Identification of mechanical parameters to be used as a firmness standard on quality evaluations of stored blueberry : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Manawatu, New Zealand

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Massey University
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Blueberry firmness is considered a relevant quality variable influencing consumer acceptability of fresh blueberries. However, the blueberry supply chain and research community have not yet adopted a standard method to measure firmness on postharvest quality evaluations. This thesis has focused on characterising the mechanical properties of blueberry ‘Nui’ and ‘Rahi’ as influenced by different factors such as storage relative humidity (i.e., fruit water loss), controlled atmosphere and harvest maturity. The mechanical parameters were obtained by using the instrumental methods of texture profile analysis (TPA) equipped with a flat plate and the penetration test equipped with a 0.39 mm round tip diameter needle probe. Mechanical parameters of hardness slope (BHS, also known as chord stiffness) of TPA and displacement at skin break (DSk) of the penetration test can be used to track water loss changes in stored blueberries. The DSk and BHS can also accurately detect quality changes induced by controlled atmosphere storage. In addition, BHS can detect maturity differences in stored blueberries, but the force at skin break (FSk) provides better detection of maturity differences at harvest evaluations. To demonstrate the relevance of chord stiffness evaluations at a commercial level, sensory evaluation of texture of hand-touch firmness using a formal sensory panel setting and trained assessors was related to instrumental mechanical parameters. Chord stiffness measured as BHS using a flat plate compression and skin break slope (SSk) measured using a needle probe were strongly related to consumer sensory perception of hand-touch firmness. A blueberry batch with an average BHS ≤0.47 kN m⁻¹ or SSk ≤0.13 kN m⁻¹ was associated with a very high likelihood of unmarketable berries (i.e., berries are ‘soft’ or ‘very soft’). In summary, BHS was an informative parameter of blueberry quality across factors inducing the textural changes and providing commercially relevant information about consumer acceptability. These results can assist the development of a standard instrumental method to measure postharvest firmness on blueberry quality evaluations for research and commercial purposes. Further studies should focus on validating the feasibility of BHS to determine blueberry quality across other sources of textural variation, such as calcium and ethylene-related treatments. In addition, threshold values for mechanical parameters related to consumer acceptance (sensory analysis) may be identified across an extensive range of blueberry genotypes and using other sensory descriptors also relevant to the consumers, such as crispness. Finally, this research identifies alternative areas for further studies, such as the blueberry firming (an increase of firmness during storage) occurring consistently on blueberries ‘Nui’ stored under high RH in regular air or a controlled atmosphere of 5 kPa CO₂ + 4 kPa O₂.
Blueberries, Mechanical properties, Postharvest physiology, Quality, Storage