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Food Microbiology 28 (2011) 1275e1283

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Food Microbiology

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Application of high hydrostatic pressure to decontaminate green onionsfrom Salmonella and Escherichia coli O157:H7

Hudaa Neetoo, Sanaz Nekoozadeh, Zheng Jiang, Haiqiang Chen*

Department of Animal and Food Sciences, University of Delaware, Newark, DE 19716-2150, USA

a r t i c l e i n f o

Article history:Received 22 December 2010Received in revised form11 May 2011Accepted 13 May 2011Available online 23 May 2011

Keywords:Green onionsHigh pressureEscherichia coli O157:H7Salmonella

* Corresponding author. Tel.: þ1 302 831 1045; faxE-mail address: haiqiang@udel.edu (H. Chen).

0740-0020/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.fm.2011.05.005

a b s t r a c t

Consumption of fecally contaminated green onions has been implicated in several major outbreaks offoodborne illness. The objectives of this studywere to investigate the survival and growth of Salmonella andEscherichia coli O157:H7 in green onions during storage and to assess the application of high hydrostaticpressure (HHP) to decontaminate green onions fromboth pathogens. Bacterial strains resistant to nalidixicacid and streptomycin were used to inoculate green onions at low (w1 log cfu/g) and high (w2 log cfu/g)inoculum levels which were then kept at 4 or 22 �C for up to 14 days. Both pathogens grew to an average of5e6 log cfu/g during storage at 22 �C and the bacterial populationswere fairly stable during storage at 4 �C.High-pressure processing of inoculated green onions in the un-wetted, wetted (briefly dipped inwater) orsoaked (immersed in water for 30 min) conditions at 250e500 MPa for 2 min at 20 �C reduced the pop-ulation of Salmonella and E. coliO157:H7 by 0.6 to>5 log cfu/g, depending on the pressure level and samplewetness state. The extent of pressure inactivation increased in the order of soaked>wetted> un-wettedstate. Thepressure sensitivityof thepathogenswas alsohigher at elevated treatment temperatures.Overall,after pressure treatment at 400e450 MPa (soaked) or 450e500 MPa (wetted) for a retention time of 2 minat 20e40 �C, wild-type and antibiotic-resistantmutant strains of Salmonella and E. coliO157:H7 inoculatedon green onionswereundetectable immediately after treatment and throughout the 15-day storage at 4 �C.Thepressure treatments alsohadminimal adverse impactonmost sensorial characteristics aswell as on theinstrumental color of chopped green onions. This study highlights the promising applications of HHP tominimally process green onions in order to alleviate the risks of Salmonella and E. coli O157:H7 infectionsassociated with the consumption of this commodity.

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1. Introduction

The increasing popularity of minimally processed fruits andvegetables has been attributed to the increasingly recognizedhealth benefits associated with the consumption of fresh produce,combined with the propensity of consumers to eat out or buyready-to-eat and convenience foods (Alzamora et al., 2000). Greenonion (Allium cepa) is one such commodity that is widely used inthe USA, either as a seasoning, herb, garnish orminor component ofa meal in oriental andMexican foods such as salads, dips and soups.Buyers are in constant demand of green onions on a year-roundbasis and as a result, shippers in North America typically sourcethe commodity from both Mexico and the USA depending onseason and availability. Unfortunately, imported green onions have

: þ1 302 831 2822.

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already been linked to three microbial foodborne outbreaks in theUSA, including four deaths and 1028 illnesses (Calvin et al., 2004).Green onions are thought to be particularly susceptible tocontamination because the plant surfaces are complex or adherentto fecal particles (Dato et al., 2003) that may act as carriers offoodborne pathogens. In addition, the unique morphology of thisvegetable, characterized by its moist hollow tube leaf, provides notonly ample opportunity for bacteria hidden in the leaf cavity toamplify to hazardous levels (FAO, 2008) but also protection fromwashing and chemical sanitizers. Furthermore, since green onionsare often used in small quantities as a garnish in various types offood, a relatively small volume of contaminated product couldpotentially lead to exposure of a large number of people (Calvinet al., 2004).

Pathogenic bacteria such as Salmonella and Shigella flexneri andviruses such as hepatitis A virus that are often transmitted via thefecal-oral route have all been linked to green onion contamination(California Department of Food and Agriculture, 2010). An outbreak

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e12831276

of S. flexneri infection in 1994 was linked to green onions grown ona single farm in Mexico and distributed through shippers in Cal-ifornia (U.S. FDA, 2001). In 2003, a hepatitis A outbreak occurreddue to the use of fecally contaminated onions as an ingredient inseveral dishes served at a Pennsylvanian restaurant (CDC, 2003).Even though the Centers for Disease Control and Prevention havenot recorded any past foodborne illness outbreaks with Salmonellain green onions, there were voluntary recalls issued in the summerof 2009 due to suspected contamination with the pathogen(California Department of Food and Agriculture, 2010), causingheightened concerns among the industry and regulatory sectors.

Foodborne pathogens can contaminate vegetables such as greenonions at any stage along the farm to table continuum e duringcultivation (pre-harvest) or during post-harvest handling (Wachtelet al., 2002; Islam et al., 2004). Green onions, like radishes andother crops that are hand-bunched, tend to involve more intensivehand labor during the harvesting and packing process than mostfruits and vegetables, thus increasing the probability of microbialcontamination. Due to the practice of hand-harvesting, qualitysorting, bunching and packing of these commodities, there arenumerous “touch points” early in the supply chain, each consti-tuting a potential opportunity for a contamination event. Hence,establishing control points at the post-harvest (handling) stage is ofparamount importance if the occurrence of such a hazard is to bereduced.

Decontamination of fresh produce is also an important stepagainst pathogenic and spoilage microorganisms. Severalresearchers have investigated the use of chemical and physicalmeans to improve the microbiological safety and quality of onionsand green onions. Fino and Kniel (2008) previously investigatedthe efficacy of UV light to inactivate hepatitis A virus on greenonions. Park et al. (2008, 2009) investigated the antimicrobialefficacy of electrolyzed water and acidic electrolyzed water againstEscherichia coli O157:H7, Salmonella Typhimurium and Listeriamonocytogenes on green onions. Kim et al. (2005) treated fresh-cutgreen onions with warm water alone or in combination withirradiation at 0, 0.5, 1.0 and 1.5 kGy in order to improve theirmicrobiological quality. Although they observed that the applica-tion of thermal treatments brought about a reduction in microbialpopulations, they reported that the beneficial effect disappearedduring storage. Fan et al. (2003) studied the effect of irradiation atdoses of 1, 2, and 3 kGy on the background microflora of fresh-cutgreen onion leaves. The authors observed that doses> 2 kGy wererequired in order to improve the microbiological quality and safetyof green onions, with such treatments, however, resulting in anincreased loss of aroma and deterioration of visual quality. On thecontrary, high hydrostatic pressure (HHP) has been demonstratedto effectively inactivate foodborne pathogens and spoilage flora ona wide range of fruits and vegetables with minimal collateraleffects on their sensory and nutritional characteristics (Yaldagardet al., 2008). Moreover with HHP, pressure transmission isuniform and quasi-instantaneous (isostatic principle), making itbetter suited to target pathogens located on the surface, subsur-face or internal tissues of produce compared to other interven-tions. Although the application of HHP to inactivate viralpathogens on green onions was previously investigated (Kingsleyet al., 2005), to our knowledge, the use of this intervention toinactivate enteric bacterial pathogens on this commodity has notbeen pursued.

The objectives of this study were therefore to investigate thegrowth and persistence of Salmonella and Escherichia coli O157:H7in green onions during storage and to assess the application of HHPas a non-thermal processing method to decontaminate greenonions from both pathogens with minimal adverse effect on theirphysical and sensorial properties.

