葉綠素在乙醇溶液中的顏色穩定性之研究

Color Stability of Chlorophyll in Ethanolic Solution

黃志能(Chih-Neng Huang)

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/碩士(2009年)

https://doi.org/10.6342/NTU.2009.02601

摘要

摘　要
葉綠素為綠色蔬果中常見的色素，但此類色素本身並不穩定，因此含有葉綠素的產品在加工及貯藏過程中容易褪色而失去綠色的色澤。目前食品領域中，對於此類色素的研究著重於實際的食物原料以及加工後的蔬菜泥及蔬菜汁等，主要目的在觀察不同加工條件下葉綠素的保留程度。此外，一些酒精飲料，例如竹葉青酒以及奇果酒等，是利用含有葉綠素的原料浸漬而成，但在酒精系統下此類色素的穩定性卻少有文獻提及。
本論文目的主要探討葉綠素於乙醇溶液中的顏色安定性並提出可能的影響機制。實驗時以不同乙醇濃度 (0%-60%) 的葉綠素模式溶液貯藏於20、30、40及50℃環境中，觀察乙醇溶液中葉綠素的顏色變化，並提出相關的動力學模式。
結果顯示，葉綠素在乙醇模式溶液中，顏色降解為一級反應，且降解速率隨著反應溫度的提高而增加；在乙醇濃度的影響方面，發現在40%乙醇濃度下，葉綠素的降解速率高於其它酒精度的組別；但是當乙醇濃度在30%以下及50%以上時，葉綠素的降解速率均低於0%的控制組，且最穩定的情況發生在60%乙醇濃度，顯示低乙醇濃度及高乙醇濃度下，乙醇對葉綠素具有護色效果，使得顏色呈現較安定的狀態。
由於乙醇會造成系統水活性的改變，為了解乙醇溶液中，水活性對葉綠素的影響，利用凍結點下降 (freezing point depression) 的方式測量不同乙醇濃度系統之水活性 (water activity; aw)，並以甘油調整系統的aw作為對照組，結果發現水活性0.80 (相當於40%乙醇) 時，葉綠素降解速率最快；但在甘油調整系統水活性的實驗中，發現水活性愈低葉綠素顏色表現愈穩定，顯示，在乙醇系統下，水活性並非影響葉綠素穩定性的主要因子。
利用光譜分析了解在不同水活性、溶劑及酒精濃度下的光譜表現，發現在40%乙醇濃度時，光譜有明顯的向紅效應 (bathochromic)，即往長波長的方向移動。顯示在此濃度下，乙醇與水分子間交互作用大於其它酒精度的組別，使得水分子中所含的氫離子較易接觸乙醇中的葉綠素，促進葉綠素的降解；而在高乙醇濃度時 (50%)，乙醇分子間能夠形成疏水性接合 (hydrophobic association)，阻礙環境中的水分子與葉綠素作用，達到保護葉綠素的效果。進一步試驗利用氯化鈉減弱乙醇分子間的交互作用，結果顯示添加氯化鈉的組別，葉綠素的光譜表現會往長波長的方向移動；而在顏色安定性方面，將添加氯化鈉的試驗組置入50℃環境貯藏時，發現在乙醇濃度50%以下的組別，一天內綠色色澤明顯消失，而在60%乙醇濃度時，數天後仍可保有綠色顏色，顯示在此濃度下，氯化鈉不易減弱乙醇分子間疏水性接合。此與文獻提及乙醇濃度在57%以上所形成的疏水性結構不易受環境因子影響有關。因此，乙醇與水分子間的交互作用力應為影響葉綠素顏色安性的主要原因。

關鍵字 : 葉綠素、乙醇、水活性、向紅效應、氯化鈉

關鍵字

葉綠素 ； 乙醇 ； 水活性 ； 向紅效應 ； 氯化鈉

並列摘要

Abstract
The presence of chlorophyll is common among green fruits and vegetables. Being an unstable pigment, chlorophyll degrades readily during the processing and storage of these commodities. The stability of chlorophyll plays an important role in their quality retention. Researches on the stability of this pigment are usually done using the produce itself or its aqueous solutions, puree and juice for example, as the experiment material. Some alcoholic beverages, such as kiwi wine-cooler and Chinese bamboo leaf liqueur, contain chlorophyll as the major color compound. However, the stability of chlorophyll in ethanolic solutions is seldom investigated.
The objective of the present study was to investigate the stability of chlorophyll in ethanolic solutions. In the experiment, solutions in different ethanol concentrations containing 4μg/mL of chlorophyll a were held at 20, 30, 40 and 50°C for 0-60 day. A Color and Color Difference Meter was used to monitor the color change. The degradation kinetics of green color in the solution was evaluated.
The results show that the degradation of chlorophyll in ethanolic solutions follows first order reaction kinetics, similar to previous reports on chlorophyll degradation in aqueous systems. The degradation rate increases with the temperature. The degradation rate of chlorophyll also varies with ethanol concentration. It reaches the maximum at 40% ethanol concentration, and a minimum at 60% ethanol concentration. At an ethanol concentration higher than 50% or lower than 30%, the green color is more stable than the 0% control group
Ethanol can cause a change in the water activity (aw) of the system. Freezing point depression method was used to estimate aw of ethanolic solutions and to understand the effect of aw on the color stability of chlorophyll. Glycerol was used for preparing reference samples. The results show that the degradation rate reaches maximum at 0.80 aw, which is at 40% ethanol concentration. However, in glycerolic solutions, the color degradation rate of chlorophyll was slower when the aw was reduced. We propose that aw was not the major factor to affect the color stability of chlorophyll in ethanolic solution.
By comparing the spectra of chlorophyll in solutions with different water activities and solvent concentrations, we postulate that the interaction between water and ethanol may be the major factor affecting the color stability of chlorophyll in ethanolic solution. At 40% ethanol concentration the bathochromic shift is most obvious, indicating that the interaction between water and ethanol is strongest, that the hydrogen ions from water molecules interact with chlorophyll most rapidly, and that the pigment degrades at the highest rate. When ethanol concentration is higher than 50%, the hydrophobic association among ethanol molecules may occur, which hinders water from interacting with chlorophyll and protects this pigment.
Sodium chloride was added to reduce the hydrophobic association among ethanol molecules. Results show that sodium chloride shifted the spectrum of chlorophyll to higher wavelength. Regarding to color stability, at 50℃ surrounding, when ethanol concentration was lower than 50%, the green color disappeared in one day in the presence of sodium chloride. The green color retained for several days at 60% ethanol concentration, indicating that the hydrophobic association among ethanol molecules at this concentration was weakened by sodium chloride but to a less extent. The effect of sodium chloride on the hydrophobic structure among ethanol molecules decreased when the ethanol concentration increased from 57%. Restated, the interaction between an ethanol molecule and water may be the major factor to affect the color stability of chlorophyll.

Key words: chlorophyll, ethanol, water activity, bathochromic shift, sodium chloride

並列關鍵字

chlorophyll ； ethanol ； water activity ； bathochromic shift ； sodium chloride

參考文獻

趙鳳鼎：玫瑰浸漬酒顏色品質之研究。國立台灣大學食品科技所碩士論文

曾國展：花青素在乙醇溶液中顏色表現之研究。國立台灣大學食品科技所博士論

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