Efficacy of electrolyzed water, chlorine dioxide and photocatalysis for disinfection and removal of pesticide residues from stone fruit

https://doi.org/10.1016/j.postharvbio.2018.10.009Get rights and content

Highlights

  • Pesticide removal varies with the treatment used and the target substance.

  • Chlorine dioxide significantly reduced tebuconazole residues.

  • Photocatalysis significantly reduced iprodione, microbiota and disease incidence.

  • Electrolyzed water decreased the superficial microbiota to undetectable counts.

Abstract

Concerns about chemicals and pesticides in food plants have increased dramatically during the last decade. Following stricter legislation and studies about toxicity and human health risks, new ways of reducing toxic residues are urgently required. In this study, oxidizing agents such as electrolyzed water (EW), chlorine dioxide (ClO2) and photocatalysis have been used during the postharvest phase in order to remove the residues of cyprodinil, tebuconazole and iprodione from the surface of peaches, nectarines and apricots. Moreover, the disinfection capability of these agents has also been tested as an alternative to sodium hypochlorite. Our results show that pesticide removal from stone fruits by oxidizing technologies significantly varies depending on the treatment used and the target substance. ClO2 significantly reduced tebuconazole residues from all the fruits (by more than 60%) and photocatalysis similarly reduced iprodione residues (between 50 and 70%). However, EW achieved a percentage of residue reduction similar to that of tap water, never exceeded 40%. In contrast, EW reduced the superficial microbiota to undetectable counts, also decreasing the percentage of rotted fruit from 32 to 7%. Photocatalysis produced similar results since it was able to decrease the microorganisms present on the fruit surface by nearly 2 log units and the incidence of disease by 50%. It was concluded that a strategy combining photocatalysis treatment during cold storage to reduce pesticide residues and spoilage microorganisms with electrolyzed water washing to reduce any remaining microbial contamination prior to commercialization will substantially reduce disease and ensure the safety of stone fruits for human consumption.

Introduction

A large number of pesticides are applied in agricultural areas all over the world. The worldwide pesticide consumption (except Northern America) reported by FAOSTAT was 3,013,970 tons of active ingredients in 2014 (Cengiz et al., 2017). As a consequence of their widespread use, they have become serious food contaminants, posing a threat to human health. Considerable laboratory studies and epidemiological evidence show that certain pesticides are related to carcinogenesis, endocrine disruptions, birth defects, reproductive disorders and cardiovascular diseases (Mostafalou and Abdollah, 2013). Therefore, a risk assessment is necessary to ascertain the health effects due to intakes of pesticide residues on food (Chen and Zhu, 2011). To reduce possible health risks from pesticides, decreasing their residues, especially in widely consumed food commodities, is crucial. Fresh fruits and vegetables are the major food source for pesticide exposure. The reason is that fresh produce accounts for about 30% of an individual diet by mass and is assumed to contain traces of residual pesticides as much of it is consumed raw (Cengiz et al., 2017).

Stone fruits are some of the most widely grown and consumed fruit in the world. In 2016, Spain exported 999,981 t of peach, nectarine, plum, apricot and cherry with a total value of €1 billion (Federation of Fruit, Vegetable, Flower and Live Plant Grower Exporter Associations (FEPEX, 2016)). Given their high economic value, it is necessary to obtain high-quality crops and fruits in order to maintain Spanish exports. Cyprodinil, iprodione and tebuconazole are widely employed during the cultivation of stone fruits. Cyprodinil [4-cyclopropyl-6-methyl-N-phenylpyrimidin-2-amine] is an anilinopyrimide also with a systemic and broad-spectrum fungicide action. It also has a low water solubility (13 mg L−1 at 20 °C) and volatility (5.1 × 10-01 mPa at 20 °C) (PPDB), and is unlikely to leach to groundwater. In soils it is moderately persistent, although this usually depends upon local conditions, and it is likely to degrade quickly in daylight. The dicarboximide iprodione [3-(3,5-dichlorophenyl)-N-isopropyl-2,4-dioxoimidazolidine-1-carboxamide] has both preventive and curative antifungal activity and is also broad-spectrum. It inhibits the germination of spores, cellular division and mycelial growth. It remains in the soil for approximately 50–160 d. Its water solubility (6.8 mg L−1 at 20 °C) and volatility (5.0 × 10-4 mPa at 20 °C) are also low. Tebuconazole [(RS)-1-p-chlorophenyl-4,4-dimethyl-3-(1H-1,2,4-triazol-1-ylmethyl) pentan-3-ol] is a broad-spectrum systemic fungicide belonging to the triazole class, used on grapes, cereals, stone fruits, peanuts, bananas and other fruits. It is rapidly absorbed into the vegetative parts of the plant, with principally acropetal translocation, to inhibit ergosterol biosynthesis by fungi (Lucini and Molinari, 2009). It has a low water solubility (36 mg L−1 at 20 °C) and volatility (1.3 × 10−03 mPa at 20 °C) (PPDB, Pesticide Properties Data Base). The chemical structures of these pesticides are shown in Fig. 1. According to the European Legislation on Maximum Residue Limits (MRL) (Commission Regulations (EU) 400/2015 and 626/2017), tebuconazole, cyprodinil and iprodione are registered for use on stone fruits in Europe with MRLs (mg kg−1) of 0.6 (apricots and peaches) and 1 (sweet cherries and plums), 2.0 for all stone fruits, and 3.0 (plums), 6 (apricots) and 10 (sweet cherries and peaches), respectively.

