Effectiveness of Acidified Sodium Chlorite and Other Sanitizers to Control Escherichia coli O157:H7 on Tomato Surfaces

Abstract

The use of a suitable sanitizer can reduce the risk of produce-related foodborne illnesses. We evaluated the effectiveness of several sanitizers to reduce inoculated Escherichia coli O157:H7 on the surface of cherry tomatoes (Solanum lycopersicum var. cerasiform). Depending on the method of inoculation (dipping/spotting), each of 80 g (eight tomatoes) of inoculated cherry tomatoes was washed in 400 mL of sanitizer solutions or 400 mL distilled water for 5 minutes. The effectiveness of sanitizers on spot-inoculated E. coli O157:H7 on tomato surfaces was found higher than on dip-inoculated tomatoes. Washing with water or chlorine water (0.1 g/L as free chlorine) could reduce 1.3 log CFU/g of E. coli O157:H7 in dip-inoculated (6.8 log CFU/g) tomatoes. Washing with lactic acid (LA) solution (1.0 g/L), phytic acid solution (1.0 g/L), calcinated seashells (oyster/sakhalin surf clam), and 1.0 g/L chitosan in 0.5 g/L LA (Chito) did not exhibit a significant higher effectiveness than that of water wash alone (1.0 log CFU/g). Acidified sodium chlorite (ASC) solution prepared from 0.5 g/L of sodium chlorite and 1.0 g/L LA or phytic acid reduced 3.5 log CFU/g of E. coli O157:H7 in dip-inoculated tomato surfaces. ASC (0.5 g/L of sodium chlorite and 1.0 g/L of LA) wash followed by a second wash with LA exhibited an additional sanitary effectiveness compared to a single wash with ASC. However, washing with ASC followed by a second wash with Chito exhibited an additional 1.0 log CFU/g reduction compared to a secondary wash with water. No significant difference of color, taste, and texture was observed among the washed cherry tomatoes.

Introduction

F resh fruits and vegetables are an important part of a healthy diet, supplying much-needed vitamins, minerals, and fiber (Matthews, 2006). Most fruits and vegetables or related ready-to-eat foods sold in the market rarely carry pathogenic bacteria under usual condition (Abadias et al., 2008; Ilic et al., 2008; Bohaychuk et al., 2009). However, these ready-to-eat foods always have the possibility of contamination with foodborne pathogens during production at the farm level or at any stages of product handling from harvest to the point of sale. According to the Center for Science in the Public Interest database of foodborne illness, 28,315 of 138,622 cases (20.4%) or 554 of 4486 outbreaks (12.3%) in the United States from 1990 through 2003 were related to the consumption of fresh produce (Dewaal et al., 2006). The pathogens of concern reported with these foodborne illness outbreaks were Escherichia coli O157:H7, Salmonella spp., Shigella spp., Listeria monocytogenes, Clostridium botulinum, Crytosporidium spp., Cyclospora spp., hepatitis A virus, and Norwalk-like viruses (Sapers et al., 2006). Tomato, lettuce, spinach, and jalapeno pepper are the major causes of these outbreaks in the United States (Zhuang et al., 1995; Sivapalasingam et al., 2004; CDC, 2007, 2008; Gupta et al., 2007; Hanning et al., 2008; Wendel et al., 2009).

Many sanitizers, including sodium hypochlorite (NaClO) (Zhuang et al., 1995), electrolyzed water (Bari et al., 2003; Deza et al., 2003; Park et al., 2008), ozonated water (Chaidez et al., 2007), acidified sodium chlorite (ASC) (Inatsu et al., 2005a; Nei et al., 2009), and calcinated calcium (Bari et al., 2002), have been evaluated for their effectiveness in killing microorganisms on different fresh produce (Gonzalez et al., 2004; Alvarado-Casillas et al., 2007; Pao et al., 2007; Stopforth et al., 2008; Velázquez et al., 2008; Keskinen et al., 2009). Chitosan suppressed the growth of low-temperature-growing bacteria such as Pseudomonas spp. or L. monocytogenes to prevent spoilage and increase the safety of food (Beverly et al., 2008).

Washing produce with sodium-chlorinated water (NaClO) is the most commonly used method to remove pathogens from fruit and vegetable surfaces. The active hypochlorite is believed to lose its effectiveness by reacting with nitrogen-containing compounds in foods, resulting in halogenated organic compounds (Wei et al., 1985). Concern over the carcinogenicity and toxicity of these compounds, particularly trihalomethanes, has prompted consideration of alternative disinfectants.

