Influence of diluent on antimicrobial activity of cinnamon bark essential oil vapor against Staphylococcus aureus and Salmonella enterica on a laboratory medium and beef jerky.

The influence of chemical diluents on the antimicrobial activity of plant essential oil (EO) vapors was evaluated. We first determined if vapors generated from 22 chemical diluents not containing EO had antimicrobial activities. Ethyl ether vapor retarded the growth of S. aureus. The minimal inhibitory concentrations (MICs) and the minimal lethal concentrations (MLCs) of cinnamon bark EO vapor, which was diluted in and generated from 21 diluents, against S. aureus and S. enterica were determined. Cinnamon bark EO vapor showed significantly (P ≤ 0.05) lower MICs against S. aureus when diluted in dimethyl sulfoxide (DMSO), ethanol, ethyl acetate, or jojoba oil, and against S. enterica when diluted in DMSO, ethanol, or jojoba oil, compared to those in other diluents. We compared antimicrobial activities of cinnamon bark EO vapor diluted in DMSO, ethanol, ethyl acetate, or jojoba oil against S. aureus and S. enterica on beef jerky as a food model. Antimicrobial activity was significantly (P ≤ 0.05) higher when vaporized from DMSO. These results indicate that antimicrobial activity of cinnamon bark EO vapor may vary significantly (P ≤ 0.05) depending on the type of diluent from which it is vaporized. These observations provide basic information when developing food and food-contact surface decontamination strategies using EO vapors.

Effects of temperature, pH and sodium chloride on antimicrobial activity of magnesium oxide nanoparticles against E. coli O157:H7.

AIM: This study was done to determine the effects of temperature, pH and sodium chloride (NaCl) on antimicrobial activity of magnesium oxide (MgO) nanoparticles (NPs) against E. coli O157:H7. METHODS AND RESULTS: Culture conditions were established by varying the pH (5.0, 7.2 and 9.0), NaCl concentration (0.5, 2.0, 3.5 and 5.0%, w/v), and incubation temperatures (4, 12, 22 and 37°C). At each condition, the antimicrobial activities of MgO-NPs (0, 1, 2 and 4 mg/ml) against E. coli O157:H7 were measured. Four-way analysis of variance indicated interactions among all factors had a significant effect (p ≤ 0.05) on the antimicrobial activity of MgO-NPs. The concentration of MgO-NPs necessary to cause a 5-log reduction of E. coli O157:H7 under the most inhibitory conditions (37°C, pH 9.0, and 5.0% NaCl) was 0.50 mg/ml of MgO-NPs. CONCLUSION: The antimicrobial activity of the MgO-NPs increased significantly (p ≤ 0.05) with increased temperature, pH and NaCl concentration in TSB. SIGNIFICANCE AND IMPACT OF THE STUDY: The influence of intrinsic and extrinsic factors on antimicrobial activity of MgO-NPs we found will contribute to the development of microbial decontamination strategies using MgO in the food industry.

Antimicrobial activities of plant essential oil vapours against Acidovorax citrulli and Xanthomonas campestris on Cucurbitaceae, Brassicaceae and Solanaceae seeds.

AIM: This study was done to develop a seed decontamination treatment for organic seeds against plant pathogens (Acidovorax citrulli and Xanthomonas campestris) using essential oil (EO) vapours without affecting the seeds' germination rate. METHODS AND RESULTS: By using a diffusion assay and determining minimum inhibitory and lethal concentrations, we screened two EO vapours which were most inhibitory to A. citrulli (cinnamon bark and garlic EO vapours) and X. campestris (onion and garlic EO vapours). After 48 h of exposure to EO vapours at 25°C and 43% or 85% relative humidity (RH), no significant decrease (p > 0.05) in germination rates was observed compared with those of control seeds. It was observed that EO vapour treatment at 25°C and 43% or 85% RH for 48 h caused significant population reductions (p ≤ 0.05) (ca. 0.3-2.6 log colony forming unit/g) compared to those of untreated seeds. CONCLUSION: Applications of EO vapours showed significant (p ≤ 0.05) antimicrobial effects against A. citrulli and X. campestris on both laboratory mediums and plant seeds without decreasing the germination rate of seeds. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides useful information for the development of natural seed sterilization treatments using EO vapours.

Inactivation of Escherichia coli O157:H7 on radish and cabbage seeds by combined treatments with gaseous chlorine dioxide and heat at high relative humidity.

