The use of colour to separate tomatoes into groups at distinct stages of ripeness is usually a subjective procedure. In an attempt to reduce variability, tomatoes previously sorted by eye into five categories from mature green to full red were reassessed using a tristimulus colour difference meter. Various methods of presenting the samples to the machine for colour testing and of transforming the results were compared, and a formula was developed so that objective limits for each ripening stage could be calculated. In order to obtain matched samples of fruit at any particular stage of ripeness, the instrument can be programmed to reject fruit falling outside the appropriate limits. The colour components of tomatoes of several cultivars chosen subjectively to be at the same stage of ripeness (orange-red) were shown to be related to fruit firmness.

The colour of subjectively-chosen tomato fruit of three contrasting cultivars at each of five stages of ripeness were measured on a tri-stimulus colour difference meter. The values were introduced into a formula previously developed for calculating objective limits for each ripening stage, but satisfactory separations were not achieved. When the colour components for areas on the side-walls of tomatoes were introduced into a revised formula, good discrimination was achieved between classes of fruit from fairly ripe to fully ripe. Similar measurements carried out on areas near the stylar scar and used with an alternative formula led to efficient separation of less ripe fruit into distinct classes. It is suggested that the revised formula can be applied to any cultivated tomato line except colour mutants.

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