Several investigations revealed that chlorine dioxide is a disinfectant equal to or more effective than chlorine (Lillard, 1979). The bactericidal effectiveness of aqueous chlorine dioxide against bacteria on the surface of lettuce, cabbage, cucumber, and green pepper has been reported (Reina et al., 1995; Zhang and Farber, 1996; Han et al., 2001). Chlorine dioxide (ClO₂) gas is much more soluble than chlorine in water and has about a 2.5 times greater oxidizing capacity than that of hypochlorous acid (Benarde et al., 1965). Chlorine dioxide forms only little amount of chlorinated organic compounds when in contact with food surfaces, but chlorine does much.

ASC, an antimicrobial agent, is prepared by mixing a sodium chlorite (NaClO₂) solution with a generally recognized as safe organic acid. This chemical reaction results in active chlorine dioxide showing bactericidal activity in combination with acidity. The U.S. Food and Drug Administration has approved ASC for use in poultry, red meat, comminuted meat products, and processed fruits and vegetables to reduce bacterial contamination (Office of the Federal Register, U.S. Government Printing Office, 1999). The bactericidal effect of ASC for surface washing of raw salmon (Su and Morrissey, 2003), beef carcass (Castillo et al., 1999), and broiler carcasses (Kemp et al., 2000) has been studied. However, few studies have been performed with ASC for surface washing of raw vegetables or fruits (Hasegawa et al., 1990; Lukasik et al., 2003). The objective of this study was to compare the effectiveness of ASC along with other sanitizers to inactivate spot- and dip-inoculated E. coli O157:H7 tomato surfaces. ASC followed by a secondary wash with distilled water (DW), LA, or Chito to see the effectiveness of the sanitizers suppressed the pathogen population during storage with a view to its potential application to foods and food contact surfaces as an antimicrobial treatment.

Materials and Methods

Media and chemicals

All media and chemicals used in this study were purchased from Nissui Pharmaceutical Co., Ltd. (Tokyo, Japan) and Wako Pure Chemicals (Osaka, Japan), except calcined seashells. A calcined oyster shell (Ca-Oy) and calcined sakhalin surf clam (Ca-SS) were purchased from Kaiho Products Co., Ltd. (Yokohama, Japan) and Surfcera Co., Ltd. (Tokyo, Japan), respectively.

Samples

Commercial cherry tomatoes sold in local markets in Tsukuba city, Japan, had been purchased and used. Each tomato weighed 10.0 ± 0.5 g as measured by an electric balance (Mettler Toledo PB303-S/FACT, Columbus, OH). Injured or dirty tomatoes were discarded.

Bacterial strains

Four Enterohemorrhagic E. coli O157:H7 strains, CR-3, MN-28, MY-29, and DT-66 isolated from bovine feces were provided by the Laboratory of Zoonosis, National Institute of Animal Health, Tsukuba, Japan. Rifampicin (Rif)-resistant strains were prepared in our laboratory by chemical mutagenesis according to the method of Inatsu et al. (2003), and the resistant strains were stored in 20% glycerol at −80°C pending further use. To minimize the presence of naturally occurring microorganisms from tomatoes on the enumeration medium, all E. coli O157:H7 strains were adapted to grow in tryptic soy broth (pH 7.3; Nissui Seiyaku, Tokyo, Japan) supplemented with rifampicin (50 μg/mL).

Preparation of inocula

A cocktail mixture of four rifampicin-resistant E. coli O157:H7 strains was used for spot or dip inoculation at room temperature. Each of four E. coli O157: H7 strains was inoculated into four test tubes of 5 mL of brain heart infusion broth containing 50 μg/mL rifampicin and incubated for 16 hours at 37°C. Cells of each strain were collected by centrifugation (4500 rpm, 5 minutes, 25°C) and re-suspended in 5 mL phosphate-buffered saline containing 0.9% sodium chloride, pH 7.2. Equal volumes of cell suspensions of four strains were combined to give approximately equal populations of each strain. The inoculum was maintained at 22°C ± 1°C and applied to the tomatoes within 1 hour of preparation. Cross-strain inhibition tests revealed that the E. coli O157:H7 strains did not interact each other and that mixtures of strains in inocula did not cause any viability loss by strain interaction.

