Chemical Intervention Technologies for Seafood Safety

Chlorine solution has been the most commonly used conventional disinfectants in the food processing industry. This is due to its wide availability, low cost, bactericidal effect and that it is easy to monitor the free residual level on food samples and food processing facilities (2, 3). The Food and Agriculture Organization (FAO) reported that up to 200 ppm chlorine is used for sanitizing clean food processing equipment and 0.2~0.5 ppm free residual chlorine level should be maintained in the distribution system (3). Despite its antimicrobial properties, chlorine suffers many limitations which include its corrosiveness to the products and processing equipment, low effectiveness against bacterial spores and protozoan oocysts, rapid decrease in bactericidal effect upon contact with organic matter and increased temperature, as well as the production of carcinogenic and teratogenic by-products such as trihalomethanes and haloacetic acids in water (2, 4–6). Due to this concern, other chlorine-based sanitizers, such as chlorine dioxide, were being studied as an alternative sanitizing agent in seafood processing. Chlorine dioxide is more stable than chlorine and possesses higher antimicrobial activity than chlorine. Studies have illustrated the effectiveness of chlorine dioxide in the depuration of V. parahaemolyticus in oyster and reduction of microorganisms in water for seafood washing and handling (7). Nevertheless, chlorine dioxide treatments decreased the sensorial properties of seafood and its by-products, such as chlorite and chlorate, continue to be a health concern (7). These factors fuelled the research in non-chlorine based sanitizers which are safe, environmental friendly and able improve the overall microbial and sensorial quality of seafood products.

The use of electrolysed oxidising (EO) water began in Japan and has been reported to possess strong antimicrobial effects on a variety of pathogenic bacteria related to food safety (8). EO water is produced by the electrolysis of a dilute (0.1~0.2%) sodium chloride (NaCl) solution (9). Hypochlorous acid (HOCl) is formed at the anode during electrolysis results in a low pH solution called acidic electrolyzed water (AcEW) with antimicrobial property. The bactericidal of AcEW against pathogens and spoilage microorganisms is stronger than that of conventional chlorine-based sanitizers due to its high oxidation reduction potential (>1,000 mV) at low pH (10~12). The cathode, on the other hand, produces hydroxyl ions, resulting in the formation of basic EO water is used for dirt and grease removal from items such as cutting boards and kitchen utensils (8, 13, 14).

The use of EO water for disinfecting bacteria in raw seafood and seafood processing equipment has been reported (Table 1). Ozer and Demirci (15) showed that treating fresh salmon with acidic EO water, AEW (pH of 2.6, redox potential of 1,150 mV and free chlorine of 90 mg/L) at 35°C for 64 min resulted in a 1.07 log CFU/g and 1.12 log CFU/g reduction in E. coli O157:H7 and L. monocytogenes, respectively. Huang and others (14) reported that AEW was effective for reducing 0.7 log CFU/cm² of E. coli and up to 2.6 log CFU/cm² of V. parahaemolyticus on tilapia skin surfaces. Another studies indicates that including basic EO water pre-treatment and mild heat increased the antimicrobial activity of AEW against V. parahaemolyticus on shrimp by 4.4 log CFU/g (16). In terms of the effect of AEW on the sensory qualities of seafood, Kim et al. (17) reported that preserving Pacific saury with EO ice enhanced the freshness by 4~5 days as compared to tap water ice.

Apart from the raw seafood commodity, seafood processing equipment could also be contaminated with pathogens such as L. monocytogenes due to poor process hygiene by food handlers and cross-contamination with raw seafood and the final products (18). Liu et al. (19) demonstrated that AEW effectively reduced L. monocytogenes contamination in seafood processing equipment. In their study, the treatment by immersion in EO water containing 50 mg/L chlorine for 5 min delivered significant reduction of L. monocytogenes on stainless steel sheet and ceramic tile (2.33 log CFU) as well as floor tile (1.52 log CFU) when compared to tap water washing. The antimicrobial efficiency of AEW against L. monocytogenes was shown to be proportional to its chlorine content and ORP. Another study also investigated the antimicrobial activity of AEW water against L. monocytogenes and Morganella morganii on seafood processing surfaces which commonly cause listeriosis and histamine fish poisoning respectively (20). AEW achieved a reduction of 7.0 log CFU on the 24-h biofilms of these microorganisms in the MBECTM Assay System. Moreover, the study also revealed strain to strain variability in AEW susceptibility by the fact that not all L. moncytogenes strains inoculated on food processing surface were sensitive to AEW. Only three out of ninety L. monocytogenes and one out of five strains of M. morganii on conveyor belt coupon were reduced by 1~2.5 log CFU/cm² upon treated with EO water for 5 min. L. monocytogenes cells attached on raw salmon was also reduced by 2.0 log CFU/g upon exposure to EO water for 5 min. Nevertheless, it was reported that applying AEW on seafood processing equipment might lead to corrosion due to the strong acidity, hence limiting the application (21). The volatile chlorine gas released by AEW at low pH may also cause harm to human and the environment (21).

