Instrumental mechanical parameters related to hand-feel touch firmness of blueberries

Abstract

Hand-feel touch firmness is a sensory method reported to be used by the blueberry industry to characterise individual fruit firmness quality. A ‘soft’ blueberry may be considered unmarketable. However, firmness assessed by a hand touch is questionable due to being highly influenced by the assessor's subjective judgment. Objective instrumental methods are preferred to measure firmness and create a reliable and consistent dataset. This research aimed to study the relationship between sensory hand-feel touch firmness measured by a trained sensory panel and instrumental mechanical parameters of blueberry ‘Centurion’. To our knowledge, this is the first attempt to relate instrumental mechanical parameters to hand firmness using a sensory panel setting and reference samples to exemplify hand-feel intensity differences. Seven assessors were trained to evaluate hand touch firmness in a sensory panel setting. References samples resembling a blueberry were made of silicone and used to calibrate assessors’ performance by exemplifying four blueberry firmness scores (‘very soft’, ‘soft’, ‘firm’ and ‘very firm’). Six sensory sessions were conducted, where each session represented a harvest date and storage time combination (3 harvest dates X 2 storage times). Each sensory session considered blueberries exposed to five storage relative humidity (71 %, 88 %, 95 %, 97 % and 100 % RH). As a result, assessors were able to discriminate differences in hand touch firmness as influenced by storage relative humidity (water loss differences). In addition, mechanical parameters measured by the slope of force-deformation curve using a plate probe on a compression test (hardness slope) or a needle probe on a penetration test (skin break slope) were best related to sensory hand-feel firmness. Average hardness slope and skin break slope higher than 1.7 kN m−1 and 0.42 kN m−1, respectively, were associated with a very low (≤5 %) likelihood of observing unmarketable (‘soft’ + ‘very soft’) 'Centurion' blueberries in a batch. Our study provides a basis for the standardisation of mechanical methods that can be used to assess blueberry quality based on its ability to represent consumer sensation preferences of texture.