Inoculation of samples

Two hundred microliters of the inoculum was spotted on the body of each tomato randomly on 10 points. Care was taken, in the spot inoculation, to avoid placing the inoculum on the stem scar. For dip inoculation, 200 mL suspensions of each pathogen were combined to give 800 mL of a four-strain mixture of cell suspension in phosphate-buffered saline. Fifty tomatoes were submerged in the inoculum with gentle agitation with a sterilized glass rod for 1 minute and then transferred to a sterilized mesh basket for 1 minute. The spot- or dip-inoculated tomatoes were placed on a mesh in a laminar flow biosafety hood and air-dried for 2 hours at room temperature. These inoculated tomatoes were used for the washing experiments.

Washing

The dried tomatoes were washed once or twice in sanitizers with gentle mixing with a glass rod at room temperature. DW, sodium hypochlorite solution (0.1 g/L as active chlorine; NaClO), lactic acid (LA) solution (0.5 or 1.0 g/L), phytic acid (Phy) solution (1.0 g/L), ASC solution [consisting of 0.5 g/L sodium chlorite and 1.0 g/L of LA or Phy; ASC (LA) or ASC (Phy)], chitosan solution (1.0 g/L in 0.5 g/L LA; Chito), and saturation concentration (1.0 g/L) of Ca-Oy or Ca-SS were used. All the sanitizers used in this study were prepared immediately before use, and 2.5 L of each washing solution was used for 50 cherry tomatoes for 5 minutes with gentle agitation. In case of secondary washing, the tomatoes were first washed with ASC (LA) and placed on a sterile mesh for 5 minutes to drain off excessive sanitizer, and then the secondary washing was done by DW, LA, or Chito. The secondary washed tomatoes were aseptically transferred to Ziploc plastic bags and stored at 10°C ± 0.2°C for 2 days. The pH of the washed solution (after use) was measured by pH-electrode method (F-55; Horiba Co., Ltd., Kyoto, Japan). Viable E. coli O157:H7 counts were determined by spread plating method at days 0 and 2.

Microbiological analysis

Before and after each washing operation, eight tomatoes were separately sampled and used for the assay (n = 8). The treated tomatoes (one tomato; weight 10 g) were placed in a stomacher bag and 90 mL of 0.1% peptone water was added and stomached for 2 minutes. The homogenate was serially diluted in 0.1% peptone water, and 1.0 mL of undiluted samples was pour plated onto Tryptic soy agar (TSA; Nissui, Tokyo, Japan) and Sorbitol MacConkey agar (SMAC; Nissui, Tokyo, Japan) medium supplemented with 50 μg/mL of rifampicin. For washed solution, 1.0 mL of washing solution after use was pour plated on nonselective TSA-Rif and selective SMAC-Rif medium. The plates were incubated at 37°C for 48 hours, and plate containing 20 to 300 colonies was counted. At least five randomly picked presumptive E. coli O157:H7 colonies were confirmed by biochemical tests (Inatsu et al., 2004). All experiments were repeated three times to confirm reproducibility. The logarithmic value of 24 data points was used for statistical analysis. Tukey–Kramer multiple comparison method was used for the detection of significant difference (level of significance was 5%).

Sensory analysis

Noninoculated tomatoes were used for sensory evaluation. Thirty panelists selected from different departments of the National Food Research Institute who were experienced with sensory panels evaluated the quality of washed and nonwashed tomatoes. The panelists were informed about the nature of the study. The test area was free of extraneous odors and sound, and panelists were instructed not to talk during testing. Panelists evaluated samples and marked score sheets in individual booths. Sensory panelists were presented with either a single pair or two pairs of tomatoes for each parameter evaluation. Panelists were instructed to evaluate samples in the order of presentation and to clear their palates between samples with the crackers and water. The presentation order of the samples was balanced among panelists to avoid a position error bias. Panelists did not have any prior knowledge concerning the direction of the difference of washed and nonwashed tomatoes. The evaluation of color, taste, and texture was based on graphic ration scale method (Stone and Sidel, 2004). The color, taste, and texture were assessed on each tomato sample. For each of the quality attributes, 0.1 m lines were marked with most preferable at the right end and most nonpreferable at the left end. The panelists were asked to mark a point in the line corresponding to their preference. The length between the left to the mark point was measured as the value of preference. Tukey–Kramer multiple comparison method was used for the detection of significant difference (level of significance was 5%).

