Inactivation of Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Senftenberg by ultrasonic waves under pressure

The resistance of Salmonella Enteritidis (ATCC 13076), Salmonella Typhimurium (ATCC 13311), and Salmonella Senftenberg 775W (ATCC 43845) to ultrasonic waves under pressure treatments, at sublethal (manosonication) and lethal temperatures (manothermosonication) in citrate-phosphate buffer and in liquid whole egg was investigated. The influence of treatment parameters on the inactivation rate of manosonication was also studied. Decimal reduction times (Dt) of Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Senftenberg 775W corresponding to a heat treatment at 60 degrees C in pH 7 buffer and in liquid whole egg were 0.068, 0.12, and 1.0 min for buffer, and 0.12, 0.20, and 5.5 min for liquid whole egg. Those corresponding to a manosonication treatment (117 microns, 200 kPa, 40 degrees C) in both media were 0.73, 0.78, and 0.84 min, and 0.76, 0.84, and 1.4 min, respectively. When the amplitude of ultrasonic waves was increased linearly, the inactivation rate of manosonication increased exponentially. The inactivation rate also increased when pressure was raised. However, the magnitude of this increase was progressively smaller at higher pressures. The magnitude of the influence of the amplitude of ultrasonic waves and static pressure on the inactivation rate of manosonication was the same in the three serotypes investigated. Whereas a heat treatment at 60 degrees C only attained a 1/2-log cycle reduction in the number of Salmonella Senftenberg 775W survivors, a manothermosonication treatment (117 microns and 200 kPa) at this temperature attained a 3-log cycle reduction.     

雾化植物精油对鼠伤寒沙门氏菌抑制效果和樱桃番茄品质的影响

本研究的目的是评测雾化液态植物(芥末、肉桂、牛至和百里香)精油对樱桃番茄表面和茎端的抑菌效果及对果实品质的影响。将樱桃番茄的表面和茎端分别接种鼠伤寒沙门氏菌(ATCC 53467和ATCC 5 3468菌种),晾干之后放到包装盒中,采用加湿器来雾化液态的植物精油,然后通过一根PE管导入到装有已经接种了鼠伤寒沙门氏菌的樱桃番茄的包装盒中。处理后的果实存放在10℃的冰箱中3个星期,存放期间每隔1个星期进行微生物指标测定,不接菌种的果实用来测定果实品质指标。结果表明,雾化的植物精油能够显著的抑制樱桃番茄表面和茎端的鼠伤寒沙门氏菌(表面及茎端菌落分别降低2.00 log CFU/fruit和1.00 log CFU/fruit左右)。随着贮藏期的延长,菌落总数也随之降低,这证明其抑菌效果仍然有效。雾化植物精油杀菌处理对果实硬度,颜色,VC等品质指标没有显著影响(p>0.05)。雾化是一种方便有效的可用于储存或运输果蔬产品消毒的新方法。      

Inactivation of Salmonella Typhimurium and Staphylococcus aureus by pulsed electric fields in liquid whole egg

In this study, the lethal effectiveness of pulsed electric fields (PEF) on the inactivation of Salmonella enterica subs. enterica ser. Typhimurium and Staphylococcus aureus in liquid whole egg (LWE) has been investigated. Maximum inactivation levels of 4 and 3 Log₁₀ cycles of the population of Salmonella Typhimurium and S. aureus were achieved with treatments of 45 kV/cm, 30 μs and 419 kJ/kg, and 40 kV/cm for 15 μs and 166 kJ/kg, respectively. The non-linear kinetics of inactivation observed for both microorganisms at all the investigated electric field strengths were described by mathematical equations based on the Weibull distribution. The developed equations enabled to compare the microbial resistance to PEF and to establish the most suitable treatment conditions to achieve a determined level of microbial inactivation. PEF treatments varying from 30 kV/cm, 67 µs and 393 kJ/kg to 45 kV/cm, 19 µs and 285 kJ/kg allow to reduce 3 Log₁₀ cycles the population of the microorganism of concern in PEF food processing of LWE, Salmonella Typhimurium.Industrial relevanceThe data presented in this investigation in terms of electric field strength, specific energy and treatment time result of relevance to evaluate the possibilities of PEF technology to pasteurize LWE with this technology. The models developed in this study can be applied to engineering design, and for the evaluation and optimization of the PEF technology as a new technique to obtain Salmonella free LWE.Based on our results it is not recommended to apply treatments of energy levels higher than 250 kJ/kg, since PEF lethality hardly increased but markedly augmented the energetic costs. For these energy values, PEF technology by itself is not sufficient (3 Log10 cycles in the best case scenario) to assure the safe security of LWE. Therefore, intelligent combinations of PEF with other preservation technologies have to be developed in order to use pulsed electric fields as an alternative to heat pasteurization of LWE.     