This study was done to develop a method to inactivate Escherichia coli O157:H7 on radish and cabbage seeds using simultaneous treatments with gaseous chlorine dioxide (ClO2) and heat at high relative humidity (RH) without decreasing seeds' viability. Gaseous ClO2 was spontaneously vaporized from a solution containing hydrochloric acid (HCl, 1 N) and sodium chlorite (NaClO2, 100,000 ppm). Using a sealed container (1.8 L), an equation (y = 5687×, R2 = 0.9948) based on the amount of gaseous ClO2 generated from HCl-NaClO2 solution at 60 °C and 85% RH was developed. When radish or cabbage seeds were exposed to gaseous ClO2 at concentrations up to 3,000 ppm for 120 min, germination rates did not significantly decrease (P > 0.05). When seeds inoculated with E. coli O157:H7 were treated with 2,000 or 3,000 ppm of gaseous ClO2 in an atmosphere with 85% RH at 60 °C, populations (6.8-6.9 log CFU/g) on both types of seeds were decreased to below the detection limit for enrichment (-0.5 log CFU/g) within 90 min. This study provides useful information for developing a decontamination method to control E. coli O157:H7 and perhaps other foodborne pathogens on plant seeds by simultaneous treatment with gaseous ClO2 and heat at high RH.

Antimicrobial activity of ClO2 gas against Salmonella Enteritidis on almonds.

Nuts, including almonds, are occasionally contaminated with Salmonella spp. In this study, we used chlorine dioxide (ClO2) gas to inactivate S. enterica subsp. Enterica serovar Enteritidis on almonds. Almonds inoculated with a single strain of S. Enteritidis (8.95 log cfu/mL) were exposed to ClO2 gas generated from 1.0 or 1.5 mL ClO2 solution in a sealed container at 50 or 60 °C (43% relative humidity) for up to 10 h. The concentration of ClO2 gas peaked at 354-510 and 750-786 ppm within 0.5 h upon deposition of 1.0 and 1.5 mL of aqueous ClO2, respectively, and gradually decreased thereafter. Population of S. Enteritidis on almonds treated at 50 °C decreased to 1.70-2.32 log cfu/sample within 1 h of exposure to ClO2 gas and decreased to below the detection limit (1.7 log cfu/sample) at all ClO2 concentrations after 8 h. At 60 °C, the microbial population fell below the detection limit within 1 h, regardless of the volume of ClO2 solution supplied. Microbial survival on almonds treated with ClO2 gas and stored at 12 or 25 °C was observed for up to 8 weeks and the organism was not recovered from the almonds treated for 10 h and stored at 12 °C for 2-8 weeks. The lightness (L value) and redness (a value) of almonds treated for 10 h were not changed by ClO2 gas treatment, but yellowness (b value) increased. Results showed that Salmonella on almonds was successfully inactivated by ClO2 gas treatment and the microbial survival did not occur during storage.

Antimicrobial activity of gaseous chlorine dioxide against Aspergillus flavus on green coffee beans.

The aim of this study was to use chlorine dioxide (ClO2) gas to inactivate Aspergillus flavus on green coffee beans. Green coffee beans inoculated with A. flavus were exposed to ClO2 gas generated from 0.75, 1, 1.25, or 1.5 mL of ClO2 solution in a sealed container at 25 or 50 °C (100% relative humidity [RH]) for up to 10 h. Numbers of A. flavus on beans treated at 25 °C decreased by 1.1-2.2 log cfu/bean within 1 h of exposure to ClO2 and decreased to below the detection limit (≤1.0 log cfu/bean) at all ClO2 concentrations after 10 h. At 50 °C, the microbial population reached the detection limit within 0.5 h regardless of the ClO2 solution concentration. Beans exposed to gas for 10 h at 25 °C were stored for 14 days under conditions of 43, 75, or 100% RH and 4, 12, or 25 °C. At 4 °C, visible mold growth was not established regardless of RH. After 12 days, mold was observed only at 25 °C. At 100% RH, beans stored under the same conditions without ClO2 gas treatment showed mold formation at 13 and 4 days at 12 and 25 °C, respectively.

Antimicrobial activities of organic acid vapors against Acidovorax citrulli, Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes on Cucurbitaceae seeds.

This study investigated the antimicrobial activities of organic acid vapors against a phytopathogen (Acidovorax citrulli) and foodborne pathogens (Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes) on the surface of Cucurbitaceae seeds. Germination percentages of cucumber, honeydew melon and watermelon seeds treated with acetic and propionic acid vapors (100 mg/L) at 50 °C and 43% or 85% relative humidity (RH) for up to 2 h did not significantly (P > 0.05) decrease. Treatment with formic acid significantly (P ≤ 0.05) decreased the germination percentage. The antimicrobial activities of acetic and propionic acid vapors (100 mg/L; 50 °C; 43% or 85% RH) were determined. A. citrulli was inactivated within 1 h on cucumber and watermelon seeds, regardless of type of organic acid or RH. The phytopathogen was reduced to levels below the detection limit (-0.5 log CFU/g) for enrichment on honeydew melon seeds treated with acetic acid vapor. S. enterica and L. monocytogenes were inactivated within 2 h at 85% RH on honeydew melon and watermelon seeds treated with acetic acid and propionic acid vapors. E. coli O157: H7 was inactivated by treatment with acetic acid vapor at 85% RH. This study provides useful information for developing a method to decontaminate Curcurbitaceae seeds using organic acid vapors as lethal agents.

