Use of optical coherence tomography and light microscopy for characterisation of mechanical properties and cellular level responses of ‘Centurion’ blueberries during weight loss
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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2023, Postharvest Biology and TechnologyCitation Excerpt :This study revealed not only the effectiveness of textural features for blueberry firmness evaluation but also specific details on the texture of the cells. Textural features were expected to be effective for firmness classification because (a) other studies on morphological changes of microstructures during blueberry softening implied possible modifications in image texture of the cells (Allan-Wojtas et al., 2001; Li et al., 2021), and (b) applications using spectra and texture from hyperspectral images were successful to measure firmness or soluble solids content in other fruits such as apples and grapes (Fan et al., 2016; Gao and Xu, 2022). Additionally, the results of this study also suggested that the texture highly effective for firmness classification can be defined with a distance between 16 and 128 pixels from a single-band image source (530 nm or 680 nm band images) with 20 × objective lens.
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2022, Postharvest Biology and TechnologyCitation Excerpt :Input operational settings of compression speed, strain distance, and time duration between compression cycles can influence TPA parameters such as hardness, cohesiveness, springiness, and chewiness (Alvarez et al., 2002; Rosenthal, 2010; Madieta et al., 2011; Rivera et al., 2021b). For blueberries, TPA has been previously conducted by selecting any of two compression strain distances, 15% (Rivera et al., 2021a; b; Li et al., 2021) or 30% (Chiabrando et al., 2009; Xie et al., 2018; Rivera et al., 2021b; Giongo et al., 2022). However, Rivera et al. (2021b) showed that TPA performed to 15 % strain deformation (approximately 2 mm) better differentiated mechanical differences on ‘Nui’ and ‘Rahi’ blueberries with different water loss levels.