Total soluble solids concentrations for ‘Fiesta Gem’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Titratable acidity values of the ‘Fiesta Gem’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Titratable acidity values of the ‘Britney Lane’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Respiration rates for ‘Fiesta Gem’ peaches over 28 d of storage expressed in mL CO2 kg−1·h−1 during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Respiration rates for ‘Britney Lane’ peaches over 28 d of storage expressed in mL CO2 kg−1·h−1 during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
‘Fiesta Gem’ peaches treated with saturated (approximately 8 mg·L−1) aqueous O3 after 21 d of cold storage at 1.7 °C and 95% relative humidity.
‘Britney Lane’ peaches treated with 30 mg·L−1 of dissolved oxygen concentration after 14 and 28 d of cold storage at 1.7 °C and 95% relative humidity.
‘Britney Lane’ peaches treated with saturated (approximately 8 mg·L−1) aqueous O3 after 21 d of cold storage at 1.7 °C and 95% relative humidity.
Oxidation-reduction potential values during the first 5 min of unit operation expressed in mV.
Dissolved oxygen concentration values during the first 12 min of unit operation expressed in mg·L−1.
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Sanitation is a crucial postharvest operation that aims to reduce the microbiological load of harvested produce. Currently, in the southeastern United States, peach packing houses use chemical oxidizers, such as sodium hypochlorite (NaOCl) in their hydrocooling systems to treat the fruit and suppress foodborne pathogens postharvest. This study aims to evaluate the effectiveness and efficiency of a novel sanitation technology called high-oxygen water (HOW) as an alternative to NaOCl. It is based on the generation of stable nanobubbles of oxygen (O2) in water in combination with gaseous ozone (O3) without the need for chemicals. The suspended solution has the potential to reduce microorganism loads during the exposure period and prevent the growth of microorganisms during storage. This technology could serve as an effective sanitation treatment for peaches during hydrocooling. Peaches were treated using HOW at various concentrations of dissolved oxygen (10, 20, and 30 mg·L−1) combined with saturated (approximately 8 mg·L−1) O3. These treatments were compared with a standard NaOCl treatment (50 mg·L−1 of free chlorine). Our team evaluated the effects of HOW by assessing postharvest fruit quality changes and decay incidence over time. The results indicated that while HOW treatments showed potential in maintaining postharvest quality, high concentrations of O3 were detrimental to fruit quality, causing increased decay incidence compared with the NaOCl treatment.
The peach production season in Georgia spans from early May to mid-August, with 7700 acres of orchards producing approximately 3.3 tons of fruit/acre (National Agricultural Statistics Service 2024). Postharvest quality management is critical for maintaining the freshness and marketability of fresh produce, particularly peaches (Lurie and Crisosto 2005). Factors such as proper temperature management, relative humidity, handling from harvest to the packing house, and the use of suitable packaging are very important to ensure high fruit quality at a store display (Giannopoulos et al. 2024). Fresh fruits and vegetables can potentially carry harmful pathogens such as Salmonella, Escherichia coli, and Listeria monocytogenes, which can be extremely dangerous to humans if consumed. Additionally, postharvest diseases such as Rhizopus rot (Rhizopus stolonifer), brown rot (Monilinia fructicola), sour rot (Geotrichum candidum), blue mold (Penicillium expansum), Alternaria rot (Alternaria spp.), and gray mold (Botrytis cinerea) may infect produce, resulting in increased decay incidence and subsequently lowering their marketability (Janisiewicz and Korsten 2002; Nabila and Soufiyan 2019). Appropriate fruit sanitation plays a crucial role in minimizing the potential for microbial contamination. Most postharvest chemical oxidizers and sanitizers in the fruit industry carry some level of hazard to human health and could be harmful to the environment. In recent years, there has been an increasing demand for safer alternative fruit sanitation methods in the produce industry (Bilek and Turantaş 2013). The most frequently used sanitizer in the produce industry is currently sodium hypochlorite (NaOCl), commonly known as chlorine, due to its wide antimicrobial spectrum and low cost (Mishra et al. 2018; Warriner et al. 2009). It is used during the hydrocooling process or in the form of water spray to reduce the microbiological loads of fresh produce.
A rapid way to evaluate the efficacy of oxidant-based sanitizers is by calculating the free residual chlorine levels using the oxidation-reduction potential (ORP) as an indicator (Haute et al. 2019). However, the efficacy of a sanitizer should be assessed by measuring the log reduction in the microbial populations of interest (Dev Kumar et al. 2020). Studies have shown that ORP monitoring can be beneficial in the postharvest handling and processing of fruits and vegetables during water disinfection (Suslow 2004). A minimum ORP level of 650 mV is generally recommended to disinfect and sterilize water. This concentration is considered to be sufficient to effectively kill or inactivate a variety of microorganisms including bacteria, viruses, and protozoa (Suslow 2004). Factors such as the aqueous solution’s pH and temperature, the sanitizer concentration, and contact time are critical in determining the effectiveness of the sanitation process, as outlined in the Food and Drug Administration’s 2015 Standards for the Growing, Harvesting, Packing, and Holding of Produce for Human Consumption (US Food and Drug Administration 2015).
