結球萵苣(Lactuca sativa L. var. capitata;Crisphead lettuce)為菊科萵苣屬一年生葉用蔬菜,是全球生產量、食用量最高的蔬菜之一。結球萵苣採收時由主莖基部切下,切面於創傷後轉為紅褐色,為影響採後外觀品質的主要因子之一。褐變發生是由創傷後引起組織內苯丙胺酸氨基裂解酶(Phenylalanine ammonia lyase, PAL)活性上升導致酚類物質開始累積,酚類物質與多酚氧化酶(Polyphenol oxidase, PPO)作用形成醌類,進一步形成褐化物質。已有研究指出施用醋酸溶液以及貯藏環境氣體組成之控制,均具有抑制結球萵苣創傷傷口褐變之效果。本研究探討市面之食用醋代替試驗用醋酸、不同處理方式、包裝材質、包裝規格與包裝處理對於延緩結球萵苣貯藏期間酵素型褐變及其他品質劣變之效益。結球萵苣主莖切片於5℃施用2%、3%醋酸溶液可抑制a*值上升以及創傷後第二天之PAL活性以及總酚含量。醋酸濃度1.5%之糯米醋、珍釀白醋、工研醋以及蘋果醋皆能有效抑制創傷後第二天PAL活性以及總酚含量,醋酸濃度1.5%蘋果醋處理結球萵苣主莖切片相較於相同濃度之工研醋、糯米醋以及珍釀白醋能完全抑制a*值上升,減緩褐化程度。使用海綿沾取蘋果醋施用於結球萵苣主莖切面5秒鐘,a*值、b*值皆未有上升之趨勢,能夠抑制褐變之發生,其結果與前人所使用之浸漬法相同,顯示海綿沾附法可做為浸漬法改進使用方法。結球萵苣主莖切塊取代主莖圓片施用蘋果醋之試驗顯示,隨著樣品體積之增加,抑制褐變所需濃度增加。處理蘋果醋並搭配包裝顯著的抑制a*值以及b*值上升,降低褐變程度,維持結球萵苣之品質。規格方面隨著包裝袋密封程度增加,更能抑制a*值與褐變程度之上升,推測因袋內氧氣濃度降低與二氧化碳濃度累積所造成。聚乙烯材質搭配蘋果醋原液能顯著的抑制褐變程度,並且低密度聚乙烯材質有較佳之結果。結球萵苣切面施用蘋果醋搭配脫氧劑、氣體組成5% O2, 0% CO2之主動氣變以及真空包裝處理後,各處理組間在a*值、b*值以及褐化指數上無明顯之差異,二氧化碳濃度也未達到造成葉球傷害之濃度,皆維持在1%以下。因此利用LDPE包裝袋處理蘋果醋搭配氣變包裝可有效解決結球萵苣莖部採收切口褐變問題。
Iceberg lettuce (Lactuca sativa L. var. capitata;Crisphead lettuce) belongs to the Asterraceae, is most yield and consumption for vegetable in the world. Harvesting lettuce causes wounding at the stem end which turned from white to reddish-brown. This phenomenon was called butt discoloration and was considered a defect in quality attributes of head lettuce. It was well known that wounding of plant tissues usually induces the activity of phenylalanine ammonia lyase (PAL) and results in the accumulation of phenolic compounds, which were then oxidized to quinones by polyphenol oxidase (PPO) and eventually condensed spontaneously to brown pigments. Several researches had indicated that acetic acid treatment or modified atmosphere packaging can prevent cut surface of lettuce from browning. The objective of this study was to use the edible vinegar instead acetic acid and investigate the influence of different treatment, packaging specification, packaging material and packaging treatment on surface browning and quality of wounded stem of head lettuce. Soaking lettuce stem discs in 2% and 3% acetic acid stored at 5℃ inhibiting of a* value , PAL activity and total phenolic content. 1.5% apple vinegar treatment compared to the same concentration of Kong yen rice vinegar, rice vinegar and premium rice vinegar were more effectively of inhibiting a* value. Using sponge soak in apple vinegar and moistening on the surface of lettuce stem for 5 seconds, a* value and b* value were been inhibited. This result is the same as soaking which used before, so we can substitute sponge moistening for soaking. The results of the cut lettuce stem replace stem discs show that the concentration of browning increased with the increase of the volume of the sample. The apple vinegar treatment with packaging can inhibit a*, b* value, reduce the degree of browning and maintain the quality of lettuce. The more sealing of package specifications the more inhibition of a * value and the degree of browning rise, presumably due to the reduction of oxygen concentration in the bag and carbon dioxide accumulation. Polyethylene material with apple vinegar solution can significantly inhibit the degree of browning, and low-density polyethylene material has better results. The apple vinegar using in surface of lettuce stem with passive modified atmosphere packaging using deoxidizer, active modified atmosphere packaging with 5% O2 and 0% CO2 and vacuum packaging, each treatment did not have obvious difference in a*, b* value, browning score. The carbon dioxide concentration did not reach the standard of injuring the leaf ball of lettuce, it maintain below 1%. This result suggested that using LDPE packaging film with the treatment of apple vinegar could effectively solve the problem of harvesting lettuce causes wounding at the stem end which turned from white to reddish-brown.
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