Use of optical coherence tomography and light microscopy for characterisation of mechanical properties and cellular level responses of ‘Centurion’ blueberries during weight loss

Postharvest blueberry softening hinders consumer acceptance and correlates with high moisture loss during storage. Such textural variations have been attributed to factors such as turgor, cell wall modifications and other microstructural changes in the outer cell layers of the fruit. This paper investigates the impact of moisture loss on blueberry quality, as well as the structure and/or function relationships associated with fruit mechanical properties during postharvest using an integrated physical and novel microstructural approach. Four different weight loss conditions [62%, 76%, 93% and 98% relative humidity (RH)] at 5.7 °C were evaluated over a three-week postharvest storage period to assess blueberry mechanical parameters using texture profile analysis (TPA), whilst simultaneously assessing microstructural changes non-destructively by optical coherence tomography (OCT) and destructively by light microscopy. Water loss affected postharvest blueberry cell morphology and resulting texture profile. Increasing weight loss resulted in a reduction in hardness, hardness slope and chewiness but an increase in cohesiveness and springiness. Blueberries stored at 98% RH also showed increasing hardness after storage, as opposed to the softening of fruit observed for the other RH conditions. The 3D OCT images revealed that the layer thickness of the top two cellular layers in the skin of blueberries increased with increasing weight loss as a result of moisture loss and had linear Pearson's correlations with TPA parameters of blueberry. Microscopy more clearly illustrated irregular cell boundaries, increased intercellular spaces, loss of cell integrity and cell collapse during moisture loss. Future work should investigate the chemical compositional changes in the epidermal and sub-epidermal layers of the blueberry stored at low water loss conditions in order to investigate the observed firming effect over the storage period. OCT has the potential to enable non-destructive real-time monitoring of the near-surface internal cellular layers of blueberries. Future work is required to validate the use of OCT as a non-destructive tool for this purpose.