Although NaOCl effectively disinfects produce, numerous reasons exist to seek a sustainable replacement. Repeated NaOCl applications can build up to high concentrations in recirculating water, which may damage sensitive produce (Suslow 2000). NaOCl is known to leave toxic residues in the water and potentially on the produce, which can create undesirable off flavors and cause discoloration of produce skin surfaces. Most importantly, chlorine by-products (trihalomethanes and haloacetic acids) are potentially carcinogenic (Choe et al. 2021). Additionally, some studies on the effectiveness of NaOCl as an antimicrobial disinfectant treatment for fresh produce have produced mixed results (Hopkins et al. 2021).
Ozone (O3) is an unstable gas that quickly decomposes into O2, leaving the treated commodities free of any harmful residues, unlike NaOCl or other common disinfectants. The solubility of O3 in water is constrained, reaching a maximum of 29.9 mg·L−1 at 20 °C. However, solutions with concentrations greater than 10 mg·L−1 are difficult to produce, and most systems are limited to producing concentrations of 5 mg·L−1 or less (Smilanick et al. 1999). O3 is a powerful oxidant that can kill harmful microorganisms such as Listeria, E. coli, Salmonella, and other pathogenic bacteria, viruses, and fungi (Smilanick et al. 1999). In both gaseous and aqueous forms, O3 continues to receive attention as a potential alternative to chlorine treatments. This is due to its superior oxidizing potential compared with other common oxidizing methods (Nabavi et al. 2022) and its effectiveness against a wide variety of microorganisms (Xue et al. 2023).
In the past, O3 had been used as a disinfectant in gaseous and aqueous forms, as well as in combination with other techniques. In 2015, a study found that exposure of peach juice to 18 mg·L−1 aqueous O3 in combination with low temperature successfully extended the shelf life of the refrigerated peach juice (Loredo et al. 2015). Gaseous O3 treatments of 0.3 mg·L−1 concentration in combination with low temperature (5 °C) successfully minimized the growth of Botrytis cinerea on table grapes and inhibited mycelial growth of Monilinia fructicola, Botrytis cinerea, Mucor piriformis, and Penicillium expansum, maintaining the quality of peaches (Palou et al. 2002). Another study focused on a low fluctuation temperature in combination with gaseous O3 application, which resulted in O3 oxidizing and damaging the peach quality later in storage (Jia et al. 2020). However, it is possible that the combination of O3 and NaOCl may form halogenated disinfection by-products (DBPs) such as dichloroacetaldehyde, which are considered to be extremely toxic and harmful to human health (Krasner et al. 2006). The presence of bromide in the water during O3 treatments can also lead to the formation of harmful DBPs such as bromate, which is carcinogenic. Therefore, the purity of water is a critical factor in the formation of carcinogenic DBPs during O3 treatments (Aljundi 2011). The novelty of nanobubble technology in combination with O3 has not been explored on peaches, and there is limited literature available on its application in this area. A technology based on the generation of high-oxygen water (HOW) was recently developed by NABAS Technologies Inc. (Rockville, MD, USA) as a promising and sustainable replacement of NaOCl. This new technology combines the powerful oxidative abilities of O3 with the added stability of nanobubbles. Dissolved oxygen (DO) concentration is one of the most important factors in water/solution quality, and it refers to the amount of oxygen present and dissolved in the water (Boyd 2020). Freshwater saturation with DO varies depending on the temperature. At 20 °C, the saturation point of DO in water is approximately 9.08 mg·L−1 (Boyd 2020). The nanobubbles are cavities of air that have a size of more than 10 nm and less than 100 nm (Alheshibri et al. 2016). The generation of nanobubbles increases the DO content of the water, giving more space for gases like O3 to diffuse into them until they burst and release it along with other by-products into the water (Lyu et al. 2019; Wang et al. 2018). Nanobubbles can bond with a chemical oxidizer, such as O3, allowing them to remain in solution for significantly longer periods, thus increasing dwell times (Lyu et al. 2019). This happens due to their small size, which slows the rate at which they rise to the surface (Wu et al. 2022). Consequently, there is a longer period in which they can react with the microorganisms that are harbored on the product surface. Hydroxyl radicals that are generated during the contact of O3-impregnated nanobubbles with the pathogens are responsible for the disinfecting properties of the technology (Seridou and Kalogerakis 2021).