Results and Discussion

Surface decontamination or produce sterilization by surface washing with common sanitizers may reduce 1.0 to 2.0 log CFU/g of attached bacteria (Matthews, 2006). Recently, consumers have been interested in the application of natural antimicrobial compounds such as organic acids, calcinated calcium, or chitosan to control pathogens in foods (Tiwari et al., 2009). We investigated the effectiveness of ASC and other sanitizers, including natural antimicrobial compounds, to kill E. coli O157:H7 on the surface of tomatoes.

E. coli O157:H7 was not recovered from uninoculated tomatoes (data not shown). Regardless of tomatoe conditions or treatments, larger populations of E. coli O157:H7 were recovered on TSA-Rif than on SMAC-Rif. E. coli O157:H7 counts were 0.20 to 0.80 log₁₀ CFU/g higher for washed tomatoes plated on TSA-Rif than for control samples plated on selective medium (Tables 1 and 2). This finding agrees with other reports describing the poor performance of selective medium in recovering pathogen from treated samples (Ellajosyula et al., 1998; Bari et al., 2003).

Table 1.

Recovery of Inoculated Escherichia coli O157:H7 from Washing Solutions and Their pH

		Spot inoculation (log CFU/mL) ^a		Dip inoculation (log CFU/mL) ^a
Used sanitizer	pH of washing solution	TSA-Rif	SMAC-Rif	TSA-Rif	SMAC-Rif
DW	6.1 ± 0.3	6.4 ± 0.4	5.1 ± 1.0	5.8 ± 0.2	5.4 ± 0.2
NaClO	9.1 ± 0.2	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0
LA	2.3 ± 0.1	6.9 ± 0.3	5.4 ± 0.7	5.2 ± 0.0	2.3 ± 0.9
Phy	1.6 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0
ASC (LA)	2.4 ± 0.1	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0
ASC (Phy)	1.8 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0
Ca-Oy	12.0 ± 0.1	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0
Ca-SS	11.9 ± 0.1	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0	1.3 ± 0.0
Chito	3.5 ± 0.0	5.7 ± 0.2	4.6 ± 0.1	4.9 ± 0.7	3.5 ± 1.4

Average value and standard deviation of three replicate experiments. The detection limit was 1.3 log CFU/mL.

DW, distilled water; NaClO, sodium hypochlorite (0.1 g/L as free chorine); LA, lactic acid (1.0 g/L); Phy, phytic acid (1.0 g/L); ASC (LA), 0.5 g/L sodium chlorite and 1.0 g/L lactic acid; ASC (Phy), 0.5 g/L sodium chlorite and 1.0 g/L phytic acid; Ca-Oy, calcined oyster shell (1.0 g/L); Ca-SS, calcined sakhalin surf clam (1.0 g/L); Chito, 1.0 g/L chitosan in 0.5 g/L lactic acid.

Table 2.

Recovery of Inoculated Escherichia coli O157:H7 on Tomatoes After Single Washing Treatment

	Spot inoculation (log CFU/g) ^a		Dip inoculation (log CFU/g) ^a
Used sanitizer	TSA-Rif	SMAC-Rif	TSA-Rif	SMAC-Rif
Unwashed	7.5 ± 0.1 A	7.1 ± 0.2 A	6.8 ± 0.1 A	6.6 ± 0.1 A
DW	4.4 ± 0.4 B	4.0 ± 0.6 B	5.5 ± 0.4 B	5.3 ± 0.2 B
NaClO	2.3 ± 0.1 C	2.3 ± 0.0 C	5.4 ± 0.4 B	5.4 ± 0.5 B
LA	4.8 ± 0.2 D	4.2 ± 0.3 B	5.3 ± 0.4 B	4.9 ± 0.4 C
Phy	2.4 ± 0.2 C	2.3 ± 0.1 C	5.4 ± 0.4 B	4.9 ± 0.4 C
ASC (LA)	2.3 ± 0.1 C	2.3 ± 0.0 C	4.2 ± 0.7 C	3.6 ± 0.6 E
ASC (Phy)	2.3 ± 0.0 C	2.3 ± 0.0 C	4.3 ± 0.4 C	3.6 ± 0.5 E
Ca-Oy	2.4 ± 0.4 C	2.4 ± 0.3 C	5.3 ± 0.3 B	5.0 ± 0.3 BC
Ca-SS	2.3 ± 0.0 C	2.3 ± 0.0 C	5.1 ± 0.5 B	4.6 ± 0.5 CD
Chito	3.7 ± 0.3 E	2.8 ± 0.5 D	5.1 ± 0.3 B	4.8 ± 0.3 CD

The values with different letters within the same column differ significantly (p < 0.05). The detection limit was 2.3 log CFU/g.

