Evaluation of a portable impact probe for rapid assessments of flesh firmness in peaches and nectarines

Flesh firmness (FF) is traditionally measured with penetrometers, although these instruments are time-consuming; their error rate increases in soft fruit; measurements cannot be rapidly and non-destructively collected in situ before harvest; and data collection and management often require scribing in a notebook or specific computer software. We assessed the accuracy and user-friendliness of a portable impact probe to non-destructively detect FF in peach and nectarine cultivars close to harvest time. The study was carried out at the Tatura SmartFarm (Victoria, Australia) during summer 2019/20 on fruit of three nectarine and four peach cultivars (n=200 per cultivar). FF was measured with a hand-held penetrometer and with a non-invasive device based on an impact probe interfaced with a smartphone via Bluetooth. The device measured the impact probe’s acceleration profile when hitting the fruit surface and generated two measures of the bounce profile – the peak acceleration (PA) and the full width at half maximum (FWHM). The relationships of PA and FWHM with FF of peach and nectarine fruit were characterized by power functions. The Akaike Information Criterion was used to select the best models and the relative standard errors of the models were used as comparable measures of error. Both PA and FWHM showed similar robustness in their relationship with FF, although the latter demonstrated higher measurement variability in soft fruit and slightly reduced prediction ability in mature fruit than the former. Our results suggested that the impact probe was a good indicator of FF when fruit were softer and more mature, but lost accuracy with hard and unripe fruit. Hence, the impact probe used in this study is less suitable for early, pre-harvest, in situ FF determination but more appropriate for FF estimation of softer fruit in the post-harvest chain.