Validation of ground color chart as indicators of maturity stage and harvest date of apples

The ground color index is one of most used parameters for maturity assessment of green or partly red varieties of apples at orchard. The validation of ground color (color on the least red part of skin) cards developed for ‘Gala’ and ‘Fuji’ apples was assessed in the present study by the significance of correlation between ground color indices (estimated by color chart) and other ripening indicators. Changes in ground color on ‘Gala’, ‘Royal Gala’ and ‘Fuji’ estimated by color chart (1 to 5 scale) related significantly with evolution of other ripening indices including
ground color measured by colorimeter, firmness, starch index, and soluble solids content. The lowest correlation indices occur between chart ground color and titratable acidity, ethylene production and red color intensity, particularly for ‘Fuji’. Data of firmness changes fitted to linear or quadratic models as functions of ground color evolution. These models
were used to estimate indices of chart ground color relative to optimum harvest period which vary from 2.8 to 4.1 for ‘Gala’, 2.5 to 3.7 for ‘Royal Gala’ and 2.2 to 3.6 for ‘Fuji’.