SURVIVAL OF ESCHERICHIA COLI, STAPHYLOCOCCUS AUREUS AND SALMONELLA ENTERITIDIS IN FROZEN CHICKEN HAMBURGER

The survival of food pathogens during frozen storage of chicken hamburgers was evaluated. Hamburgers were contaminated with Escherichia coli (ECHC), Staphylococcus aureus (SAFH) and Salmonella Enteritidis (SE86), separately, and stored at −18C for 28 days. Results demonstrated reductions of 0.63 log₁₀ cfu/g, 0.73 log₁₀ cfu/g and 0.27 log₁₀ most probable number/g in the populations of ECHC, SAFH and SE86, respectively. These microorganisms were also stored at −18C in 0.1% peptone water and presented significantly different reductions ( P <  0.05) when compared with frozen hamburgers, suggesting that components of chicken hamburger may exert a cryoprotective effect on microbial cells.

PRACTICAL APPLICATIONS

Even though Brazilian poultry meat industries present high levels of quality control, the presence of bacterial pathogens in hamburgers may occur. In Brazil, chicken hamburgers are frequently commercialized frozen, and it is well established that frozen temperatures are able to avoid microbial growth. However, frozen temperatures can also be able to inactivate part of the bacterial population, and the magnitude of its effect will vary according to the type of food and the process that it was submitted. This study evaluated the survival of Escherichia coli , Staphylococcus aureus , and Salmonella Enteritidis in frozen hamburgers, aiming to verify if this process could provide an additional margin of safety to this product.     

SURVIVAL OF SALMONELLA TYPHIMURIUM, ESCHERICHIA COLI O157:H7 AND LISTERIA MONOCYTOGENES DURING STORAGE ON BEEF SANITIZED WITH ORGANIC ACIDS

Sterile beef tissue was inoculated with either Salmonella typhimurium, Escherichia coli O157:H7 or Listeria monocytogenes Scott A and washed with 23C distilled water, 1% lactic acid or 1% acetic acid. The washed tissue was subjected to simulated dry chilling or spray chilling followed by storage at 5C. The washed tissue was stored at 5C for up to 21 days at 26% relative humidity, and total bacterial populations were determined by plating on nonselective and selective agars. There was no significant difference in the surviving populations of S. typhimurium, Escherichia coli O157:H7, or L. monocytogenes after storage, irrespective of chilling method. The surviving populations of bacteria were significantly lower on acid washed adipose tissue, when compared to the comparable water washed tissue. These results indicate that although injury and recovery of pathogenic bacteria may occur as a result of organic acid carcass sanitizing treatments, there was no practical significance of this phenomenon after 3 days of storage.     

SURVIVAL AND GROWTH OF ACID ADAPTED ESCHERICHIA COLI STRAINS IN BROTH AT DIFFERENT PH LEVELS

Acid resistance of Escherichia coli O157:H7 strain UT 10 and Escherichia coli ATCC 25922 was determined in brain heart infusion broth at pH 7.4, 4.5 and 2.5. Variations due to acid stress in the counts of both strains were also determined. Acid adaptation enhanced the survival of both strains at pH 4.5, but neither strain could survive after 4 h at pH 2.5. At optimum growth conditions (pH 7.4), E. coli ATCC 25922 exhibited increased viability over E. coli UT 10. At pH 4.5, E. coli UT 10 was more tolerant to low pH than E. coli ATCC 25922. An increase in saturated fatty acids of both AA strains was observed, indicating the importance of lipid modification in enhancing survival at low pH. The results of this study indicated that the food industry should therefore adapt their processing/preservation procedures by taking the most acid tolerant pathogenic E. coli strains into consideration in order to ensure the safety of their products.

PRACTICAL APPLICATIONS

The study will enlighten the industry on the survival of acid adapted pathogens such as E. coli O157:H7 at low pH. It also indicates that current measures used in the preservation of low pH foods can trigger acid adaptation. For this reason, the effectiveness of current preservation measures in controlling foodborne pathogens should be reassessed. The importance of using cells adapted to different pH values in food challenge studies is also highlighted. Since fermented foods, which are generally regarded as safe, have been implicated in foodborne disease outbreaks, more attention should be given to the prevention of contamination of foods with pathogens such as E. coli O157:H7 since they may survive in low pH foods and cause disease.     

EFFECT OF SPICES ON GROWTH AND SURVIVAL OF SALMONELLA TYPHIMURIUM DT 104 IN GROUND BEEF STORED AT 4 AND 8C

Few studies have addressed the use of spices against pathogens associated with meat. The effects of garlic, ginger and turmeric were evaluated against Salmonella Typhimurium DT 104 that were inoculated either in spice paste or in buffered peptone water (BPW) or in heat‐treated ground beef and stored at 4 and 8C for 10 days. Data from the spice pastes study showed a decrease in Salmonella Typhimurium DT 104 counts, and the greatest reduction (3.39 log) was observed in garlic paste stored at 4C. Garlic in BPW data showed a reduction of 1.5 and 1.0 log in Salmonella Typhimurium counts at 4 and 8C, respectively. Ground beef stored at 4C showed no growth or a slight reduction in growth in samples with spice, while all samples at 8C showed an increase in Salmonella Typhimurium counts. Results show that the spices inhibit or inactivate Salmonella Typhimurium DT 104 when they are in direct contact. However, when spices are added to a complex food system such as ground beef, the inhibitory activity of these spices considerably decreases.     

