of phenolic compounds, which in turn is oxidized by polyphenol oxidase (PPO) and eventually leads to tissue browning. The objective of this work was to study factors affecting butt discoloration and to develop appropriate treatments to alleviate browning. Butt discoloration development increased with storage temperature and 5℃ was selected for this experiment. Stem discs, 2 cm dia x 5 mm thickness, prepared from ‘No. 6’ head lettuce grown in Yun-Ling County, Taiwan were used for experiments. Heat shock treatments at 50℃ for 90 sec inhibited disc browning effectively and maintained their color and texture for 9 days. Treating discs with sulfhydryl reagents, including cysteine (Cys), cysteine hydrochloride (CysH) and N-acetyl cysteine (NAC) could inhibit disc browning, but the their efficacy varied depending on the concentration as well as duration. When soaking in each solution for 5 min, 1% NAC delayed but did not inhibit browning; 1% Cys and CysH also delayed browning but caused the disc to turn yellow or yellow-green. 2% CysH showed better inhibition but discs still turned yellow-green. 3% NAC showed the best result of browning inhibition and maintained disc color through the experiment. When soaking the discs in 1% Cys and 2% CysH for 1 min, 30 sec or 5 sec, or in 2% CysH for 1 min or longer all delayed disc browning but caused the discs to turn yellow-green too. Reducing the soaking time for 3% and 2% NAC could delay disc browning, but they turned yellow eventually. Soaking discs in 1%, 1.5% and 3% acetic acid for 5 min and stored at 5℃ delayed browning effectively. 1.5% and 3% acetic acid treatments maintain the white color for 10 days but caused the texture of discs to turn soft. Reducing the duration of 3% and 1.5% acetic acid treatments to 30 sec and 5 sec showed similar inhibitory effect and the latter treatment did not cause tissue softening. For 1% acetic acid, minimum of 30 sec was required to obtain similar result. Brushing 3% acetic acid to the stem butt of lettuce head and stored at high humidity environment could inhibited butt discoloration but had the problem of microorganism growth. Addition of 1% chitosan to the solution did not inhibit the growth of microorganism. Brushing the butts with 3% acetic acid with 0.1% sodium benzoate and wrapped the heads in perforated plastic bags and stored at 0℃ inhibited butt discoloration as well as microorganism growth effectively. The PAL activity in the untreated cut surface started to increase 12 h after wounding and reached its peak at 48 h then started to decline. The color of the cut surface started to turn pink 72 h after wounding and became reddish-brown later. 3% acetic acid treatment reduced PAL activity effectively and maintained the inhibition all the time. 3% NAC treatment delayed the raise in PAL activity but did not reduce its activity. The phenolic compounds were analyzed with HPLC and A320 was monitored. Six separation peaks including the chlorogenic acid were observed in extracts from untreated reddish-brown discs, no separation peaks were found in extracts from white discs treated with 3% NAC and acetic acid. While none of the above peaks were found in extracts from yellow-green discs treated with NAC and CysH, two other peaks were obtained. The above results indicated that the possible action of acetic acid was to inhibit PAL activity which resulted in reduced phenolic contents in the butt; and the possible action of sulfhydryl compounds was to modify the structure of phenolic compounds rendering them unavailable for PPO action.
Subjects
結球萵苣
切面褐化
褐變
半胱胺酸
醋酸
Type
thesis
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