Synergistic antimicrobial activity of oregano and thyme thymol essential oils against Leuconostoc citreum in a laboratory medium and tomato juice.

This study was done to identify combinations of essential oils (EOs) that elicit synergistic antimicrobial effects against Leuconostoc citreum, a spoilage bacterium in vegetable and fruit juices. Twenty-four EOs were tested for antimicrobial activity against L. citreum using an agar well diffusion assay. Ten EOs showed relatively strong antimicrobial activity. Among those, cinnamon bark, oregano, and thyme thymol EOs showed the strongest activity (minimal inhibitory concentration = 1.25 µL/mL). It was confirmed that a combination of oregano and thyme thymol EOs had a synergistic antimicrobial activity (fractional inhibitory concentration index = 0.3750) using a checkerboard assay. This combination also had a synergistic antimicrobial activity against L. citreum in tomato juice. An initial population of L. citreum in tomato juice (5.3 log CFU/mL) increased to ≥ 8.6 log CFU/mL within 48 h at 15 °C and 24 h at 25 °C. Populations reached 6.8-7.2 log CFU/mL in tomato juice containing either 0.156 mL/mL oregano or thyme thymol. However, in the presence of both EOs (0.156 µL/mL in total), populations of L. citreum were significantly lower (5.7-6.5 log CFU/mL) (P ≤ 0.05). Findings will be useful when developing non-thermal food preservation technologies to increase the shelf-life of juices and other foods not containing synthetic preservatives.

Synergistic activities of gaseous oregano and thyme thymol essential oils against Listeria monocytogenes on surfaces of a laboratory medium and radish sprouts.

We investigated combinations of gaseous essential oils (EO gases) for their synergistic inhibitory activities against Listeria monocytogenes on a laboratory medium and radish sprouts. The minimum inhibitory concentrations and minimum lethal concentrations of oregano, thyme thymol, and cinnamon bark EO gases against L. monocytogenes were 0.0781 µL/mL on nutrient agar supplemented with glucose and bromocresol purple (NGBA). A checkerboard assay showed that combinations of oregano and thyme thymol EO gases and of oregano and cinnamon bark EO gases exert the strongest synergistic antilisterial activity (fractional inhibitory concentration index [FICI] = 0.3750). A combination of thyme thymol and cinnamon bark EO gases also had a synergistic effect (FICI = 0.5000) on L. monocytogenes on NGBA. Combinations of oregano and thyme thymol EO gases were tested for synergistic antimicrobial activity against L. monocytogenes on radish sprouts. A combination of these gases, each at 0.313 µL/mL, caused a significant (P ≤ 0.05) reduction in the number of L. monocytogenes on radish sprouts compared with reductions caused by treatment with oregano or thyme thymol EO gas alone at the same concentration. Our findings provide information that will be useful when developing antimicrobial applications using EO gases to control L. monocytogenes in the food industry.

Inhibition of Listeria monocytogenes using biofilms of non-pathogenic soil bacteria (Streptomyces spp.) on stainless steel under desiccated condition.

Of the 1648 microbial isolates from 133 soil samples collected from 30 diverse locations in the Republic of Korea, two isolates exhibited strong antilisterial activity and ability to grow to high populations (>8.0 log CFU/ml) in Bennett's broth. Isolates were identified as Streptomyces lactacystinicus (strain Samnamu 5-15) and Streptomyces purpureus (strain Chamnamu-sup 4-15). Both isolates formed biofilms on the surface of stainless steel coupons (SSCs) immersed in Bennett's broth within 24 h at 25 °C. Cells retained antilisterial activity after biofilm formation and showed significantly (P ≤ 0.05) enhanced resistance against dry conditions (43% relative humidity [RH]) compared to the cells not in biofilm. An initial population (ca. 3.2 log CFU/cm2) of Listeria monocytogenes inoculated on SSCs lacking Streptomyces biofilm decreased to 1.4 log CFU/cm2 within 48 h at 25 °C and 43% RH. In contrast, L. monocytogenes (3.3 log CFU/cm2) inoculated on SSCs containing Streptomyces biofilm decreased to populations below the theoretical detection limit (0.5 log CFU/cm2) within 24 h. The results indicate that biofilms formed by the Streptomyces spp. inhibitory to L. monocytogenes showed enhanced resistance to desiccation condition (43% RH) and effectively inhibited the growth of L. monocytogenes on the surfaces of SSCs. Antilisterial biofilms developed in this study may be applicable on desiccated environmental surfaces in food related environments such as food storage, handling, and processing facilities to enhance the microbiological safety of foods.