2. Materials and methods

2.1. Bacterial strains and inoculum preparation

Threewild-type E. coli O157:H7 strains (250, Cider strain and DD3795) and five wild-type Salmonella enterica strains (Typhimuriumstrains (T43, T45 and TDT 104), WorthingtonW35 and MontevideoMo57) from the University of Delaware culture collection wereused. The individual wild-type strains were adapted to be able togrow in the presence of 100 mg/ml of nalidixic acid (Fisher Scien-tific, Hampton, NH, USA) and 100 mg/ml of streptomycin (Sigma, St.Louis, MO, USA). Individual cultures were grown in tryptic soybroth (Difco Laboratories, Sparks, MD, USA) plus 0.6% yeast extract(Difco) (TSBYE) (for wild-type strains) or TSBYE supplemented with100 mg/ml of nalidixic acid and 100 mg/ml of streptomycin (TSBYE-NS) (for mutant strains) overnight at 35 �C. Cultures were thentransferred into 10 ml of fresh TSBYE (for wild-type strains) orTSBYE-NS (for mutant strains) and incubated at 35 �C for 24 h.Equal volumes of individual cultures were mixed to form a three-strain cocktail of E. coli O157:H7 mutants, a three-strain cocktailof E. coli O157:H7 wild-types, a five-strain cocktail of Salmonellamutants, and a five-strain cocktail of Salmonella wild-types. Dilu-tions of the four cocktails were prepared by diluting appropriatevolumes of the culture composites into sterile 0.1% peptone water.

2.2. Survival and growth of Salmonella and E. coli O157:H7 in greenonions during storage

Fresh “clipped tops” green onions (Allium cepa) were purchasedfrom a local grocery store and stored at 4�1 �C and used within 2days of purchase. The full-length green onions were cut intosegments weighing w5 g and these portions partly included thecompressed stem and the foliage cluster. A volume of 50 ml of thediluted cocktail of Salmonella or E. coli O157:H7 mutants was spot-inoculated using a modified method of Long et al. (2010) onto theupper and lateral surfaces of cut sections to achieve a final concen-trationof approximately1 log cfu/g (low inoculum load) or 2 log cfu/g (high inoculum load). The onions were air-dried in a bio-safetyhood at ambient temperature (22�1 �C) for 24 h. The inoculatedsamples were then placed into sterile bags and stored at either4�1 �C for 14daysor ambient temperature for 8days. Samplesweremicrobiologically analyzed at selected time intervals by transferringthem into stomacher bags to which 20 ml of sterile 0.1% peptonewater was added and subsequently stomached for 2 min (Seward400 Stomacher, SewardMedical Co., London, U.K.). Themixturewasserially diluted in sterile 0.1% peptone and surface-plated in dupli-cate on tryptic soyagarwith0.6%yeast extract (Difco) supplementedwith 100 mg/ml of nalidixic acid and 100 mg/ml of streptomycin(TSAYE-NS). TSAYE-NS plates were incubated for 3 days at 35 �C.Colonies formed on the plates were enumerated.

2.3. Pressure inactivation of Salmonella and E. coli O157:H7 in un-wetted, wetted or soaked green onions

Green onions were inoculated with the mutant cocktail ofSalmonella or E. coliO157:H7 as described above to afinal populationdensity ofw5 log cfu/g and then air-dried in the bio-safety hood for24 h prior to high-pressure treatment. Inoculated samples (5 g)were subjected to one of the three treatments: un-wetted (nowatertreatment), wetted (dipped in sterile deionized (DI) water for 30 s)and soaked (immersed in sterile DI water for 30 min). The wateractivity of greenonion samples in the un-wetted,wettedand soakedstateswasdeterminedusingadewpointhygrometer,AqualabSeries3B (Decagon Devices, Pullman, Washington, USA) at 23e24 �C. Thesamples were then inserted into sterile pouches, heat-sealed and

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e1283 1277

placed in an outer pouch, which was then double-sealed to avoidleakage during pressure treatment. The inoculated samples weretreated at pressure levels ranging from 250 to 500 MPa (in 50-MPaincrements) for 2 min at 20 �C (initial sample temperature) usinga high-pressure unit with temperature control (Model Avure PT-1,Avure Technologies, Kent, WA). The pressure-come-up rate wasapproximately 22 MPa/s. The pressure-release was almost imme-diate (<4 s). Pressurization time reported in this study did notinclude the pressure-come-up or release times. Temperatureincreases during pressure treatment of green onion samples were2.8, 3.0 and 3.3 �C/100 MPa at 20, 30 and 40 �C, respectively.

After pressure treatments, the treated and untreated sampleswere stomached and subjected to microbiological analysis onTSAYE-NS as described in the previous section. The green onionsamples were also directly enriched in 80 ml of TSBYE-NS andincubated for 48 h at 35 �C to allow selective resuscitation of sub-lethally injured cells. Samples were then streaked onto SorbitolMacConkey (Difco) supplemented with nalidixic acid (100 mg/ml)and streptomycin (100 mg/ml) (SMAC-NS) for E. coli O157:H7 andonto Xylose Lysine Desoxycholate agar (Difco) supplemented withnalidixic acid (100 mg/ml) (XLD-N) for Salmonella. A preliminarystudy in our laboratory indicated that XLD-N and SMAC-NS wereoptimal selective media for accurate detection of survivors ofSalmonella and E. coli O157:H7 antibiotic-resistant mutants sincethey did not yield false-positive or false-negative results. After 24 hof incubation, presence of growth exhibiting morphological andbiochemical characteristics typical of E. coli O157:H7 or Salmonellawas determined by visually inspecting the plates.

2.4. Effect of treatment temperature on pressure inactivation ofSalmonella and E. coli O157:H7 in wetted and soaked green onions

Since soaking was generally more effective than wetting for theinactivation of both pathogens, lower pressure levels(250e450 MPa) were used for the soaked state and higher pressurelevels (450e550 MPa) for the wetted state. Soaked green onionsinoculated with the cocktail of mutant strains of Salmonella or E. coliO157:H7andpackaged in sterilepoucheswere submerged for10 minin the water bath surrounding the pressure cell set at 4, 20, 30 or40 �C for samples to equilibrate to thewater bath temperature beforebeing treated at 250e400 MPa for 2 min. After pressure treatment,treated and untreated samples were immediately placed in icedwater to instantly cool the samples prior to determination of countsand enrichment. Since low temperature (4 �C) did not enhancepressure inactivation of the bacteria,wetted green onions inoculatedwith Salmonella andE. coliO157:H7were treated at450e550 MPa for2 min only at higher temperature levels of 20, 30 and 40 �C.

2.5. Validation of HHP treatments in ensuring safety of green onionsthroughout its refrigerated shelf-life

The experiments conducted above enabled the identification offour pressure conditions that resulted in complete inactivation ofboth Salmonella and E. coli O157:H7 mutants in wetted or soakedgreen onions: 500 MPa at 20 �C or 450 MPa at 40 �C in the wettedstate, and 450 MPa at 20 �C or 400 MPa at 40 �C in the soaked state,all for 2 min. To ensure that these treatments could consistentlyeliminate the pathogens and prevent any subsequent outgrowthduring their refrigerated shelf-life, green onions (w5 g) were inoc-ulated with the cocktail of mutant strains of Salmonella or E. coliO157:H7 to a final density of w5 log cfu/g and pressure treated atthe four aforementioned conditions. Immediately after pressuretreatment, samples were cooled in an ice-water mixture. Thepressure-treated green onions were microbiologically analyzedimmediately after pressure treatment (day 0) and stored at 4 �C for

5, 10 and 15 days and analyzed as described before. In order todetermine if the development of nalidixic acid and streptomycinresistance could have significantly altered the pressure resistance ofthe marker strains when compared to their wild-type counterparts,the aforementioned processing conditions were also validated withwild-type strains of Salmonella and E. coli O157:H7. Briefly, greenonions were inoculated with wild-type strains of Salmonella orwild-type strains of E. coli O157:H7 to a density of w5 log cfu/g andpressure-treated at the four identified conditions. To test for thepresence of Salmonella and E. coli O157:H7 in the treated samples,the official methods were used. Briefly, Salmonella-inoculated andprocessed samples were pre-enriched in Lactose broth (80 ml) for24 h at 35 �C. After the pre-enrichment step, 0.1 ml of the mixturewas transferred to 10 ml of RappaporteVassiliadis (RV)medium andincubated for another 24 h at 42 �C. After the secondary enrichmentstep, the samples were then streaked onto XLD plates and incubatedat 35 �C (U.S. FDA, 2007). E. coli O157:H7-inoculated samples wereenriched in 80 ml of EHEC Enrichment Broth for 24 h at 35 �C. Afterthe enrichment step, the sampleswere then streaked onto CT-SMACplates and incubated for 24 h (U.S. FDA, 2002).

