IN VITRO STUDIES TO CONTROL THE GROWTH OF MICROORGANISMS OF SPOILAGE AND SAFETY CONCERN IN HIGH-MOISTURE, HIGH-pH BAKERY PRODUCTS

Initial agar plate studies were done to determine the effects of various levels (0 to 2,000 ppm) of potassium sorbate (KS) and sorbic hydroxamic acid (SHA) over a wide pH range (5 to 9) on the growth of microorganisms of spoilage and safety concern in high-moisture, high-pH bakery products. While growth of most microorganisms was inhibited for > 28 days on agar plates containing ∼1,000 ppm of KS at pH 5 and incubated at 30C, growth of all microorganisms occurred in plates at pH 7 and 9, regardless of the concentration of KS. SHA was equally effective at pH 5, however, it proved to be a more effective inhibitor against most microorganisms at higher pH (9).
Subsequent agar plate studies were done with water-ethanol (WE) and mastic oil-ethanol (ME) emitters. While WE emitters failed to control the growth of all microorganisms under investigation, ME emitters controlled the growth of most microorganisms, with the exception of Listeria monocytogenes, for ∼12 to 28 days on agar plates packaged in high-gas-barrier Cryovac or metallized bags, respectively. Inhibition was not simply due to the levels of ethanol, which ranged from ∼1.2 to 2.8% v/v, but rather, the mastic volatiles in the package headspace.
This study has demonstrated the potential of SHA and ME emitters to control the growth of several microorganisms of spoilage and safety concern in high-moisture, high-pH bakery products. However, the type of packaging material influenced the antimicrobial efficacy of this vapor-phase inhibitor.     

Effects of mastic resin and its essential oil on the growth of proteolytic Clostridium botulinum

Studies were done to determine the effect of mastic resin and its essential oil, alone and in conjunction with ethanol, on the growth of proteolytic strains of Clostridium botulinum in media, and on neurotoxin production in challenge studies with English-style crumpets. Preliminary studies, using a spot-on-the-lawn method, indicated that high levels of mastic resin in ethanol ( approximately 8% w/w) were required for complete inhibition of all strains of C. botulinum tested, but mastic resin in ethanol had a greater anti-botulinal effect than ethanol alone. However, only low levels of mastic oil ( approximately 0.3% v/v) were required for inhibition of proteolytic strains of C. botulinum. Both studies showed a strain specific inhibition, with C. botulinum type A strains being more sensitive to mastic resin and its essential oil than type B strains. However, mastic resin in ethanol proved to be more effective when used as a vapor phase inhibitor applied to cotton pads and placed inside inoculated plates than when added directly to media. While both mastic resin and its essential oil inhibited the growth of proteolytic strains of C. botulinum in vitro, they failed to inhibit neurotoxin production in challenge studies with C. botulinum in English-style crumpets.     

EFFECT OF ETHANOL VAPOR ON GROWTH AND TOXIN PRODUCTION BY CLOSTRIDIUM BOTULINUM IN A HIGH MOISTURE BAKERY PRODUCT

To determine the effect of ethanol vapor on toxin production by Clostridium botulinum, studies were done in English style crumpets (a_w 0.990, pH 6.5) challenged with 500 spores/g C. botulinum types A and proteolytic B and packaged in high gas barrier bags [ethanol transmission rate (ETR) 0.21 g/m²/day @ 25 C]. Crumpets were packaged in air with either commercially available ethanol vapor generators (Ethicap® 2, 4 or 6G) or cotton wool pads saturated with 2, 4 or 6 g of 95% food grade ethanol and stored at 25C. Toxin was detected in all inoculated control crumpets (0% ethanol) after 5 days at ambient temperature (25C). Ethicap® 2G delayed toxicity for 10 days while complete inhibition (>21 days) was observed in all crumpets packaged with 4 or 6G Ethicap® or with 2, 4 or 6 g of ethanol per pad. However, all crumpets were overtly spoiled by this time. Both headspace ethanol and absorption of ethanol by crumpets increased as a function of Ethicap® size/weight of ethanol. Based on these preliminary studies, ethanol vapor would appear to be an effective additional barrier to control the growth and toxin production by C. botulinum in high moisture bakery products and ensure the safety of these products at ambient temperature.     

