Effect of Temperature on Color of Concord Grape Juice

Studies on Qualities of Grape Juice Part X

Abstract

The influence of temperature on the color of 10-30% Concord grape(Vitis labrusca L.)juice products was studied. Pale purple grape juice at room temperature(25°C)gave dark purple and became turbid at low temperature(5°C). The color density at 520nm of grape juice was increased with the fall of temperature. The increment of absorbance at 520nm of malvidin-3, 5-diglucoside solution with the change from 32°C to 7°C corresponded to the enrichment of absorbance with the decrease of 0.2pH units in pH range 1.0-3.0. However, the pH change with temperature did not explain the enrichment of absorbance accompanied by pH change. Potassium and tartaric acid were qualitatively detected in the precipitate which was separated from grape juice (pH>3.2)at low temperature. From these results, it was thought that the color change of grape juice with temperature resulted from the nucleophilic substitution reaction on anthocyanin pigment with water (hydration reaction)and a small quantity of tartar deposit.