Fruit Surface Colour as Related to Quality Attributes in Two Plum (Prunus domestica L.) Cultivars at Different Maturity Stages

The fruit of 'Opal' and 'Victoria' plums (Prunus domestica L.) were harvested at commercial harvest time at the Ullensvang Research Centre, western Norway. The fruit were sorted into three maturity groups (MG1-3) according to their visual surface colour: green, pink and bluish-red for 'Opal', and green, pink and violetred for 'Victoria', combined with their initial firmness, as evaluated by finger pressure. Their size, weight, soluble solids content, sensory firmness assessment, and colour parameters L* (lightness), a* (green/red) and b* (blue/yellow) were measured, and the Chroma and Hue angle indices and the colour index for red grapes (CIRG) were calculated. The fruit of increasing maturity were established across the maturity groups, as indicated by both increased soluble solids content and decreased the sensory firmness assessment from MG1 to MG3. The differences in the colour parameters and indices between the fruit in the three maturity groups were more or less similar for both cultivars. MG2 contained fruit that appeared to be a mixture of MG1 and MG3 fruits in terms of the colours, which resulted in large standard deviations for the colour parameters/indices in MG2. This appeared to cause the problem of sorting these plum cultivars by colour, whereby surface colour development during on-tree ripening along with the colour parameters/indices did not allow visual sorting of the fruit into three maturity groups. None of the colour parameters/indices correlated systematically and significantly with the quality parameters of weight, size, soluble solids content and the sensory firmness assessment within each of the maturity groups. However, high and systematic correlations were found between soluble solids content and the sensory firmness assessment on the one hand, and all of the colour parameters/indices on the other, when calculated over the total test material of each cultivar.