25 20 15 10 5 0 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 Decay IEC 3. Controll<strong>in</strong>g accuracy <strong>of</strong> harvest date <strong>in</strong>dexes <strong>in</strong> Ingrid Marie apples. 1 = Two weeks earlier 2 = optimum harvest period 3 = Two weeks later
FIRMNESS IN PLUMS – COMPARISONS OF THREE DIFFERENT TESTING METHODS Lars Sekse¹, Ursula Wermund², Sigrid Flatl<strong>and</strong>¹ <strong>and</strong> Eiv<strong>in</strong>d Vangdal¹ ¹Planteforsk, Ullensvang Research Centre, N-5781 L<strong>of</strong>thus, Norway e-mail: lars.Sekse@<strong>plant</strong>eforsk.no ²Imperial College at Wye, University <strong>of</strong> London, Wye, Ashford, Kent TN25 5AH, UK Firmness is an important quality factor <strong>in</strong> plums, <strong>of</strong>ten related to taste <strong>and</strong> shelf life. Different <strong>methods</strong> can be used to measure firmness <strong>in</strong> stone <strong>fruit</strong>s. At Ullensvang Research Centre three different <strong>in</strong>strumental <strong>methods</strong> to measure firmness were tested dur<strong>in</strong>g the 2001-season, <strong>and</strong> compared to a method judg<strong>in</strong>g firmness press<strong>in</strong>g the <strong>fruit</strong>s between two f<strong>in</strong>gers. The Dur<strong>of</strong>el ® is a manual Shore-type non-destructive penetrometer that has a plunger <strong>of</strong> diameter 0.25 cm² connected to a sensor measur<strong>in</strong>g the superficial contraction <strong>of</strong> the <strong>fruit</strong> surface. The PNR10 ® penetrometer has a needle with a sphaeric end with diameter 0.50 mm <strong>and</strong> a total weight <strong>of</strong> 115 g. The needle is placed <strong>in</strong> vertical position tangentially to the <strong>fruit</strong> surface <strong>and</strong> is then allowed to penetrate <strong>in</strong>to the <strong>fruit</strong> surface for 15 seconds, driven by its total weight. The distance measured represents a measure <strong>of</strong> the firmness <strong>of</strong> the <strong>fruit</strong> surface. Fruit firmness <strong>in</strong> stone <strong>fruit</strong>s can also me measured us<strong>in</strong>g the f<strong>in</strong>gers to gently squeese the <strong>fruit</strong>, giv<strong>in</strong>g scores for firmness on a scale rat<strong>in</strong>g from e. g. 1-5, where 1 is regarded very hard, 3 medium <strong>and</strong> 5 very s<strong>of</strong>t. Fruit firmness <strong>of</strong> ‘Victoria’ plums were measured first with the Dur<strong>of</strong>el ® <strong>in</strong>strument on the two opposite “cheeks” <strong>of</strong> the <strong>fruit</strong>s. Firmness <strong>of</strong> the same <strong>fruit</strong>s were then measured on approximately the same location on the <strong>fruit</strong> surface by PNR10 ® . The same <strong>fruit</strong>s were then tested for firmness by “the two f<strong>in</strong>gers” method. Comparisons by regression analyses showed that <strong>in</strong>dividual <strong>fruit</strong>s had approximately the same firmness on each <strong>of</strong> the two cheeks, measured with both Dur<strong>of</strong>el ® <strong>and</strong> PNR10 ® . When compar<strong>in</strong>g Dur<strong>of</strong>el ® with “the two f<strong>in</strong>gers” method, the results were highly significant as well, but gave lower value <strong>of</strong> the square <strong>of</strong> the correlation coefficient (R²). Even lower was the straight l<strong>in</strong>e regression between the results <strong>of</strong> the two f<strong>in</strong>ger method <strong>and</strong> the means <strong>of</strong> the paired PMR10 ® <strong>measurement</strong>s, but still highly significant. When compar<strong>in</strong>g the results from the two penetrometers, a clearcut curvature <strong>in</strong> the regression plot was observed, the straight l<strong>in</strong>e fitt<strong>in</strong>g was high <strong>and</strong> significant, but the observed results fitted much better to a quadratic regression model account<strong>in</strong>g for the l<strong>in</strong>e curvature, <strong>and</strong> even better to a model <strong>in</strong>corporat<strong>in</strong>g the logaritmic values <strong>of</strong> the PMR10 ® <strong>measurement</strong>s. The results <strong>in</strong>dicate that <strong>fruit</strong> firmness <strong>in</strong> plums measured by the two penetrometers correlates more or less well with “the two f<strong>in</strong>gers” method, while compar<strong>in</strong>g the two penetrometers showed that convert<strong>in</strong>g results from one type <strong>of</strong> <strong>measurement</strong> to another can not similarly be made on the basis <strong>of</strong> straight l<strong>in</strong>e regression fitness.
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