Color Stability of Chlorophyll in Ethanolic Solution

Abstracthe 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 groupthanol 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.y 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.ey words: chlorophyll, ethanol, water activity, bathochromic shift, sodium chloride