Disinfection of water in the fresh-cut produce industry is a critical step to minimize the potential transmission of pathogens from a water source to produce, among produce within a given batch and from batch-to-batch over time. Foodborne illnesses happen from eating contaminated food. These illnesses rank among the most common forms of disease in the world and cause more than one million deaths per year. According to the Centers for Disease Control and Prevention (CDC), foodborne illnesses affect an estimated 48 million people each year (one out of six), resulting in 128,000 hospitalizations and 3,000 deaths in the United States. Foodborne disease outbreaks can be prevented by proper application of industrial food decontamination techniques including pasteurization, thermal sterilization, washing, irradiation and the addition of antimicrobial agents.
Why is a New Sanitation Technology Needed?
Thermal processing is the most commonly used sanitation technique in the food industry. However, it adversely affects the quality of processed foods. Therefore, this approach cannot be used for raw products in the industry. Disinfecting cleaning agents that are commercially available and approved for food are utilized in the fresh-cut produce industry to maintain food quality and safety. The sanitizers typically used are as follows:
- Chlorine
- Potassium persulphate
- Isopropanol
- Hydrogen peroxide
- Sodium dichloro isocyanurate
- Ethanol
- Phenol derivatives
- Quaternary ammonium compounds
The potential for chemical residues, limited effectiveness and high cost associated with these agents makes alternative disinfection processes attractive. Therefore, in the past decade, many novel disinfection technologies such as electrolyzed water (EW), ozone and UV light have been proposed by scientists. Electrochemical disinfection has been receiving more attention from the fresh-cut produce industry.
What is Electrolyzed Water?
Electrolyzed water, also referred to as electrolyzed oxidizing (EO) water, is generated by applying electricity to water with dilute salt content. It is produced by commercially available electrolysis devices (Figure 1).
Dilute salt water (NaCl) passes through an electrolysis cell, which contains inert, positively charged and negatively charged electrodes separated by a membrane (Figure 2). Chloride ions (Cl-) migrate to the positively charged anode as sodium ions (Na+) migrate to the negatively charged cathode. Both pass through an ion transfer membrane that allow only chloride ions or only sodium ions into the respective chambers.
The negatively charged cathode area produces basic water, while the positively charged anode area produces acidic water. The current passing through the electrolyzed water generator and the voltage between the electrodes are set at 8–10 amperes and 9–10 volts, respectively. The pH and oxidative reduction potential (ORP) record by a pH/ion meter. ORP refers to the ability of a solution to oxidize or reduce another material, and it has been used as a measure of disinfection capability.
Mainly two types of EW, known as acidic electrolyzed water (AEW) and basic electrolyzed water (BEW), are produced simultaneously by electrolysis as explained in Table 1.
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| Figure 1. Commercial-scale 200L capacity electrolyzed water generator (Hoshizaki Electric Co. Ltd., Japan). | Figure 2. Schematic of electrolyzed water generator and resulting compounds (Huang et al., 2008). |
| Table 1. Characteristics of acidic and basic electrolyzed water. | ||
| Electrolyzed water type | Basic electrolyzed water (BEW, Catholyte) | Acidic electrolyzed water (AEW, Anolyte) |
| Electrode | Cathode (-) | Anode (+) |
| pH | 10.0 to 11.5 | 2.5 to 3.5 |
| ORP | -800mV to -900mV | +1000mV to +1200mV |
| Voltage | -700 to +200mV | +200 to +820mV |
| End products | Sodium Hydroxide (NaOH) | Hypochlorus Acid (HOCI); Dilute hydrochloric acid (HCl) |
How Does Electrolyzed Water Act Against Microorganisms?
Antimicrobial activity of EW is still not fully understood. However, a significant number of scientists believe that the presence of chloride ions, low pH and high ORP of AEW kill the existing microorganisms. Research findings have shown that mainly two mechanisms kill microorganisms. Generally, bacteria grow in a pH range of 4–9, ORP +200 to +800mV (aerobic bacteria) and -700 to +200mV (anaerobic bacteria). Therefore, outside of the ORP range of AEW could damage the cell membranes, disrupt the metabolic fluxes and ATP production in bacterial cells. In low pH, the outer membrane of bacterial cells increases the entry of HOCl into bacterial cells, which inhibits the carbohydrate metabolism. These deteriorations kill the microbial cells. BEW has strong reducing potential due to high pH and low ORP. It leads to the reduction of free radicals in microorganisms.
In addition to that, researchers have proposed six mechanisms that control the microbial growth:
- Inhibition of glucose oxidation
- Disruption of protein synthesis
- Reaction with nucleic acids
- Unbalance of metabolism after destruction of key enzymes
- Induction of DNA lesions
- Inhibition of oxygen uptake
- Oxidative phosphorylation of microorganisms
Types of Commercially Available Electrolyzed Water Systems
The EW systems are divided into three major types based on their automatic control systems in different companies. EO water generator types include the following:
- Users select the brine-flow rate, while the machine adjusts voltages and/or amperages accordingly.