2.6. Effect of pressure treatment on the color and texture of greenonions

Surface color readings of untreated and pressure-treated greenonion bulbs, stems and leaf blades were independently performedwith a chromameter (Minolta CR-400, Minolta, Osaka, Japan) every5 days throughout the 15-day storage period. Color parameterswere quantified in the Hunter L, a, and b color space where L refersto lightness, ranging from 0 (blackness) to 100 (whiteness), positivea means red and negative a means green, and positive b meansyellow and negative b means blue.

The texture of untreated and pressure-treated green onionbulbs, stems and leaf blades was independently determined usingthe TA.XT2i texture analyzer and a TA-91 Kramer shear probe(Texture Technologies Corp., N.Y., USA). The samples were placedon the TA-91 platform and penetration tests were carried out withthe probe set to move down at a pre-test speed of 6.0 mm/s, testspeed of 3.0 mm/s and post-test speed of 10 mm/s and indent thesample through a test distance of 5 mm. The maximum force (Fmax)needed to compress the samples was recorded using the TextureExpert Exceed software (Texture Technologies) and used as a firm-ness measurement (Liu and Li, 2006).

2.7. Sensory evaluation

Sensory evaluation (overall visual appearance, color, aroma andtexture) of chopped green onions was performed using a nine-pointhedonic scale (9¼ excellent quality and 1¼ extremely poor quality)by 7 judges every 5 days throughout the 15-day storage period. Theuntrained panelists independently conducted subjective assess-ments on each sampling day for all samples. Reference samples thatcorresponded to each scale point were available for judges to ratethe samples. The cut-off score was fixed at 5 (Liu and Li, 2006).

2.8. Statistical analyses

All experiments were replicated at least three times. Whereappropriate, statistical analyses were performed on datausing Minitab� Release 15 (Minitab Inc., University Park, PA, USA).One-way analysis of variance (ANOVA) and Tukey’s one-waymultiple comparisons were used to determine differences in thebacterial populations and the physical and sensorial properties ofpressure-treated green onions. Significant differences wereconsidered at the 95% confidence level (P< 0.05).

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e12831278

3. Results and discussion

3.1. Survival and growth of Salmonella and E. coli O157:H7 in greenonions during storage

At refrigeration temperature, Salmonella and E. coli O157:H7were unable to grow in green onions, but able to survive at bothhigh (w2 log cfu/g) and low inoculum load (w1 log cfu/g) (Fig. 1).Microorganisms often survive at refrigerated temperatures eventhough these conditions reduce or eliminate the ability of theorganisms to multiply (U.S. FDA, 2001). Survival of foodbornepathogens on produce is significantly enhanced especially if theprotective epidermal barrier has been broken either by physicaldamage, such as punctures or bruising, or by degradation by plantpathogens (bacteria or fungi) (U.S. FDA, 2001). Since the inoculawere applied superficially on the uncut epidermal surface as well asinjected into the transversely cut (injured) ends, we expected thepathogens to adhere and survive better on the tissues. Otherresearchers have similarly reported that microorganisms tend topersist at refrigeration temperatures on the surface of cut, choppedor injured tissue of various produce items including apples(Dingman, 2000), cantaloupes (Del Rosario and Beuchat, 1995),lettuce (Davis et al., 1988), tomatoes (Asplund and Nurmi,1991) andsprouts (Castro-Rosas and Escartin, 2000). With fresh-cut produce,the action of cutting can cause release of plant cellular fluids,providing a nourishing environment for microorganisms.

When green onions were stored at ambient temperature (Fig. 1),thepopulationofE. coliO157:H7 increasedby3.8e4.7 log cfu/gwhile

7

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FU /

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Fig. 1. Growth curves of Escherichia coli O157:H7 (A) and Salmonella enterica (B) ingreen onions artificially inoculated at a low (w1 log cfu/g) or high (w2 log cfu/g) levelduring refrigerated (4 �C) and ambient temperature (22 �C) storage. Error bars shownrepresent one standard deviation.

the population of Salmonella rose by 3.6e4.6 log cfu/g. The ability ofpathogens to proliferate is influenced by environmental (storage)parameters such as temperature as well as the intrinsic character-isticsorphysiological stateof the freshproduce item.Conditions suchas the presence of available moisture (Beattie and Lindow, 1995,1999) and nutrients can greatly influence bacterial populations andpromote the multiplication of pathogens, especially at non-refrigerated temperatures (U.S. FDA, 2001). Although the physicalenvironmentof thegreenonion leaf bladesand stemsurfacesmaybeinhospitable for the growth of enteric bacteria due to the presence ofthe plant’s natural barriers such as the cell walls and wax layers aswell as lack of nutrients and free moisture, the higher moisturecontent and nutrient availability in the injured (cut) areas couldpromote relatively faster microbial growth. Numerous authors havesimilarly demonstrated the increase in population density of E. coliO157:H7 and Salmonella during storage at ambient temperatures(22e26 �C) on various fresh-cut product items such as melons(Fredlund et al., 1987), papaya cubes (Castillo and Escartin, 1994),apples (Dingman, 2000), tomatoes (Weissinger et al., 2000) andjuices (Mutaku et al., 2005). Ma et al. (2010) and Liao et al. (2010)showed that Salmonella was able to survive on whole jalapenopeppers without growth but proliferated on inoculated chopped(injured) pieces of pepper. Hence our study shows that fresh greenonions that may become accidentally exposed to Salmonella andE. coliO157:H7 as a result of agricultural pre-harvest or post-harvestpracticesmayenable the pathogens to growor survive dependingonthe storage temperature.

3.2. High-pressure inactivation of pathogens in un-wetted, wettedor soaked green onions

Our results for the high-pressure inactivation of Salmonella andE. coli O157:H7 are shown in Table 1. The efficacy of pressureinactivation varied as a function of the pressure magnitude for bothpathogens. When green onions were treated in each of the threedifferent states, a steady decrease in the populations of Salmonellaand E. coli O157:H7 was observed at increasing pressure levels. Inaddition, the efficacy of HHP also depended on the state of thegreen onions and ranked in the order of soaked>wetted> un-wetted state. Green onions in the un-wetted state, just like mostfruits and vegetables, had a water activity of ca. 0.95 while thewater activity of green onions in the wetted and soaked states wascloser to one (0.995). The water activity of the product in differentstates of wetness is thought to differentially affect the resistance ofinoculated pathogens to pressure with greater inactivation athigher water activity as observed by Neetoo et al. (2008, 2009).Oxen and Knorr (1993) showed that a reduction of water activityfrom 0.98e1.00 to 0.94e0.96 resulted in a marked reduction ininactivation rates for microbes suspended in a food. The increasedbaro-resistance of microorganisms at low water activity may beattributed to a partial cell dehydration due to the osmotic pressuregradients between the internal and external fluids, causing cells toshrink, acquire thicker membranes, and consequently an increasedpressure resistance. The baro-protective effect of reduced wateractivity reveals that inactivation of microorganisms by high pres-sure depends not only on pressure and extent of the treatment, butalso on the interactions with other intrinsic variables such as wateractivity (Palou et al., 1997a,b).

