Impacts of Electrolyzed Water Treatments on Bioactive Compounds and Microbial and Visual Quality of Minimally Processed ‘Granny Smith’ Apples

Abstract

Ready-to-eat fresh-cut apples deteriorate rapidly in visual quality due to browning, leading to consumer rejection and food waste. In addition, minimal processing induces tissue damage and releases organic substrates, which could accelerate microbial growth. The present study evaluated the impacts of alkaline and acidic electrolyzed water (AIEW and AEW) on natural microbial load and bioactive compounds on fresh-cut ‘Granny Smith’ apples. Minimally processed apples were dipped for 10 min in AEW and AIEW solutions (200 mg L⁻¹), packed in PET containers with lids, and stored for 9 days at 2 °C. Overall, fresh-cut ‘Granny Smith’ apples treated with AEW significantly (p < 0.05) maintained higher total phenolics (99.4 ± 4.3 mg GAE L⁻¹) and antioxidant capacity (79.5 ± 6.5 mg VitCE L⁻¹) compared to the non-treated control samples (42.9 ± 5.1 mg GAE L⁻¹, 31.9 ± 8.1 mg GAE L⁻¹, respectively). Similarly, pretreatment with AIEW maintained the highest total flavonol content (55.71 ± 1.5 mg QE L⁻¹) compared to the AEW-treated samples and control (p < 0.05). AEW pretreatment led to a 2 Log and a 1 Log decline in total aerobic mesophilic bacteria and yeasts and moulds, respectively. The best visual quality and highest visual score was maintained by AEW and followed by AIEW. This study further demonstrated the effectiveness of electrolyzed water treatments in minimizing browning and enhancing bioactive compounds in fresh-cut ‘Granny Smith’ apples.

1. Introduction

Globally, within the last ten (10) years, fresh-cut fruits have gained popularity with consumers for being ready-to-eat, providing convenience, and being highly nutritious with health-benefiting compounds. The increasing consumption of these kinds of fruits can be attributed to the shifts in consumers’ lifestyles and their preference for new and healthy natural products, as well as advancements in processing [1]. Apples rank high among the most popular fruits and are a significant source of organic acids, phenolic compounds, dietary fibre, vitamins, and starch content as well as micro- and macronutrients [2,3,4]. When apples undergo minimal processing, they encounter various operation steps including sanitizing, peeling, shredding, cutting, and packaging. These processes result in fruit tissue disruption, increasing free organic substrates and accelerating the susceptibility to microbial deterioration, which in turn shortens the produce shelf life [5]. Due to ethylene generation, enzymatic and non-enzymatic browning, respiratory activity, and nutritional release from cells that are activated by injuries, fresh-cut apples shelf life is limited to a maximum of 7 days [5,6,7]. Furthermore, quality loss in fresh-cut apples could be attributed to physical injury, water loss, and microbiological decay. Tissue softening, surface browning and discolorations, reduced nutritional content, texture loss, translucency, exudation, formation of an off-flavour and off-odour, and microbial proliferation are the characteristics of the ensuing spoilage [8].

Over the years, to prevent browning, agents such as calcium sorbate and ascorbic acid have commonly been used by the industry [9]. Nevertheless, extended usage of these solutions may cause them to get contaminated with microorganisms, which facilitates the growth of bacterial pathogens. Chlorine-based solutions with hypochlorite as the active agent (between 50 and 200 mg L⁻¹) are generally suggested for pretreating whole and minimally processed fruits. However, hypochlorite- or chlorine-based solutions bind to organic matter in the solution to form disinfectant by-products, which also diminishes the available active chlorine concentration. This could result in the sanitizing solutions needing to be changed every two or three uses [10] or repeated over-dosing with chlorine leading to microbial resistance [11,12,13]. Because of these safety concerns and the quality deterioration of fresh-cut products, there is an urgent need for alternative, environmentally friendly pretreatments such as the electrolyzed water treatment to develop safe, and highly nutritional.