Ozone (O₃) is an allotropic form of oxygen (O₂). In 2007, ozone has been approved as GRAS (Generally Recognized as Safe) chemical and FDA in 2001 officially approved ozone-containing chemicals for use in the food industry including seafood (22, 23). According to the United States Occupational Safety and Health Administration (OSHA), the Short Term Exposure Limit (STEL) to ozone is 0.3 ppm for a maximum of 15 min exposure time no more than three times a day (24). Research on ozone has been undergoing as a chlorine-alternative since then for inhibiting spoilage and pathogenic bacteria in seafood processing. Nevertheless, conclusive evidence regarding the sanitizing effect of ozone is insufficient. This is due to process variability in experiments as well as the effect of microbial population, temperature, pH, commodity surface characteristics, and the presence of organic substrates to the antimicrobial effect of ozone (25). Despite the variability, the study of ozone as an antimicrobial agent is still ongoing due to its strong oxidizing power. Ozone gas can be produced by a domestic ozone generator utilising atmospheric air as the oxygen source (23). Aqueous ozone is generated by pulling ozone into a water stream under negative pressure with the aid of an injection system (2, 26). The bactericidal effect of ozone is due to its ability to diffuse through microbes’cell membrane, its high oxidation potential and its reaction with organic material is up to 3,000 times faster than chlorine (27). However, ozone is highly unstable in water and decomposes rapidly to oxygen in which the half-life of ozone activity may be less than 1 min in processing water with suspended organic matter (26). Hence, ozone has to be regenerated during the sanitization process.

Research investigating the efficacy of ozone spray application mechanisms for ensuring microbial safety and chemical quality attributes of high lipid content salmon fillet was carried out by Crowe et al. (25). Their studies indicated that ozone sprays (1.5 mg/L) effectively decreased the initial counts of aerobic bacteria and inoculated L. innocua without significantly increased in lipid oxidation levels in salmon fillets stored under 4°C. In another similar study, Vaz-Velho et al. (23) evaluated the efficacy of gaseous ozone (0.1 × 10^—3 g/L) and reported a reduction of 1.0 log CFU/g L. innocua after 3 week-storage of cold-smoked salmon fillet in vacuum packs at 5°C. Manousaridis et al. (28) immersed mussels in 1.0 mg/L aqueous ozone and discovered significant reductions in aerobic plate count (up to 2.1 log reduction) coupled with a 35% extension in shelf-life of treated mussels. The treatment also did not induce significant changes in lipid oxidation and sensory attributes of treated mussels. Since the application of ozone for direct contact with seafood products has been approved by FDA in 2001, special ozone application practices have been developed. Sterile and ozonized water holds potential in replacing seawater for fresh fish washing and ice manufacturing to conserve the chemical, microbiological and sensorial aspects of fresh fish (29).

Organic acids were found to be effective against psychrophilic and mesophilic microorganisms in fresh produce in the recent years (30, 31). Out of all the organic acids, lactic acid is most commonly used to preserve and disinfect poultry and meat products, but the inhibitory effect of lactic acid on seafood is still under studied (30, 32). Nevertheless, current research suggested that lactic acid is useful for commercial applications for effective decontamination of seafood, such as shrimp (30) and catfish (33). It is found that treatment with 1~3% lactic acid delivered a V. parahaemolyticus reduction of 2~3 log CFU/g without deteriorating the sensory aspect of the seafood commodity (30, 34).

Synergistic bactericidal effect was observed by Kim et al. (35) when combining irradiation and chlorine treatments in the disinfection of mussel and squids. A combination of 1 kGy gamma radiation and 100 ppm of chlorine further reduced the total aerobic count of mussels by 2.66 log CFU/g and squid by 2.46 log CFU/g as compared to when both treatments were used alone. A higher concentration of chlorine (150 ppm) was required when the radiation source was changed to an electron beam to achieve comparable synergistic effects. This was attributed to the difference in penetration depth and dose rate of the two radiation sources. The same authors also reported enhanced synergistic effects of irradiation and chlorine treatment when 1,000 mg/L of vitamin B₁ was added to the sodium hypochlorite solution. In particular, the addition of thiamine increased the synergistic effect of 2 kGy electron beam radiation combined with 100 ppm chlorine to 2.41 log CFU/g from 0.18 log CFU/g in oysters (36).