ACID PH PRODUCED BY LACTIC ACID BACTERIA PREVENT THE GROWTH OF BACILLUS CEREUS IN BOZA, A TRADITIONAL FERMENTED TURKISH BEVERAGE

The growth and survival of Bacillus cereus, a known pathogen commonly found in cereals, during lactic acid fermentation of boza, a traditional Turkish cereal beverage, was studied. In the boza base inoculated with both the starter culture and B. cereus, the acidity developed to pH 2.6 and 0.8% titratable acidity after 72 h; the growth of B. cereus was reduced from 3.9 log cfu/mL to 1 log cfu/mL within 72 h. The control boza base to which starter was not added had a pH of 3, titratable acidity of 0.8%. The B. cereus in this boza base to which no starter culture was added dropped to 1 log cfu/mL after 72 h. No strains of lactic acid bacteria were found to produce bacteriocins antagonistic to B. cereus. Low pH and acidity were found to be the major factors inhibiting growth of B. cereus in boza.     

INCIDENCE AND CHARACTERIZATION OF BACILLUS CEREUS IN MEAT AND MEAT PRODUCTS CONSUMED IN TURKEY

A total of 100 retail samples of meat and meat products were examined for Bacillus cereus using mannitol egg yolk polymyxin (MYP) agar as a selective isolation medium. Only 22.4% of the samples contained detectable levels of B. cereus, with counts ranging from log₁₀ 0.69 to 4.80 cfu/g, but a large number of other organisms up to log₁₀ 9.06 cfu/g were sometimes observed on the plates and may have masked the presence of B. cereus or inhibited growth. Two samples of soudjouck contained significant levels of B. cereus, sufficient enough to create a public health hazard. Selected isolates were tested for diarrheal enterotoxin production by a reversed passive latex agglutination (RPLA) test kit. Results showed no difference in the toxin production of B. cereus between beef, ground meat, soudjouck and pastrami samples. Plasmid‐profile analysis and susceptibility to the six commonly used antimicrobial agents were done on selected B. cereus isolates. About 96.4% of the isolates contained plasmids. Plasmid sizes ranged between 2.5 and 3.5 kb. The isolates showed a high rate of resistance to oxacillin (OXA) and amoxicillin (AMC) and a low frequency of resistance to the other antimicrobial agents, with all of them being susceptible to vancomycin (VAN). Approximately 54% of the isolates showed multiple resistance. There was no apparent relationship between drug resistance and carriage of plasmids.     

Effect of pH and CO2 on growth and toxin production by Clostridium botulinum in English-style crumpets packaged under modified atmospheres.

The effect of pH and CO2 on both growth of and toxin production by Clostridium botulinum in English-style crumpets, packaged under modified atmospheres was investigated using a 2 x 2 factorial experiment. English-style crumpets (water activity, 0.990; pH 6.5 and 8.3) were inoculated with C. botulinum spores types A and proteolytic B (500 spores/g), packaged in either 60% CO2 (balance N2) or 100% CO2, stored at ambient temperature (25 degrees C), and monitored daily for toxicity. Toxin was detected after 4 days in crumpets packaged in 60% CO2, irrespective of initial product pH. Toxin production was delayed 1.5 to 3 days in crumpets packaged under 100% CO2. Analysis of variance indicated a significant interaction effect of pH and %CO2 on time of earliest toxin detection. Delay of toxin production was greatest for high pH (8.3) crumpets. All products were organoleptically acceptable at the time of toxigenesis, and therefore, high moisture-high pH bakery products, if contaminated with spores of C. botulinum, could become hazardous if packaged in atmospheres containing CO2.     