Fresh produce harvested from the fields must be safe and new alternatives for removing pesticide residues need to be developed and evaluated. Although some oxidation technologies are increasingly efficient for pesticide degradation, a series of transformation products (TPs) with unknown physical-chemical properties may be produced before the parent compounds are completely mineralized. Numerous studies have reported that various TPs have more polarity, toxicity or persistence than the parent itself, which suggests the TPs may carry an environmental risk (González-Rodríguez et al., 2011; Sharma et al., 2014; Xu et al., 2014; Nicol et al., 2016). Thus, apart from their effectiveness, the toxicity of the degradation products from the parent compounds, if they are likely to be formed, is probably the key factor for deciding whether a process is appropriate for a certain application.

After harvest, fresh produce is often washed with tap water to remove dirt and debris and to reduce microbial counts but this washing has limited effects on pesticide residue removal because many pesticides are hydrophobic (Iizuka and Shimizu, 2014). Water is a useful tool for reducing potential contamination but its disinfection is necessary to maintain its microbial quality, avoiding cross-contamination between clean and contaminated products. The fruit and vegetable industry has used chlorine as one of the most effective sanitizers to assure the safety of its products, sodium hypochlorite (NaOCl) being the most commonly used source of chlorine. Several studies have demonstrated the efficacy of chlorine in degrading pesticides (Pugliese et al., 2004; Duirk et al., 2009; Hao et al., 2011). Recently, Qi et al. (2018) established that electrolyzed oxidizing (EO) water, as an advanced chlorine-based solution, is superior to regular sodium hypochlorite-based sanitizers in removing pesticide residues from fresh produce. Therefore, given that there are many studies that support its antimicrobial activity on different fruits (Graca et al., 2011; Torlak, 2014; Ding et al., 2015) and vegetables (Ding et al., 2011; Gómez-López et al., 2013; Hao et al., 2015; Mansur and Oh, 2015), it is of great interest to investigate its efficacy in degrading pesticides.

Washing solutions with the addition of strong oxidizing agents such as ozone (Ong et al., 1996; Hwang et al., 2001; Wu et al., 2007; Chen et al., 2013) or chlorine dioxide (Hwang et al., 2001; Hwang et al., 2002) have also been shown to be effective in removing residual pesticides from produce samples. In recent years, chlorine dioxide (ClO2) has gained attention as an alternative disinfectant to NaClO for the fresh and fresh-cut produce industry (Artés et al., 2009; Gómez-López et al., 2009; Van Haute et al., 2017). ClO2 is highly effective for the preservation of products such as carrots (Gómez-López et al., 2008), tomato (Guo et al., 2014), strawberry (Trinetta et al., 2013) and blueberry (Sun et al., 2014). ClO2 oxidizes but, unlike chlorine, it does not chlorinate and, as such, chlorinated disinfection by-products (DBPs) are not significantly produced (López-Gálvez et al., 2010; Tomás-Callejas et al., 2012). However, when ClO2 oxidizes water matrix constituents, it is reduced principally to chlorite (ClO2) and, to a lesser extent, to chlorate (ClO3) and chloride (Hua and Reckhow, 2007). ClO2 and ClO3 can cause anaemia in some animals and high levels are harmful to the thyroid function (Hebert et al., 2010). ClO2 are classified as noncarcinogenic products but are regulated DBP in the United States at a maximum level of 1 mg L−1 in drinking water (United States Environmental Protection Agency (USEPA), 2009). ClO3 is a substance that is no longer approved as a pesticide according to the European Commission Decision 2008/865/EC (EC, 2008) but no specific MRLs have been established and, therefore, a default MRL of 0.01 mg kg−1 is applicable. The EFSA Panel on Contaminants in the Food Chain (EFSA, 2015) reported that the source of the chlorate residues detected in food could arise from the use of chlorinated water for food processing and the disinfection of food processing equipment. Gil et al. (2016) made some practical recommendations to minimize the chlorate accumulation in the water washing process and the residues found in fresh cut lettuce when sodium hypochlorite was used as a washing aid, for example using a more diluted chlorine solution and shorter storage times of the stock solution, together with proper rinsing. Chen et al. (2011) did not detect ClO2, ClO2, or ClO3 residues in mulberry fruits treated by 60 mg L−1 ClO2 for 15 min and attributed this effect to the potable water rinse applied after treatment. However, data on the transfer of ClO2- and ClO3 to any specific commodity is needed before ClO2 can be recommended as a water washing treatment.