SURVIVAL OF SALMONELLA TYPHIMURIUM IN ROASTING CHICKENS COOKED IN A MICROWAVE, CONVECTION MICROWAVE, AND A CONVENTIONAL ELECTRIC OVEN

Fresh whole roasting chickens were inoculated with Salmonella typhimurium and cooked either in a microwave, convection microwave, or conventional electric oven to an internal temperature of 74°C, 77°C, 79°C, or 85°C. Cooked chickens were tested for the presence of viable Salmonella organisms using an Enzyme Linked Immunosorbent Assay (ELISA). No chicken (0/12) cooked to ± 79°C in the convection microwave or conventional oven tested positive for Salmonella. Two out of three chickens cooked to 79°C and all three chickens cooked to 85°C in the microwave tested Salmonella positive. Tetrathionate broth dilutions from all 11 microwave cooked chickens produced isolated colonies on Brilliant Green Agar. No growth was observed on Brilliant Green Agar plates inoculated with tetrathionate broth dilutions from chickens cooked to an internal temperature of 79°C or 85°C in the conventional oven or to 85°C in the convection microwave.     

IN VITRO COMPARISON OF ANAEROBIC AND AEROBIC GROWTH RESPONSE OF SALMONELLA TYPHIMURIUM TO ZINC ADDITION

Zinc supplemented diets have been used to provide zinc as a nutrient and higher concentrations have been used to induce molt in laying hens. It is not known if the zinc in these diets would inhibit Salmonella spp growth. This study examines the effects of zinc compounds on the growth of S. typhimurium poultry isolate under aerobic and anaerobic conditions. The aerobic growth response of S. typhimurium was determined either in tryptic soy broth (TSB) or minimal (M9) broth containing five different concentrations (0.67, 2.01, 3.35, 4.69, and 6.03% [wt/vol]) of either Zn acetate [Zn(C₂H₂O₂)₂2H₂O] or Zn sulfate [ZnSO₄7H₂O] while anaerobic growth response was determined in M9 broth with or without reductants (L-cysteine hydrochloride [C₃H₇NO₂SHCl], and sodium sulfide [Na₂S 9H₂O]). Aerobic growth rates inhibited (P < 0.05) by Zn acetate than by Zn sulfate in TSB medium. The Zn source and concentration decreased (P < 0.05) aerobic growth response of S. typhimurium poultry isolate in M9 medium. The growth rates of S. typhimurium under anaerobic growth conditions were less responsive to Zn salts but were generally lower (P < 0.05) in the presence of reductant than in the absence of reductants at each concentration of Zn compound. The results in this study provide evidence that Zn may inhibit S. typhimurium under in vitro aerobic or anaerobic atmospheric conditions and S. typhimurium grows less optimally under anaerobic growth conditions.     

EFFECT OF MICROWAVE HEATING ON SURVIVAL OF SALMONELLA TYPHIMURIUM IN ARTIFICIALLY CONTAMINATED READY-TO-EAT FOODS

This research evaluated the destruction of Salmonella typhimurium during reheating of foods in two different types of microwave ovens: a conventional 750W oven and 700W oven with preset controls. The heating times in the conventional microwave oven were established as 50 s for baby food and 75 s for mashed potatoes as well as for the beef stroganoff samples, while for the preset oven those periods of time were determined by a built-in temperature sensor. The percentage of food samples positive for S. typhimurium in conventional microwave was 47.8% while in microwave with preset controls it was 93.3%. Our results suggest that reheating contaminated foods in microwave ovens may, therefore, not be sufficient to destroy S. thyphimurium in order to assure food safety .     

DEATH KINETICS OF ESCHERICHIA COLI IN A COMBINED TREATMENT WITH HEAT AND MONOLAURIN

A kinetic study was performed on the combined effect of monolaurin and heat on the death of Escherichia coli. The following results were obtained: (1) Monolaurin was about eleven times more active than that of sodium laurate; (2) Temperature enhanced the effect of monolaurin. True enthalpyentropy compensation effect was shown in this death reaction. The value of 341.9°K was obtained from the formula of ΔH*= T_c·ΔS*+ b and 331.5°K from Arrhenius plots as a compensation temperature; (3) The apparent minimum enhancing concentration of monolaurin ranged from 0.0056 mM to 0.013 mM, varying with the heating temperature. It may be concluded from the results of this study that the enhancing effect of monolaurin on the thermal death of E. coli corresponds to that of the amphoteric surfactant type (S type) agent defined in our previous report.     