- Users select the amperages and/or voltages, while the machine adjusts the brine-flow rate accordingly.
- Users select a preset chlorine concentration level of EO water from a display panel, and the machine changes the brine-flow rate and amperages and/or voltages automatically.
Why Is Electrolyzed Water Important to the Food Industry?
EW shows a broad spectrum of microbial decontamination as disinfectants (Table 2). AEW is more effective as a sanitizer than chlorine due to low pH (2.5 to 4), high ORP (+1000mV to +1200mV) and the concentration of free chlorine (11.3mg/L to 86.3mg/L). BEW works well as a cleaning agent due to its high pH and low ORP. Therefore, EW is used in the medical, dental, food processing, agriculture and dairy industries as a disinfecting technique.
EW was introduced to the food industry as a novel disinfecting agent in the late 1800s. It has more than a 150-year-long history. The concept of EW for water decontamination, water regeneration and surgical disinfection in medical institutions was originally developed in Great Britain during the late 1800s and early 1900s. Since the 1980s, EW also has been used in Japan. The first type of EW used was the acidic type, and it was accepted quickly by the Japanese food industry. It is effective in decontaminating fruits and vegetables, poultry, meat, egg, seafood and food processing equipment and food contact surfaces such as cutting boards, ceramic tiles, floors, stainless steel and glassware in food processing plants. Research has shown that EW was very effective on vegetables like carrot, spinach, bell pepper, lettuce, potato, cucumber, alfalfa seeds and fruits such as tomato, peach, apple, strawberry and also seafood like raw salmon, tilapia and yellow-fin tuna.
Currently EW is starting to receive more attention in the U.S. food industry. Electrolyzed water generators have been approved for applications in the food industry by the U.S. Environmental Protection Agency (EPA), U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA).
| Table 2. Microorganisms susceptible to electrolyzed water. | |
| Organism | |
| Bacteria | Fungi |
| Escherichia coli (Venkitanarayanan et al. (1999b), Bari et al. (2003), Park et al. (2004) Liao et al. (2007)) | Aspergillus (Buck et al. (2002), Hara et al. (2003c)) |
| Salmonella enteritidis (Venkitanarayanan et al. (1999b), Liao et al.( 2007)) | Botrytis (Buck et al.(2002)) |
| Salmonella typhimurium (Fabrizio and Cutter (2003), Liao et al. (2007) | Burkholderia glumae (Oomori et al. (2000b)) |
| Pseudomonas aeruginosa (Vorobjeva et al. (2003)) | Botryosphaeria (Al-Haq et al. (2000, 2001a, 2002a)) |
| Pseudomonas solanacearum (Matsuoka and Kawasaki (1994)) | Cladosporium (Buck et al. (2002)) |
| Citrobacter freundii (Vorobjeva et al. (2003)) | Colletotrichum (Al-Haq et al. (2003 a,b)) |
| Flavobacter sp. (Vorobjeva et al. (2003)) | Curvularia (Buck et al. (2002)) |
| Proteus vulgaris (Vorobjeva et al. (2003)) | Fusarium (Grech and Rijkenberg (1992)) |
| Alcaligenes faecalis (Vorobjeva et al. (2003)) | Helminthosporium (Buck et al. (2002)) |
| Aeromonas liquefaciens (Vorobjeva et al. (2003)) | Magnaporthe (Tamaki et al. (2001)) |
| Campylobacter jejuni (Park et al. (2002a), Liao et al. (2007)) | Monilinia (Al-Haq et al. (2001a, 2002a)) |
| Enterococcus faecalis (Vorobjeva et al. (2003)) | Penicillium (Buck et al. (2002)) |
| Listeria monocytogenes (Venkitanarayanan et al. (1999b), Fabrizio and Cutter (2003)) | Phytophthora (Grech and Rijkenberg (1992)) |
| Staphylococcus aureus (Vorobjeva et al. (2003), Park et al. (2002b)) | Tilletia indica (Bonde and Nester (2002)) |
| Bacillus cereus (Vorobjeva et al. (2003)) | |
| Enterobacter aerogenes (Park et al. (2002b)) | |
| Erwinia carotovora (Robbs et al. (1995)) | |
| Xanthomonas (Lazarovits et al. (2004)) | |
| Vibrio parahaemolyticus (Huang et al. (2006a), Kimura et al. (2006)) | |
Benefits and Limitations
No disinfection system is perfect. Each has its merits and limitations. Table 3 outlines the merits and limitations of electrolyzed water.
| Table 3. Merits and limitations of electrolyzed water. | |
| Merits | Limitations |
| Only uses water and salt. | No residual disinfection; antimicrobial activity is quickly lost. |
| Low operating cost. | High equipment cost. |
| Does not change ingredients, texture, scent or flavor of food. | Water is corrosive and can rust some metals. |
| No need for transport, handling and storage of hazardous chemicals. | Reacts with protein, reducing its effectiveness. |
References
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