Inactivation of Salmonella Typhimurium in orange juice containing antimicrobial agents by pulsed electric field

Combinations of different hurdles, including moderately high temperatures (<60 degrees C), antimicrobial compounds, and pulsed electric field (PEF) treatment, to reduce Salmonella in pasteurized and freshly squeezed orange juices (with and without pulp) were explored. Populations of Salmonella Typhimurium were found to decrease with an increase in pulse number and treatment temperature. At a field strength of 90 kV/cm, a pulse number of 20, and a temperature of 45 degrees C, PEF treatment did not have a notable effect on cell viability or injury. At and above 46 degrees C, however, cell death and injury were greatly increased. Salmonella numbers were reduced by 5.9 log cycles in freshly squeezed orange juice (without pulp) treated at 90 kV/cm, 50 pulses, and 55 degrees C. When PEF treatment was carried out in the presence of nisin (100 U/ml of orange juice), lysozyme (2,400 U/ml), or a mixture of nisin (27.5 U/ml) and lysozyme (690 U/ml), cell viability loss was increased by an additional 0.04 to 2.75 log cycles. The combination of nisin and lysozyme had a more pronounced bactericidal effect than did either nisin or lysozyme alone. An additional Salmonella count reduction of at least 1.37 log cycles was achieved when the two antimicrobial agents were used in combination. No significant difference (P > 0.05) in cell death was attained by lowering the pH value; only cell injury increased. Inactivation by PEF was significantly more extensive (P < 0.05) in pasteurized orange juice than in freshly squeezed orange juice under the same treatment conditions. This increase might be due to the effect of the chemical composition of the juices.     

Quantification of contamination of lettuce by GFP-expressing Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium

The primary objective of this study was to determine the possibility of internalization of GFP-expressing Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium (S. Typhimurium) strains MAE 110 (multi-cellular morphology) and 119 (wild type morphology) into lettuce seedlings (Lactuca sativa cv. Tamburo) grown in an inoculated hydroponic and soil system. The second aim was to quantify the level of contamination with the use of a proper surface sterilization method. Silver nitrate was superior in reducing the number of viable bacteria on leave surfaces compared to sodium hypochlorite and ethanol. With the hydroponic system internal colonization of lettuce only occurred at high densities with S. Typhimurium MAE 119. With the soil system E. coli O157:H7, S. Typhimurium 110 and S. Typhimurium 119 were found at considerable densities in sterilized leaf samples (respectively, 3.95, 2.57 and 2.37 log cfu/g on average) with prevalences of 0.29, 0.23 and 0.15, respectively. No statistical differences were observed between the Salmonella strains. A negative correlation was observed between shoot weight and leaf contamination. The observed presence of the pathogens in lettuce, after thorough surface sterilization, demonstrates the possible presence of human pathogens in locations were they are unlikely to be removed by the actions of consumer washing and therefore pose a serious threat when occurring in field situations.     

Inactivation of Salmonella Enteritidis PT30 on the surface of unpeeled almonds by cold plasma

The contamination of nut products, like almonds, with human pathogens is a reoccurring concern in the food industry. In this study the inactivation of Salmonella Enteritidis PT 30 (ATCC BAA-1045) on the surface of unpeeled almonds by cold atmospheric pressure plasma was investigated. Air, O₂, N₂, CO₂ and 90% CO₂ + 10% Ar were used as process gas. Inoculum preparation and inoculation of almonds was done according to the guidelines recommended by the Almond Board of California. Furthermore, impact of plasma treatment on product color was measured. All plasma treatments inactivated Salmonella. Maximum achieved inactivation depends on the used process gas. Air plasma inactivated > 5.0 log₁₀, O₂ plasma 4.8 and N₂ plasma 2.0 log₁₀ after 15 min treatment. The plasma treatment with air and N₂ resulted in a browning of the unpeeled almond's surface color. Whereas the other used plasma did not alter the color considerably.Industrial RelevanceThe contamination of raw nuts with human pathogens is an on-going food safety concern. The application of cold atmospheric pressure plasma has a high potential as a gentle technology for the surface decontamination. The results of this study suggest that a cold plasma treatment could be an alternative technology for the pasteurization of almonds. The use of air plasma achieved more than a 5 log₁₀, which is in general required by the U.S. Food and Drug Administration for the approval as an alternative inactivation technology for food products. However, the scale up to commercial treatment levels requires the complete understanding of the involved product-plasma interactions.     