Considerable research on electrolyzed water (EW) as a microbial decontaminant or disinfectant for fresh-cut fruits has been conducted. For example, Lopes et al. [14] found that treating fresh-cut mangoes with various doses (0, 75, 150, 225, and 300 mg L⁻¹) of neutral electrolyzed water (NEW) effectively inactivated bacteria, coliforms, yeasts, and moulds. Salmonella enterica, Escherichia coli, and Listeria spp. levels declined on fresh-cut ‘Rocha’ pears when treated with AEW and NEW, according to Graça et al. [15]. Furthermore, without sacrificing any nutrients or antioxidant activity, a 40.2% decrease in S. Typhimurium was observed in fresh-cut red cabbage treated with AEW [16]. Moreover, it was discovered that utilizing AEW and NEW rather than a sodium hypochlorite (SH) solution had a higher impact on yeasts, namely Pichia fermentans, Metschnikowia pulcherrima, Candida sake, and Hanseniaspora uvaram, for fresh-cut ‘Royal’ apples [17]. However, the role of electrolyzed water as an anti-browning and/or antimicrobial agent has not been considerably explored or fully understood.

Moreover, fruit quality requirements for fresh-cut apples are different from those of the fresh market. The soluble-solids-to-acid ratio is often regarded as a significant factor in apple flavour [1,4,9]. ‘Granny Smith’ apples have uniquely vibrant green mesocarp and a crispy texture, with a total-soluble-solid-to-acid ratio that has a varying degree of tartness (sweetness and acidity). Furthermore, ‘Granny Smith’ apples are one of the most commercially available globally with desired culinary attributes for ready-to-eat fresh-cut mixes and for baking or cooking [3,4]. Hence, ‘Granny Smith’ apples were selected for this work.

Consequently, the presumption that guided our investigation was that pretreatment with AEW and AIEW would boost antioxidant properties, induce the concentration of polyphenols, retain visual quality appearance, and significantly reduce the microbial load of fresh-cut ‘Granny Smith’ apples during cold storage. In this context, the study objectives were to evaluate the impact of AEW and AIEW as pretreatments: (i) on the phytonutrients (antioxidant activity, phenolics, and total flavonols); and (ii) the efficacy in reducing natural microbial load in fresh-cut ‘Granny Smith’ apples.

2. Materials and Methods

2.1. Plant Material

Apples (Malus domestica, ‘Granny Smith’) were harvested from the Agricultural Research Council (ARC) Elgin Research Farm, Grabouw, South Africa at commercial maturity based on an average (n = 15) weight of 250 ± 2.85 g, 10.9% total soluble solids (TSS), and 0.89 g 100 mL⁻¹ titratable acidity (TA, malic acid). After being soaked with tap water to get rid of dirt and organic matter, the bulk harvest was allowed to air dry under pilot-scale pack house conditions. The fruit was subsequently driven to the Agri-Food Systems and Omics Laboratory, ARC Infruitec-Nietvoorbij, Stellenbosch, South Africa, in well-refrigerated trucks, under cool (4 °C) conditions. Upon arrival, fruit that had been mechanically damaged was separated from the batches. Fruit without apparent wounds or rots was kept at 0.5 °C and 95% RH under regular atmosphere conditions.

2.2. Electrolyzed Water Generation

The ELA-12 000ANW system (ECA Technologies, Envirolyte, Pretoria, South Africa) was used to generate acidic (AEW) and alkaline (AEW) electrolyzed water. The electrolyzed water was generated via electrolysis of hydrochloric acid in the range of 0.05%, sodium chloride (0.26%), and water (99.69%). The electrolyte flow passed through an electrolytic cell at a rate of 2 mL min⁻¹, at a setting current of 3.8–3.9 V and amperage 10 A. The pH and oxidation reduction potential (ORP) were measured immediately after preparation and right before experiments to confirm that they were not significantly changed using a pH meter (D-22, Horiba, Kyoto, Japan) and ORP meter (HM-60V, TOA Electronics Ltd., Tokyo, Japan), respectively. The EW was setup with an initial free available chlorine concentration (ACC) of 200 mg L⁻¹, and the set pH and ORP for AEW was a pH of 2.0–3.0 and an ORP of >750 mV; while the AIEW was pH = 11–13 and ORP > −900 mV. Descriptions of treatments used in this experiment are shown in

Table 1. Electrolyzed water concentration used to treat fresh-cut ‘Granny Smith’ apple fruit.

Treatment(s)
Active Compound(s)	Concentration (mg L−1)	Dipping Duration (min)	Abbreviation(s)
KCl	200	10	AEW
NaCl	200	10	AIEW
Non-treated	-	-	Control

AIEW (alkaline electrolysed water), AEW (acidic electrolysed water).