Solutions of O2 nanobubbles with entrapped gaseous O3 molecules can be delivered through the nozzles of a typical NaOCl disinfection system. This solution could potentially reduce microorganism loads, without any significant impacts on produce quality and thus serve as an alternative and effective sanitation treatment for fresh produce. Based on personal communication with the manufacturer, no significant changes in the current packing line would be needed other than the addition of the HOW generator in place of the NaOCl injection system. Packers could see significant reductions in their operating costs, resulting in higher profitability.
Peaches are climacteric stone fruit that continue to ripen after harvest when picked at a mature stage (Lurie 2021). To extend postharvest life, metabolic activities like respiration and ethylene production need to be slowed. The most common method of delaying the rate of senescence is by reducing the fruit temperature to 1 °C. Therefore, peach fruit needs to be cooled after harvest to the target temperature as soon as possible using an appropriate precooling system. Currently, in the southeastern United States, hydrocooling with 50 mg·L−1 concentration of NaOCl is the preferred cooling and sanitation system for peaches by the method of immersion or drenching for 15 to 30 min. Peach sanitation is a resource-intensive method that could be improved by amending the process, including phasing out chemical oxidizers and adopting a nonchemical approach. In this case, HOW treatments in combination with O3, which do not involve a complicated application process, chemical residues, or high input expenses, could potentially be an alternative sanitation method for the fresh peach industry. The adoption of HOW could reduce the need for recurring applications of chemical oxidizers that are costly, have harmful residues, and often negatively affect fruit quality.
This study examines the application of HOW nanobubble technology and O3 treatments in comparison with NaOCl application (industry standard). The objective of this study was to evaluate the effects of HOW and O3 nanobubble treatments on the postharvest physicochemical quality and decay incidence of fresh peaches. By assessing these key quality parameters, we aimed to determine and investigate the viability of this technology as a sustainable alternative to traditional chemical treatments in postharvest peach handling.
Two popular commercial, yellow-fleshed clingstone peach cultivars, Fiesta Gem and Britney Lane, were evaluated. The selection of ‘Fiesta Gem’ and ‘Britney Lane’ was made due to their significance in the region’s market and their genetic differences in firmness characteristics. Generally, the Fiesta Gem cultivar produces slightly firmer fruit compared with ‘Britney Lane’, which tends to have a softer texture. Peaches were harvested at the commercial optimum maturity stage from commercial orchards in central Georgia (Fort Valley, GA, USA). Harvesting was done manually by trained crews, in the early morning hours to minimize heat stress. Fruit were procured from the packinghouses before they underwent the standard hydrocooling process and were transported immediately to the Vidalia Onion Research Laboratory in Tifton, GA. The transport took approximately 1 h, and the temperature of the vehicle interior was kept at 20 °C to preserve fruit quality attributes. Fruit were sorted to eliminate misshapen, infected, or damaged peaches.
The study was carried out in the summer of 2021 with both cultivars tested simultaneously. The peaches were treated using aqueous solutions of different DO concentrations (10, 20, and 30 mg·L−1) or ozonated water (saturated concentration of approximately 8 mg·L−1), compared with a NaOCl solution of 50 mg·L−1 free chlorine (control) as the standard agricultural practice in the industry. Fresh clean tap water was used to prepare the NaOCl solutions, with pH values ranging between 6.5 and 7.0. Fruit treatment methods and concentrations were based on previous studies that tested similar technologies and were modified accordingly (Shiroodi et al. 2021; Smilanick et al. 2002).
The quality attributes (visual appearance, visual evaluation, respiration rate, skin/flesh color, firmness, total soluble solids, titratable acidity, and decay incidence) were the dependent variables studied. A total number of 3,200 peaches (1600 per cultivar) were used for the purpose of this study, with 80 individual peaches per treatment (NaOCl 50 mg·L−1; 10, 20, and 30 mg·L−1 DO; and ozonated water) per measurement day (0, 7, 14, 21, and 28 d). The 80 peaches were then randomly divided into three groups, each representing a replicate for the subsequent evaluations.
After the sorting process, fruit were stored overnight in a controlled temperature room at 1.7 °C and 90% relative humidity (RH) to avoid inducing chilling injury symptoms (Crisosto et al. 2008). The following day, the peaches were allowed to reach ambient temperature, to simulate the typical handling process where fruits are treated directly as they are received from the field. The peaches remained in ambient conditions until the temperature of the fruit’s interior reached 22 °C, aided by commercial fans that were used to increase air circulation. Following the treatment applications, the surface of the fruit was dried for approximately 1 h using fans at ambient conditions.
The fruit were transferred to a cold room maintained at 1.7 °C and 90% RH and stored for up to 28 d. Following the cold storage period (measurement days: 7, 14, 21, and 28 d), a subset of fruit was transferred at ambient conditions (20 °C and 70% RH) for an additional 3 d, to simulate typical retail and/or home conditions and to allow for the appearance of quality defects and decay.