Inactivation kinetics of inoculated Escherichia coli O157:H7 and Salmonella enterica on lettuce by chlorine dioxide gas

The purpose of this investigation was to study inactivation kinetics of inoculated Escherichia coli O157:H7 and Salmonella enterica on lettuce leaves by ClO(2) gas at different concentrations (0.5, 1.0, 1.5, 3.0, and 5.0 mg l(-1)) for 10 min and to determine the effect of ClO(2) gas on the quality and shelf life of lettuce during storage at 4 degrees C for 7 days. One hundred microliters of each targeted organism was separately spot-inoculated onto the surface (5 cm(2)) of lettuce (approximately 8-9 log CFU ml(-1)), air-dried, and treated with ClO(2) gas at 22 degrees C and 90-95% relative humidity for 10 min. Surviving bacterial populations on lettuce were determined using a membrane transferring method, which included a non-selective medium followed by a selective medium. The inactivation kinetics of E. coli O157:H7 and S. enterica was determined using first-order kinetics to establish D-values and z-values. The D-values of E. coli and S. enterica were 2.9+/-0.1 and 3.8+/-0.5 min, respectively, at 5.0 mg l(-1) ClO(2) gas. The z-values of E. coli and S. enterica were 16.2+/-2.4 and 21.4+/-0.5 mg l(-1), respectively. A 5 log CFU reduction (recommended by the United States Food and Drug Administration) for E. coli and S. enterica could be achieved with 5.0 mg l(-1) ClO(2) gas for 14.5 and 19.0 min, respectively. Treatment with ClO(2) gas significantly reduced inherent microflora on lettuce and microbial counts remained significantly (p<0.05) lower than the uninoculated control during storage at 4 degrees C for 7 days. However, treatment with ClO(2) gas had a significantly (p<0.05) negative impact on visual leaf quality. These results showed that treatment with ClO(2) gas significantly reduced selected pathogens and inherent microorganisms on lettuce; however, the processing conditions would likely need to be altered for consumer acceptance.     

Inactivation and occurrence of sublethal injury of Salmonella Typhimurium under mild heat stress in broth

Thermal processing of food products continues to be the standard commercial method used for enhancing microbiological safety. The influence of cell characteristics of Salmonella Typhimurium strains, such as growth phase, cell density, and the availability of nutrients and metabolites was evaluated regarding a 3?Log₁₀ reduction of surviving cells when exposed to a mild heat treatment of 55?°C. Using nonselective and selective agars it was revealed that under thermal stress Salmonella Typhimurium suffered from injuries on both outer and cytoplasmic membrane, which consequently led to cell death. Compared to stationary cells, those in exponential growth phase were more prone to thermal injury, especially on the cytoplasmic membrane. Occurrence of thermal inactivation and injury decreased with the increase of cell density, and with the increased availability of metabolites produced during cell growth, but showed no apparent correlation with nutrients in the culture broth. The results enlighten the importance of cell growth phase, cell density and metabolite availability in Salmonella Typhimurium thermal resistance. Consequently these factors should be well considered when models are constructed and used to predict thermal effects on the survival of the pathogen. Moreover, our data show that attention should be paid when selective media are used to detect sublethally injured cells.     

The effects of active and passive modified atmosphere packaging on the survival of Salmonella enterica serotype Typhimurium on washed romaine lettuce leaves

Modified atmosphere packaging (MAP) is commonly used to preserve the quality of ready-to-eat lettuce by inhibiting oxidative browning and growth of microbial populations. The efficacy of MAP is improved by the initial displacement of air with a gas mixture of a desirable composition (active MAP). The present work focused on the effects of MAP on indigenous microbial populations and the survival of Salmonella enterica on the surface of the lettuce. Chlorine-washed leaves of romaine lettuce were inoculated with S. enterica serotype Typhimurium or not inoculated and packaged in one of the three systems: (a) passive MAP in polyethylene bags; (b) active MAP in the same bags with a gas mixture of 10% O₂, 10% CO₂, and 80% N₂; and (c) control without MAP. Active MAP ensured the more rapid establishment of a steady-state atmosphere and favorable conditions during the transitional period preceding the steady state. The active MAP had an antimicrobial effect on indigenous lettuce microflora, but not on Salmonella and even favored the survival of the pathogen, possibly due to the elimination of its natural antagonists. The effects of the passive MAP were less pronounced. The results obtained draw attention to potential safety risks of ready-to-eat fresh produce kept in active MAP and require further investigation.     

