Effect of penetration speed on flesh firmness measured on stored kiwifruit

The effect of penetration speed on flesh firmness (FF) measurement by motorised penetrometer was examined for 'Hayward' (Actinidia deliciosa var. deliciosa) and 'Hort16A' (Actinidia chinensis Planch. var. chinensis) kiwifruit. Data was collected for penetration speeds varying from 4 to 40 mm s(-1) using stored fruit of FF similar to 10 N; a typical minimum FF threshold for export from New Zealand. Measurements were made on a number of instruments (Instron, CUSS FTA, HortPlus. TA.XTplus), using fruit from different orchards and in each of two different seasons. As expected, FF values increased with increasing penetration speed. A firmness-speed model was developed, based on the Maxwell theological model for viscoelastic materials, which proved adequate in describing the FF data in terms of the effect of penetration speed. The effect of penetration speed was not adversely influenced by cultivar, season or instrument type. Within the range of fruit firmness examined - stored fruit below 20N - it was concluded that the firmness-speed model could be successfully used to compare firmness values generated using instruments operating at different penetration speeds. (C) 2011 Elsevier BM. All rights reserved.