.

2.3. Fruit Preparation and Treatment

‘Granny Smith’ apples with 10.9% TSS, 0.89 g 100 mL⁻¹ TA, malic acid, and with an average whole fruit size of 7.3 ± 0.45 cm diameter were cut/sliced into 6 sections using an electric slicer (ARC-Intrude, Stellenbosch, South Africa). The bulk sliced apples were then divided into three batches based on the treatments, and each batch was dipped for 10 min into the respective test solutions with ACC 200 mg L⁻¹: i.e., AEW (pH = 2.0–3.0), AIEW (200 mg L⁻¹, pH = 11–13), and the control samples (C) were not treated. After dipping, the batches were drained, and samples (≈100 g) were packed in 84 mm × 55 mm PET containers with lids and stored at 2 °C (optimum storage) for 9 days. PET packaging materials were chosen as sustainable materials for their strength and toughness, as well as being 100% recyclable in South Africa. Packed samples were taken out in triplicate per treatment for analysis at regular interval on days 0, 3, 6, and 9.

2.4. Visual Quality Assessment

To assess the impacts of AEW and AIEW on the development of browning and appearance, a visual quality assessment was conducted on minimally processed ‘Granny Smith’ apples. The evaluation was conducted by untrained panellists (n = 5) who frequently consume whole and freshly cut apples and who are familiar with the desirable quality attributes of ‘Granny Smith’ apples. Moreover, each sample day included taking high quality photographs, and the individual apple slices’ fruit surface were inspected for the following: (i) appearance of dryness; (ii) changes in colour; and (iii) appearance of deterioration. Visual assessment scores ranged from 1 to 5; where 5 = 0–20%, 4 = 21–40%, 3 = 41–60%, 2 = 61–80%, and 1 = 81–100% for fruit that showed total colour change and dryness. Appearance of decay was calculated as percentage of total fruit pieces. Visual assessment was observed on day 0, 3, 6, and 9.

2.5. Bioactive Compounds

2.5.1. Extraction Preparation

A 1 g tissue portion was cut from each apple slice at the exterior of the core area to just below the skin at a location halfway from the calyx to the core. Samples were then promptly stored until the analysis at −18 °C. On the day of analysis, the frozen tissue was homogenized with cold methanol (4 °C). The samples were subsequently centrifuged at 5 000 rpm and 4 °C after being allowed to thaw at ambient temperature (20 °C). Following filtration of the samples and dilution of the solution with distilled water, the supernatants were carefully added to the appropriate extracts in the pill vials.

2.5.2. Total Phenolics

To quantify total phenolic content (TPC), the Folin–Ciocalteu (FoC) procedure was used as explained by Nyamende et al. [11]. A shake incubator (Solab, SL 222, Piracicaba, Brazil) was used to mix the extracts with sodium carbonate solution and FoC reagent. Thereafter, the extracts were centrifuged at 5000 rpm for 10 min at 25 °C. The reaction plate was left for 2 h in the dark at ≈20 °C. Utilizing a microplate spectrophotometer, absorbance of the sample was measured at 750 nm. TPC was extrapolated using the gallic acid equivalent (GAE) standard curve and represented as mg GAE L⁻¹.

2.5.3. Total Flavonols

Total flavonols content was determined using a adjusted method outlined by Pavun et al. [18]. The extracts (2 mL) were homogenized for 30 s, and the supernatants were subjected to an extraction via rotation for 10 min and then centrifuged at 5000 rpm for 5 min; thereafter, samples were incubated at room temperature in the dark. Furthermore, 12.5 μL of hydrochloric acid (0.1%) in ethanol (95%), 12.5 mL of quercetin, and 225 μL HCl (2%) were added into the well plate and the mixture was incubated for 30 min at room temperature. At 415 nm, the absorbance was measured with a microplate spectrophotometer. Quercetin standard curves were used to extrapolate total flavonols and expressed as QE g⁻¹.