All postharvest evaluations were performed at the Vidalia Onion Research Laboratory (Tifton, GA, USA) and were separated into destructive and nondestructive. Initial fruit quality measurements were performed on day 0.
Peach firmness was acquired using a digital force gauge (AFG 250N; Mecmesin/PPT Group UK, West Sussex, UK) attached to a texture analyzer tower (Multitest 2.5-d; Mecmesin/PPT Group UK). Puncture tests were performed using an 8-mm cylindrical probe for nine peaches per treatment after removing a thin layer of the skin using a scalpel. During the measurement, the texture analyzer tower loading speed was set at 2 mm·s−1 and a maximum puncture depth of 10 mm (Contador et al. 2016). To ensure the consistency of the measurements, the peaches were placed on a stable surface, and the maximum force was expressed in Newtons (N).
Composite samples were prepared by homogenizing 10 g from samples of five different peaches in three replicates per treatment. Peach samples were homogenized using a homogenizer (PowerGen 500; Fisher Scientific, Waltham, MA, USA) and centrifuged using a centrifuge (Allegra 25R; Beckman Coulter, Brea, CA, USA) for 35 min at 4800 rpm at 4 °C to separate the juice from the insoluble cell material. The supernatant was filtered through a cheesecloth, and the samples were stored frozen at −20 °C. The total soluble solids concentration was measured by using a pocket refractometer (PAL-1; ATAGO Co., Ltd., Tokyo, Japan), and the results were expressed as a percentage of total soluble solids (TSSs) (at 20 °C). The juice sample was titrated to pH 8.2 with 0.1 M NaOH using an autotitrator (DL15; Mettler Toledo, Greifensee, Switzerland) and was expressed as a percentage of malic acid equivalents in juice.
Respiration rate (RR) measurements were performed using a CO2/O2 MA package gas analyzer (900141; Bridge, Bedford Heights, OH, USA). Ten fruit were randomly selected from each treatment and used for each of the three replications. The groups of ten fruit were enclosed for 1 h in 2-gal plastic buckets (United Solutions Inc., Leominster, MA, USA), and the differential CO2 from the respiration was determined by the following formula (Fonseca et al. 2002):
where R is respiration (consumption/production) rate (mL⋅kg−1⋅h−1), V is the free volume (mL), y is the volumetric concentration (% v/v), M is the mass (kg), and t is time (h).
Fifteen peaches were randomly selected from each treatment for visual appearance assessments, and each fruit served as a replicate. High-quality pictures were taken on each measurement day (0, 7, 14, 21, and 28 d) using a digital camera (DS126741 EOS Rebel; Canon, Tokyo, Japan) in an enclosed photobooth to ensure consistent lighting conditions. A total of 40 peaches from each treatment were visually inspected for the presence of external decay per measurement day. The χ2 test was used to compare each treatment with NaOCl (control) for the decay incidence observations:where Oi is the observed value, and Ei is the expected value.
To assess the skin and flesh color, 20 peaches were randomly selected from each treatment. Measurements were taken at three random spots from the surface of the peach skin and the flesh parts (after removing a 1-mm thick skin slice using a scalpel), and the results were averaged. The average of the three measurements was considered an individual measurement.
A digital colorimeter (CR-400; Konica Minolta, Tokyo, Japan) was used to measure the skin color of peach samples. A common white reflector plate was used to calibrate the apparatus. The CIE L*a*b* color scheme was used to acquire the readings. The L* represents lightness (L = 0 for total black, L = 100 for pure white); a* represents the red–green index (+a for redness, −a for green); and b* is the yellow–blue index (+b for yellow, −b for blue) (Long et al. 2021). The total color difference (ΔE) was calculated using the formula where L2 − L1*, a2 − a1*, and b2 − b1* are the color differences between the control group and the treated groups. The average value was calculated according to the values measured at three randomly selected positions of the peach flesh according to the following formula (Pathare et al. 2013):
The DO and O3 treatments were produced by a proprietary technology [Nano Air Bubble Aeration System (NABAS), Rockville, MD, USA], in which tap water was treated in a 40-gal (151.4 L) water tank. A commercial NaOCl solution (Clorox, Oakland, CA, USA) of 5% (w/v) was diluted with tap water to achieve a concentration of 50 mg·L−1 of free chlorine. The concentration of the NaOCl levels was monitored using a chlorine colorimeter water tester (YSI 900, Yellow Springs, OH, USA). Also, tap water samples were analyzed to ensure no excess chlorine was present. During the generation of the aqueous solution, the 40-gal tank was continuously monitored by two water quality testing instruments (optical and analog) (ProQuatro, Yellow Springs, OH, USA). ORP levels served as an indicator of the oxidizing capacity of the solution, which is important for maintaining and understanding the efficacy of the O3 and nanobubble levels. Similarly, DO levels, also very important for the oxygenation of the solution, were monitored in real-time. The combined monitoring of ORP and DO ensured that these parameters remained within the optimal range for effective disinfection, while also maintaining the stability and reactivity of the nanobubbles. The unit is designed to measure xmultiple parameters simultaneously, including DO, conductivity, pH, temperature, and ORP. The average temperature of the solutions was kept at 24 °C by adding ice when needed. The water solutions were transferred to 20-gal (75.7 L) buckets, in which the peaches were immersed for 30 min.