DEVELOPMENT OF STRESS AND SURVIVAL OF SALMONELLA TYPHIMURIUM ATCC 7136 IN A TAPIOCA SOYSTARCH PRODUCT CONTAINING POTASSIUM SORBATE

The effects of reduced water activity and 0.1% sorbate on Salmonella typhimurium ATCC 7136 in a soy-starch product were examined. The formulated product (final pH 6.7), was inoculated with cells to give a final concentration of approximately 2 × 10⁶ cells/g. Ten gram samples were placed in a series of desiccators over saturated salt solutions and stored at either 22°C or 40°C. Water activities ranged from 0.79 to 0.97. Samples were withdrawn at prescribed intervals and appropriate dilutions made. A dual plating procedure was used to monitor growth (tryptic soy agar) and development of injury (Levine's Eosin Methylene Blue Agar supplemented with 2% NaCl and 0.05% sodium desoxycholate). Cells stored at 22°C remained viable longer at an a_w of 0.79 than at a_w values of 0.96 to 0.97. As storage time increased, cells were stressed at a faster rate at higher a_w values. This effect was accelerated at 40°C and as storage time increased.     

EFFECTS OF TRISODIUM PHOSPHATE ON POPULATIONS OF SALMONELLA TYPHIMURIUM AND SALMONELLA ENTERITIDIS IN VITRO

This study determined the bactericidal activity of different concentrations of trisodium phosphate (TSP) against Salmonella typhimurium and Salmonella enteritidis in vitro. One milliliter of each TSP solution (0.5, 1, 1.5 and 2% w/v) was mixed with 1 mL of each bacterial culture and left to stand at room temperature (18  ±  1C) for 0, 15, 30, 45, 60 or 90 min before enumeration of the surviving pathogens.
The effects of 1.5 and 2% TSP were apparent on both bacterial species from 0 min and bacterial counts reached undetectable ( < 1.0  ×  10¹ cfu/mL) levels immediately. The 1% TSP solution effected close to 4.20–5.72 log reduction after 15 min. However, the reduction effect of 0.5% TSP was a function of treatment time, and some culturable bacteria remained even after 90 min of treatment. These results suggest that the bactericidal effect of TSP solution on these pathogens depends on its concentration and on the duration of treatment.

PRACTICAL APPLICATIONS

Poultry has been identified as one of the most important reservoirs of Salmonella typhimurium and Salmonella enteritidis in food chain. Efforts to eliminate or reduce Salmonella spp. and other pathogens attached to poultry carcasses during processing have been made for many years. In 1992, the U.S. Department of Agriculture approved the use of trisodium phosphate (TSP) as a processing aid to reduce Salmonella spp. on raw poultry carcasses. The aim of the present study was to determine the bactericidal effects of different concentrations of TSP on pure cultures of these pathogens in vitro .     

INACTIVATION OF ESCHERICHIA COLI SUSPENDED IN LIQUID EGG USING PULSED ELECTRIC FIELDS

Liquid egg inoculated with Escherichia coli was exposed to a 26kV/cm pulsed electric field with 2 and 4 μs pulse duration, 1.25 and 2.50 Hz pulsing rates, up to 100 pulses/unit volume, and stepwise and continuous recirculation treatment schemes while maintaining a bulk temperature below 37C. The inactivation of E. coli was a function of the pulse duration and the number of pulses. The destruction of Escherichia coli in liquid egg followed a first order kinetic and the treatment was more effective when the applied pulses were of a 4 μs pulse duration. A 6D reduction was obtained for viable E. coli using both pulsing rates and treatment schemes with no protein coagulation.     

RAPID DETECTION OF SALMONELLA ENTERITIDIS AND ESCHERICHIA COLI USING SURFACE PLASMON RESONANCE BIOSENSOR

The Biacore surface plasmon resonance (SPR)‐based biosensor was used to detect Salmonella enterica serovar Enteritidis and Escherichia coli in spiked skim milk using specific antibodies. The optical SPR biosensor registers changes in the refractive index during binding of the analyte (bacteria) to the ligand (antibody) on the gold‐coated sensor chip surface. Immobilized protein A‐captured specific antibodies were used to detect the bacteria in less than an hour. The reaction was found to be specific, and the limits of detection of the assay were found to be 25 cfu/mL for E. coli and 23 cfu/mL for Salmonella. Because the assays were shown to be sensitive, reproducible and very specific, the SPR‐based Biacore biosensor has the potential for near real‐time, yet specific, detection and presumptive identification of bacteria in food and water.     

SPECIFICITY OF ESCHERICHIA COLI LYSINE AUXOTROPH GROWTH KINETICS AND LYSINE ASSAY RESPONSE TO VARIOUS SOLUBLE MAILLARD REACTION PRODUCTS

Lysine availability in foods or feeds can be rapidly assessed by an Escherichia coli lysine auxotroph (lys) based assay. The overall objective of these studies was to examine if the accuracy of this lysine assay was affected by the presence of different soluble Maillard reaction products (MRP) in a model system. The standard curve without addition of MRP was parallel to the standard curves with addition of histidine-MRP (histidine with glucose) or arginine-MRP (arginine with glucose). The y-axis intercept of the standard curve without addition of MRP was significantly (P < 0.05) greater than the y-axis intercepts of the standard curves with addition of MRP. This data indicated that MRP inhibited lys growth when limited by lysine concentration, and accurate determination of lysine availability in food or feed sample may require an internal standard curve where known lysine increments are added to the samples to be analyzed.