2.5.4. Antioxidant Capacity

With a few minor adjustments, the FRAP assay was performed in accordance with Commisso et al. [19]. Ascorbic acid was used as the standard. A functional FRAP solution was created by combining HCl (40 mM) and FeCl₃ (20 mM) with 2, 4, 6-tripyridyl-s-triazine (TPTZ, 10 mM) and 300 mM acetate buffer. FRAP reagent was added after the extracts were centrifuged (I206, fitted with ST-720 m rotor, Hermle Labortechnik, Wehingen, Germany) for 5 min at 5000 rpm and 25 °C. Thereafter, the reaction plate was left in the dark for 30 min at 20 °C. Using a microplate spectrophotometer, the absorbance was measured at 593 nm. To determine the extracts’ antioxidant capacity, a standard curve was extrapolated. The results were expressed as µM TE g⁻¹.

2.6. Microbiological Analysis

Total aerobic mesophilic bacteria (TAMB) and yeasts and moulds (Y&M) on apple slices were quantified via the total plate count method [11]. This was conducted in order to evaluate the antimicrobial effectiveness of both AEW and AIEW. The total exposed surface area of all sliced apples was based on the geometry and dimensions of sliced apples [20]. The areas of both sides of the half-circular slice (radius ≈ 3.25 cm) and the epidermal side were added together. The total exposed surface area was approximated at 39.72 cm². Initially, each fresh-cut apple slice was added into a sterile saline solution and vortexed gently for 90 min. Threefold dilutions were made by adding 1 mL of diluent to 9 mL of PS solution. Yeasts and moulds were incubated on plates for three to five days at 28 °C, whereas TAMB was incubated for two days at 37 °C. The number of colonies on the plates was calculated as the total colony forming unit on the complete surface of the apple slice, which was then converted to Log CFU cm⁻².

3. Results and Discussion

3.1. Visual Quality

Fresh-cut ‘Granny Smith’ apples were visually inspected for changes in colour and surface appearance during storage. After 9 days of storage, the non-treated control group received the lowest scores, with 81–100% of the apple slice surface showing dryness and colour change (Figure 1A). The AEW-treated fresh-cut apples showed better visual surface appearance with 0–20% change in appearance and dryness. While AIEW-treated fresh-cut apples had scores of ≈3 with 41–60% showing changes in appearance and dryness. According to the visual surface appearance score, AEW-treated samples had the highest scores throughout the storage duration compared to the control that became brown (Figure 1A).

As shown in Figure 1B, untreated samples promoted browning with a high degree of dryness on the surface of apple slice, while AIEW fresh-cut apple slices demonstrated slight dryness on the surface (Day 9). On the other hand, AEW-treated fresh-cut apples demonstrated surface colour retention when compared to the AIEW and control (Figure 1B). The observed browning in control samples could be associated with chilling injuring due to the low storage temperature [13].

Browning in apples is primarily due to enzymatic oxidation involving the polyphenol oxidase (PPO) enzyme and phenolic compounds [5,6,13]. When apple tissue is damaged or exposed to air, PPO catalyses the oxidation of phenolic compounds to quinones, which then polymerizes into brown pigments [5,6]. PPO activity is generally higher at neutral to slightly acidic pH levels. Therefore, by increasing and or reducing the pH of the electrolyzed water, this can create conditions less favourable for PPO activity, thereby slowing down or inhibiting browning [13]. Alkaline electrolyzed water can inhibit this process due to the presence of hydroxyl ions (OH⁻) and its high pH which can neutralize or disrupt the function of PPO. In contrast, acidic electrolyzed water, with its low pH, can also prevent browning by reducing the pH environment due to the presence of hypochlorous acid (HOCl), which can inhibit PPO enzyme activity and decrease the stability of quinones [13]. The results, therefore, suggest that both AEW and AIEW treatment could minimize chilling-induced browning at low storage temperature.

3.2. Bioactive Compound

3.2.1. Total Phenolics Content

Storage duration and pretreatment had a significant impact on TPC for treated and non-treated fruit samples (p ≤ 0.05). The TPC of fruit samples treated with AEW increased significantly from 67.94 mg GAE L⁻¹ (day 0) to 99. 39 mg GAE L⁻¹ (day 9), while the untreated samples (control) declined continuously and significantly from 67.94 mg GAE L⁻¹ to 42.99 mg GAE L⁻¹ (Figure 2). Samples treated with AEW had the highest total phenolic concentration at 9 days of storage (Figure 2). This result agrees with other studies [14,21]. For instance, it was demonstrated that minimally processed apples treated with AEW treatments maintained a substantially higher TPC than both the control and organic acid-treated samples [21]. Moreover, Lopes et al. [14] demonstrated that NEW treatments improved the quality of minimally processed mangoes for up to 12 days stored at 3 °C, preserving nutritional components. Minimally processed kiwi fruit kept at 4 °C for 8 days showed a delay in the decline in total phenolic content when treated with SAEW [22].