The experimental setup was a two-factor complete randomized design with treatment and storage duration as factors and a two-way arrangement of treatments. The effects of these factors were examined independently. For this purpose, a one-way analysis of variance test was used to evaluate the effects of the treatments on each measurement day. The Tukey honestly significant difference test was used to test for differences between means. A 95% confidence interval was used for all calculations (P ≤ 0.05). A statistical software (RStudio; Posit, PBC, Boston, MA, USA) was used to perform all statistical analyses.
The change in firmness during storage varied between days for ‘Fiesta Gem’, whereas ‘Britney Lane’ showed fewer fluctuations in firmness. Peaches from both cultivars softened over time across all treatments. ‘Fiesta Gem’ peaches experienced a 38.6% reduction in firmness from day 0 to day 7 (Supplemental Data). This most likely occurred due to the development of mealiness, which appeared in the 21-d measurement for ‘Fiesta Gem’, a cultivar known to be more prone to the disorder. Previous studies have demonstrated that numerous fruits and vegetables, including peaches, tend to soften gradually during cold storage. Specifically, in fresh-market peaches, abnormal softening may occur after prolonged storage, altering juicy flesh into a dry and mealy texture (Arefi et al. 2015). Despite the changes mentioned above, statistical analysis showed no statistically significant differences between treatments for all days of storage for both cultivars.
The assessment of TSSs in ‘Britney Lane’ showed no significant differences between treatments during cold storage (Supplemental Data), indicating consistent ripeness and sweetness regardless of the treatment applied. Conversely, significant differences in TSS between treatments were observed for ‘Fiesta Gem’, however, only after 14 d of storage. More specifically, peaches treated with ozonated water or 20 mg·L−1 DO solution had higher concentrations of TSS compared with the control (Fig. 1). There were no treatment differences on the other days.
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
Treatments influenced titratable acidity (TA) levels on some dates for both cultivars. ‘Fiesta Gem’ peaches treated with 10 mg·L−1 DO solution had significantly higher TA compared with those treated with 20 and 30 mg·L−1 DO and O3 (Fig. 2). On the last day of storage, the NaOCl-treated peaches had higher TA values compared with the 10 and 20 mg·L−1 DO treatments.
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
‘Britney Lane’ peaches treated with 10 mg·L−1 DO solution had higher TA values compared with the 20 mg·L−1 treatments on day 14 of their storage (Fig. 3). Although significant differences in TA were observed between treatments at 14 and 28 d for both cultivars, there was no significant pattern. It is important to note that the acidity levels decreased over time for both cultivars.
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
Treatment influenced respiration rates of ‘Fiesta Gem’ peaches on days 7 and 21. An initial increase in respiration rates was expected during the first days of their storage life. After 7 d of storage, ‘Fiesta Gem’ fruit treated with 30 mg·L−1 DO solution had elevated respiration rates compared with those treated with 10 mg·L−1 DO, O3, and NaOCl (Fig. 4). The respiration rates of ‘Britney Lane’ were affected by treatments on days 7 and 21; however, at 14 and 28 d, there were nonstatistically significant differences, and no pattern was observed (Fig. 5).
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
Differences in decay incidence were significant only for ‘Brittney Lane’ fruit. ‘Fiesta Gem’ peaches exhibited an increased decay incidence earlier in the storage period compared with ‘Britney Lane’. After 7 d, all treatments except the 30 mg·L−1 DO resulted in up to 5% decay incidence, which increased over time (Table 1). The highest decay incidence for ‘Fiesta Gem’ peaches treated with O3 was 10% after 21 d of storage (Table 1 and Fig. 6) with no significant difference across the treatments.
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
‘Britney Lane’ peaches treated with 30 mg·L−1 of DO solution had a slight increase in the incidence of decay starting at 14 d of cold storage, which increased over time until after 28 d (Fig. 7) of storage (Table 1). ‘Britney Lane’ peaches treated with O3 developed serious skin burns that progressed to decayed skin tissue after 21 and 28 d of storage. The decay incidence of peaches treated with O3 increased to 20% after 14 d of storage (Table 1 and Fig. 8). Fruit treated with NaOCl had a 0% incidence of decay at the same sampling times. Both ‘Britney Lane’ and ‘Fiesta Gem’ peaches that were treated with O3 had increased external decay starting at 14 d of storage. ‘Fiesta Gem’ and ‘Britney Lane’ treated with NaOCl (control) after 28 d of cold storage had a low decay incidence of 2.5% for ‘Fiesta Gem’ and 0% for ‘Britney Lane’.