The method evaluated involved four-strain mixtures of each pathogen, which is consistent with currently accepted practices for use in studying the survival and growth of pathogens in food (Ellajosyula et al., 1998; Bari et al., 2003) and with Scientific Advisory Panel recommendations (Federal Register, 1997). All strains examined were adapted to grow in the presence of 50 μg of rifampicin per milliliter, one of several markers used to evaluate the survival of bacterial pathogens in food products with potentially large numbers of interfering background microbiota. Antibiotic-resistant markers have been widely used in studies to determine the fate of pathogens in nonsterile foods, including fresh produce (Lin and Wei, 1997), meat (Dorsa et al., 1997), and milk (Palumbo et al., 1997).

The mean number of E. coli O157:H7 applied in spot inocula to each tomato was 7.5 log CFU, and 6.8 log CFU/g of E. coli O157:H7 population was detected on cherry tomatoes after dip inoculation.

The pH of washing solutions after use is shown in Table 1. LA (1.0 g/L), Phy (1.0 g/L), ASC (LA) or ASC (Phy) (containing 0.5 g/L of sodium chlorite and 1.0 g/L of LA or Phy), and Chito exhibited pH less than 3.5. The pH of saturated concentration (1.0 g/L) of calcinated seashells made of oyster (Ca-Oy) or Ca-SS was around 12.0. DW and sodium hypochlorite (NaClO: 0.1 g/L as free chlorine) exhibited weakly acidic (6.1) or basic (9.3) pH, respectively.

The DW, LA, and Chito washing solution contained approximately 6.0 or 5.0 log CFU/mL of viable cells enumerated by TSA-Rif or SMAC-Rif, respectively, in spot-inoculated samples (Table 1). In case of dip-inoculated samples, 5.0 log CFU/mL or a lower value of viable cells was enumerated by TSA-Rif or SMAC-Rif, respectively. In contrast, irrespective of inoculation method, the viable cell count enumerated by TSA-Rif was less than the detection limit (1.3 log CFU/mL) in NaClO, Phy, ASC (LA), ASC (Phy), Ca-Oy, and Ca-SS wash solutions (Table 1). The difference in the results between LA and Phy was thought to reflect not only the difference of acidity but also the chelating effect of Phy (Urbano et al., 2000). These results suggested that the free E. coli O157: H7 cells once removed from the surface of tomatoes were easily killed in the washing solution except for DW, LA, and Chito. On the contrary, these washing solutions may be a cause of cross contamination once the input of pathogen occurs.

The results of single washing treatment on reducing the viable E. coli O157:H7 cells on tomato surfaces are shown in Table 2. In case of spot inoculation, washing with DW and LA reduced the viable cells approximately 3.0 log CFU/g, and washing with Chito could reduce approximately 4.0 log CFU/g of E. coli O157:H7 populations. However, washing with NaClO, ASC, Ca-Oy, and Ca-SS reduced the population of E. coli O157:H7 to counts equal or very close to the detection limit (2.3 CFU/g). In contrast, only 1.5 log CFU/g (DW, NaClO, LA, Phy, Ca-Oy, Ca-SS, and Chito) or 2.5 log CFU/g (both ASCs) reduction was achieved for dip-inoculated samples (Table 2). ASC has been reported to exhibit similar or higher effectiveness than that of sodium hypochlorite in leafy green vegetables (Gonzalez et al., 2004; Inatsu et al., 2005b; Stopforth et al., 2008; Keskinen et al., 2009; Nei et al., 2009). Similar experimental results were obtained in this study with dip-inoculated tomatoes.

The effectiveness of sanitizers in reducing E. coli O157:H7 populations was found less in dip-inoculated samples compared to spot-inoculated samples, because when tomatoes were dip inoculated, cells in suspensions that came in contact with the stem scar tissue would be expected to adhere in higher numbers than would those on the skin and also microorganisms may infiltrate the stem scar tissue and blossom ends of tomatoes (Lang et al., 2004).