In another study by Li et al. [23], SAEW-treated minimally processed eggplants stored for 8 days at 4 °C maintained a greater phenolic content with AEW-treated samples when compared to distilled water. The greater concentration of TPC observed during the storage of fresh-cut ‘Granny Smith’ apples treated with AEW in comparison with AIEW-treated could be ascribed to higher ACC and its lower pH. Thus, it can be explained that the variation in total phenolic content seen in the fresh-cut apple samples could have been caused by the ACC of AEW and AIEW influencing the activity of specific enzymes.

In contrast, for control samples, there was a degradation of TPC from 82.94 mg GAE L⁻¹ on day 3 to 42.99 mg GAE L⁻¹ at the end of storage on day 9. Lopes et al. [14] showed that TPC for non-treated fresh-cut mango (control) stored for 12 days at 3 °C declined during storage compared to samples treated with neutral electrolyzed water treatments (NEW) that maintained the TPC of fresh-cut mango samples. The comparative influence of the treatments on TPC was AEW > AIEW > control in this study.

3.2.2. Total Flavonol

The interaction type of treatment by storage duration significantly impacted the TFC (p ≤ 0.05), as shown in Figure 3. Total flavonol content of fruit samples treated with AIEW showed a higher flavonoid content with an increase from 48.65 mg QE L⁻¹ (day 0) to 55.71 mg QE L⁻¹ at (day 9). Fruit samples under the different treatments exhibited a progressive increase in total flavonoid concentration on day 6; however, by the end of day 9 of storage, the control samples had decreased. This resulted from the fruit’s entire surface accumulating phenolic compounds due to the mechanical damage subjected to the whole fruit after cutting, as reported by Zhao et al. [22] for fresh-cut kiwi slices. Apple slices treated with AEW and AIEW had no significant difference at the end of day 6 of storage. The relative influence was AEW > AIEW > control for total flavonol content.

Li et al. [23] found that treating fresh-cut eggplant t with SAEW increased total flavonoids on day 9, showing 11.31% increase with respect to tap water. Similar observations were confirmed by Li et al. [24] for jujube fruit treated with slightly acidic EW. The authors reported an initial increase in total flavonoids for treated jujube fruit until day 30, and SAEW treatment helped maintain relatively high total flavonoids compared to other treatments at the end of storage. Chen et al. [25] reported that AEW treatment inhibited the degradation of flavonoids in the pericarp of longan fruit during storage.

Increased ACC is known to further modify the flavonoid concentration under EW treatments by regulating essential enzymes and phenylalanine ammonia-lyase (PAL), which are involved in phenylpropanoid metabolism [12,13]. Flavonoids are important indices for evaluating nutritive properties of fruit and contribute to minimizing decay development and improving resistance [16]. The above results indicated that both AEW and AIEW treatment could retain higher contents of flavonoids for fresh-cut ‘Granny Smith’ apples during cold storage.

3.2.3. Antioxidant Capacity

The antioxidant capacity based on FRAP for treated and un-treated fresh-cut Granny Smith apples were greatly impacted by the combination of storage time and treatment methods (p ≤ 0.05). For both AEW and AIEW treatments, a continuous increase was observed in FRAP, while this was in contrast to the control samples which did not change significantly during storage. The lowest FRAP was recorded for the control, in comparison to treated fresh-cut apples (Figure 4). AEW-treated fruit maintained the highest antioxidant capacity in fresh-cut ‘Granny Smith’ apples followed by AIEW-treated samples at the end of storage day 9. As the polyphenol content in apples increases, the overall antioxidant capacity of the fruit also tends to increase (Supplementary Figures S1 and S2). This is because polyphenols are effective at donating electrons to free radicals, thereby neutralizing them and reducing oxidative stress [13]. Furthermore, by reducing oxidative stress, antioxidants help protect cells from damage, which can contribute to potentially lowering the risk of chronic diseases and improving overall health [4].

