Effectiveness of Hexanal and its mode of action on the post-harvest quality of Banana fruits (MUSA SPP)

Abstract

Banana is one of the major fruits produced in Kenya mainly by smallholder farmers for the domestic market. Despite its importance and potential, the banana value chain is faced with several constraints including high post-harvest losses estimated to be 40%. Application of suitable and affordable post-harvest technologies is key in reducing losses and enhancing banana profitability across the value chain. The present study was conducted to assess the effectiveness of hexanal, a naturally occurring compound in prolonging the shelf-life of ‘Grand nain’ and ‘sweet banana’ varieties produced under two agro-ecological zones (AEZ) of Kenya, namely Meru County (AEZ II) and Machakos County (AEZ IV). Further, the study sought to validate the effect of hexanal treatments on the fruits’ postharvest quality and elucidate the molecular basis of its mode of action. To realize these objectives, three separate but related experiments were conducted between July 2016 and May 2018. In the first objective, the best method of hexanal application (pre-harvest spray and post-harvest dip); the effective concentration (2% and 3%); and the effective treatment duration (2.5 minutes or 5 minutes) were evaluated. The treated and untreated fruits were compared for postharvest longevity based on ripening-related changes under ambient room conditions (25 ± 1oC and RH 60 ± 5%). The ripening changes determined at regular intervals from randomly sampled fruits included rate of respiration, rate of ethylene production, percentage cumulative weight loss, peel color, peel and pulp firmness. A Complete Randomized Design with factorial arrangements was used for the shelf life studies. In the second objective, the best treatments in experiment one were selected for analysis of the effect of hexanal treatment on selected quality attributes including Total Soluble Solids (TSS), Total Titratable Acidity (TTA), Vitamin C and simple sugars (fructose, glucose and sucrose). The TSS level was determined using a digital handheld refractometer while vitamin C and simple sugars were analyzed using the high performance liquid chromatography (HPLC). The TTA content was determined by titration. Sensory evaluation was conducted using untrained panelists to ascertain consumer perception viz a viz machine-determined quality attributes. In the third objective, mature-green ‘Grand nain’ banana fruits were treated with either hexanal, ethylene, 1-Methylcyclopropene (1-MCP), a combination of 1-MCP and hexanal or left untreated (control). After the treatments, all the fruits were left to undergo ripening under ambient room conditions. Three fruits from each treatment combination were randomly sampled at two-day interval for analysis of selected physical (peel color, peel and pulp firmness) and physiological parameters (rate of respiration and ethylene production). The middle portion of both the peel and the pulp of each sampled fruit was diced and quickly frozen in liquid nitrogen and stored at -80oC for quantitative polymerase chain reaction analysis (qPCR) and sequencing using next generation sequencing technique. Experiment one results showed that hexanal applied twice as a pre-harvest spray significantly (p < 0.05) delayed time to fruit harvesting by 12 days and 18 days in ‘Grand nain’ variety produced in AEZ IV and AEZ II, respectively and 12 days in ‘sweet banana’ variety in both zones compared to the untreated fruits. Effectiveness of hexanal was significantly affected by duration of application with the post-harvest dip for 5 minutes enhancing fruit shelf life by 9 and 6 days compared to 6 and 3 days in fruits dipped for 2.5 minutes in ‘Grand nain’ and ‘sweet banana’ variety respectively. In the second objective experiments, the results showed that hexanal significantly reduced the rate of increase of the various quality attribute parameters analyzed. The TSS levels in both varieties and AEZs, were not significantly affected by hexanal treatment. However, variety had a significant effect on the TSS levels with ‘sweet banana’ having higher levels (31 -33.8 o brix) compared to ‘Grand nain’ fruits (27-30o brix) regardless of hexanal treatment and AEZs. Hexanal treated fruits maintained relatively higher vitamin C levels throughout the storage period in both AEZs compared to the untreated controls. ‘Grand nain’ fruits dipped in hexanal had lost only 49 % and 22 % of the initial vitamin C content in fruits from AEZ IV and AEZ II respectively, 9 days later compared to 54 % and 51 % in the untreated controls. Hexanal treatment significantly delayed the rate of increase of simple sugars in both varieties and AEZs compared to the untreated ones. Results of sensory evaluation showed no significant differences in the various quality attributes scored between the treated and untreated control fruits. Objective three results showed that, hexanal and 1-MCP treatments suppressed the expression of various cell wall genes such as xyloglucan endotransglucosylase, Polygalacturonase, Pectin Lyase and Pectin Methylesterase as well as 1-Aminocyclopropane-1-Carboxylic Acid Oxidase by 4- 6 days and 6-10 days respectively compared to fruits treated with ethylene and the controls. However, later expression of these genes occurred 2- 6 days earlier in the hexanal treated fruits as compared to those treated with either 1-MCP or a combination of 1-MCP and hexanal. Sequencing results showed that hexanal treatment suppressed the expression of phospholipase D genes, xyloglucan endotransglucosylase, Polygalacturonase and 1-aminocyclopropane-1-carboxylate synthase by a fold change of 3.3 – 6.7 in day four of storage contrary to the observed induction of the same genes in fruits treated with ethylene. The best results were obtained from hexanal applied at a concentration of 2% or 3% as either a pre-harvest spray at 30 days and 15 days before harvest or as a post-harvest dip for 5 minutes. These results indicate that hexanal technology is effective to delay ripening and extend the shelf-life of banana fruit without affecting the quality attributes of the fruit. These results further suggest that hexanal mode of action may be through temporal suppression of genes involved in the banana ripening process. The technology should be promoted for adoption by banana farmers and traders in Kenya to prolong the post-harvest shelf-life thereby extending the marketing period and reduce postharvest losses in the banana value chain.