Radiant catalytic ionization (RCI) or photocatalysis stands out among recently developed advanced oxidation processes (AOP). The technology of RCI is still not well known, but its popularity is gradually increasing. It uses the appropriate wavelength and the phenomenon of photooxidation in the presence of UV radiation and appropriate photo-catalysts such as TiO2. This leads to the production of superoxide ions and hydroxides, and the generation of plasma based on hydrogen peroxide (Skowron et al., 2018). This process involves the generation of the free hydroxyl radical (radical dotOH), a powerful and non-selective chemical oxidant (Heponiemin and Lassi, 2012). Generally, the reaction of hydroxyl radicals and organic compounds will produce water, carbon dioxide and salts. However, the attack of the radical dotOH radical in the presence of oxygen generates a complex series of oxidation reactions in which the exact routes of these reactions to complete mineralisation of the organics are still not quite clear. The advantage of this technology is the ability to perform constant disinfection of the ambient air in food processing plants (Skowron et al., 2018). RCI uses very low levels of ozone and in the catalytic process breaks ozone down forming other oxidation products.

In this study, the objective was to evaluate the effectiveness of three oxidizing technologies (electrolyzed water, chlorine dioxide and photocatalytic process) in removing the widely used pesticides cyprodinil, iprodione and tebuconazole from stone fruits (peach, nectarine and apricot) as well as their antimicrobial disinfectant efficacy. Moreover, the impact of the treatments on the physico-chemical quality of the products was investigated. To our knowledge, this is the first study where EW, ClO2 and photocatalysis have been applied for the removal of these pesticide residues from fresh stone fruit.

Section snippets

Fruit samples

The experiment involved 60 trees randomly selected from an experimental orchard in the Campus of Aula Dei, Montañana (Aragón, Spain) located 10 min from our laboratory. All were organically grown and it was ascertained that the samples were not initially contaminated with residues. The absence of pesticide residues was verified by chromatographic analysis. For each species, peaches (cv. Spring Flame), nectarines (cv. Nectafun) and apricots (cv. Pink Cot), fruits of similar size, colour and an

Pesticide removal by electrolyzed water (EW) and chlorine dioxide (ClO2) washings and by photocatalysis treatment

Fig. 2, Fig. 3, Fig. 4 show the experimental results of pesticide degradation by EW and ClO2 treatments after 5, 15 and 25 min washing for peach (cv. Spring Flame), nectarine (cv. Nectafun) and apricot (cv. Pink Cot), respectively.

The efficacies of EW for cyprodinil and iprodione removal from all the fruits were very low (about 15–20 %) and slightly higher in the case of tebuconazole (about 30%) being the removal efficacy, in most of the cases, lower to that obtained with tap water rinsing

Conclusions

Pesticide removal from stone fruits by oxidizing technologies significantly varied with the treatment used and the target substance. EW seems to be ineffective regardless of the pesticide tested (tebuconazole, cyprodinil or iprodione) whereas ClO2 significantly reduced tebuconazole levels and photocatalysis was active against cyprodinil and iprodione. However, electrolyzed water as a washing treatment and photocatalysis as a complementary cold storage technology have been demonstrated to have

Acknowledgements

This work was supported by Grant LIFE12/ENV/ES/000902 (Zero Residues Project) from the Environment Life Programme of the European Commission. Funding from Diputación General de Aragón (T41) and Fondo Social Europeo is acknowledged. H. Calvo is the beneficiary of a pre-doctoral grant C195/2015 from the Aragón Regional Government (Spain).

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