CHALLENGE STUDIES WITH PROTEOLYTIC CLOSTRIDIUM BOTULINUM IN YEAST AND CHEMICALLY LEAVENED CRUMPETS PACKAGED UNDER MODIFIED ATMOSPHERES

Challenge studies were done with proteolytic Clostridium botulinum (10³ spores/g) in yeast-and chemical-leavened crumpets (50-g) packaged in air with an ethanol vapor (2-G Ethicap®) generator or in 100% CO₂ and stored at ambient temperature (25C) for 30 days. Neurotoxin was detected in all gas- (CO₂) packaged crumpets after 5 days regardless of the method of leavening. While neurotoxin was delayed for 10 days in chemical-leavened Ethicap®-packaged crumpets, it was not detected in any similarly packaged yeast-leavened crumpets throughout storage. This inhibition of growth and neurotoxin production by C. botulinum was attributed to the production of ethanol by Saccharomyces cerevisiae in the yeast leavened crumpets, in conjunction with the ethanol vapor generated by the Ethicap® sachets (2-G), to levels to inhibitory to the growth of C. botulinum (>2.8% v/v). Subsequent challenge studies in sterile crumpets inoculated with either C. botulinum (10³ spores/g) or a co-inoculum of C. botulinum (10³ spores/g) and S. cerevisiae (10⁵ CFU/g) confirmed the significant role (p<0.001) of S. cerevisiae in enhancing the antibotulinal efficacy of ethanol vapor. These studies showed that the method of crumpet leavening could have a profound effect on the growth of and neurotoxin production by C. botulinum in crumpets packaged under modified atmospheres.     

EFFECTS OF BIFIDOCIN B AND LACTOCOCCIN R ON THE GROWTH OF LISTERIA MONOCYTOGENES AND BACILLUS CEREUS ON STERILE CHICKEN BREAST

In the study, the efficacies of bifidocin B and lactococcin R produced by Bifidobacterium bifidum and Lactococcus lactis subsp. cremoris R to control Listeria monocytogenes or Bacillus cereus in irradiated raw chicken breast during storage at 5–8C for 28 days or at 22–25C for 24 h were determined. Each irradiated raw chicken breast was inoculated with 10⁶ cfu/g L. monocytogenes or B. cereus. It was found that both bacteriocins were more effective against L. monocytogenes or B. cereus at 5–8C than at 22–25C, and that lactococcin R was more inhibitory than bifidocin B. Challenge study analysis demonstrated that incorporation of bifidocin B or lactococcin R at a level of 1,600 or 3,200 AU/g could effectively inhibit the growth of L. monocytogenes or B. cereus for 3–4 weeks at 5–8C or 6–12 h at 22–25C.     

Effects of process parameters on growth and metabolism of<Emphasis Type="Italic"> Lactobacillus sanfranciscensis</Emphasis> and<Emphasis Type="Italic"> Candida humilis</Emphasis> during rye sourdough fermentation

The effects of temperature, pH, inoculum level, and NaCl on the growth and metabolism of Lactobacillus sanfranciscensis and Candida humilis in rye sourdough were determined. The temperature optima for growth of C. humilis and L. sanfranciscensis were 28 and 32 °C, respectively. Yeast growth was inhibited at 35 °C. The pH did not affect yeast growth in the range 3.5–5.5, whereas growth of L. sanfranciscensis was inhibited at pH 4.0. A NaCl concentration of 4% (flour base) inhibited growth of L. sanfranciscensis but not C. humilis. The effects of the process parameters on the formation of lactate, acetate, ethanol, and CO₂ by the organisms were generally in agreement with their effects on growth. However, decreased formation of acetate by L. sanfranciscensis was observed at 35 °C although lactate and ethanol formation were not affected. In conclusion, the study provides a rationale for the stable persistence of L. sanfranciscensis and C. humilis in traditional sourdoughs and will facilitate the optimisation of sourdough fermentations in traditional and new applications.     

