Sanitizer efficacy in preventing cross-contamination of heads of lettuce during retail crisping

Highlights

• Soaking with electrolyzed water prevented cross-contamination among lettuce heads and controlled bacterial populations during a triple use of crisping water.

• Soaking of inoculated Romaine lettuce with electrolyzed water significantly reduced the population of Salmonella, E. coli O157:H7, and L. monocytogenes compared to tap water and commercial acid sanitizers.

• No difference was noted between tap water and commercial acid sanitizers in reducing foodborne pathogens on lettuce.

Abstract

This study was conducted to provide information regarding mitigation of cross-contamination through the use of sanitizer during crisping at retail outlets. Seven non-inoculated heads and one inoculated head (≈5 log CFU/g) of lettuce were placed into commercial sink filled with 76 L of tap water (TW), electrolyzed water (EW, free chlorine: 43 ± 6 ppm), lactic acid and phosphoric acid-based sanitizer (LPA, pH 2.89), or citric acid-based sanitizer (CA, pH 2.78) and soaked for 5 min. Two subsequent batches (eight non-inoculated heads per batch) were soaked in the same solution. Soaking with EW significantly reduced the population of S. enterica (2.8 ± 1.5 log CFU/g), E. coli O157:H7 (3.4 ± 1.1 log CFU/g), and L. monocytogenes (2.6 ± 0.7 log CFU/g) inoculated on Romaine lettuce compared to TW, LPA, and CA (p < 0.05). On Red leaf lettuce, EW significantly reduced populations of S. enterica and E. coli O157:H7, but not L. monocytogenes compared to other treatments. No significant difference was noted between TW, LPA, and CA in reducing foodborne pathogens (p > 0.05) or preventing cross-contamination. Soaking with EW prevented cross-contamination among lettuce heads and controlled bacterial populations in crisping water for three consecutive batches. EW may be an effective option as a sanitizer to minimizing the cross-contamination of leafy greens during the retail crisping.

Introduction

Leafy vegetables are considered to present the greatest concern with regard to microbiological hazards of fresh produce (FAO/WHO, 2008) and were identified as presenting the greatest risk for food regulated by the U.S. Food and Drug Administration (CSPI, 2009). According to the Center for Science in the Public Interest (CSPI), produce was the top food category responsible for 643 outbreaks (19% of total outbreaks) and 20,456 illnesses (24% of total illnesses) between 2004 and 2013 in the United States (CSPI, 2015). Shiga toxin-producing Escherichia coli (especially E. coli O157:H7), S. enterica, and Listeria monocytogenes are the primary pathogens of concern for leafy vegetables such as lettuce and spinach (Castillo and Rodríguez-García, 2004, Heaton and Jones, 2008, Herman et al., 2015).

Microbiological safety and quality of leafy vegetables can be improved through washing. Typically produce is washed with potable water with/without chemical antimicrobial agents for the primary purpose of removing soil and debris. Washing can reduce the population of pathogenic and spoilage microorganisms, improve appearance, and potentially extend shelf-life of produce (Artés et al., 2009, Herdt and Feng, 2009). According to the U.S. Food Code (section 3–302.15 Washing Fruits and Vegetables), chemical sanitizing agents approved by the FDA (e.g. sodium hypochlorite, specified in 21 CFR 173.315) and as specified under § 7–204.12 of the Food Code may be used for washing fruits and vegetables (FDA, 2013). Wash water can serve as a vehicle for microbial cross-contamination of product, especially when the water is reused without addition of sanitizing agents (Gil et al., 2009). The addition of a sanitizing agent to water intended for the washing of fresh vegetables has been recommended to prevent transfer and survival of pathogens during washing (Sapers, 2001, FDA, 2009, Gil et al., 2009, Luo et al., 2012).

Chlorine is the most widely used sanitizer in the food industry because of its broad spectrum of antimicrobial activity, relative low cost, and minimal negative effect on product quality (Luo et al., 2012, Parish et al., 2003, Sapers, 2014). There are many alternatives to the use of chlorine including peroxyacetic acid, organic acids (lactic, citric, and acetic acids), and electrolyzed water. There are different types of EW: acidic electrolyzed water (AEW), neutral electrolyzed water (NEW), and basic electrolyzed water (BEW; alkaline electrolyzed water) (Kim et al., 2000, Hsu, 2005, Huang et al., 2008, Artés et al., 2009). AEW has been shown to be effective in reducing or inactivating foodborne pathogens on lettuce (Park et al., 2001, Park et al., 2008, Koseki et al., 2004, Keskinen et al., 2009), spinach (Park et al., 2008), cabbage, lemons, tomatoes (Pangloli et al., 2009), and apples (Graça et al., 2011). NEW or slightly acidic electrolyzed water (pH 5.0–6.5) has received much greater attention by the food industry since skin irritation is reduced, chlorine odor limited, and increased stability at near-neutral pH compared to AEW (Len et al., 2002, Ayebah and Hung, 2005, Abadias et al., 2008, Waters et al., 2014).

Although sales of fresh-cut/bagged salad has steadily increased in popularity due to its convenience, the sales of heads of lettuce are still one of the top 10 vegetables purchased in US supermarkets (Cook, 2012). Most major retail grocery stores and supermarkets employ a crisping practice for leafy greens to improve consumer acceptance. Crisping typically involves the submersion of commodities in water (with or without sanitizers) for up to 10 min followed by refrigeration, which gives products a fresh look and crisp texture. Leafy greens such as Romaine lettuce, loose leaf lettuce, and kale may require crisping since they are easily dehydrated during storage and distribution (California Department of Public Health, 2012, Defense Commissary Agency, 2012). Crisping may result in cross-contamination through the transfer of pathogens from contaminated commodities. Indeed, crisping has been suspected of facilitating cross-contamination of lettuce linked with an E. coli O157:H7 outbreak that occurred in Montana in 1995 (Ackers et al., 1998). A paucity of research has focused on the microbiological safety of retail crisping practices.

Studies have demonstrated the efficacy of sanitizers including chlorine, organic acids, and EW in reducing microbial loads on leafy vegetables (Gil et al., 2009, Nou and Luo, 2010, Barrera et al., 2012, Luo et al., 2012, Davidson et al., 2013; Palma-Salgado et al., 2014). Most studies have focused on industry type processing and reduction of microbiological population on commodity rather the control of microbes in the wash water. Often fresh-cut/shredded lettuce is the test commodity of choice and processing done under pilot scale for evaluating a sanitizers’ efficacy against foodborne pathogens. In the present study, the efficacy of commercially available electrolyzed water concentrate and acid-based sanitizers in preventing cross-contamination during crisping in a retail setting (e.g., supermarkets) was investigated. The ability of the sanitizers to reduce foodborne pathogens (E. coli O157:H7, S. enterica, and L. monocytogenes) on whole heads of lettuce was also investigated.