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
The nanobubble O3 solution reached an ORP level of 813 mV after 2 min of the unit’s operation (Fig. 9). After 5 min, the ORP increased and plateaued at 937.9 mV. The solution was monitored for 88 h to provide ORP kinetic mapping. The ORP levels remained at the 517.2-mV level for 1 d (Supplemental Data). DO concentration followed a similar pattern to the DO levels. It increased to 50.71 mg·L−1 within 10 min of the unit operation (Fig. 10) and then decreased to the initial concentrations after 88 h (Supplemental Data).
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
Citation: HortTechnology 35, 1; 10.21273/HORTTECH05557-24
Microorganisms can infect peaches on the tree (preharvest infection) or during the harvesting, handling, and marketing operations (postharvest infection). Postharvest diseases like brown rot (Monilinia fructicola), sour rot (Geotrichum candidum), Rhizopus rot (Rhizopus stolonifer), blue mold (Penicillium expansum), Alternaria rot (Alternaria spp.), and gray mold (Botrytis cinerea) have the potential to infect produce via wounds, resulting in reduced quality and a shorter shelf life (Adaskaveg and Förster 2023; Janisiewicz and Korsten 2002; Yaghmour et al. 2012). Also, bacteria risky for humans like Salmonella enteritidis have the potential to contaminate fruit during harvest through contact (Fatica and Schneider 2011).
The postharvest disinfection step for fresh produce is crucial because it reduces the pathogenic microbiological load, diminishing the risks of fruit inoculation and further disease incidence (Xue et al. 2023). Chemical oxidizers such as NaOCl and O3 play a significant role in this process because they increase the ORP levels in the water. ORP is the ability of a solution to oxidize or reduce other substances (Myers 2019). It is measured in millivolts and has been recommended to fresh produce packers and shippers as an easily standardized approach to water disinfection for harvest and postharvest handling. ORP values of 650 to 700 mV have the potential to prevent spoilage by bacteria such as E. coli and Salmonella by eliminating them within a few seconds (Suslow 2000). The equipment we used had the potential to increase the ORP levels between 950 and 1000 mV for the first 30 min, validating the sanitation properties of the solution.
A study focusing on the sanitization of peeled onions using 4.5 mg·L−1 aqueous O3 for 8 min without the presence of nanobubbles found that O3 treatments resulted in overall better quality than chlorinated treatments for whole peeled onions (Aslam et al. 2021). Another study on green bell pepper shreds found that 2.4 mg·L−1 O3 for 5 min prolonged the shelf life and maintained the postharvest quality (Ummat et al. 2018). Although extensive research has been conducted on gaseous O3 and its combination with water solutions, researchers have not thoroughly investigated the use of O3 in combination with the nanobubble technology for peach disinfection.
This research was conducted on a new proprietary technology that has not been commercially used for fresh produce. The equipment had the potential to increase the DO levels in the water up to approximately 70 mg·L−1, indicating the high presence of nanobubbles in the solution. The properties of the bubbles in the water solution vary based on their size (Crisosto et al. 2008). Although researchers disagree on the size boundaries of macrobubbles, most suggested a minimum size of approximately 10 μm, whereas the maximum size suggested was 100 μm (Temesgen et al. 2017). Macrobubbles have a poor mass transfer capacity compared with nanobubbles, because they rise rapidly and directly to the solution’s surface, where they volatilize (Takahashi et al. 2003). Contrastingly, nanobubbles have a higher mass transfer capacity compared with larger bubbles and can remain in aqueous solutions for more than 2 weeks (Ohgaki et al. 2010). Nanobubbles offer a unique mechanism for maintaining postharvest quality through their ability to effectively attach to and penetrate microbial cells. When nanobubbles collapse, especially when infused with gases like O2 or O3, they can generate several by-products depending on the environmental conditions. A recent study found that collapsing gas microbubbles in charged water with iron particles form solid shells around nanobubbles, stabilizing them by preventing bursting and extending the lifespan of the nanobubbles in water (Takahashi et al. 2024). Previous studies have proven the free-radical generation by the collapse of 50-μm size microbubbles, which is useful for the decomposition of organic chemicals and wastewater treatment (Takahashi et al. 2007b). On the other hand, when the nanobubbles combine with gaseous O3, under acidic conditions, they burst and collapse, releasing hydroxyl radicals, a key by-product in advanced oxidation processes (Khuntia et al. 2015; Takahashi et al. 2007a). Hydroxyl radicals can inactivate E. coli by targeting the cell wall and membrane, breaking down essential proteins, and causing the leakage of vital substances, which eventually leads to structural collapse and cell degradation (Hou et al. 2012).