Inactivation of Salmonella enterica serovar Typhimurium and Quality Maintenance of Cherry Tomatoes Treated with Gaseous Essential Oils

The antimicrobial activity of the essential oils (EOs) from cinnamon bark, oregano, mustard, and of their major components cinnamaldehyde, carvacrol, and allyl isothiocyanate (AIT) was evaluated as a gaseous treatment to reduce Salmonella enterica serovar Typhimurium in vitro and on tomatoes. In vitro tests showed that mustard EO and AIT had the greatest inhibition of Salmonella, followed by cinnamon EO and cinnamaldehyde, while oregano and carvacrol showed the least inhibition. Scanning electron microscopy images of S. Typhimurium on tomatoes suggest that the EOs and their major components damaged the bacteria, and the damage was more obvious after posttreatment storage at 10 °C for 4 and 7 d. Salmonella on inoculated tomatoes was reduced by more than 5 log colony forming units (CFU)/g by mustard EO and AIT, by 4.56 and 3.79 log CFU/g following cinnamon EO and cinnamaldehyde treatments, respectively, and 1.54 and 3.37 log CFU/g after oregano EO and carvacrol treatments, respectively. Mustard EO and AIT induced discoloration, softening, and loss of the vitamin C and lycopene during 21 d of storage at 10 °C, while treatment with cinnamon EO and cinnamaldehyde did not result in significant changes in tomato quality. Tomatoes treated with oregano EO had better quality than nontreated samples after storage. Therefore, treatment with cinnamon and oregano EO and their major components appeared to be feasible for inactivation of Salmonella on tomatoes and maintaining quality.     

Biofilm formation and the survival of Salmonella Typhimurium on parsley

Although several studies provide evidence that the formation of biofilms by human pathogens on plant tissue is possible, to date there is no direct evidence that biofilms enhance the resistance of plant-associated pathogens to disinfectants or biocides. We hypothesized that biofilm formation would enhance the adhesion and survival of Salmonella on leafy vegetables. To test our hypothesis, we compared the adhesion and persistence of Salmonella Typhimurium and its biofilm-deficient isogenic mutant. Following inoculation of parsley and rinsing with water or chlorine solution, both strains had similar survival properties, and up to 3-log reduction were observed, depending on chlorine concentration. This indicates that the biofilm matrix of Salmonella likely does not play a significant role in initial adhesion and survival after disinfection. After a week of storage the biofilm producing strain survived chlorination significantly better than the biofilm-deficient mutant. However, the recovery of the mutant was still elevated, indicating that although the biofilm matrix has a role in persistence of Salmonella after chlorination treatment of parsley, this is not the most important mechanism, and other mechanisms, probably the ability to penetrate the plant tissue or the pre-existing biofilms, or production of different polysaccharides other than cellulose, provide the protection.     

Reduction of Salmonella Typhimurium and Listeria monocytogenes on produce by trisodium phosphate

The application of trisodium phosphate (TSP) on produce against food-borne bacteria has not been extensively evaluated. This research studied the effect of 20 and 50 mg/ml TSP in reducing Salmonella Typhimurium (ST) and Listeria monocytogenes (Lm) on model produce (lettuce and peppers). Washed and air-dried Iceberg lettuce (3 × 3 cm²) and Jalapeno peppers (25–30 g) were spiked with S. Typhimurium or L. monocytogenes (～7 log₁₀ CFU/ml). Samples were treated with 20 or 50 mg/ml TSP, 200 mg/L sodium hypochlorite (for comparison with traditional washes) or water (control rinse) for 15 or 30 s. Treatments were immediately neutralized with trypticase soy broth (TSB) containing 30 mg/ml beef extract, serially diluted, plated on Xylose Lysine Tergitol 4 (XLT4) agar for ST and Tryptic Soy Agar (TSA) for Lm and incubated at 37 °C for 24–48 h. Results showed that 20 mg/ml TSP and 200 mg/L sodium hypochlorite caused 5–6 log₁₀ CFU/ml reduction in ST on both lettuce and peppers after 15 s and 30 s, while 50 mg/ml TSP reduced ST to undetectable levels on both produce at both contact times. For overnight Lm cultures, a mere reduction of 0.21 and 0.28 log₁₀ CFU/ml was obtained using 20 and 50 mg/ml TSP after even 5 min, while 200 mg/L sodium hypochlorite decreased Lm by ～1 log₁₀ CFU/ml after 30 s and 1 min on lettuce, and decreased pure culture Lm to undetectable levels after 3 min. Thus, 50 mg/ml TSP appears effective against ST with negligible effects against Lm, whose mode of action needs to be understood.     

