Microbial reduction and storage quality of fresh-cut cilantro washed with acidic electrolyzed water and aqueous ozone

Abstract

Efficacy of decontamination treatments in reducing microbial populations on cilantro and in improving its storage quality was investigated. Fresh-cut cilantro samples were washed with one of the five treatments: tap water, acidic electrolyzed water (AEW), aqueous ozone, chlorinated water, and aqueous ozone followed by AEW (sequential wash). Treated cilantro was packaged in polyethylene bags prepared with films of selected oxygen transmission rate of 6200 mL/(d m²) and stored at 0 °C for 14 days. The total aerobic bacterial population, total enterobacteriaceae, electrolyte leakage and sensory qualities were examined every 4 days. Test results indicated that the sequential wash is effective in initial microbial count reduction. This treatment also maintained low microbial growth during storage. However, the higher electrolyte leakage may indicate cilantro tissue damage in this treatment. Using AEW alone also resulted in moderate control of aerobic bacterial growth during storage. Ozone treatment, on the other hand, achieved the highest overall quality of cilantro during storage and also maintained the typical cilantro aroma.

Introduction

Cilantro has been widely used in Mexican dishes, particularly salsa and ceviche, as well as in Asian and Indian specialties. However, the safety of cilantro has raised concerns. Surveys conducted by the Food and Drug Administration (FDA) reported that 1.6% cilantro samples were contaminated by both Salmonella and Shigella (FDA, 1999, FDA, 2001) and 5.0% by Cryptosporidium oocysts, a parasite (Monge & Chinchilla, 1996). In 1999 there was an outbreak caused by S. Thompson on cilantro in California (Anon., 2002). Fresh-cut cilantro is more susceptible than whole cilantro to spoilage microbial and pathogen contaminations due to tissue damage.

Conventional fresh-cut cilantro production uses rinse water, usually chlorinated at 100 mg/L, to decontaminate cilantro leaves. However, the relatively low inactivation rate of chlorine at concentrations limited by regulation, as well as the adverse effect of chlorine by-products, has raised concerns in the food industry. Scientists have been searching for alternative methods to protect fresh-cut produce from decaying, to prolong shelf life, and to secure product safety.

Fan, Niemira, and Sokorai (2003) used ionizing radiation at different doses to treat cilantro that resulted in 2.2- to 3-log reductions in initial total aerobic plate count (TPC) and relatively good retention of sensorial quality and shelf life. Luo, Wachtel, McEvoy, Kim, and Hung (2004) developed a modified atmosphere packaging system for fresh-cut cilantro leaves with acceptable quality and a 14-day shelf life. Ozone as an aqueous disinfectant was declared to be generally recognized as safe (GRAS) for food contact applications in 1997 (Graham, 1997). Ozone’s primary advantages include fast decomposition in water to oxygen, no residue, and improved microbial reduction efficacy against bacteria, viruses, and fungal spores than hypochlorite (Khadre, Yousef, & Kim, 2001). Aqueous ozone has been tested for its efficacy in the decontamination of lettuce (Kim, Yousef, & Chism, 1999; Singh, Singh, Bhunia, & Stroshine, 2002), and alfalfa seeds and sprouts (Sharma et al., 2002a, Sharma et al., 2002b; Singh, Singh, & Bhunia, 2003; Wade et al., 2003). Microbial studies in laboratory testing typically show a 2-log reduction in total microbial counts and significant reduction of spoilage species commonly found in fruits and vegetables (Khadre et al., 2001). Large scale testing, however, usually yields only a 90% microbial reduction (Anon., 1999).

Acidic electrolyzed water (AEW) has a strong bactericidal effect against pathogens and spoilage microorganisms, more effective than chlorine, due to its low pH, high oxidation reduction potential (ORP) and the presence of residual chlorine (Bari, Sabina, Isobe, Uemura, & Isshiki, 2003; Izumi, 1999; Kiura et al., 2002; Park, Hung, Doyle, Ezeike, & Kim, 2001; Venkitanarayanan, Ezekike, Hung, & Doyle, 1999). Koseki, Yoshida, Isobe, and Itoh (2001) applied AEW (pH 2.6, ORP 1140 mV, 30 mg/L available chlorine) to surface treat lettuce and reported a 2-log reduction in viable aerobes that was higher than the 1.5-log reduction using ozonated water (5 mg/L). AEW was also tested for its efficacy in inactivating Salmonella on alfalfa seeds and sprouts by Kim, Hung, Brackett, and Lin (2003) and Stan and Daeschel (2003). Electrolyzed water at high pH (pH 6.8, 20 mg/L available chlorine) was tested as a disinfectant and the research found that it did not affect tissue pH, surface color, or general appearance of fresh-cut vegetables (Izumi, 1999). Inactivation of Listeria monocytogenes biofilm formers on stainless steel surfaces with AEW treatment reported a 9-log reduction in viable count (Kim, Hung, Brackett, & Frank, 2000). Inactivation of Salmonella and Escherichia coli O157:H7 on alfalfa seeds was generally enhanced by a simultaneous treatment of ultrasound, chemicals and heat (Scouten & Beuchat, 2002). However, Singh et al. (2003) found that no individual treatment among aqueous ClO₂, ozonated water, and thyme oil was able to completely eliminate E. coli O157:H7 inoculated on alfalfa seeds. They employed a sequential wash strategy (thyme oil followed by ozonated water and aqueous chlorine dioxide) to inactivate E. coli O157:H7 and found only the sequential wash controlled the growth of microbial populations during sprouting (Singh et al., 2003). The use of combined or sequential treatment in produce decontamination is an approach worthy of further investigation. It is hoped that a combination of AEW and ozonated water will be an effective way to wash fresh-cut produce. The objective of this study was to examine the effect of AEW, ozonated water and sequential wash (ozonated water followed by AEW) on texture, color, electrolyte leakage, sensory, and microbial reduction of cilantro during 14-day storage at 0 °C.