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

https://doi.org/10.1016/j.jfoodeng.2021.110596Get rights and content

Highlights

  • Water loss affected postharvest blueberry cell morphology and resulting texture profile.

  • Water loss reduced hardness and chewiness but increased cohesiveness and springiness.

  • Near-surface cell layer thickness can be quantified using OCT non-destructively.

  • Layer thickness had linear Pearson's correlations with TPA parameters of blueberry.

  • Microscopy showed increased size, irregular shape and collapse of cells during water loss.

Abstract

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.

Introduction

Blueberries originate from North America and belong to the genus Vaccinium (Eck et al., 1990). Species of major economic importance include rabbiteye (V. ashei Reade), lowbush (V. angustifolium Aiton), and highbush (V. corymbosum L) blueberry (Hancock et al., 1996). Increasing demand for blueberries worldwide resulting from high nutritional value and health benefits has resulted in stability of high prices within the fresh and processed blueberry market. As a result, the worldwide growth in metric tonnes produced increased by 139% from 2008 to 2016 (Brazelton and Young, 2017). The total production of blueberry in 2018 was over 600,000 tonnes with most of the fruit harvested in the North American region (FAOSTAT, 2018).

Fresh blueberries rapidly deteriorate as a result of decay, shrivel and softening (Forney, 2008). A large number of cultivars are available to growers, with large variability in growing conditions and postharvest behaviour (Strik, 2007). Highbush cultivars have been reported to have larger stem scars than rabbiteye cultivars and tend to soften faster during the postharvest chain (Makus and Morris, 1993). Fresh berries have a limited shelf life of up to 1 month, with shrivel, extensive softening, and rots being the main postharvest quality issues. Extending storage life enables year-round availability and large volume of export to distant markets. It is recommended that postharvest temperature throughout the supply chain remain between 0 and 5 °C to maintain commercial quality (Kader, 2001; Mitcham et al., 2002; Perkins-Veazie, 2004).

Relative humidity (RH) affects the rate of moisture loss from blueberries and consequently quality variables such as fruit shrivel and softening. By storing blueberry fruit at 90–95% RH acceptable berry quality can be maintained for up to 2 and 4 weeks in highbush and rabbiteye cultivars respectively (Kader, 2001; Perkins-Veazie, 2004). Paniagua et al. (2013) demonstrated that moisture loss measured as weight loss has a large influence on blueberry firmness (i.e. maximum force to 1 mm deformation). Storage in low RH conditions at 4 °C resulted in 15.06% weight loss and extensive berry softening, while high RH postharvest conditions help to prevent fruit moisture loss and maintain texture characteristics at suitable consumer acceptability. Similarly, a reduction in the maximum force to berry deformation when storage weight loss was higher than 2–4% was reported for ‘Bonita’ blueberries compressed to 1 mm deformation (Ferraz et al., 2000), and for ‘Bluecrop’ and ‘Sierra’ blueberries compressed to 75% deformation (Liu et al., 2019). Contrarily, Chiabrando et al. (2009) found a weight loss of approximately 6.2% after 35 days (d) storage in 90–95% RH at 0 °C resulted in an increase in the maximum force to 30% deformation (i.e. hardness) and a decrease in cohesiveness, springiness and resilience for ‘Coville’ blueberries.

Different texture responses obtained under different conditions of blueberry moisture loss implies a question of causality that still needs to be addressed. Forney et al. (1998) proposed reduced turgor as a result of blueberry moisture loss was related to the softening of fresh fruit, however, no direct experimental evidence has been reported in the literature to support this hypothesis. Increased firming of blueberries has also been observed regularly at low weight loss levels of 1–2% (Miller et al., 1993; Forney et al., 1998; Duarte et al., 2009; Chiabrando and Giacalone, 2011; Paniagua, 2012). This firming has been proposed to be the thickening of parenchyma cell walls (Allan-Wojtas et al., 2001) and the corrugation of epidermal cell walls (Bünemann et al., 1957) observed microscopically, although the thickening of parenchyma cell walls was not observed in the following blueberry season (Allan-Wojtas et al., 2001).

The objective of this research is to evaluate the relationship between moisture loss and the resulting mechanical properties and observed cellular changes on stored ‘Centurion’ blueberries. Cellular structural changes between blueberries treated at different humidity conditions stored for different periods up to three weeks are monitored first non-destructively using optical coherence tomography (OCT), and then destructively using light microscopy. Originally developed for medical applications, OCT is a non-contact, non-invasive and real-time technique capable of assessing the near-surface microstructures of plant tissue. It has been increasingly used in the assessment of horticultural products such as apples (Verboven et al., 2013) and kiwifruit (Li et al., 2015, 2016). The feasibility of OCT and microscopy to determine microstructural differences in terms of the size, area and number of large parenchyma cells, will be investigated. Comparison amongst storage conditions and storage period may provide an explanation of the possible quality outcomes under manipulated conditions and enable control practices to be implemented during the postharvest supply chain.

Section snippets

Fruit sourcing

Two kilograms of late-season ‘Centurion’ rabbiteye blueberries (Vaccinium ashei Reade) were obtained from Omaha Organic Blueberries located near Matakana, Auckland, New Zealand and collected from a single block with homogenous plant conditions and common orchard management (i.e. irrigation, nutrition, pest control and pruning). Fully mature berries were hand-harvested on February 21, 2018 by farm pickers according to commercial practises and normal harvest index (i.e. 100 % blue surface

Relationship between weight loss and mechanical properties

Weight loss increased during storage for all treatments with increasing air flow rates (i.e. reducing RH). The cumulated values of weight loss after 3 week's storage (Fig. 4) were 0% (98% RH, 0 L s−1), 6.01% (93% RH, 2.5 × 10−4 L s−1), 11.14% (76% RH, 5 × 10−4 L s−1) and 14.67% (62% RH, 10−3 L s−1). Paniagua et al. (2013) recorded very similar weight loss values (between 1 and 15% after 3 weeks storage) using the same air flow treatments for “Centurion” blueberries, and claimed that these

Conclusions and recommendations

This work adds to the growing body of knowledge on the causes and mechanisms for blueberry softening. A high correlation between weight loss and TPA parameters were obtained for blueberries stored under different humidity conditions. High weight loss conditions resulted in a reduction in hardness, hardness slope and chewiness but an increase in cohesiveness and springiness. The cellular microstructural changes in the near skin regions of blueberries resulting from water loss during storage were

Credit author statement

Mo Li:Conceptulisation, Methodology, Formal analysis, Resources, Writing – Original draft, Writing – Review & Editing, Supervision, Project administration, Funding acquisition. Sebastian Rivera: Methodology, Formal analysis, Writing – Review & Editing, Visualisation. Deena Franklin: Investigation, Formal analysis, Writing – Original draft, Visualisation. Emilia Nowak: Writing – Review & Editing, Supervision. Ian Hallett: Resources, Writing – Review & Editing, Supervision. Sylwia Kolenderska:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank Omaha Organic Blueberries, Auckland for their help in sourcing of the fruit, and the PG Support from the School of Food and Advanced Technology, Massey University, Palmerston North for providing funding for the experimental work of this study. The authors also acknowledge technicians, Peter Jeffery and Kenneth Teh from the School of Food and Advanced Technology, Massey University for their generous help that contributed to the experimental work of this study.

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