減輕結球萵苣切面褐化之處理技術

Abstract

結球萵苣 (Lactuca sativa L. var. capitata )為國內冬季重要外銷蔬菜。結球萵苣採收時由主莖基部切下，此切面因創傷緣故會由白色轉為紅褐色，稱為底部褐化(Butt discoloration)，影響商品外觀。創傷會誘導苯丙胺酸氨基裂解酶 (phenylalanine ammonia lyase, PAL)活性，造成酚類化合物增加，進而受多酚氧化酶作用，最後形成褐色物質。本研究探討影響切面褐化之因子並發展適當之處理技術，減低切面褐化之發生。切面褐化速度會隨貯藏溫度升高而增加，本次試驗選擇5℃作為試驗溫度。台灣生菜村‘6號’結球萵苣主莖圓片使用以50℃處理90秒熱水處理能抑制褐化褐化並維持質地完整達9天，顯示有良好抑制褐變的效果。以含硫氫基(sulfhydryl bond)之化合物－半胱胺酸 (cysteine, Cys)、半胱胺酸鹽酸鹽(cysteine hydrochloride, CysH)、N-乙醯-L-半胱胺酸(N-acetyl cysteine, NAC)，在抑制褐化的效果方法依化合物種類與處理條件而異。以各溶液浸泡5分鐘，1% NAC可延緩但無法有效抑制切面褐化，1% Cys及CysH亦有延緩褐化效果，但兩者皆會轉呈黃色或黃綠色；2% CysH效果較佳，但仍會生成黃綠顏色，3% NAC 溶液效果最好且維持原片初始白色色澤。使用1% CysH浸泡1分鐘、30秒及5秒或2% CysH浸泡1分鐘以上皆有延緩紅褐色產生的效果，但會使圓片轉為黃綠色。縮短3%及2% NAC施用時間，雖有延緩褐化效果，但最後形成偏黃的色調。以1%、1.5%及3%之醋酸溶液浸泡處理5分鐘後，貯藏於5℃有延緩褐化之效果，以1.5%及3%處理可維持正常色澤達10天，但會造成組織軟化。將3%及1.5%縮短處理時間至30秒及5秒皆有相同抑制效果，後者無軟化現象；1%醋酸溶液則需浸泡30秒以上才能維持效果。將3%醋酸塗刷於完整葉球切面，在高濕環境下可抑制褐化但有微生物孳生問題，添加1%幾丁聚醣無法有效抑制微生物孶長，以0.1% 苯甲酸鈉與3% 醋酸混合施用後以打孔塑膠袋包裝貯藏於0℃，可抑制褐變及微生物生長至貯藏後12天，有最佳外觀品質。未處理切面於5℃下貯藏12小時後PAL活性即開始增加，至48小時達高峰後開始緩慢下降，切面顏色在72小時後逐漸變為粉紅色，隨貯藏時間增加再轉為紅褐色。3% 醋酸可有效降低PAL活性且不隨貯藏時間增加，顯示其抑制反應為不可逆；3% NAC無法降低PAL活性，但可延緩PAL上升時間。以HPLC分析圓片中酚類化合物並檢測320 nm之吸光值：褐化圓片中可分離得到包括綠原酸在內的六個波峰；以3% NAC及醋酸處理之白色圓片未偵測到上述酚類物質；處理NAC及CysH之黃綠色圓片中可測得另二個不同的酚類物質波峰，此結果顯示醋酸處理之作用為抑制PAL活性導致酚類物質生成受抑制，含硫氫基化合物之作用為改變酚類物質之構造，使其無法被酵素作用。

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.