The findings from this work agree with the observation from Plesoianu et al. [21]. The authors reported that antioxidant activity for fresh-cut ‘Florina’ and ‘Lonathan’ apples treated with AEW was higher than the non-treated samples. Furthermore, Akther et al. [26] evaluated the influence of SAEW treatments on fresh-cut cauliflower. The authors reported that the SAEW effectively preserved fresh-cut cauliflower antioxidant activity. Other studies focused on EW treatments on fresh fruit agree with the findings from this study. They demonstrated that various EW treatments better maintained or elevated levels of antioxidant capacity compared to control samples [27]. Gao et al. [27] investigated minimally processed apples treated with AEW. The authors showed that AEW-treated apple slices had higher antioxidant activity than the control and organic acid treated samples. The authors attributed this measured response to a lower pH which slows down the loss of nutrients and inhibits enzyme activities. Additionally, it has been reported that blueberries treated with AEW had higher antioxidant enzyme activities than samples treated with distilled water [28], which may explain some of the observed variations in antioxidant activity observed in this study.

3.3. Microbiological Analysis

The interaction effects of the treatments and storage duration showed a significant influence in the decline observed for both TAMB and Y&M on the fresh-cut ‘Granny Smith’ apples (p ≤ 0.05). Results obtained showed that both AEW and AIEW exhibited a significant decontamination activity on the fresh-cut ‘Granny Smith’ apples (Figure 5). Initial microbial load was 2.3 Log CFU cm⁻² and 2.1 Log CFU cm⁻² for TAMB and Y&M, respectively. At the end of storage day 9, TAMB count declined to ≈0.7 Log CFU cm⁻² under AEW treatment, and to ≈1.3 Log CFU cm⁻² under AIEW treatment, respectively (Figure 5A). Similarly, Y&M was reduced from ≈2.1 Log CFU cm⁻² to ≈0.7 Log CFU cm⁻² under AEW treatment and reduced to ≈1.1 Log CFU cm⁻² under AIEW treatment (p > 0.05) (Figure 5B). In contrast to this work, a previous study reported higher microbial load (>5 Log) at storage for fresh-cut apples treated with SAEW [27]. These differences could be due to the higher storage condition investigated by the author, and the initial microbial diversity on the produce [11]. Thus, the inhibition of both TAMB and Y&M growth in this study could be attributed to the combination of EW and the low storage temperature. This emphasizes how crucial the ideal cold chain temperature is to ensure the safety of ready-to-eat produce.

Research has confirmed the antimicrobial efficacy of electrolyzed water treatments on fresh-cut apples. For instance, the impact of AEW and NEW treatments on yeasts on fresh-cut ‘Royal Gala’ apples was studied by Graça et al. [15]. Both treatments with AEW and NEW produced greater yeast reductions compared to sodium hypochlorite. Gao et al. [27] reported that the total number of surface colonies on fresh-cut apples declined by 2.8 log compared to the controls. Rahman et al. [29] reported on the effectiveness of alkaline electrolyzed water on the disinfection efficacy of fresh-cut carrots. Compared to the control, the authors found that treatment with AIEW showed the greatest reduction in bacteria, yeast, and moulds. Furthermore, acidic and neutral electrolyzed water was able to inactivate Cronobacter sakazakii and Escherichia coli found in fresh-cut ‘Tommy Atkins’ mangoes [30]. The highest reduction counts for E. coli were observed on mangos treated with AEW, while Cronobacter sakazakii also displayed similar results. Kuljaroensub et al. [31] investigated the effects of AEW on the quality and microbiological control of minimally processed “Tanee” and “Klauay Namwa” banana leaves. The authors discovered that using AEW successfully eliminated microbial growth and eradicated all bacteria on both cultivars. These findings also agree with other studies [32,33,34,35]. According to previous studies, acidic electrolyzed water has a higher biocidal effect than alkaline electrolyzed water [11,12,13,27,36]. Wang et al. [37] reported that AEW primarily causes oligopeptide destruction at the bacterial cell wall’s alanine and N-acetylmuramic acid joints which results in the bacterial cell wall becoming perforated, which in turn breaks down the disulphide bonds in the cell membrane proteins, increasing the permeability of the cell membrane, thus causing intracellular fluid to dissipate and AEW invasion to break down nucleic acids and other bioactive substances, ultimately causing cell death [37]. However, the disinfection mechanism of AIEW is still not known; alkaline electrolyzed water can be used for cleaning cutting boards and reducing surface particles or organic matter before acidic electrolyzed water is applied with greater efficacy [12].

