FIRMNESS OF STRAWBERRIES AS MEASURED BY A PENETROMETER

Abstract

Strawberries are subjected to compression, bruising, and shearing effects during harvesting, packing, and shipping. The resistance of the fruit to mechanical injury is related to firmness; hence there is needed a suitable method for measuring firmness in order to evaluate varietal differences, stage of maturity, or degree of ripeness, effects of environmental conditions during fruit development on firmness, and effects of different shipping and storage conditions on changes in firmness. The improved types of pressure testers (6, 10) designed for pomaceous fruits are not sufficiently sensitive for evaluating differences in firmness of strawberries. No satisfactory pressure tester has been reported for use on strawberry fruit (6). Resistance of strawberries to wounding was measured by Hawkins and S ando (8) as the pressure required to puncture the epidermis with a needle 636 microns in diameter. Culpepper et al, (3) measured the resistance of the strawberry to penetration by a small needle (0.032 inch in diameter). The depth of penetration was not indicated, however ; the sudden penetration of the needle was used as the end point. This needle tester gave an index of maturity but did not measure varietal differences in firmness or resistance to crushing in shipping or handling. This device did not give a measure of the resistance of the first to deformation or flattening as does the pressure or squeeze tester developed by Haller et al. (7). This tester which measured the resistance to flattening of the fruit is a modification of the type described by Verner (14). A 2,000-gram scale was used and the plunger disc moved through a distance of i inch. Full ripe berries of approximately the same stage of maturity were used. A direct correlation was found to exist between the dry weight and firmness of the different varieties.