結球萵苣採收成熟度指標與低壓包裝技術之研究

Abstract

結球萵苣(Lactuca sativa var. capitata L.)為國內主要外銷蔬菜，每年11月至隔年2月為主要外銷季節，目前產業以葉球剖面內葉緻密度或以手指按壓葉球緊實度進行成熟度判定，仰賴農民經驗，因此常造成產銷雙方爭議，需要建立量化採收成熟度指標。‘3號’、 ‘常利6號’、 ‘大將’結球萵苣於10月定植於雲林地區，9週後葉球球徑成長趨緩、按壓具有彈性，葉球剖面內葉緻密度3級，產量合乎商業標準，總可溶性固形物含量與乾物率下降趨勢緩和，且呼吸率降至最低適合貯運，因此建議採收定植55∼60天、葉球硬度合適且葉球重量達500∼700 g為最適成熟度。定植9週成熟度合適葉球採收經修整後以慣行厚度低密度聚乙烯袋包裝，貯藏於貯運推薦1℃，葉球貯藏6週期間失重皆在2%以內，外觀無萎凋皺縮現象，‘3號’葉球球形扁圓整齊，有利裝箱作業，且貯藏期間葉球呼吸率最低生理代謝較慢，貯藏末期總可溶性固形物含量顯著維持較高(3.4∼3.7%)，貯藏6週腐爛率12.5%顯著低於其他2品種（腐爛率＞20%），且無中肋褐變現象，維持良好外觀達4∼6週，為三個參試品種中貯運性最佳品種。為改善貯後褐變現象，選用符合外銷規格之‘3號’結球萵苣葉球進行低壓包裝後貯於1℃，可顯著抑制中肋與莖部採收切口褐變，且貯藏10週仍維持高脆度與總可溶性固形物含量(2.8%)，並將腐爛率降至25%以下，而慣行包裝處理已幾乎全數腐爛；葉球經此技術包裝仍可於1℃貯藏期間保持良好外觀達8∼10週。但低壓包裝葉球貯後二氧化碳累積達10%以上，產生無氧呼吸導致乙醛(47∼81 μL∙L-1)、乙醇(974∼2028 μL∙L-1)累積，貯藏8週後袋內產生明顯異味且葉球開始出現心葉傷害，整體品質因而逐漸下降，包裝技術仍待改進。

Crisphead lettuce (Lactuca sativa var. capitata L.) is an important export vegetable in Taiwan. The major export season is from November through February annually. Up to date, the industrial-used maturity determination of crisphead lettuce is totally based on the degree of leaf compactness of head profile or the firmness of head compressed with hands, both are highly rely on personal experiences; as an consequence, the maturity of this commodity is ambiguous and usually caused a disputation between growers and wholesalers. Therefore, it is necessary to establish a quantify maturity index for crisphead lettuce. ‘No. 3’, ‘Chang-Li No. 6’, and ‘Da-Jiang’ lettuce heads were considered as the optimal maturity stage for harvest at 9 weeks after transplant in Yunlin County, Taiwan, on October, because the heads exhibited fully expansion in size, firm solidity by hand compression, appearance of Class 3 leaf compactness, stabilizations of the total soluble solids and dry weight, as well as a minimum of respiration rate . A 55- to 60-day-old lettuce head after transplant with appropriate firmness and 500-700 g is recommended as the optimum maturity. After harvest and trimming, the lettuce heads were packaged with low density polyethylene (LDPE) bags and store at 1℃, the temperature commercially used for transportation of crisphead lettuce. The percentage of weight loss of the produce was less than 2% and displayed no wilting and wrinkled on appearance. Among the three cultivars investigated, ‘No.3’ was the best cultivar for package operation due to its spheroid shape. Moreover, since the respiration rate of ‘No.3’ head had the lowest respiration rate, and thus, metabolism, and decay accidence (12.5% vs.＞20% of other two after 6 weeks storage), maintained the highest total soluble solids (3.4∼3.7%), no disorder of rib browning, as well as good visual quality among the cultivars tested for 4∼6 weeks at 1℃, it is the best cultivar for storage and export. To alleviate the browning problem of stored crisphead lettuce, ‘No. 3’ lettuce heads with export quality were treated by hypobaric packaging and then stored at 1℃. By comparing with conventional packaging, hypobaric packaging significantly retarded midrib browning and butt discoloration, and was able to retain crispy texture and high total soluble solids (2.8%), lower decay percentage (＜25%) up to 10 weeks when compared with those treated with conventional packaging, which were almost completely decay. However, the hypobaric package treatment prone to cause problems of carbon dioxide (＞10%), acetaldehyde (47∼81 μL∙L-1), and ethanol (974∼2028 μL∙L-1) accumulations due to anaerobic respiration. Because of marked off-odor formation and increase of heart-leaf injury percentage after 8 weeks storage, the overall quality of the product declined gradually. Further modification and improvement are required before this technique is applied in the postharvest handling of crisphead lettuce industry.