Furthermore, the decline in microbial counts on the fresh-cut apples in this study was associated with the available chlorine concentration [12]. At the end of 9 days storage, decontamination efficacy revealed that AEW > AIEW > control at the end of storage for TAMB. Similarly for yeast and mould count, AEW was more effective compared to AIEW treatments. The final AMB and Y&M were below the allowable limits, set by the Foodstuffs, Cosmetics, and Disinfectants (FCDA) Act 54 of 1979 in South Africa and the International Commission for Microbiological Specifications for Foods. As a result of treatment, the highest microbial load in the treated samples was significantly below the maximum limits permitted for fresh fruit; thus, the apples could be deemed safe at the end of their storage.

3.4. Correlation Analysis

The relationship between total flavonoid content, phenolic content, antioxidant capacity, TAMB, and Y&M were evaluated using Spearman’s Rank correlation. Spearman’s correlation was used to determine the relationship between the mentioned variables. Other than that, the strength of the relationship between the relative movements of the variables was estimated using the general rule for evaluating the size of a correlation coefficient. The results of the correlation interaction between antioxidant capacity and total flavonol showed a low positive correlation at 0.347 (Figure 6). This supports previous studies in which SAEW treatment improved the antioxidant capacity of fresh-cut kiwifruit by increasing the contents of flavonoids and total phenols [22]. Moreover, the correlation between antioxidant capacity and phenolic content showed a low positive correlation at 0.234 (Figure 6). On the other hand, the interaction between total flavonol and phenolic content had a negative correlation (Figure 6).

Antioxidant capacity and TPC showed a gradual increase for AEW-treated samples, while the TAMB and yeasts and moulds declined rapidly as storage time increased (Supplementary Figures S1 and S2). Correlation analysis indicated that the antioxidant capacity was significantly (p < 0.01) negatively correlated with the TAMB (correlation coefficient r = −0.384, in Supplementary Figure S1A) and yeasts and moulds (correlation coefficient r = −0.809, Supplementary Figure S1B). Furthermore, phenolic content was significantly (p < 0.05) negatively correlated with the TAMB (correlation coefficient r = −0.492) and yeasts and moulds were not significant (correlation coefficient r = −0.298), as shown in Supplementary Figure S2. For the changes in flavonols, correlation to TAMB was negative and not significant (r = −0.199), while yeast and mould had a negative correlation and was not significant (r = −0.413). Therefore, this could be explained by the fact that the continuous decline in TAMB and yeasts and moulds was associated with the accumulation of total phenolics and the increase in antioxidant capacity. The flavonol content from the AIEW-treated fresh-cut ‘Granny Smith’ apples was higher than the control and AEW-treated samples with a significant (p < 0.01) difference from day 6 to day 9. These findings demonstrated that both AEW and AIEW treatment can efficiently delay degradation and inhibit microbial spoilage. This is in line with a previous work by Graça et al. [17] and Chen et al. [25] that by maintaining the bioactive content the incidence microbial spoilage in fresh-cut apples and longan fruit, respectively, in slowed down.

4. Conclusions

The presented study demonstrated that fresh-cut apples treated with AEW and stored under cold storage provided effective reduction of TAMB and yeasts and moulds. Overall, both EW treatments maintained good quality and visual acceptability. Treatments with AIEW best maintained flavonoid content of the fresh-cut apples. Based on the findings reported, comparing the effectiveness AEW to that of AIEW: (i) AEW had a better visual representation, disinfection efficacy, and maintained the phenolic concentration as well as antioxidant activity of the fresh-cut apples; and (ii) AIEW was more effective as an anti-browning agent and extending shelf life through pH alteration with mild antimicrobial action. The advantages of AEW are primarily as a disinfectant with strong antimicrobial properties, and the ability to remove residues. Electrolyzed water treatment offers a practical application potential for the fresh-cut apple industry. Therefore, future study that will compare a broader spectrum of acidic, neutral, and alkaline EW to commercial and chemical-based disinfectants and anti-browning agents used for fresh-cut apples is needed. Furthermore, based on available studies, EW is generally regarded as safe with no harmful substances. In this work no harmful products were reported; however, full evaluation of the effects of EW in a fresh-cut fruit matrix should be undertaken to provide more information.