Effect of heat treatment, pH, sugar concentration, and metal ion addition on green color retention in homogenized puree of Thompson seedless grape

Homogenized puree of Thompson seedless (Vitis vinifera 'Thompson Seedless') grape was treated under different conditions, including heating time (5-30min), temperature (20-80°C) and pH (2-10). Treatments with separate additions of glucose, fructose, and sucrose at concentrations of 100-600g/L and salts of MgCl₂, ZnCl₂, GuSO₄, and KCl at concentration of 1g/L were also tested at 20°C up to 30min. Relative chlorophyll pigment retention (CR) was measured at 664nm. The chlorophyll pigments were the most stable at 20°C and the nature pH of grape puree (pH 3.4). Increasing temperature and changing pH reduced CR. After 30min treatment at 80°C, CR decreased to lower than 20% and 50% at pH 2 and 5, separately. Increasing fructose and sucrose concentrations reduced CR at pH 3, but increased it at most levels at pH 4. Increasing glucose concentration protected chlorophyll pigment from reduction in some cases at pH 3 and 4. CR increased with addition of metal ions increased at pH 4, but decreased at pH 3. Conditions of 23.3°C, pH 4.5, and 120g/kg fructose and 210g/kg glucose produced the highest CR value.