The equipment tested, according to the manufacturer, can generate nanobubbles predominantly smaller than 5 μm in diameter. These nanobubbles are invisible to the naked eye and preserve the gas (O3) within them longer, producing high levels of activity for up to 4 d, as found in our study. Although nanobubble technology appears to be promising in enhancing postharvest disinfection processes, a large-scale industrial application would present several challenges that must be addressed. The generation of nanobubbles typically requires precise control over gas injection and pressure conditions to achieve the right bubble sizing (Burfoot et al. 2017). Maintaining such conditions across large industrial volumes of water can be challenging. Ensuring the stability and consistency of nanobubbles size is crucial for their effectiveness in disinfection (Seridou and Kalogerakis 2021). Moreover, assessing the O3 treatments in combination with the nanobubble technology for large-scale industrial use is necessary, considering the investment in specialized and novel equipment and the ongoing maintenance required periodically. Modifications to current industrial systems, such as those used for hydrocooling in peach packing houses, might be necessary to ensure the technology aligns, works efficiently, and is compatible with existing protocols. Future research should focus on optimizing nanobubble generation methods to improve scalability, as well as conducting comprehensive cost–benefit analyses to determine the feasibility of widespread adoption in the food industry.
The tested unit generated high levels of oxygenated and ozonated water solutions but lacked the ability to regulate O3 concentration levels. The high levels of O3 emitted can make its use risky for produce disinfection because such high levels of O3 can cause serious quality disorders. O3 is a strong oxidizer; hence, high concentrations may harm skin tissues, causing burns or discolorations and initiating decay in fruit. We observed serious skin burnings on both ‘Fiesta Gem’ and ‘Britney Lane’ fruit that were manifested as white uneven spots on peach surfaces during the first 7 d. During the latter part of cold storage, the white spots progressed into necrotic areas, which in turn developed severe decay incidence. ‘Britney Lane’ is a nonmelting cultivar; thus, it does not contain the endopolygalacturonase enzyme that causes fruit flesh to break down over time (Belisle et al. 2017). This made ‘Britney Lane’ peaches more resilient to decay, with most of the treatments showing no decay during the first 2 weeks of storage. On the other hand, ‘Fiesta Gem’ peaches started to exhibit decay symptoms earlier, especially when treated with 10 and 20 mg·L−1 DO. Nevertheless, after 28 d of cold storage, O3-treated ‘Britney Lane’ peaches showed a significantly higher incidence of decay (20%) when compared with ‘Fiesta Gem’ (7.5%). For both cultivars, DO treatments did not affect the decay incidence when compared with NaOCl (control). Decay incidence was only significantly different for ‘Britney Lane’ when compared with O3 after 28 d of cold storage. Both peach cultivars displayed unique decay patterns when subjected to different treatments. A study by Contigiani et al. (2018) showed that washing strawberries with ozonized water for 5 min was effective in reducing fungal growth by about 22% to 25% compared with untreated fruit. However, longer treatment times (10 or 15 min) did not significantly improve fungal control and even increased weight loss compared with untreated fruit (Contigiani et al. 2018).
Overall, the 30-min treatment of 8 mg·L−1 O3 concentration had a notable impact on the visual appearance and decay incidence parameters; however, it did not significantly affect other quality parameters of peaches. Although O3 shows promise in maintaining certain aspects of peach quality, such as TSS, TA, RR, firmness, and color, it poses risks to the structural integrity of the fruit, underscoring the need for careful optimization and control of treatment conditions to avoid adverse effects. Findings confirmed similar results to Smilanick et al. (2002) with peaches immersed in 5 mg·L−1 ozonated water for 15 min, resulting in injured fruit surfaces. Another study on strawberries showed that a 5-min dip in 3.5 mg·L−1 O3 water treatments did not impair the physicochemical properties or the sensory quality of the strawberries (Contigiani et al. 2020). Balawejder et al. (2021) studied saturated concentration of ozonated treatments by rinsing apples for 10, 15, and 30 min. The results showed that O3 treatments did not negatively affect the appearance or sensory properties of apples (Balawejder et al. 2021). Considering research and other experiments conducted on other commodities using similar techniques and strategies, it is evident that fruits with varying textures and quality characteristics respond differently when exposed to different concentrations and durations of O3 treatment. Further research is essential to understand the genotypic differences and their potential interactions with the use of DO as growers develop integrated management systems. The investigation of both postharvest quality characteristics and the potential pathogen elimination will provide valuable information for the fresh-market peach industry.