3.3. Effect of temperature on HHP inactivation of Salmonella andE. coli O157:H7 in green onions

Table 2A shows the outcomes of pressure inactivation ofSalmonella and E. coliO157:H7 in green onions in the soaked state atlow (4 �C) and elevated temperatures (>20 �C). An increase in the

Table 1Pressure inactivation of Salmonella enterica and Escherichia coli O157:H7 in green onions in the un-wetted, wetted and soaked states.

Treatments Salmonella Escherichia coli O157:H7

Un-wetted Wetted Soaked Un-wetted Wetted Soaked

Initial inoculum 5.5� 0.3A 5.4� 0.4A 5.5� 0.3A 5.0� 0.1A 5.0� 0.1A 5.1� 0.1A

Wetting/soaking only N/A 5.1� 0.4A 4.8� 0.3A N/A 4.7� 0.1A 4.3� 0.1A

HHP at 250 MPa/20 �C 4.2� 0.3A 3.5� 0.7A 3.1� 0.7A 4.4� 0.2A 3.7� 0.3A 3.6� 0.3A

HHP at 300 MPa/20 �C 3.8� 0.4A 3.1� 0.3A 2.4� 0.6A 4.2� 0.3A 2.8� 0.1A 2.9� 0.9A

HHP at 350 MPa/20 �C 3.3� 0.7A 2.3� 0.5A 1.7� 0.7A 3.6� 0.5A 2.0� 0.4B 1.8� 0.9B

HHP at 400 MPa/20 �C 2.8� 0.5A <0.7 (1/3)B <0.7(0/3)B 3.3� 0.5A 1.3� 0.5B <0.7(1/3)C

HHP at 450 MPa/20 �C 2.0� 0.4A <0.7 (1/3)B <0.7(0/3)B 2.8� 0.4A <0.7(0/3)B <0.7(0/3)B

Data representing mean log survivors (cfu/g)� standard deviation. Numbers in parentheses represent the number of samples testing positive after enrichment out of a total of3 trials. “Wetting/soaking only” means “Wetting or soaking before HHP”. “N/A” means Not Applicable. The limits of detection were 0.7 log cfu/g for the plating method and1 cfu/5 g for the enrichmentmethod. Values in the same rowwithin the same category (Salmonella or Escherichia coliO157:H7) followed by the same letter are not significantlydifferent (P> 0.05).

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e1283 1279

processing temperature above ambient temperature (30e40 �C)generally enhanced pressure inactivation achieving elimination ofboth enteric pathogens at a pressure level of 400 MPa and treat-ment temperature of 40 �C. On the contrary, pressurization ata reduced temperature of 4 �C did not enhance bacterial pressureinactivation. In the light of findings reported in Table 2A, we thensubsequently investigated the efficacy of the conjunct applicationof high pressure at a magnitude of�450 MPawith mild heating (30and 40 �C) on wetted green onions (Table 2B). Under all theaforementioned conditions shown in Table 2B, the load of Salmo-nella and E. coli O157:H7 was reduced by over 5 logs with elimi-nation at pressures ranging from 450 to 550 MPa and temperaturesof 30e40 �C.

Several authors have reported that the temperature duringpressurization can significantly affect the lethality of the processand that pressure inactivation of microbes is greatest at tempera-tures higher or lower than normal growth temperatures. Inparticular, temperatures above 30 �C have been shown to enhancethe pressure inactivation of pathogens such as Salmonella Enter-itidis (Ponce et al., 1999) and E. coli (Ponce et al., 1998a). Similar toother authors, we also observed that moderate heating enhancedmicrobial inactivation of inoculated microorganisms in greenonions when treated at 30 and 40 �C at pressures ranging from 250to 450 MPa. Although there are several reports in the literatureshowing the correlative effect of reduced treatment temperaturesand increased pressure sensitivity of bacteria such as Pseudomonasfluorescens (Lopez-Caballero et al., 2002) and Listeria innocua(Ponce et al., 1998b), our findings did not reveal an increase in theinactivation rate at the sub-ambient treatment temperature of 4 �C.On the contrary, sub-ambient treatment temperature increased thepressure resistance of both pathogens at pressures of250e400 MPa. These observations are congruent with our previousfindings on the limited extent of pressure inactivation of E. coliO157:H7 on alfalfa seeds at refrigeration temperature (Neetoo et al.,2009).

Table 2APressure inactivation of Salmonella enterica and Escherichia coli O157:H7 in green onions

Treatments Salmonella

4 �C 20 �C 30 �C 40

Initial inoculum 5.9� 0.4A 5.9� 0.4A 5.9� 0.4A 5.9Soaking only 5.4� 0.1A 5.4� 0.1A 5.4� 0.1A 5.4HHP at 250 MPa 3.7� 0.3A 3.5� 0.2A 3.2� 0.4A 2.9HHP at 300 MPa 2.9� 0.5A 2.5� 0.4A 1.7� 0.0A 1.7HHP at 350 MPa 2.2� 0.6A 1.5� 0.5AB 1.1� 0.0AB <0HHP at 400 MPa 0.4� 1.0 (2/3)A <0.7 (0/3)A <0.7 (0/3)A <0

Data representing mean log survivors (cfu/g)� standard deviation. Numbers in parenthes3 trials. “Soaking only” means “Soaking before HHP”. The limits of detection were 0.7 logsame row within the same category (Salmonella or Escherichia coli O157:H7) followed b

Hence moderate heating at 40 �C can potentiate microbialinactivation by pressure and potentially reduce the severity andcost of the high-pressure treatment. Indeed, the practice ofcombining preservation technologies (hurdle concept) tocompensate for shortcomings of individual processes, and tocircumvent the use of extremely high levels of a single treatmenthas proven to be very effective for various applications of HHP infood (Shearer et al., 2000) and may have considerable potential forcost-effective processing of green onions for safety.

3.4. Efficacy of HHP treatments to ensure absence of entericpathogens in green onions during refrigerated storage

Green onions have an estimated shelf-life of 15 days at 4 �C andsuch an extended low temperature storage can potentially allowthe recovery and outgrowth of pressure-injured cells (Carlez et al.,1994). To ensure that the aforementioned selected HHP treatmentscan eliminate both pathogens and ensure no outgrowth duringstorage, green onions inoculated with a w5 log cfu/g initial load ofSalmonella mutants or E. coli O157:H7 mutants were subjected topressure treatment for 2 min at (i) 450 MPa and 20 �C (soaked), (ii)500 MPa and 20 �C (wetted), (iii) 400 MPa and 40 �C (soaked) or(iv) 450 MPa and 40 �C (wetted). Samples analyzed immediatelypost-HHP and at various intervals during storage produced nodetectable survivors after enrichment for all three trials conducted(data not shown). The pressure treatments were also able toeliminate wild-type Salmonella and E. coli O157:H7 strains in greenonions since no survivors were detected throughout the 15-dayrefrigerated storage (data not shown).

Our findings therefore provide greater assurance that pressuretreatments eliminated the pathogens and that subsequent refrig-eration of pressure-treated green onions did not provide anopportunity for recovery or outgrowth of injured pathogenicbacterial cells. Indeed, the significance of injured microorganismsin pressure-treated foods should not be overlooked. Pathogenic

in the soaked state at reduced or elevated temperatures.

