Methods of Testing Strawberry Fruit Firmness

Abstract

Fruit firmness testing of strawberries was carried out with five methods. Four instruments, Instron, Texture Analyser, penetrometer PNR10 (SUR Berlin) and Bareiss HHP-2001-Fff, were tested. In addition "Eisenhuth's method" was tried, which involves putting the fruits into a cylinder, placing a load on the top and measuring the compression. Eisenhuth's method was found to be cheap, time-saving and easy to use, and is a promising method for firmness testing of strawberries. Bareiss is a small instrument, which was found to be easy and quick to use, and gave reliable results. The results were easily transmitted to a personal computer. Instron and Texture Analyser are good and advanced instruments with reliable results, but they are expensive, complicated and time-consuming. The PNR10 is quite easy to use but time-consuming, and the results showed large variations. Relatively large natural variability in strawberry firmness was found; therefore, the sample size should be at least 25 fruits. Strawberry fruit firmness was tested in 18 cultivars by two methods. Considerable cultivar differences were found and the ranking of the cultivars was different by the two methods. Large variations in fruit firmness and in soluble solids between individual fruits within samples were observed, especially for cv. 'Korona'. The correlations between firmness and soluble solids content were not significant. Small fruits tended to be firmer than large ones. Ripe fruits were softer than pink ones, but the difference was reduced during storage. The fruits were softer when the internal fruit temperature was 20°;C than at lower temperatures. Fruit colour measured by a Minolta Colorimeter showed smaller variations than in firmness and soluble solids. Only low correlations between colour and other fruit characteristics were found.