Fruit Flesh Firmness in Two Plum Cultivars: Comparison of Two Penetrometers

Plums (Prunus domestica L.) of cultivars 'Opal' and 'Victoria' were sampled at commercial harvest time in the experimental field at Bioforsk Ullensvang, southwestern Norway. Samples were taken from the south-facing side of trees 1-1.5 m above ground, brought to the laboratory and graded into three different degrees of fruit development (1 = less coloured, 3 = best coloured) based on a combination of their surface ground and blush colour. Colour (LDF), weight, size and soluble solids content (TSS) were measured in representative fruit samples. Fruit flesh firmnesses were measured using two different penetrometers; Durofel (Copa-Technologie S.A., Paris, France), a hand-held shore-type penetrometer equipped with a plunger connected to a sensor that measures specifically the elasticity of the outer fruit skin when the plunger is pressed against the fruit, and PNR 10 (Sommer & Runge KG, Berlin, Germany) equipped with a plunger penetrating into the fruit surface by means of its own weight (115 g) over a 15 s period. Both cultivars showed highly significant differences in amount of colour, but only minor differences in fruit weight and size between the three groups. TSS increased significantly from group 1 to group 3, for 'Opal' more distinct than for 'Victoria'. The resulting firmness measurements were compared by means of regression analysis. In 'Opal' group 2 and 3 and in all three groups of 'Victoria' the measures from the two penetrometer types fitted significantly to a regression line; a quadratic better than a linear one, indicating that the two penetrometers measured slightly different properties of the fruit skin and outer fruit flesh.