Incidence,Antibiotic Susceptibility,and Toxin Profiles of Bacillus cereus sensu lato Isolated from Korean Fermented Soybean Products

Korean fermented soybean products, such as doenjang, kochujang, ssamjang, and cho‐kochujang, can harbor foodborne pathogens such as Bacillus cereus sensu lato (B. cereus sensu lato). The aim of this study was to characterize the toxin gene profiles, biochemical characteristics, and antibiotic resistance patterns of B. cereus sensu lato strains isolated from Korean fermented soybean products. Eighty‐eight samples of Korean fermented soybean products purchased from retails in Seoul were tested. Thirteen of 26 doenjang samples, 13 of 23 kochujang samples, 16 of 30 ssamjang samples, and 5 of 9 cho‐kochujang samples were positive for B. cereus sensu lato strains. The contamination level of all positive samples did not exceed 4 log CFU/g of food (maximum levels of Korea Food Code). Eighty‐seven B. cereus sensu lato strains were isolated from 47 positive samples, and all isolates carried at least one enterotoxin gene. The detection rates of hblCDA, nheABC, cytK, and entFM enterotoxin genes among all isolates were 34.5%, 98.9%, 57.5%, and 100%, respectively. Fifteen strains (17.2%) harbored the emetic toxin gene. Most strains tested positive for salicin fermentation (62.1%), starch hydrolysis (66.7%), hemolysis (98.9%), motility test (100%), and lecithinase production (96.6%). The B. cereus sensu lato strains were highly resistant to β‐lactam antibiotics such as ampicillin, penicillin, cefepime, imipenem, and oxacillin. Although B. cereus sensu lato levels in Korean fermented soybean products did not exceed the maximum levels permitted in South Korea (<10⁴ CFU/g), these results indicate that the bacterial isolates have the potential to cause diarrheal or emetic gastrointestinal diseases.     

Effect of extrusion cooking on structure and functional properties of pea and kidney bean proteins

Pea (Pisum sativum L cv Ballet) and kidney bean (Phaseolus vulgaris L cv Pinto) seeds were extruded at 148 and 156 °C respectively. Protein solubility at various pH values and in various solvents was determined and analysis of protein fractions was carried out by SDS‐PAGE. Also, sulphhydryl and disulphide groups, water‐holding capacity (WHC), water solubility index (WSI) and oil absorption capacity (OAC) were determined. No changes in total nitrogen content of pea and kidney bean seeds occurred as a result of thermal treatment. Protein solubility from raw and extruded legumes was significantly higher in saline solutions than in water in the pH range 2–10. The solubility of proteins from extruded pea and kidney bean flours was greatly decreased with respect to native flours when extraction was in buffer (pH 7.0) alone. Extraction with buffer containing 2‐mercaptoethanol (2‐ME) or sodium dodecyl sulphate (SDS), alone or in combination, greatly increased protein extractability. As a result, the relative solubility was nearly 100% in buffer with SDS and 2‐ME for both raw and extruded samples. Total and free sulphhydryl group and disulphide contents decreased significantly (P < 0.05) after extrusion cooking. Moreover, extrusion treatment caused major changes in the band patterns of the albumin and globulin fractions obtained by SDS‐PAGE. WHC and WSI of extrudates increased significantly in both peas and kidney beans. A significant reduction in OAC was observed in extruded kidney bean flour. © 2000 Society of Chemical Industry     

Growth/no growth interfaces of Bacillus cereus, Staphylococcus aureus and Salmonella enteritidis in model systems based on water activity, pH, temperature and ethanol concentration

This study focuses its attention on the boundary between the growth and no growth of three strains ofSalmonella enteritidis , Bacillus cereus and Staphylococcus aureus in the presence of growth controlling factors such as temperature, pH, water activity (A_w) and ethanol concentration. Preliminarly, the minimal values of pH, A_wand temperature, and the maximum ethanol concentrations allowing the growth of the considered micro-organisms were determined. The calculation of these values enabled the use of logistic model to evaluate the growth/no growth boundary for the bacteria in relation to the considered independent variables. The location of the growth/no-growth boundaries for S. enteritidis and Staph. aureus were strongly affected, at the same ethanol concentration, by temperature, pH and A_w. Among the considered species, Staph. aureus was endowed with the highest sensitivity to low pH values whileB. cereus's growth/no growth interface, was quite unaffected by the combination of the stresses, when the physico–chemical conditions were above the minimum for growth. The effects of temperature, A_wand ethanol on the limitation of growth of the considered species were not merely additive. It was possible to identify the combinations of such factors preventing the growth of Salmonella enteritidis, Staph. aureus and B. cereus.     