Reduction of Salmonella Typhimurium in experimentally challenged broilers by nitrate adaptation and chlorate supplementation in drinking water

The effects of two feed supplements on Salmonella Typhimurium in the ceca of market-age broilers were determined. Broilers orally challenged 6 days before slaughter with a novobiocin- and nalidixic acid-resistant strain of Salmonella Typhimurium were divided into one of four groups (20 birds each). The first group (the control group) received no treatment, the second group received sodium nitrate (SN) treatment (574 mg of NaNO3 per kg of feed), the third group received experimental chlorate product (ECP) treatment (15 mM NaClO3 equivalents), and the fourth group received ECP treatment in combination with SN treatment. The SN treatment was administered via feed for 5 days immediately before slaughter, and ECP was provided via ad libitum access to drinking water for the last 2 days before slaughter. Cecal contents were subjected to bacterial analysis. Significant (P < 0.05) Salmonella Typhimurium reductions (ca. 2 log units) relative to levels for untreated control broilers were observed for broilers receiving ECP in combination with SN. The ECP-only treatment resulted in significant (P < 0.05) reductions (ca. 0.8 log) of Salmonella Typhimurium in trial 2. We hypothesize that increasing Salmonella Typhimurium nitrate reductase activity resulted in increased enzymatic reduction of chlorate to chlorite, with a concomitant decrease in cecal Salmonella Typhimurium levels. On the basis of these results, preadaptation with SN followed by ECP supplementation immediately preharvest could be a potential strategy for the reduction of Salmonella Typhimurium in broilers.     

Resistance changes in Salmonella enterica serovar Typhimurium treated by High Hydrostatic Pressure and Pulsed Electric Fields and assessment of virulence changes by using Caenorhabditis elegans as a test organism

The goal of this study was to evaluate the development of Salmonella enterica subsp. enterica serovar Typhimurium resistance against Pulsed Electric Fields (PEF) and High Hydrostatic Pressure (HHP), and to study the possible virulence changes of the resistant subpopulations using Caenorhabditis elegans. Results showed that S. Typhimurium became resistant to both non-thermal technologies after four consecutive treatments. Survival probability of the worms (C. elegans) increased significantly when they were fed with S. Typhimurium treated by PEF four consecutive times in comparison with the other two subpopulations, untreated and treated once. For HHP, results indicated that up to percentile 50%, the survival probability of worms fed with treated microorganisms (once and four times) was greater than that of worms fed with untreated ones. Also, the C. elegans egg-laying pattern was modified and there were significant differences in the number of eggs laid in the first two days among the three subpopulations studied. Consequently, although S. Typhimurium develops microbial resistance against PEF or HHP treatments, when it is applied repeatedly, its virulence against C. elegans appears to decrease.Industrial relevanceAmong the non-thermal technologies, Pulsed Electric Fields and High Hydrostatic Pressure have a great potential. Nevertheless, it is necessary to validate them from a safety point of view because of the huge amount of damaged cells that can be obtained if sublethal treatments are applied. These studies have industrial relevance in food safety ensuring proper processing when using these non-thermal technologies.     

Inactivation of Salmonella spp. on tomatoes by plant molecules

The efficacy of carvacrol (CAR), trans-cinnamaldehyde (TC), eugenol (EUG) and β-resorcylic acid (BR) as a wash treatment for reducing Salmonella spp. on tomatoes was investigated. Plum tomatoes inoculated with a six-serotype mixture of Salmonella (10⁸ CFU) were subjected to washing in sterile deionized water (control) or deionized water containing chlorine (100 ppm), CAR (0.25 and 0.75%), TC (0.5 and 0.75%), EUG (0.25 and 0.75%), or BR (0.75 and 1.0%) for 15 sec, 1 min, and 3 min. The plant molecules were more effective (P < 0.05) in reducing Salmonella on tomatoes compared to washing in water and chlorine. Both concentrations of CAR and TC, and 0.75% EUG decreased Salmonella counts on tomatoes by ~ 6.0 log CFU/ml at 1 min. Both concentrations of BR decreased the pathogen on tomatoes to undetectable levels at 3 min of exposure. Washing of tomatoes in deionized water and chlorine for 3 min reduced Salmonella by ca. 2.0 and 4.0 log CFU/ml, respectively. No Salmonella was detected in the wash water containing the plant molecules or chlorine, whereas a substantial population of the pathogen survived in the control wash water. Moreover, none of the dipping treatments had any effect on the red color of tomatoes (P > 0.05). Results indicate that CAR, TC, EUG and BR could effectively be used to kill Salmonella on tomatoes, but additional studies on sensory and quality characteristics of tomatoes treated with plant molecules are warranted.     

