| Summary: | 碩士 === 國立臺灣大學 === 園藝暨景觀學系 === 104 === Iceberge lettuce (Lactuca sativa L. var. capitata), belonging to the Asteraceae family, is an important cool-season crop for export in Taiwan. Iceberg lettuce is harvested at the bottom of stem and the cut surface turns from white to reddish-brown after storage. Browning of cut surface causes the quality loss and market value loss of head lettuce during postharvest. Wound-induced phenolic compounds in wound lettuce stem tissue are oxidized to O-quinones by polyphenol oxidase (PPO) and lead to the formation of browning pigment at the cut surface. Researches indicated that acetic acid or sulfhydryl compounds treatment can prevent the browning of cut surface, but sulfhydryl compounds treatment could cause the yellowish appearance of cut surface. In this study, the changes in phenolic compounds content that most contributed to the browning of cut surface after wounding were investigated. Furthermore, the effect of different chemical treatment on alleviating cut surface browning, and the changes in phenolic content of stem disk after wounding were studied in relation to browning. When stem disk of iceberg lettuce were stored at 5°C after wounding, browning appeared first in vascular bundle of stem disk, and then the cut surface turned red-brown. At the same time, the change in a* value of stem disk was related to the total phenolic content and the caffeic acid derivatives content. Chlorogenic acid, chicoric acid and isochlorogenic acid were the main phenolic compounds that accumulated in the control stem disk after wounding, and were good substrate for iceberg lettuce polyphenoloxidase (PPO) in vitro. After oxidized by PPO, chicoric acid quinone could cause the deeper browning color than chlorogenic acid quinone. During in vitro oxidizing process, chlorogenic acid and chicoric acid were oxidized independently by PPO. However, flavonoids which are not good substrate for iceberg lettuce PPO could enhance the browning because flavonoids could be non-enzymatically oxidized by caffeic acid derivatives quinones when caffeic acid derivatives exist. In the stem disk, chlorogenic acid were synthesized within 2 days after wounding, and then chicoric acid and isochlorogenic were synthesized 2 days after wounding. The content of the three caffeic acid derivatives were peak at 4 days after wounding. Four days after wounding, the content of chlorogenic acid declined, whereas the content of chicoric acid and isochlorgenic acid remained. The appearance of 3% acetic acid treated-stem disk maintained white after 8 days of storage, and the total phenolic content and the caffeic acid derivatives content remain low content after wounding. 3% acetic acid treatment could inhibit the phenolic synthesis which was induced by wounding; therefore, it was effective on inhibiting browning. After the stem disks were treated with sulfhydryl compounds, including N-acetyl-L-cysteine (NAC) and L-cysteine hydrochloride (CySH) at concentration of 2% and 3%, the color of wound stem disk turned orange or yellow during 8 days of storage. In 2% NAC treatment, when a* value of the stem disk increased at 4 days after wounding, the stem disk appeared browning on cut surface, and the stem disk turned orange at 8 days after wounding. In 3% NAC, 2% CySH and 3% CySH treatment, which showed better inhibition of the browning, a* value of the stem disk remained low during 8 days of storage, and finally the color of the stem disk turned yellow. The NAC and CySH treatments had different effect on delaying the increase in chlorogenic acid, chicoric acid and isochlorogenic acid content. The CySH treatment could delay the increase in chicoric acid content till 8 days after wounding, while the NAC treatment was less effective. 2% CySH treatment was more effective on delaying the increase in chlorogenic acid than 2% NAC treatment. On the delaying the increase in isochlorogenic acid content, the CySH treatments and the NAC treatments had the same effect. Therefore, the main difference of the effect on delaying browning between CySH and NAC treatment was the ability of delaying the increase in chlorogenic acid and chicoric acid content. 3% CySH treatment delayed the increase in flavonoid content to 6 days after wounding; In other sulfhydryl compounds treatment and control, the flavonoid content increased at 4 days after wounding. At 8 days after wounding, the flavonoids content in the disk treated with 3% CySH and 3% NAC was 5 times more than that in control; and the flavonoids content in the disk treated with 2% CySH was 9.5 times more than that in control, while the disk treated with 2% NAC treatment was 3 times more than in control. The result of sulfhydryl compounds treatments suggest that it delayed the browning of stem disk by delaying the increase in caffeic acid derivatives content ; however, the treated-disk had higher flavonoid content which affected the browning of stem disk, and finally had yellowish appearance.
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