Growth and toxin production by Clostridium botulinum in English-style crumpets packaged under modified atmospheres.

To determine the safety of a high moisture bakery product, packaged under modified atmospheres, challenge studies were done on English-style crumpets (water activity [a(w)] 0.990, pH 6.5) inoculated postbaking with Clostridium botulinum types A and proteolytic B spores (5 X 10(2) spores/g). Products were packaged either in air, in air with an Ageless FX200 oxygen absorbent, or in a CO2/N2 (60:40) gas mixture, stored at ambient temperature (25 degrees C), and monitored for toxicity daily. All inoculated crumpets were toxic within 4 to 6 days and were organoleptically acceptable at the time of toxigenesis. Counts of C. botulinum increased to approximately 10(5) CFU/g at the time of toxicity. To determine the effect of baking on product safety, subsequent challenge studies were done on crumpets inoculated with 5 x 10(2) spores/g (baked weight basis) prior to baking. All crumpets were toxic after only 6 days, irrespective of packaging conditions, and toxigenesis again preceded spoilage. Temperature profile studies showed that the maximum internal temperature reached during baking was 97 degrees C, and the total baking process was equivalent to 0.03 min at 121 degrees C. The actual time to toxin production in both studies (4 to 6 days) correlated well with the predicted time (3.4 days) using the U.S. Department of Agriculture Pathogen Modeling Program (version 5.1) for proteolytic strains of C. botulinum. These studies confirm that high moisture bakery products, if contaminated with C. botulinum spores either pre- or postbaking, could pose a public health hazard, if packaged in air (in a high gas barrier package where O2 was depleted and CO2 was generated during storage) or under modified atmosphere packaging conditions and stored at ambient temperature.     

Controlling Listeria monocytogenes and Leuconostoc mesenteroides in Uncured Deli‐style Turkey Breast Using a Clean Label Antimicrobial

Interest in natural/organic meat products has resulted in the need to validate the effectiveness of clean label antimicrobials to increase safety and shelf life of these products. A Response Surface Methodology (RSM) was used to investigate the effects of varying levels of moisture, pH, and a commercial “clean‐label” antimicrobial (cultured sugar‐vinegar blend; CSVB) on the growth rate of Listeria monocytogenes and Leuconostoc mesenteroides in uncured turkey stored at 4 °C for 16 wk. Twenty treatment combinations of moisture (60% to 80%), pH (5.8 to 6.4), and CSVB (2.5% to 5.0%) were evaluated during phase I to develop growth curves for both microbe types, whereas the interactive effects of pH (5.8 to 6.4) and CSVB (0.0 to 4.75) were tested in 16 treatment combinations during Phase II at a single moisture level using L. monocytogenes only. CSVB inhibited L. monocytogenes growth in 14 of the 20 treatments tested in Phase I and in 12 of the 16 treatments in Phase II through 16 and 8 wk, respectively. In contrast, CSVB had little effect on L. mesenteroides, with growth inhibited in only 4 of 20 treatments in Phase I and was therefore not tested further in Phase II. Significant interactions of the RSM design coefficients yielded a predictive model for L. mesenteroides growth rate, but due to lack of growth, no growth rate model was developed for L. monocytogenes. CSVB was found to be an effective antilisteral antimicrobial, while having little effect on a spoilage microorganism.     