Postharvest sanitation and disease management are of utmost importance to the fresh produce industry. A sustainable, low-cost sanitation method that is both broad spectrum and less likely to result in resistance can provide a unique tool to suppress postharvest diseases and retain produce quality. Our study results indicate that although O3 technology is an effective sanitation method, its use may negatively affect fruit quality. To our knowledge, this is the first report studying the application of nanobubbles in combination with HOW and ozone in fresh-market peach operations. It is important to mention that HOW is a relatively new technology, which has not yet been fully tested at an industrial scale. Although this technology is very adaptable, managing tissue damage and resultant decay is a serious limitation for its adoption and scaling. Potential ways to mitigate this issue would be to expose fruit to varying time intervals (5, 10, and 15 min) and/or different regulated/lower concentrations of O3. Further investigations on the possible positive effects of the HOW technology in combination with controlled O3 concentrations should be conducted.
Total soluble solids concentrations for ‘Fiesta Gem’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Titratable acidity values of the ‘Fiesta Gem’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Titratable acidity values of the ‘Britney Lane’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Respiration rates for ‘Fiesta Gem’ peaches over 28 d of storage expressed in mL CO2 kg−1·h−1 during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Respiration rates for ‘Britney Lane’ peaches over 28 d of storage expressed in mL CO2 kg−1·h−1 during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
‘Fiesta Gem’ peaches treated with saturated (approximately 8 mg·L−1) aqueous O3 after 21 d of cold storage at 1.7 °C and 95% relative humidity.
‘Britney Lane’ peaches treated with 30 mg·L−1 of dissolved oxygen concentration after 14 and 28 d of cold storage at 1.7 °C and 95% relative humidity.
‘Britney Lane’ peaches treated with saturated (approximately 8 mg·L−1) aqueous O3 after 21 d of cold storage at 1.7 °C and 95% relative humidity.
Oxidation-reduction potential values during the first 5 min of unit operation expressed in mV.
Dissolved oxygen concentration values during the first 12 min of unit operation expressed in mg·L−1.
Contributor Notes
This research was funded in part by the Georgia Peach Council. The equipment was provided by NABAS Group Inc. We acknowledge Ioannis Gallios, Sunaab Kukal, and Canicius Mwitta for support on the statistical analysis, as well as Anthony G. Bateman for technical support throughout this project. Additionally, we acknowledge Drs. Cary Rivard and Patrick Conner for proofreading and grammatically editing the manuscript. Last, we thank Ben Lee, J.D. (President & CEO, NABAS Group, Inc.) and Bill Cannon (Chairman, Fertile Ground, LLC) for providing the technology and assistance with the unit operation.
Conceptualization, A.D. and F.L.; methodology, A.D. and F.L.; formal analysis, O.G.; investigation, O.G., R.C., F.L. and A.D.; resources, A.D. and D.C.; data curation, O.G.; writing—original draft preparation, O.G.; writing—review and editing, A.D., D.C., and F.L.; visualization, O.G.; supervision, A.D. and F.L.; project administration, A.D. and F.L.; funding acquisition, A.D. All authors have read and agreed to the published version of the manuscript.
A.D. is the corresponding author. E-mail: adeltsidis@uga.edu.
Total soluble solids concentrations for ‘Fiesta Gem’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Titratable acidity values of the ‘Fiesta Gem’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Titratable acidity values of the ‘Britney Lane’ peaches expressed as percentages (%) during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Respiration rates for ‘Fiesta Gem’ peaches over 28 d of storage expressed in mL CO2 kg−1·h−1 during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
Respiration rates for ‘Britney Lane’ peaches over 28 d of storage expressed in mL CO2 kg−1·h−1 during 28 d of cold storage at 1.7 °C and 95% relative humidity. Treatments included NaOCl (control) at a concentration of 50 mg·L−1; dissolved oxygen concentrations of 10, 20, and 30 mg·L−1; and saturated (approximately 8 mg·L−1) O3 aqueous solutions. Different letters on top of a bar indicate significant differences at P ≤ 0.05. Error bars denote the standard error variability. ns = not significant.
‘Fiesta Gem’ peaches treated with saturated (approximately 8 mg·L−1) aqueous O3 after 21 d of cold storage at 1.7 °C and 95% relative humidity.
‘Britney Lane’ peaches treated with 30 mg·L−1 of dissolved oxygen concentration after 14 and 28 d of cold storage at 1.7 °C and 95% relative humidity.
‘Britney Lane’ peaches treated with saturated (approximately 8 mg·L−1) aqueous O3 after 21 d of cold storage at 1.7 °C and 95% relative humidity.
Oxidation-reduction potential values during the first 5 min of unit operation expressed in mV.
Dissolved oxygen concentration values during the first 12 min of unit operation expressed in mg·L−1.