Escherichia coli O157:H7

�C 4 �C 20 �C 30 �C 40 �C

� 0.5A 5.2� 0.3A 5.2� 0.3A 5.2� 0.3A 5.2� 0.3A

� 0.1A 4.4� 0.2A 4.4� 0.2A 4.4� 0.2A 4.4� 0.2A

� 0.5A 3.9� 0.0A 3.5� 0.5A 3.1� 0.3A 3.2� 0.2A

� 0.2A 3.1� 0.5A 2.6� 0.8A 2.6� 0.3A 2.6� 0.4A

.7 (0/3)B 2.9� 0.2A 1.5� 1.2A 1.5� 0.7A 1.2� 1.1A

.7 (0/3)A 2.6� 0.2A <0.7 (1/3)B <0.7 (1/3)B <0.7 (0/3)B

es represent the number of samples testing positive after enrichment out of a total ofcfu/g for the plating method and 1 cfu/5 g for the enrichment method. Values in they the same letter are not significantly different (P> 0.05).

Table 2BPressure inactivation of Salmonella enterica and Escherichia coli O157:H7 in green onions in the wetted state at elevated temperatures.

Treatments Salmonella Escherichia coli O157:H7

20 �C 30 �C 40 �C 20 �C 30 �C 40 �C

Initial inoculum 5.9� 0.4A 5.9� 0.4A 5.9� 0.4A 5.2� 0.3A 5.2� 0.3A 5.2� 0.3A

Wetting only 5.6� 0.1A 5.6� 0.1A 5.6� 0.1A 4.7� 0.2A 4.7� 0.2A 4.7� 0.2A

HHP at 450 MPa <0.7 (2/3) <0.7 (1/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3)HHP at 500 MPa <0.7 (0/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3)HHP at 550 MPa <0.7 (0/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3) <0.7 (0/3)

Data representing mean log population (cfu/g)� standard deviation. Numbers in parentheses represent the number of samples testing positive after enrichment out of a totalof 3 trials. “Wetting only” means “Wetting before HHP”. The limits of detection were 0.7 log cfu/g for the plating method and 1 cfu/5 g for the enrichment method.

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e12831280

cells that are damaged or injured and initially unable to recovermay be present in the post-processing stage and may still posea concern since theymay be capable of repair and toxin production.Bozoglu et al. (2004) reported that the fate of pressure-injured cellsdepends on the prevailing conditions after HHP. Pressure-injuredcells can repair in a medium containing necessary nutrients whengiven appropriate growth conditions. This can be a problem espe-cially in low-acid food such as fresh vegetables as the recovery ofinjured cells during storage may possibly result in food-bornedisease or spoilage. Studies on high-pH foods that contain nomicrobial inhibitors have shown recovery of microorganismsduring storage following HHP treatment at less than 600 MPa(Carlez et al., 1994). Carlez et al. (1994) reported that Pseudomonasspp. were not detected immediately after HHP treatment in mincedmeat treated at 400 MPa for 20 min but were detected after 6 daysat 3 �C storage, hence underscoring the importance of microbio-logical analysis at various time-points through its storage life toensure that the commodity is still safe for consumption.

The use of antibiotic-resistant strains generally facilitatesprocess validation studies in which inoculated bacteria are easilydistinguished from background flora present in the food product.Antibiotic-resistant double mutants used as marker strains in thisstudy facilitated recovery on non-selective media especially in thepresence of background flora; however, antibiotic-resistantmutants may potentially exhibit differences in their serotype,sensitivity to stresses or overall growth characteristics (Eisenstadtet al., 1994). Mann (2005) mentioned that during selection of

Table 3L values (whiteness), b values (yellowness) and a values (greenness) of untreated and presat 4 �C.

Treatments

0 5

L value of bulbsControl 48.7� 0.4aA 51.1Soaked/400 MPa/40 �C 54.5� 4.1aA 52.1Soaked/450 MPa/20 �C 53.5� 4.0aA 54.2Wetted/500 MPa/20 �C 47.2� 4.0aA 50.6Wetted/450 MPa/40 �C 54.9� 4.1aA 51.5

b value of compressed stemsControl 17.4� 3.0aA 14.9Soaked/400 MPa/40 �C 20.4� 1.1aA 13.8Soaked/450 MPa/20 �C 20.8� 0.6aA 17.9Wetted/500 MPa/20 �C 18.7� 2.1aA 18.6Wetted/450 MPa/40 �C 17.7� 1.5aA 13.8

a value of leaf bladesControl �2.9� 3.6aA �4.3Soaked/400 MPa/40 �C �5.1� 1.2aA �4.1Soaked/450 MPa/20 �C �3.8� 0.9aA �1.9Wetted/500 MPa/20 �C �6.1� 0.3aA �4.9Wetted/450 MPa/40 �C �2.8� 0.8aA �1.5

Data representing mean color values� standard deviation. For the same color parameteicantly different (P> 0.05). Values in the same row followed by the same upper case let

strains for drug resistance, significant alterations may occur inphenotype and growth characteristics. Thus, it is important that themutant and wild-type strains used either behave similarly on thesubstrate or produce easily identified differences in their relativesensitivity to pressure so that correct inferences can be made.Results from this validation step demonstrated that the mutantstrains were feasible alternatives to use in lieu of wild-type strainssince they displayed comparable baro-sensitivity.

3.5. Efficacy of HHP treatments on the color and texture of greenonion

The L, a, and b values of pressure-treated and untreated greenonion samples were determined every 5 days during the 15-daystorage duration (Table 3). Green onions can be susceptible toa variety of color defects including browning, yellowing, loss ofgreen pigmentation, fading etc. Since white bulbs are prone tobrowning or darkening (Kasim, 2009), compressed stems are proneto yellowing (Kasim, 2009; Cantwell et al., 2001) and leaf blades aresusceptible to loss of the green pigmentation (Cantwell et al., 2001)or bleaching (Kingsley et al., 2005) after treatment or duringstorage, only the L values (whiteness) of bulbs, b values (yellow-ness) of compressed stems and a values (greenness) of leaf bladesare presented in Table 3. Immediately after pressure treatment andduring storage, there was little noticeable difference in the L valuesof pressure-treated and untreated bulbs (P> 0.05). There was alsono significant difference in the b values of pressure-treated vs.

sure-treated green onion bulbs, compressed stems and leaf blades respectively stored

Days of storage

10 15

� 3.6aA 50.1� 3.0aA 48.7� 0.5aA

� 0.8aA 47.1� 1.5aA 42.5� 2.0aB

� 0.3aA 51.3� 2.1aA 47.8� 1.6aA

� 1.6aA 47.8� 4.7aA 48.4� 3.0aA

� 2.0aA 52.5� 1.2aA 48.5� 0.3aA

� 1.5aA 15.4� 1.5aA 12.6� 3.0aA

� 2.9aAB 14.7� 0.5aAB 10.3� 2.1aB

� 1.9aAB 17.6� 2.7aAB 12.4� 1.4aB

� 4.1aA 14.6� 2.5aAB 11.0� 2.5aB

� 3.9aAB 19.8� 2.8aA 9.3� 2.8aB

� 1.0aA �4.0� 1.5aA �2.5� 0.6abA

� 0.7aA �4.0� 0.8aA �2.6� 1.0abA

� 0.4aA �0.9� 0.6bA �0.5� 0.8bA

� 2.1aAB �3.4� 1.0abB �3.7� 0.9aB

� 0.7aA �2.6� 0.5abA �1.3� 1.5abA

r, values in the same column followed by the same lower case letter are not signif-ter are not significantly different (P> 0.05).

Table 4Firmness of untreated and pressure-treated green onions stored at 4 �C.

