Impact of electrolyzed water treatments on the physiological responses, phytonutrients, and overall quality of ‘Granny Smith’ apples

Abstract

‘Granny Smith’ apples are the main export of pome fruit in South Africa. The fruit are stored for an extended period under normal or controlled atmosphere conditions to take advantage of main export markets such as the United Kingdom (UK) and the European Union (EU). However, during this time the apples are susceptible to various post-harvest pathological disorders such as grey and blue mould, which are mainly due to Botrytis cinerea and Penicillium expansum, respectively. In addition, ‘Granny Smith’ apples are susceptible to various storage physiological disorders such as superficial scald. Current commercial practice to control pathological disorders has been the use of chemical fungicides. However, several fungicides are not used for post-harvest treatment or have been removed from the market due to possible toxicological risks to humans and the environment. Most pack houses in South Africa use a chlorine-based solution prepared commercially from sodium hypochlorite (NaOCl) with concentrations ranging from 5 mg L-1 - 200 mg L-1 as a decontamination agent for washing the surfaces of fresh produce including apples and has been authorized for use by the fresh produce industry. However, the reactivity of the chlorine species present in chlorine-based sanitizers could be different than compared to other chlorine-based sanitizers because of the reactive and complex nature of chlorine. In addition, the produce industry has raised concerns regarding the additional regulatory barriers, limitations and safety, and regulations for the use of chlorine in its present form. This has therefore made it necessary to urgently investigate alternative non-chemical, safe, environmentally- friendly, and low-cost post-harvest treatment strategies to control the incidence of pathological and physiological disorders in apple fruit. Electrolyzed water (EW) is an emerging hurdle technique and has excellent antimicrobial properties against several microbial pathogens and it is gaining popularity in the food industry. In addition, it has no adverse impacts to the environment. Therefore, the overall aim of this study was to investigate the impact of electrolyzed water treatments on the physicochemical and biochemical quality attributes, as well as changes in phytonutrients and natural microbial load of ‘Granny Smith’ apples. Electrolysed water (EW) was effective as a curative agent against B. cinerea and P. expansum. The slightly alkaline electrolysed water (SAI-EW) treatments against B. cinerea resulted in significant (p ≤ 0.05) reduction in lesion zones of decay across all concentrations in comparison to the control stored up 21 days at 5 °C plus two days of accelerated storage at 24 °C. The curative efficacy of the treatments was most effective at the highest concentration of 500 mg L-1, followed by 400 mg L-1, and 300 mg L-1 for treated apples. The acidic electrolysed water treatments (AEW) against P. expansum and B. cinerea resulted in a significant (p <0.05) reduction in lesion zones compared to the control samples stored up to 9 days at 15 °C. The AEW curative efficacy was most effective at 300 and 200 mg L-1. ’Granny Smith’ apples were further treated with alkaline electrolysed water (AIEW) and compared to sodium hypochlorite at the current industry standard of 200 mg L-1. The impact of these treatments on the overall fruit quality, physiological disorder (superficial scald) and microbial load was investigated. Apples treated with AIEW maintained low pH, titratable acidity (TA) and total soluble solids (TSS) compared to other treatments (p < 0.05). The interaction of treatments and storage duration had a significant impact on total polyphenols and total flavonoids (p ≤ 0.05). At the end of storage day 21, AIEW treated apples better maintained the antioxidant capacity compared to control and sodium hypochlorite (NaOCl, 200 mg L-1) (p ≤ 0.05). Treatments with AIEW and sodium hypochlorite had no effect on scald incidence. Treatments with AIEW resulted in ≈ 2 Log reduction in total aerobic mesophilic bacteria (from 4.1 Log CFU cm2 to 2.2 Log CFU cm-2) and < 1 Log reduction for yeast and mould (from 3.9 Log CFU cm2 to 2.7 Log CFU cm2) count. At the end of storage, AIEW treated apples (with 200 mg L-1 for 15 min) maintained the lowest total aerobic mesophilic bacteria and yeast and mould count compared to the control samples. Based on the microbial analysis it was expedient to further characterize the microbial community isolated from the AIEW treated and non-treated ‘Granny Smith’ apples during 21 days post-harvest storage. Based on initial morphological identification about (87) pure colonies (56 possible bacteria and 31 possible yeasts) were isolated across all treated samples from 0 to 21 post-harvest storage. Out of the total 87 isolates, 56 isolates were identified as bacteria via the enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR), and only 27 out of these isolates resulted in genetically diverse bacteria populations. Based on post-harvest treatment selection, a total of eight different dominant bacteria (Staphylococcus epidermidis, S. capitis, Erwinia aphidicola, Enterobacter bugandensis, Curtobacterium flaccumfaciens, Pseudomonas graminis, Ochrobactrum soli and Pantoea agglomerans) were identified on the surface of treated and non-treated apples via the 16S rDNA. In addition, based on random amplification of polymorphic DNA (RAPD-PCR) using primers 1283, a total of ten fungal isolates were tentatively identified. However, only five out of the ten fungal isolates could be amplified via their ITS1 and ITS4 primers within the 100-1500 bp region. Identified fungi included Rhodotorula nothofagi, Aspergillus inuii, Debaryomyces hansenii and Phialemoniosis curvata, which predominated in different treatments.

Results obtained in this study suggest that SAI-EW and AEW can be an effective alternative for the post-harvest management of B. cinerea and P. expansum during the storage of ‘Granny Smith’ apples. Due to greater susceptibility to superficial scald, prevention measures such as treatment with 1-Methylcyclopropene (1-MCP) could be applied after EW treatment for long term storage. The overall desirable effects described in this study provides a guiding tool for the fruit industry on the postharvest sanitation alternative for deciduous fruits. The outcome provides an alternative to chlorine-based sanitisers.