Rapid and sensitive detection of Salmonella Typhimurium on eggshells by using wireless biosensors

This article presents rapid, sensitive, direct detection of Salmonella Typhimurium on eggshells by using wireless magnetoelastic (ME) biosensors. The biosensor consists of a freestanding, strip-shaped ME resonator as the signal transducer and the E2 phage as the biomolecular recognition element that selectively binds with Salmonella Typhimurium. This ME biosensor is a type of mass-sensitive biosensor that can be wirelessly actuated into mechanical resonance by an externally applied timevarying magnetic field. When the biosensor binds with Salmonella Typhimurium, the mass of the sensor increases, resulting in a decrease in the sensor's resonant frequency. Multiple E2 phage-coated biosensors (measurement sensors) were placed on eggshells spiked with Salmonella Typhimurium of various concentrations (1.6 to 1.6 × 10(7) CFU/cm(2)). Control sensors without phage were also used to compensate for environmental effects and nonspecific binding. After 20 min in a humidity-controlled chamber (95%) to allow binding of the bacteria to the sensors to occur, the resonant frequency of the sensors was wirelessly measured and compared with their initial resonant frequency. The resonant frequency change of the measurement sensors was found to be statistically different from that of the control sensors down to 1.6 × 10(2) CFU/cm(2), the detection limit for this work. In addition, scanning electron microscopy imaging verified that the measured resonant frequency changes were directly related to the number of bound cells on the sensor surface. The total assay time of the presented methodology was approximately 30 min, facilitating rapid detection of Salmonella Typhimurium without any preceding sampling procedures.     

Salmonella sensitivity to sodium hypochlorite and citric acid in washing water of lettuce residues

Salmonella spp. is one of the main lettuce pathogens and should be inactivated during the disinfection of these vegetables before consumption. In minimally processed vegetable industries, residues of organic matter can prevent the inactivation of this pathogen by disinfectants. The objective of the present work was to evaluate the inactivation of Salmonella isolated from organic lettuce to sodium hypochlorite (25 and 50 ppm) and citric acid (0.5 and 1%) in washing water added with lettuce residues. To do so, a washing water with lettuce residues was elaborated, and Salmonella was added in the order of 10⁶ CFU/ml. Thereafter, each sanitizer was added separately to evaluate its effect on reducing Salmonella counts. After 1, 2, 3, 4, 5, 10, and 15 min of contact with the sanitizers, serial dilutions using neutralizer (0.5% sodium thiosulfate) were performed and each dilution was sown in Xylose-Lysine-Desoxycholate medium. Total aerobic mesophilic counts of wash water with lettuce residues before testing (without Salmonella) and after 15 min of exposure to each sanitizer (with Salmonella) were also performed. In addition, the free chlorine still present in the samples after the contact of sodium hypochlorite with lettuce residues for 15 min. The results demonstrated that 50 and 25 ppm sodium hypochlorite could reduce 6 log CFU/ml of Salmonella in 1 and 3 min of contact, respectively, while 0.5 and 1% citric acid was able to reduce 1.26 and 1.74 log CFU/ml respectively from the same microorganism within 15 min of contact. The total aerobic mesophilic counts of the wash water before being tested were, on average, 1.5 log CFU/ml. After addition of Salmonella, with 15 min of contact with the sanitizer, the results of total counts showed the same magnitude as the Salmonella counts. Organic matter may have reacted with the free chlorine present, reducing chlorine concentrations, since values of 30.4 ppm were observed when the initial concentration should be 50 and 17.1 ppm when the initial concentration should be 25 ppm. Based on the results, sodium hypochlorite demonstrated a greater microbial reduction capacity in wash water with lettuce residues, indicating that it is more appropriate to avoid cross-contamination between batches during sanitation of lettuce in washing tanks.