褐化抑制物對結球萵苣主莖創傷後酚類含量變化之影響

Effect of Browning Inhibitors on Phenolic Contents in Wound Stem of Iceberg Lettuce

張乃文(Nai-Wen Chang)

指導教授 ： 王自存

國立臺灣大學/生物資源暨農學院/園藝暨景觀學系/碩士(2016年)

https://doi.org/10.6342/NTU201610272

摘要

結球萵苣 (Lactuca sativa L. var. capitata) 為菊科萵苣屬作物，為國內冬季重要的外銷蔬菜。結球萵苣採收時由主莖基部切下，採收切面會於創傷後轉為紅褐色，當結球萵苣以完整葉球販售時，此切面褐化為影響外觀品質與商品價值的重要因素之一。已知創傷萵苣主莖內，受創傷誘導生成之酚類經酵素氧化後會聚合產生褐色物質，因而造成主莖切面褐化；以醋酸或含硫氫基化合物處理後，具有延緩主莖切面褐化之效果，而以含硫氫基化合物處理後會使外觀呈現偏黃色色澤。本研究探討創傷後主要造成結球萵苣主莖切片褐化的酚類物質種類與其含量變化，並進一步比較以不同延緩褐化藥劑處理後，其延緩外觀褐化之效果，以及褐化與酚類物質含量變化之關係。貯藏於5°C下之結球萵苣主莖切片，於創傷後第二天維管束即出現褐化徵狀，持續褐化過程中切面逐漸轉為紅褐色，其外觀a*值變化趨勢與切片之總酚類含量、總咖啡酸含量變化趨勢相同，主要於創傷第四天以內增加。創傷後主要生成酚類為綠原酸(chlorogenic acid)、菊苣酸(chicoric acid)與異綠原酸(isochlorogenic acid)，為易被萵苣多酚氧化酶氧化的基質，並且造成褐化，同濃度下的菊苣酸經氧化後造成褐化的程度大於綠原酸。當綠原酸與菊苣酸同時存在時造成的褐化程度與個別濃度相關，彼此間並無交互關係；類黃酮並非萵苣多酚氧化酶易催化的基質，但與咖啡酸衍生物同時存在時，類黃酮可被醌類氧化並造成褐化。主莖切片創傷後首先生成綠原酸，菊苣酸與異綠原酸則是於創傷第二天後開始生成，三者含量皆於第四天以後達到最高，第四天後綠原酸含量開始下降，其於兩者含量則維持一定。萵苣主莖切片經3%醋酸處理後，其外觀可維持白色色澤至貯藏第八天，貯藏期間總酚類、總咖啡酸含量皆不因創傷而上升，顯示3%醋酸可抑制創傷後酚類物質生成，因而具有延緩切面褐化的效果。萵苣主莖切片經含硫氫基化合物N-acetyl-L-cysteine (NAC)與L-cysteine hydrochloride (CySH)以不同濃度處理後，經2% NAC處理之切片於創傷後第四天開始褐化且其外觀a*值於第四天後開始上升，第八天切面為橘色；延緩褐化效果較佳的3% NAC、2% CysH與3% CySH處理，皆且可延緩切片外觀a*值不上升至第八天，第八天外觀具黃色色澤。主莖切片經NAC與CySH處理後，其延緩創傷後綠原酸、菊苣酸、異綠原酸含量開始上升時間的效果不同，兩種濃度下之CySH處理延緩菊苣酸含量上升的效果皆優於NAC處理，且2% CySH處理延緩綠原酸含量上升的效果也優於同濃度下的NAC處理，而CySH與NAC處理對延緩異綠原酸含量上升之效果相同。CySH處理延緩萵苣主莖切片褐化的效果優於NAC處理的原因為其延緩菊苣酸與綠原酸含量上升的效果較佳。主莖切片經NAC與CySH處理後，其總類黃酮含量，除了3% CySH處理可延緩至第六天後開始上升，其餘三組皆於創傷第四天後開始上升，與對照組同時。延緩褐化效果較佳的2% CySH與3% NAC處理組的第八天含量約為對照組的5倍，3% CySH處理組的含量約為對照組的9.5倍，而延緩褐化效果較差的2% NAC處理組則為對照組的3倍。由含硫氫基化合物處理結果顯示，此類化合物處理可藉由延緩咖啡酸衍生物含量上升而延緩外觀褐化，但處理後切片之類黃酮含量高於對照組，並且貯藏後外觀呈現黃色色澤。

關鍵字

結球萵苣 ； 創傷 ； 褐化 ； 咖啡酸衍生物 ； 醋酸 ； 半胱胺酸衍生物

並列摘要

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.

並列關鍵字

Iceberge lettuce ； wounding ； browning ； caffeic acid derivatives ； acetic acid ； cysteine derivatives

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