Oxygen nanobubbles and ozone treatment, potential technology to mantain peach quality

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.

End of the Abstract.

Introduction

The peach production season in Georgia spans from early May to mid-August, with 7700 acres (aprox. 3000 ha) of orchards producing approximately 3.3 tons of fruit/acre, 8 t/ha, (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).

Pathogenes and potharvet diseases

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).

Fruit sanitation

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).

Chlorine

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

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.

Ozone and low temperature

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).

Undesirable byproducts

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).

Nanobubble technology, a solution?

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 

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.

Objective of the study: HOW treatments in combination with O3

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.

Discussion

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).

Postharvest disinfection, a crucial step

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.

Evaluation of a new technology based on nanobubbles

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 mechanisms

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).

New equipment and challenges

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.

Conclusion

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.

Picture is Fig. 7 of the paper - ‘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.

Source

Evaluating the Effects of Ozone Nanobubble Treatments on Postharvest Quality of Fresh Peaches
Orestis Giannopoulos, Ramsey Corn, Dario Chavez, Francisco Loayza, and Angelos Deltsidis
HortTechnology Volume/Issue: Volume 35: Issue 1
DOI: https://doi.org/10.21273/HORTTECH05557-24
https://journals.ashs.org/horttech/view/journals/horttech/35/1/article-p90.xml