Djulis is a Taiwanese cereal plant containing betacyanin pigment. The aim of this study was to elucidate the relative effect of ethanol (10–80%) and pH (3.5 and 5.5) on pigment stability in aqueous extracts of Djulis. Organic acids were further used to investigate their effects on color protection. All samples were thermally processed (60C, 30 min) and stored (25C, 21 days) after pigment regeneration (4C, 24 h). Results showed that ethanol promoted the pigment degradation in Djulis extract. The hypsochromic shift (decrease of λmax) and isomerization of the pigment were more pronounced in the systems with higher ethanol concentrations. At a fixed concentration of ethanol, betacyanin was more stable at pH 5.5 than that at pH 3.5. A stronger dependence of the discoloration on the ethanol concentration was found and explained by the competing result between ethanol–water mixing and ethanol–betacyanin interaction. This effect was lowest in pH 5.5 samples at 20% ethanol concentration. Statistical analysis indicated that pigment degradation was affected more by ethanol concentrations than by pH. Addition of 1% ascorbic acid exhibited the best pigment protection and explained by its best interaction with ethanol in the system.
Djulis, high in nutrient and physiochemical attributes, is a traditional food for aboriginals in Taiwan. They are used as a wine starter due to their high amylase activity. Their beautiful color, mainly comes from betacyanin, in the hull was not stable and was discarded as wastes. Due to high demand by the consumer for more natural food products, the trend toward replacement of synthetic colorants by natural product has been increasing. Betacyanin pigment could be a good candidate for natural colorants in some alcoholic beverages. Our findings in this study provided the information of how to maintain the pigment stability in betacyanin-containing beverages by proper alcohol and pH control, which is necessary for the Djulis wine industry.