Treatments Days of storage

0 5 10 15

BulbsControl 18.9� 1.2aA 17.0� 1.3aA 15.4� 0.9aA 14.6� 0.9aA

Soaked/400 MPa/40 �C 12.8� 1.8bcA 11.8� 1.1bA 11.7� 0.5abA 10.7� 0.7bA

Soaked/450 MPa/20 �C 11.6� 0.5bcA 10.2� 0.3bA 10.4� 0.4bA 10.2� 0.1bA

Wetted/500 MPa/20 �C 10.8� 0.4cA 10.8� 0.3bA 9.1� 0.4bA 9.5� 0.6bA

Wetted/450 MPa/40 �C 14.3� 1.0bA 13.0� 0.3bA 12.7� 2.4abA 10.2� 0.6bA

Compressed stemsControl 13.8� 2.1aA 11.9� 1.7aA 11.5� 1.3aA 10.0� 1.3aB

Soaked/400 MPa/40 �C 9.9� 1.3bA 8.6� 0.6aA 8.4� 1.6aA 8.7� 0.5aA

Soaked/450 MPa/20 �C 9.1� 1.9bA 9.4� 0.2aA 9.5� 0.6aA 8.6� 0.4aA

Wetted/500 MPa/20 �C 9.1� 0.3bA 8.9� 0.9aA 8.5� 1.5aA 8.8� 0.6aA

Wetted/450 MPa/40 �C 10.7� 0.6bA 9.4� 1.3aA 10.3� 0.4aA 9.2� 1.5aA

Leaf bladesControl 7.5� 0.6aA 6.7� 1.5aA 6.6� 0.2aA 5.9� 0.3aA

Soaked/400 MPa/40 �C 6.2� 0.5aA 5.3� 0.7aA 4.9� 1.7aA 4.5� 1.0aA

Soaked/450 MPa/20 �C 6.4� 0.5aA 4.5� 0.5aA 4.2� 1.7aA 3.5� 0.6aA

Wetted/500 MPa/20 �C 5.9� 1.4aA 4.4� 0.1aA 4.7� 0.4aA 5.0� 0.1aA

Wetted/450 MPa/40 �C 6.1� 0.5aA 5.2� 0.1aA 4.5� 0.2aA 4.5� 0.5aA

Data representing mean firmness values� standard deviation. For the same green onion section, values in the same column followed by the same lower case letter are notsignificantly different (P> 0.05). Values in the same row followed by the same upper case letter are not significantly different (P> 0.05).

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e1283 1281

untreated green onion stems on the different sampling days. Withregard to the leafy or foliage parts, we observed the acquisition ofa slight darker green color after pressure treatment as evidenced bythe lower (more negative) a values immediately after HHPcompared to the untreated control, although the difference was notstatistically significant (P> 0.05). There are many examples in theliterature illustrating the ability of fruit and vegetable products toretain their color characteristics after pressure treatment (Micheland Autio, 2001). Previously, Kingsley et al. (2005) alsomentioned that pressure treatment of green onions resulted in

Table 5Sensorial quality of pressure-treated and untreated green onions stored at 4 �C.

Treatments

0 5

Overall appearanceControl 8.2� 1.1aA 5Soaked/400 MPa/40 �C 7.0� 1.4aA 6Soaked/450 MPa/20 �C 8.2� 1.1aA 6Wetted/500 MPa/20 �C 7.0� 1.4aA 5Wetted/450 MPa/40 �C 6.6� 1.7aA 6

ColorControl 8.6� 1.7aA 5Soaked/400 MPa/40 �C 7.4� 1.1aA 5Soaked/450 MPa/20 �C 7.8� 2.6aA 7Wetted/500 MPa/20 �C 6.6� 1.7aA 5Wetted/450 MPa/40 �C 7.4� 1.7aA 6

AromaControl 8.6� 0.9aA 5Soaked/400 MPa/40 �C 7.0� 2.0aA 5Soaked/450 MPa/20 �C 7.8� 1.8aA 7Wetted/500 MPa/20 �C 8.2� 1.1aA 8Wetted/450 MPa/40 �C 7.4� 1.7aA 7

TextureControl 8.2� 1.1aA 7Soaked/400 MPa/40 �C 5.8� 1.1aA 5Soaked/450 MPa/20 �C 6.6� 2.6aA 5Wetted/500 MPa/20 �C 6.6� 1.7aA 6Wetted/450 MPa/40 �C 7.0� 1.4aA 5

Data representing mean sensory scores� standard deviation. Hedonic scale: 9¼ excellecolumn followed by the same lower case letter are not significantly different (P> 0.05).different (P> 0.05).

virtually no change in their color. During storage, there wasa general trend towards lower L values (i.e. increased darkening ofbulbs) and less negative a values (i.e. decreased greenness of leaves)although the trend was not consistent across all samples. Dark-ening is thought to be due to oxidative browning reactions orincreased pigment concentrations (Kasim, 2009). Indeed, injury tothe tissue has been reported to induce phenylalanine ammonialyase activity resulting in browning in several vegetables (Kasim,2009). The decreased greenness could be attributed to the degra-dation of the green pigment chlorophyll during storage (Fang et al.,

Days of storage

10 15

.3� 2.4aA 4.7� 2.4aA 4.4� 1.9aA

.1� 2.0aA 5.0� 2.0aA 3.9� 2.3aA

.7� 2.4aA 5.9� 2.0aA 4.7� 1.8aA

.9� 2.0aA 6.1� 2.3aA 5.9� 1.6aA

.7� 1.4aA 6.7� 1.8aA 6.7� 1.4aA

.3� 1.8aA 5.3� 1.6aA 4.7� 2.2aA

.9� 2.4aA 4.7� 1.6aA 4.4� 0.8aA

.6� 3.6aA 5.6� 2.3aA 5.3� 1.5aA

.9� 2.0aA 5.9� 1.4aA 6.1� 1.0aA

.7� 2.0aA 6.1� 2.5aA 6.1� 1.6aA

.6� 2.4aA 5.3� 1.4aA 4.4� 1.5aB

.6� 2.5aA 5.3� 1.8aA 6.4� 2.2aA

.0� 1.6aA 6.7� 1.8aA 6.4� 1.5aA

.4� 1.0aA 7.3� 0.8aA 6.7� 1.8aA

.3� 1.4aA 7.6� 1.0aA 7.0� 1.2aA

.3� 1.8aA 6.1� 1.6aA 6.4� 2.2aA

.3� 2.4aA 3.0� 1.6aA 3.3� 0.8aA

.9� 3.6aA 3.9� 2.3aA 3.6� 1.5aA

.1� 2.0aA 4.7� 1.4aA 4.4� 1.0aA

.0� 1.6aA 5.0� 2.3aA 4.7� 1.8aA

nt, 1¼ extremely poor quality. For the same sensorial property, values in the sameValues in the same row followed by the same upper case letter are not significantly

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e12831282

1998). Except for HHP of soaked green onions at 450 MPa and 20 �C,the other treatments had no significant impact on the a value ofgreen onion leaves (P> 0.05) after 15 days of storage.

Treatments under the various conditions of pressure-levels,temperature and sample preparation reduced the firmness of greenonion bulbs, stems and leaves to different extents (Table 4). Pressure-treated bulbs underwent significant texture loss immediately aftertreatment and during storage (P< 0.05). Pressurized stems and leavesalsopartly lost their textural integrity although thefirmness of treatedand untreated samples was not significantly different over storage(P> 0.05). The decreased firmness of untreated green onions duringstorage has also been reported by Fan et al. (2003). Several authorshave also shown that pressure treatment of vegetables can causesoftening of the tissue to varying degrees (Basak and Ramaswamy,1998). Michel and Autio (2001) attributes it to the vast number ofintercellular air spaces that are compressedduringpressure treatmentor possibly due to the liberation of pectic (softening) enzymes duringpressurization. Taken together, we can infer that pressure treatmentresulted in lowered firmness of green onions although pressuretreatmentofwettedsamplesat450MPaand40 �Cresulted in the leasttexture loss overall. Other authors have also affirmed that pressure-treated onions partly lost their structural integrity although much ofthefluidwas retainedwithin the choppedpieces (Kingsleyet al., 2005;Butz et al., 1994). Presently, pressure-processed chopped Spanishonions are already marketed in the United States by Winsoms� anddestined foruse asgarnish andflavorenhancers (Kingsleyet al., 2005).

