Disinfection effects of electrolyzed oxidizing water on suppressing fruit rot of pear caused by Botryosphaeria berengeriana

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Abstract

Chlorination presents one of the few chemical options available to help manage postharvest decay. Electrolyzed oxidizing (EO) water, containing free chlorine, is the product of a new concept developed by scientists in Japan. The effectiveness of pear (Pyrus communis L.) immersion in EO water on the control of Bot. rot on European pear, cv. La-France, was investigated. Four independent experiments were carried out. A wound was found necessary for infection. Wounded fruit were inoculated with 20 μl spore suspension of 5×105 conidia/ml of Botryosphaeria berengeriana, incubated for 4 h, immersed in EO water, and held at 20 °C, ⩾90% relative humidity (simulated retail conditions) for ripening and disease development. No chlorine-induced phytotoxicity was observed on the treated fruit. EO water suppressed the incidence and disease severity. The minimum incidence and severity were recorded for a 10-min immersion period. This study revealed that EO water is an effective surface sanitizer.

Introduction

Postharvest fungal decay of pear fruits (Pyrus sp.) causes substantial economic loss to the fruit industry. Botryosphaeria species cause canker and fruit rots on both pome and stone fruits. B. dothidea (Moung. Ex. Fr.) Ces & de Not. (syn. B. ribis Gross. & Dug.) and B. obtusa [(Schw.) Shoremaker] cause diseases of apple and peach. B. dothidea has undergone a number of name changes. Now some taxonomists believe B. dothidea should be called B. berengeriana (Brown & Britton, 1986). B. berengeriana attacks on pear fruit grown in Japan (Tanaka, 1990).

The control of fungal rot is essential to maximize the storage potential of pears. Although rotting may be reduced by CA storage, control of rotting in pears stored commercially in many countries largely depends on fungicides applied as a postharvest dip or drench (Colgan & Johnson, 1998). The use of fungicides with the same mode of action in the orchard and in the postharvest treatments has led to the development of resistance in some fungal pathogens. Resistance leads inevitably to failure to control rotting and thus need to find alternative strategies for minimizing the damage caused by the fungal rots. In certain countries, like Japan, postharvest application of fungicides is prohibited. This has necessitated the development of other approaches to control storage rots such as selective harvesting and the use of orchard sprays late in the season (Berrie, 1993). Chlorination presents one of the few chemical options available to help manage postharvest decay.

Electrolyzed oxidizing (EO) water has been reported by scientists in Japan to have strong bactericidal effects on most pathogenic bacteria (Hayashibara et al., 1994, Iwasawa et al., 1993, Iwasawa et al., 1993). A major advantage of using EO water for disinfection is that it is produced using pure water with no added chemicals except sodium chloride (NaCl). Therefore, it has less adverse impact on environment (Kim, Hung, & Brackett, 2000). In the process, two types of water possessing different characteristics are generated. An electrolyzed basic aqueous solution (pH 11.4 and oxidation-reduction potential [ORP], −795 mV), hereinafter called electrolyzed reducing (ER) water, is produced from the cathode side and has a reducing potential that leads to a reduction of free radicals in biological systems and may be useful in the treatment of organ malfunctions. An electrolyzed strong acid solution, hereinafter called electrolyzed oxidizing (EO) water, is produced from the anode side and has a high ORP (from +950 to +1180 mV), presence of hypochlorous acid (HOCl), and strong bactericidal effect. HOCl is produced during electrolysis of saline-added water, and the amount of HOCl increases in response to the amount of added NaCl (Anonymous, 1997). However, production of EO water can be modified to reduce the presence of HOCl and still maintains its effectiveness for microbial inactivation to reduce the health concern as chlorinated water (Kim et al., 2000). EO water may contain chlorine gas, HOCl, and OCl ions, all contribute to free available chlorine (FAC) i.e. uncombined chlorine radicals. Chlorine (Cl2) is generated in the anode (oxidizing, also known as acidic) water, and hydrogen (H2) in the cathode (reducing, also known as alkaline) water. The chlorine reacts with the water to form HOCl and HCl. The EO water at low pH undergoes virtually no hydrolysis to the much less effective hypochlorite ion (OCl; Kohno, 1996, White, 1992). Molecular chlorine, probably in equilibrium with the HOCl, and known to be present in EO water, is also an extremely effective sanitizer (White, 1992). Some machines have a septum (membrane) and produce EO water with pH 2–3. Neutral water at pH 6.8 is generated by electrolysis of NaCl solution without a septum because HCl formed at the anode side neutralizes the NaOH at cathode side (Hirano & Ueda, 1997). We used a machine of a non-septum type.

EO water is being widely used for disinfection purposes in Japanese hospitals and dental clinics. Its effects on food (Izumi, 1999, Venkitanarayanan, 1999a, Venkitanarayanan, 1999b) and in agricultural industries (Bonde et al., 1999, Grech & Rijkenberg, 1992, Kim et al., 2000, Yamaki, 1998) have been studied. But these reports mainly dealt with bacteria. Information is lacking on its effect on the fungi on harvested fruit. This is the first report concerning the postharvest application of EO water as fungicide on pear fruit. The objective of this study was to assess the impact of EO water on the control of Bot. rot of pear fruit artificially inoculated by different methods with conidial suspension of B. berengeriana at postharvest stage.

Section snippets

Fruit material

Mature fruit of pear (Pyrus communis L., cv. La-France), were obtained from the Yamagata Prefecture (Japan) and kept in a cold room at 1 °C until used for the experiment (up to 3 weeks). Fruit were placed on plastic holders (one layer) in commercial cardboard boxes with their stem-end facing uppermost and were preconditioned for 3 to 6 h prior to inoculation with B. berengeriana in a chamber at 20 °C, ⩾90% relative humidity (RH).

Pathogen inoculum

B. berengeriana, strain M-D1-1, obtained from the Laboratory of

Results

We used a machine of non-septum type and by controlling the flow rate of anode and cathode side water, EO water of desired pH were obtained. At the setting of 8–10 A and 9–10 V, the ORP remained 1170±20 mV at pH range from 2.6 to 3.6, while thereafter it followed a downward trend from pH 3.6 to 6.8 (Fig. 1). Regression analysis showed the non-linear pattern and significant association (R2=0.912) between pH and ORP.

Discussion

Inoculation of pear fruit by using filter paper disks (FPD), dipped in spore suspension, did not prove successful. The probable reasons could be the relatively hard skin of pear fruit which might have not allowed the B. berengeriana to penetrate or the amount of inoculum absorbed by the FPD might be insufficient to cause the infection. Non-wounded apples were not infected at any stage of development by B. dothidea (Kohn & Hendrix, 1983). Brown and Brtitton (1986) stated that wounding is

Conclusions

The study provides information on the application of EO water of different properties and its influence on the suppression of postharvest fruit rot on European pear cv. La-France. A wound is necessary for B. berengeriana to cause Bot. rot of pear. Size (width as well as depth) of a wound is also important for infection. We suppose that EO water might also have immediately reacted with the microbes present on the surface and in the first few layers of the fruit and it did not prove effective for

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