The effect of layer-by-layer edible coating on the shelf life and transcriptome of ‘Kosui’ Japanese pear fruit
Introduction
Japanese pear (Pyrus pyrifolia Nakai) comprises both climacteric and non-climacteric fruit cultivars (Itai et al., 1999). The main cultivar, however, is ‘Kosui’ which occupies approximately 40 % of the total pear cultivated area in Japan (Ministry of Agriculture, Forestry and Fisheries, 2015), primarily due to its attractive crunchiness, juiciness, and high sugar levels. Fruit of ‘Kosui’ cultivar produce moderate levels of ethylene during ripening (Itai et al., 1999), and this climacteric phenotype predisposes them to rapid postharvest softening which eventually reduces shelf life. As ‘Kosui’ fruit are usually harvested at the pre-climacteric stage, technologies that retard postharvest endogenous ethylene production and subsequently, fruit ripening would improve their storage potential.
Edible coatings offer an eco-friendly technology to control and preserve fruit quality along the postharvest value chain (Baldwin et al., 2011). The technology simply involves deposition of thin layer(s) of natural edible substances such as polysaccharides, proteins, lipids, and resins on surfaces of crops (Mohamed et al., 2020). Such coatings act as physical barriers that decrease permeability of the fruit surface to oxygen, carbon dioxide, and water vapor, leading to low rates of respiration and evapotranspiration, and eventually to retardation of the natural physiological ripening process. Furthermore, edible coatings are known to offer additional benefits such as protection against mechanical and microbial damage, provision of aesthetic appearance and prevention of desirable aroma volatiles from escaping (Baldwin et al., 2011). Polysaccharide-based coatings, particularly chitosan and alginate, are attracting interest because they are cheap, readily available, highly biodegradable, and easy to apply. In this sense, chitosan-based edible coatings have been widely shown to suppress ethylene production and respiration rates, and thereby delay fruit ripening processes such as peel color change and fruit softening in tomato (Won et al., 2018), mangoes (Jongsri et al., 2016), and apples (Gardesh et al., 2016). Alginate-based coatings have also been shown to maintain postharvest quality in various fruit species including tomato (Salas-Méndez et al., 2019), plums (Valero et al., 2013) and peaches (Li et al., 2019).
Despite the above-mentioned benefits of edible coatings towards maintenance of postharvest quality, practical application of monolayers of each material is constrained by such factors as poor adhesion ability, uneven distribution on fruit surfaces and low water vapor permeability (Poverenov et al., 2014), which reduce performance. Thus, rationally designed multicomponent edible coatings are being sought after to improve the overall performance of edible coatings (Falguera et al., 2011). The layer-by-layer (LBL) electrostatic deposition originated in materials science (Decher, 1997), and is one of the techniques employing thin multi-layers to enhance the performance of edible coatings. The LBL technique involves alternate deposition of oppositely charged polyelectrolytes onto fruit surface, which may result in the efficient control of coating properties and functionality. Indeed, improved protection and shelf extension has been achieved by using the LBL approach based on chitosan/pectin (Brasil et al., 2012; Medeiros et al., 2012) or chitosan/alginate (Poverenov et al., 2014; Souza et al., 2015; Nair et al., 2020) alternate multilayers.
The mechanism of action and synergistic effects of individual materials used in LBL edible coatings, which result in the curbing of postharvest deterioration, have been well elaborated in previous studies. For instance, LBL edible coatings have been found to act as oxygen and water vapor barriers, possess both antioxidant and antimicrobial activities, and inhibit respiration, ethylene production and fruit ripening (Nair et al., 2020). LBL edible coatings were also found to influence the overall metabolic process and fruit/vegetable quality attributes such as texture, metabolite profile and antioxidant potential (Yan et al., 2019; Maringgal et al., 2020). However, there is relatively little information regarding the effect that LBL coatings might have on the overall fruit transcriptome, which might shed light on the underlying molecular bases and subsequently allow for more precise maintenance of postharvest quality. In the present study, we attempted to examine the transcriptional changes that are associated with prolonged shelf life in ‘Kosui’ Japanese pear fruit following postharvest application of chitosan- and alginate-based LBL edible coatings.
Section snippets
Plant material
‘Kosui’ pear fruit at a pre-climacteric stage were obtained from a commercial market in Okayama city, Japan. Careful sorting was then carried out to ensure uniform size, color, and shape as well as the absence of defects and mechanical damage. All fruit were washed with deionized water before dividing them into groups based on the different treatments and sampling points.
Preparation of edible coating solutions
Medium molecular weight chitosan (190–310 kDa) was obtained from Sigma-Aldrich (St. Louis, MO, USA). All the other chemicals
Postharvest quality and physiological characteristics of ‘Kosui’ pears
Both coated and uncoated ‘Kosui’ pear fruit depicted a steady increase in weight loss during the entire storage period (Fig. 1). Notably, there were little or no meaningful differences in weight loss among the uncoated, chitosan or alginate monolayer-coated, and LBL-coated fruit groups.
The respiration rate (calculated as CO2 production rate) increased during storage in uncoated fruit (Fig. 2A), signifying the climacteric rise typically displayed by the climacteric fruit group to which ‘Kosui’
Utility of chitosan-alginate LBL edible coating to extend shelf life of ‘Kosui’ pears
Edible coatings are gaining great interest lately as a safe, eco-friendly, and effective technology for controlling and preserving the postharvest quality of fruit and vegetables (Baldwin et al., 2011; Nair et al., 2020). Thus far, different edible coating formulations have been used successfully to maintain fruit quality, and thus extend shelf life in various fruit species, such as chitosan in apples (Gardesh et al., 2016), alginate in plums (Valero et al., 2013), and cross-linked starch in
Conclusions
The chitosan/alginate based LBL coatings developed in the present study displayed superior properties than monolayer coatings in terms of inhibition of ethylene production, respiration, and overall fruit ripening in ‘Kosui’ fruit, most likely due to low oxygen permeability. The LBL coatings, however, had little effect on water vapor permeability and hence did not affect weight loss during postharvest storage. By transcriptome analysis, we demonstrated that improved shelf life in ‘Kosui’ pears
Author contributions
NH and YK designed the project. RO and MS performed most of the experiments under the supervision of YK. KM, NF, and KU performed data analysis of RNA-Seq including clustering, GO and KEGG analysis. OWM and TA took part in data interpretation and writing of the manuscript. All authors have read and approved the final version of the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing interests.
Acknowledgement
This study was supported in part by a Grant-in-Aid for Scientific Research (grant no. 20H02977) by the Japan Society for the Promotion of Science, Japan (JSPS) and a joint research project with NACOSTI, Kenya and JSPS.
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