Ethanol Production From Raw Sago Starch Under Unsterile Condition

In order to utilize raw sago starch as fermentation material, mold strain (Aspergillus niger N-10) which produces raw sago starch hydrolyzing enzyme, was isolated. Maximum raw sago starch consumption rate by the strain was obtained when the initial pH of the medium was 3.5 and the enzyme showed high stability and activity at lower pH values. Direct ethanol production from raw sago starch was investigated using a mixture of the strain and ethanol producing yeast, Saccharomyces cerevisiae IFO 0309. Neither the growth of the strains nor ethanol production was inhibited in the mixed culture of the two strains. Moreover, when fed-batch mixed culture was conducted under unsterile condition (both the medium and the apparatus were not sterilized), about 30g/1 of ethanol was produced with ethanol yield from raw sago starch Y_p/s of 0.40g ethanol/g starch.     

Sake and Beer Spoilage Lactic Acid Bacteria — A Review

Japanese rice wine, sake, is a traditional alcoholic beverage in Japan. Similar to the case with beer, sake is known to be microbiologically stable and most microorganisms fail to grow in sake. This is principally due to its high ethanol concentration that reaches approximately 20% (v/v) in undiluted sake products and 15% (v/v) in finished products. Despite the high level of ethanol content, spoilage incidents occasionally occur in sake, due to the presence of highly ethanol‐tolerant lactobacilli, known as hiochi‐bacteria. Hiochi‐bacteria are generally composed of two groups of lactobacilli, hiochi‐lactobacilli and true hiochi‐bacilli. The former group of lactobacilli is less ethanol‐and heat‐tolerant, and therefore rarely poses a problem to sake products. In contrast, the true hiochi‐bacilli exhibit extraordinarily high ethanol tolerance and cause spoilage incidents in sake, conferring acidity and off‐flavors, such as diacetyl, in spoiled products. From a taxonomic standpoint, the true hiochi‐bacilli mainly consist of two Lactobacillus species, L. fructivorans and L. homohiochi. The strains of true hiochi‐bacilli prefer sake‐like environments, and the presence of ethanol and mevalonic acid, in combination with low pH milieu, is essential or stimulatory for the growth of these bacteria. Interestingly, the type strain of L. fructivorans does not show such characteristics, suggesting the true hiochi‐bacilli are profoundly adapted to sake environments. Although beer spoilage lactic acid bacteria do not have close taxonomic relationships with true hiochi‐bacilli, there are striking similarities between these two groups of spoilage lactic acid bacteria. In this review, unique features of sake and beer spoilage lactic acid bacteria are discussed in comparative terms.     

INCIDENCE AND DIARRHEGENIC POTENTIAL OF BACILLUS CEREUS IN PASTEURIZED MILK AND CEREAL PRODUCTS IN THAILAND

ABSTRACT

Bacillus cereus, bacteria that commonly occur in foods, can potentially cause foodborne illness. Two important factors that contribute to the illness are the number of B. cereus in food and the ability of the organism to produce enterotoxins. This study investigated the number of B. cereus cells in dairy and cereal products in Thailand, using the plate count method and the presence of diarrheal‐enterotoxin genes in the isolates through the polymerase chain reaction (PCR). The genes encoding hemolysin BL (hblA, hblC, hblD), nonhemolytic enterotoxin (nheA, nheB, nheC), cytotoxin K (cytK) and enterotoxin FM (entFM) were the targets of the PCR. B. cereus was found in all pasteurized milk samples and in 37.7% of the cereal product samples, ranging from 50 to 1.7 × 10³ cfu/g. PCR results revealed that each gene occurred in more than half of the foodborne isolates tested. A large proportion (96%) of the isolates harbored enterotoxin genes and is considered to be potentially diarrhegenic.

PRACTICAL APPLICATIONS

Significant frequency of Bacillus cereus contamination in pasteurized milk and cereal products, and the large proportion of diarrhegenic strains among foodborne B. cereus indicate the high risk of foodborne illness that could be caused by consumption of these foods in Thailand. This suggests that B. cereus should not be disregarded in its significance in disease control and prevention programs. Also, as complete elimination of this organism from pasteurized milk and most of the cereal products through the processing steps is not possible, proper handling and storage of these foods should be strictly applied by the food industry. This is necessary in order to prevent the growth of the organism to levels that can cause foodborne illness. This research is also relevant to other developing countries having similar situations as Thailand, where data concerning the number of B. cereus in foods, frequency of contamination and proportion of enterotoxigenic B. cereus are limited, and where B. cereus gastroenteritis could be underestimated.     