3.6. Sensory evaluation

Table 5 summarizes the results of the sensory evaluation ofuntreated and treated chopped green onions. Generally speaking,pressure treatment of wetted samples had higher sensory scores(overall visual appearance, color, aroma and texture) compared tosamples that were soaked prior to HHP. By the end of the storageperiod, the scores of wetted pressurized samples were consistentlyhigher than those of soaked samples. Although untreated samplesscored higher in all respects initially (at day 0), samples that werepressure-treated earned higher scores in terms of overall visualappearance, color and aroma after 15 days of storage at 4 �C. Thedecreased color scores of all samples (treated and untreated) withprolonged storage are thought to be due to the gradual loss ofthe bright coloration characteristic of fresh green onions bya natural process of senescence (Fang et al., 1998). The higher aromascores of pressurized samples relative to untreated samples couldbe due to the development of desirable pungent flavor compounds.Butz et al. (1994) reported that pressure-treatment of bulb oniondices resulted in a decrease in the “natural” pungency compound(dipropyldisulphide) reminiscent of fresh (uncut) onionsbut caused an increase in the concentration of pressure-inducedflavor compounds (propyltranspropenyldisulfie and 3,4-dimethylthiophene). The textural scores partly agreed with thefirmness data determined by physical measurements. Althoughpressure-treated samples consistently earned lower textural scoresthan untreated samples, treatment of wetted samples at 450 MPa/40 �Cwas themost desirable. Taken together, we can infer that HHPof wetted green onions at 450 MPa/40 �C resulted in highestsensory scores overall.

4. Conclusions

Our results show that green onions contaminated at a low orhigh rate can grow and pose a significant food safety risk and thuswarrants an effective decontamination step prior to retailing. High-pressure processing of green onions (Allium cepa) in the un-wetted,wetted and soaked states at pressure magnitudes ranging from 250

to 550 MPa and temperatures of 4e40 �C brought about differentialinactivation of Salmonella and E. coli O157:H7 depending on thepressure level, treatment temperature and state of the samples. Inaddition, processing of green onions in the wetted or soaked statesunder treatments that combine moderate pressure levels with mildheating eliminated the inoculated pathogens in green onionsthroughout the refrigerated shelf-life of the commodity. Physicaland sensorial measurements revealed that high-pressure process-ing had little to no adverse impact on the overall appearance, colorand aroma of chopped green onion, although the texturewas partlyaffected. HHP may thus be a viable technology for the minimalprocessing of chopped green onions, which can be used as aningredient in side dishes such as salsa, a garnish or flavorenhancers.

Acknowledgement

This project was supported by the National Integrated FoodSafety Initiative of the USDA Cooperative State Research, Educationand Extension Service, grant number # 2008-51110-04346.

References

Alzamora, S.M., Tapia, M.S., Lopez-Malo, A., 2000. Minimally Processed Fruits andVegetables: Fundamental Aspects and Applications. Maryland AspenPublishers, Inc., Gaithersburg, pp. 1e11.

Asplund, K., Nurmi, E., 1991. The growth of salmonellae in tomatoes. Int. J. FoodMicrobiol. 13, 177e182.

Basak, S., Ramaswamy, H.S., 1998. Effect of high pressure processing on texture ofselected fruit and vegetables. J. Text. Stud. 29, 587e601.

Beattie, G.A., Lindow, S.E., 1995. The secret life of foliar bacterial pathogens onleaves. Annu. Rev. Phytopathol. 33, 145e172.

Beattie, G.A., Lindow, S.E., 1999. Bacterial colonization of leaves: a spectrum ofstrategies. Phytopathology 89, 353e359.

Bozoglu, F., Kaletunc, G., Alpas, H., 2004. Injury recovery of foodborne pathogens inhigh hydrostatic pressure treated milk during storage. FEMS Immunol. Med.Microbiol. 40, 243e247.

Butz, P., Wolf, S., Tauscher, B., Koller, W.D., 1994. Ultra-high pressure processing ofonions: chemical and sensory changes. LWT Food Sci. Technol. 27, 463e467.

California Department of Food and Agriculture, 2010. Commodity specific foodsafety guidelines for the production, harvest, post-harvest, and value-addedunit operations of green onions. Available from: www.U.S.FDA.gov/downloads/Food/FoodSafety/Product./UCM203114.pdf.

Calvin, L., Avendaño, B., Schwentesius, R., 2004. The economics of food safety: thecase of green onions and hepatitis A outbreaks. Economics Research Service,USDA. Available from: http://www.ers.usda.gov/publications/vgs/nov04/vgs30501/vgs30501.pdf.

Cantwell, M.I., Suslow, T.V., Hong, G., 2001. Heat treatments control extensiongrowth and enhance microbial disinfection of minimally processed greenonions. Hort Sci. 36, 732e737.

Carlez, A., Rosec, J.P., Richard, N., Cheftel, J.C., 1994. Bacterial growth during chilledstorage of pressure-treated minced meat. Lebensm. Wiss. Technol. 27, 48e54.

Castillo, A., Escartin, E.F., 1994. Survival of Campylobacter jejuni on sliced water-melon and papaya. J. Food Prot. 57, 166e168.

Castro-Rosas, J., Escartin, E.F., 2000. Survival and growth of Vibrio cholerae O1,Salmonella typhi and Escherichia coli O157:H7 in alfalfa sprouts. J. Food Sci. 65,162e165.

Centers for Disease Control and Prevention (CDC), 2003. Hepatitis A outbreakassociated with green onions at a restaurant e Monaca, Pennsylvania. Morbid.Mort. Wkly. Rep. 52, 1155e1157.

Dato, V.A., Weltman, K., Waller, M., Ruta, A., Highbaugh-Battle, C., Hembree, S.,2003. Hepatitis A outbreak associated with green onions at a restau-rantdMonaca, Pennsylvania. Morb. Mort. Wkly. Rep 52, 1155e1157.

Davis, H., Taylor, J.P., Perdue, J.N., Stelma, G.N., Humphreys, J.M., Rowntree, R., 1988.A shigellosis outbreak traced to commercially distributed shredded lettuce. Am.J. Epidemiol. 128, 1312e1321.

Del Rosario, B.A., Beuchat, L.R., 1995. Survival and growth enterohemorrhagicEscherichia coli O157:H7 in cantaloupe and watermelon. J. Food Prot. 58,105e107.

Dingman, D.W., 2000. Growth of Escherichia coli O157:H7 in bruised apple (Malusdomestica) tissue as influenced by cultivar, date of harvest and source. Appl.Environ. Microbiol. 66, 1077e1083.

Eisenstadt, E., Carlton, B.C., Brown, B.J., 1994. Gene mutation. In: Gerhardt, P.,Murray, R.G.E.,Wood,W.A., Krieg, N.R. (Eds.), Methods for General andMolecularBacteriology. American Society for Microbiology, Washington, D.C, pp. 52e57.

Fan, X., Niemira, B.A., Sokorai, K.J.B., 2003. Use of ionizing radiation to improvesensory and microbial quality of fresh-cut green onion leaves. J. Food Sci. 68,1478e1483.

H. Neetoo et al. / Food Microbiology 28 (2011) 1275e1283 1283

Fang, Z., Bouwkamp, J.C., Solomos, T., 1998. Chlorophyllase activities and chlorophylldegradation during leaf senescence in non-yellowing mutant and wild type ofPhaseolus vulgaris L. J. Exp. Bot. 320, 503e510.