The effect of washing with ASC (LA) followed by a second wash with DW, LA, or Chito on reducing viable cells of dip-inoculated E. coli O157:H7 is shown in Table 3. Secondary wash was performed at day 0, and thereafter these samples were stored at 10°C for 2 days to see the effectiveness of the sanitizers used that suppressed the pathogen population during storage. ASC (LA) wash followed by a second wash with LA exhibited additional 0.7 (enumerated by TSA-Rif) or 0.9 (enumerated by SMAC-Rif) log CFU/g reduction of E. coli O157:H7 populations compared to single washing (p < 0.05). ASC (LA) wash followed by a second wash with Chito exhibited additional 1.3 log CFU/g (enumerated by TSA-Rif or SMAC-Rif) sanitary effectiveness compared to single washing (p < 0.05) (Table 3). No significant (p > 0.05) increase in population was observed after 2 days of storage at 10°C (Table 3) as enumerated by TSA-Rif. However, an increased number of injured cells were observed after 2 days of storage as enumerated by SMAC-Rif. This result indicates that ASC (LA) washing followed by a second wash with LA or Chito could reduce >3.0 log E. coli O157:H7 populations in dip-inoculated samples and that the populations remain constant during 2 days of storage at 10°C.

Table 3.

Recovery of Inoculated Escherichia coli O157:H7 on Tomatoes After Primary and Secondary Washing Treatment ^a

		Recovered Escherichia coli O157:H7 (log CFU/g) ^b
	Samples	TSA-Rif	SMAC-Rif
	Unwashed	6.8 ± 0.1 A	6.6 ± 0.1 A
Day 0	1st washing ASC (LA)	4.2 ± 0.7 B	3.6 ± 0.6 B
	2nd washing DW	4.1 ± 0.3 B	3.5 ± 0.4 B
	LA	3.5 ± 0.4 C	2.7 ± 0.4 CD
	Chito	2.9 ± 0.3 D	2.3 ± 0.1 E
Day 2	2nd washing DW	4.1 ± 0.2 B	2.9 ± 0.3 C
	LA	3.5 ± 0.4 C	2.3 ± 0.1 E
	Chito	3.1 ± 0.6 CD	2.5 ± 0.3 DE

Dip-inoculated samples were first washed by ASC (LA). Secondary washing was performed by DW, LA, or Chito. Each of washed samples was stored at 10°C for 2 days.

The values with different letters within the same column differ significantly (p < 0.05). The detection limit was 2.3 log CFU/g.

For the overall quality determination of treated and nontreated tomatoes, color, taste, and texture parameters were analyzed and presented in Table 4. All the three preference parameters (color, texture, and taste) evaluated by the judges of 30 panels were within the preferable range (7.7 to 6.2). The value corresponding to most preferable was 10.0, most nonpreferable was 0.0, and the rejection value was set as 5.0 (Table 4). There was no significant difference observed among the washing treatment group (p > 0.05). From this result, washing with ASC (LA) followed by a second wash with either by LA or by Chito showed similar quality evaluation compared to washing tomatoe with NaClO followed by a second wash with DW.

Table 4.

Sensory Evaluation of Washed Tomatoes

Primary and secondary washing	Color ^a	Texture ^a	Taste ^a
NaClO → DW	7.5 ± 2.4 A	7.1 ± 2.1 A	6.9 ± 3.2 B
Ca-SS → DW	7.5 ± 2.2 A	7.1 ± 2.3 B	7.2 ± 2.7 A
Phy → DW	7.6 ± 2.0 B	6.7 ± 2.4 A	6.7 ± 2.6 B
ASC → DW	7.7 ± 2.1 C	7.4 ± 2.1 A	6.6 ± 2.6 C
ASC → LA	7.1 ± 2.2 A	6.8 ± 2.3 C	6.8 ± 2.2 A
ASC → Chito	7.4 ± 1.9 B	6.6 ± 2.6 A	6.2 ± 2.7 A

The values with different letters within the same column differ significantly (p < 0.05). The value corresponding to most preferable was 10.0, most nonpreferable was 0.0, and the rejection value was set as 5.0.

“→” means the order of twice washing.

In conclusion, a single washing with ASC was more effective than sodium hypochlorite and other sanitizers to reduce E. coli O157:H7 population on dip-inoculated cherry tomatoes. For spot-inoculated tomatoes ASC provided the same level of inactivation as sodium hypochlorite, Ca-Oy, and Ca-SS. ASC followed by a second wash with chitosan or LA gave approximately an additional 1.0 log inactivation, and E. coli O157:H7 population was maintained at the same level during storage at 10°C for 2 days.

Footnotes

Acknowledgments

This work was supported by a grant from the Ministry of Agriculture, Forestry and Fisheries of Japan (research project for ensuring food safety from farm to table DI-7203).

Disclaimer

Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.

Disclosure Statement

No competing financial interests exist.

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