Combination Inhibition Activity of Nisin and Ethanol on the Growth Inhibition of Pathogenic Gram Negative Bacteria and Their Application as Disinfectant Solution

Nisin and ethanol have been used as antimicrobial agents in food industry. However, nisin alone could not inhibit the growth of gram‐negative bacteria, except in combination with a chelating agent, EDTA, or organic acid. This research aimed to study the survival of Escherichia coli O157: H7, Salmonella Typhimurium TISTR 292 and Salmonella Enteritidis DMST 17368 after treatment with nisin at 100, 200, 300, 500, 800, or 1000 IU/mL and ethanol at 70%, 50%, 30%, 20%, or 10% (v/v) alone and in combination. None of all nisin concentrations could reduce the growth of target strains. While 20% ethanol (v/v) having no negative effect on human health, could slightly reduce the growth of target strains. However, the combination of nisin at 500, 800 or 1000 IU/mL and 20% ethanol displayed significant growth reduction at 15 min were below 1 log CFU/mL. Thus, the minimum inhibitory concentration of nisin and ethanol was 500 IU/mL and 20% (v/v), respectively. The release of fatty acid, genetic materials and scanning electron microscope suggested that nisin‐ethanol treated cells have altered permeability causing bacterial growth inhibition. Comparison treatment of combined solution and commercial chloride based sanitizer were done for all target strains on stainless steel surface. Survivals of three target strains were below 1 log CFU/mL. The result suggested that combined solution of nisin and ethanol may be a beneficial sanitizer for food industry to inhibit the growth of E. coli O157:H7 and Salmonella sp.     

INHIBITION OF LISTERIA MONOCYTOGENES BY NATIVE MICROFLORA OF WHOLE CANTALOUPE1

Fresh‐cut cantalcupe has been recalled due to the possible presence of Listeria monocytogenes. Several studies have reported that naturally occurring microflora of vegetable surfaces may be antagonistic to pathogen attachment, growth or survival. To test this possibility for L. monocytogenes and cantaloupes, whole melon were treated with water, ethanol (70%) or chlorine (200 ppm) to reduce the native microflora on the melon surfaces. Treated or untreated melons were immersed in a six strain cocktail of L. monocytogenes (10⁷ CFU/mL) for 10 min and then allowed to dry for 1 h inside a biosafety cabinet followed by storage at 5, 10 and 20C for 15 days. Fresh‐cut pieces prepared from the treated or untreated melons and directly inoculated with L. monocytogenes (3.48 log CFU/g) were stored under the same conditions listed above. Populations of L. monocytogenes and five classes of native microflora were investigated. Growth of L. monocytogenes in sterile or nonsterile rind and fresh‐cut homogenates was also studied. The population of L. monocytogenes recovered from inoculated (10³ to 10⁸ CFU/mL) whole melons given no disinfection treatment or washed with water was significantly less (P < 0.05) than that recovered from melons treated with chlorine or EtOH. In general, populations of L. monocytogenes declined on the surface of treated and untreated whole melons and on fresh‐cut pieces over the 15 days storage period at the temperatures tested. However, the decline in pathogen populations was less rapid in the presence of reduced populations of native microflora. Higher populations of L. monocytogenes were attained in sterile tissue homogenates than in nonsterile homogenates. Addition of yeast and mold to sterile rind homogenates was highly inhibitory to growth and survival of the pathogen. The results of this study indicate that native microflora of whole cantaloupe inhibited attachment to rind surfaces as well as survival and growth of L. monocytogenes on cantaloupe surfaces and homogenized fresh‐cut pieces. Thus, L. monocytogenes recontamination of melons having a reduced level of native microflora following application of a disinfection treatment may be a food safety concern.