含抗壞血酸乙醇溶液中褐變情形之研究

Browning of ascorbic acid-containing ethanolic solutions

莊培梃(Pei-Ting Chuang)

指導教授 ： 吳瑞碧

國立臺灣大學/生物資源暨農學院/食品科技研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.00994

摘要

色澤是含酒精 (乙醇) 飲料 (酒品) 的品質要素之一，非酵素性褐變在酒品中容易發生而降低其價值，而抗壞血酸降解可能是含果汁成分酒品的主要褐變路徑之一。本研究以抗壞血酸為主要實驗對象，先分析市面上含果汁成分之酒品的組成，再根據此組成，建構不同濃度之乙醇溶液模式系統來模擬酒品。測定抗壞血酸在模式系統中之降解速率、分析降解產物，並探討酒品之組成分對抗壞血酸褐變之影響。
結果顯示，本研究模式系統中，抗壞血酸主要降解途徑為有氧降解，主要降解產物包括2-furoic 及3-hydroxy-2-pyrone。在貯藏過程中，2-furoic 及3-hydroxy-2-pyrone 的總量隨著乙醇濃度增加而增加，而模式系統中之褐變情形與上述降解產物含量間之相關性不佳。在糖、胺基酸、單寧類、金屬離子及有機酸中，以兒茶素代表之單寧類為最主要影響抗壞血酸造成褐變的成分。不同乙醇濃度導致抗壞血酸降解產物之含量有所差異，主要原因來自於水活性的差異。高乙醇濃度 (低水活性) 利於抗壞血酸降解中間產物L-xylosone 之脫水而繼續降解。L-xylosone 已被認定是抗壞血酸降解過程中唯一能與兒茶素作用產生褐變之物質，它的加速降解，減少了本身與兒茶素反應的機會，也就導致了抗壞血酸-兒茶素褐變速率的下降。
綜合以上結果，建議含抗壞血酸酒品，在長時間儲藏時應減少其單寧類化合物含量，以避免過度褐變。高乙醇濃度雖然造成低水活性，導致抗壞血酸降解速率增加，但卻可能因抗壞血酸-單寧褐變反應速率之減緩而降低褐變。

關鍵字

乙醇 ； 褐變 ； 抗壞血酸 ； 兒茶素 ； 水活性 ； 酒精飲料

並列摘要

Color is a major quality attribute of alcoholic beverages (wine and related products). Nonenzymatic browning occurs in alcoholic beverages readily and downgrades their value. Ascorbic acid degradation can be a major browning pathway in alcoholic beverages that contain fruit juice as an ingredient. The present study took ascorbic acid as the major object in the experiments. The composition of various juice-containing alcoholic beverages on the market was analyzed for composition first. Model systems that contain the similar composition in proper buffer solutions at various ethanol concentrations were then constructed to simulate the real products. The degradation rate, the degradation products, and the relation between ascorbic acid and browning index were investigated.
In the result, the aerobic degradation pathway was found to dominate over the anaerobic pathway in all model solutions. The compounds 2-furoic and 3-hydroxy-2-pyrone were among the major degradation products. Higher ethanol concentrations corresponded with higher 3-hydroxy-2-pyrone plus 2-furoic acid contents among solutions. Bad correlation was found between browning index and the amount of degradation products. Tannins, as represented by catechin, were found to be more influential than other components, including sugars, amino acids, metal ions and organic acids. Different ethanol concentrations resulted in the formation of different profiles of ascorbic acid degradation products. It can be attributed to the difference in water activity (Aw) as the major cause. High ethanol concentrations, or low Aw, favor the dehydration and further degradation of L-xylosone, an intermediate compound in ascorbic acid degradation. L-xylosone has been recognized as the only substance that may react with catechin to cause browning. The accelerated degradation of L-xylosone reduces its own availability to react with catechin in ascorbic acid-catechin browning.
The above-described results suggest that the tannin contents in ascorbic acid-containing alcoholic beverages would better be decreased for avoiding over-browning in long-term storage. A high ethanol concentration corresponds to a low Aw that although promotes the degradation of ascorbic acid may actually reduce browning as a consequence of the decreased rate in ascorbic acid-tannin reaction.

並列關鍵字

Ethanol ； Browning ； Ascorbic acid ； Catechin ； Water Activity ； Alcoholic Beverage.

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