Fino, V.R., Kniel, K., 2008.UV light inactivationofhepatitisAvirus, Aichi virus, and felinecalicivirus on strawberries, green onions, and lettuce. J. Food Prot. 71, 908e913.

Food and Agriculture Organization of the United Nations World Health Organiza-tion Meeting Report, 2008. Microbiological hazards in fresh fruits and vegeta-bles. Available from: http://www.who.int/foodsafety.

Fredlund, H., Back, E., Joberg, L., Tornquist, E., 1987. Watermelon as a vehicle oftransmission Shigellosis. Scand. J. Inf. Dis. 19, 219e221.

Islam, M., Doyle, M.P., Phatak, S.C., Millner, P., Jiang, X.P., 2004. Persistence ofenterohemorrhagic Escherichia coli O157:H7 in soil and on leaf lettuce andparsley grown in fields treated with contaminated manure composts or irri-gation water. J. Food Prot. 67, 1365e1370.

Kasim, R., 2009. The effects of cut-type and heat treatment on fresh-cut greenonions quality. Am-Euras. J. Agric. Environ. Sci. 5, 428e433.

Kim, H.J., Hao, F., Toshkov, S.A., Fan, X., 2005. Effect of sequential treatment of warmwater dip and low dose gamma irradiation and hot water wash on the quality offresh-cut green onions. J. Food Sci. 70, 179e185.

Kingsley, D.H., Hoover, D.G., Guan, D., 2005. Pressure inactivation of hepatitis Avirus in strawberry puree and sliced green onions. J. Food Prot. 68, 1748e1751.

Liao, C.H., Cooke, P.H., Niemira, B.A., 2010. Localization, growth and inactivation ofSalmonella Saintpaul on Jalapeño peppers. J. Food Sci. 75, 377e382.

Liu, F., Li, Y., 2006. Storage characteristics and relationships between microbialgrowth parameters and shelf life of MAP sliced onions. Postharvest Biol.Technol. 40, 262e268.

Long, W., Pao, S., Inserra, P., Westbrook, E., Ahn, S., 2010. Efficacy of ozone producewashers in reducing natural and artificially inoculated microorganisms onRoma tomatoes and green onions. IFT Annual Meeting 2010, Chicago, IL.

Lopez-Caballero, M.E., Carballo, J., Jimenez-Colmenero, F., 2002. Microbial inactivationin meat products by pressure/temperature processing. J. Food Sci. 67, 797e801.

Ma, L., Zhang, G., Gerner-Smidt, P., Tauxe, R.V., Doyle, M.P., 2010. Survival andgrowth of Salmonella in salsa and related ingredients. J. Food Prot 73, 434e444.

Mann, K.M., 2005. The repercussions of exposure to nalidixic acid for Salmonellatyphimurium: alterations in phenotype, multi-drug resistance and lactic acidsusceptibility. Doctoral Dissertation. Texas Tech University.

Michel, M., Autio, K., 2001. Effects of high pressure on protein- and polysaccharide-based structures. In: Hendrickx, M.E.G., Knorr, D. (Eds.), Ultra-high PressureTreatment of Foods. Kluwer Academic/Plenum Publishers, New York, Boston,pp. 189e214.

Mutaku, I., Ashenafi, M., Erku, W., 2005. Growth and survival of Escherichia coliO157:H7 in fresh tropical fruit juices at ambient and cold temperatures. Int. J.Food Sci. Nut. 56, 133e139.

Neetoo, H., Ye, M., Chen, H., 2008. Potential application of high hydrostatic pressureto eliminate Escherichia coli O157:H7 on alfalfa sprouted seeds. Int. J. FoodMicrobiol. 128, 348e353.

Neetoo, H., Pizzolato, T., Chen, H., 2009. Elimination of Escherichia coli O157:H7 fromalfalfa seeds through a combination of high hydrostatic pressure and mild heat.Appl. Environ. Microbiol. 75, 1901e1907.

Oxen, P., Knorr, D., 1993. Baroprotective effects of high solute concentrations againstinactivation of Rhodotorula rubra. LWT Food Sci. Technol. 26, 220e223.

Palou, E., Lopez-Malo, A., Barbosa-Canovas, G.V., Welti-Chanes, J., Swanson, B.G.,1997a. High hydrostatic pressure as a hurdle for Zygosaccharomyces bailiiinactivation. J. Food Sci. 62, 855e857.

Palou, E., Lopez-Malo, A., Barbosa-Canovas, G.V., Welti-Chanes, J., Swanson, B.G.,1997b. Effect of water activity on high hydrostatic pressure inhibition ofZygosaccharomyces bailii. Lett. Appl. Microbiol. 24, 417e420.

Park, E.J., Alexander, E., Taylor, G.A., Costa, R., Kang, D.H., 2008. Fate of foodbornepathogens on green onions and tomatoes by electrolyzed water. Lett. Appl.Microbiol. 46, 519e525.

Park, E.J., Alexander, E., Taylor, G.A., Costa, R., Kang, D.H., 2009. The decontaminativeeffects of acidic electrolyzed water for Escherichia coli O157:H7, Salmonellatyphimurium, and Listeria monocytogenes on green onions and tomatoes withdiffering organic demands. Food Microbiol. 26, 386e390.

Ponce, E., Pla, R., Capellas, M., Guamis, B., Mor-Mur, M., 1998a. Inactivation ofEscherichia coli inoculated in liquid whole egg by high hydrostatic pressure.Food Microbiol. 15, 265e272.

Ponce, E., Pla, R., Mor-Mur, M., Gervilla, R., Guamis, B., 1998b. Inactivation of Listeriainnocua inoculated in liquid whole egg by high hydrostatic pressure. J. Food Prot61, 119e122.

Ponce, E., Pla, R., Sendra, E., Guamis, B., Mor-Mur, M., 1999. Destruction of Salmo-nella enteritidis inoculated in liquid whole egg by high hydrostatic pressure:comparative study in selective and non-selective media. Food Microbiol. 16,357e365.

Shearer, A.E.H., Hoover, D.G., Sikes, A., Dunne, C.P., 2000. Bacterial spore inhibitionand inactivation in foods by pressure, chemical preservatives, and mild heat.J. Food Prot. 63, 1503e1510.

U.S. Food and Drug Administration, 2001. Analysis and evaluation of preventivecontrol measures for the control and reduction/elimination of microbialhazards on fresh and fresh-cut produce. Available from: http://www.U.S.FDA.gov/Food/ScienceResearch/ResearchAreas/SafePracticesforFoodProcesses/ucm090977.htm.

U.S. Food and Drug Administration, 2002. Bacteriological Analytical Manual.Available from: http://www.fda.gov/Food/ScienceResearch/LaboratoryMethods/BacteriologicalAnalyticalManualBAM/ucm070080.htm (Chapter 4a).

U.S. Food and Drug Administration, 2007. Bacteriological Analytical Manual. Avail-able from: http://www.fda.gov/Food/ScienceResearch/LaboratoryMethods/BacteriologicalAnalyticalManualBAM/ucm070149.htm (Chapter 5).

Wachtel, M.R., Whitehand, L.C., Mandrell, R.E., 2002. Association of Escherichia coliO157:H7 with preharvest leaf lettuce upon exposure to contaminated irrigationwater. J. Food Prot. 65, 18e25.

Weissinger, W.R., Chantarapanont, W., Beuchat, L.R., 2000. Survival and growth ofSalmonella Montevideo on tomatoes and disinfection with chlorinated water.Int. J. Food Microbiol. 62, 123e131.

Yaldagard, M., Mortazavi, S.Y., Tabatabaie, F., 2008. The principles of ultra high-pressure technology and its application in food processing/preservation:a review of microbiological and quality aspects. Afr. J. Biotechnol. 7,2739e2767.