Development of a decontamination method to inactivate Acidovorax citrulli on Cucurbitaceae seeds without loss of seed viability.

BACKGROUND: Acidovorax citrulli is a plant pathogen causing bacterial fruit blotch in Cucurbitaceae family. Applying high concentration of disinfectants to seeds containing plant pathogen may substantially decrease the germination rate of seeds. Therefore, it is necessary to develop a hurdle technology which can inactivate plant pathogens without decreasing seed viability. This study was conducted to develop a decontamination method to inactivate the plant pathogen Acidovorax citrulli on Cucurbitaceae seeds by sequential treatments with aqueous chlorine dioxide (ClO2 ), drying, and dry heat. RESULTS: The maximum ClO2 concentration that did not lower germination rates of cucumber, honeydew melon, and watermelon seeds was ca. 100 µg mL-1 of ClO2 for 5 min. Optimal incubation conditions for drying seeds that had been treated with aqueous ClO2 were determined as 25 °C and 43% relative humidity (RH) for 48 h. The maximum dry-heat temperature that did not reduce germination rates of seeds, which had been treated with ClO2 and dried at 25 °C, was 60 °C at 43% RH for 24 h. When seeds containing A. citrulli (6.4-7.0 log CFU g-1 ) were treated with aqueous ClO2 (50 µg mL-1 , 5 min), dried (25 °C, 43% RH, 24 h), and dry heated (60 °C, 43% RH, 24 h), the pathogen was inactivated to below the detection limit from all three seed types (<-0.5 log CFU g-1 ). CONCLUSION: The decontamination conditions to inactivate A. citrullii from Cucurbitaceae seeds without decreasing the seed viability were determined (sequential treatment with ClO2 [50 µg mL-1 , 5 min], dried [25 °C, 43% RH, 24 h], and dry heated [60 °C, 43% RH, 24 h]). The results of this study may also be applicable to other plant pathogens on other types of seeds. © 2019 Society of Chemical Industry.

Inactivation of Salmonella Typhimurium on red chili peppers by treatment with gaseous chlorine dioxide followed by drying.

Chili pepper (Capsicum annuum L.) powder and other powdered spices containing chili peppers are occasionally contaminated with foodborne pathogens. We applied chlorine dioxide (ClO2) gas treatment to chili peppers prior to drying to inactivate Salmonella Typhimurium. Chili peppers inoculated with S. Typhimurium were exposed to ClO2 gas generated from 0.77 mL of aqueous ClO2 for 6 h at 25 °C and 100% relative humidity, followed by air drying at 55 °C for up to 24 h. Populations of S. Typhimurium and total aerobic bacteria (TAB) on peppers, chromaticity values, and moisture content were determined after various treatment times. S. Typhimurium (ca. 5.6 log cfu/g) was reduced to <0.7 log cfu/g of peppers treated with ClO2 gas for 0.5 h at 25 °C, but was detected by enrichment (>1 cfu/10.8 g) after treatment for 4 h. The pathogen was not detected after treatment with ClO2 gas for 6 h. S. Typhimurium did not decrease significantly (P > 0.05) on peppers exposed to air at 25 °C for 6 h, but rapidly decreased to 1.6 log cfu/g after subsequent drying for 6 h at 55 °C. The initial number of TAB (ca. 6.5 log cfu/g) decreased to 1.4 log cfu/g after treatment with ClO2 gas for 0.5 h and was less than the detection limit (0.7 log cfu/g) after 2 h. TAB counts were not reduced by treating peppers with air for 6 h, but decreased to <0.7 log cfu/g after subsequent drying for 24 h at 55 °C. The lightness (L value) of treated and untreated chili peppers did not change during drying, but redness (a value) and yellowness (b value) decreased. Results showed that treatment of chili peppers containing S. Typhimurium (5.6 log cfu/g) with ClO2 gas for 6 h prior to drying at 55 °C reduced the population to < 1 cfu/10.8 g.

Development of non-pathogenic bacterial biofilms on the surface of stainless steel which are inhibitory to Salmonella enterica.

Non-pathogenic bacterial biofilms were developed on the surface of stainless steel possessing desiccation tolerance and antimicrobial activity against Salmonella enterica. Three bacteria exhibiting strong antimicrobial activities against S. enterica were isolated from various soils, foods, and food-contact surfaces. Isolates were identified as Pseudomonas extremorientalis (strain Lettuce-28), Paenibacillus peoriae (strain Lettuce-7), and Streptomyces cirratus (strain Geumsan-207). These bacteria grew rapidly and formed biofilms within 24 h on the surface of stainless steel coupons (SSCs) immersed in laboratory media (tryptic soy broth or Bennet's broth) at 25 °C. Cells in biofilms had enhanced tolerance to desiccation (exposure to 43% atmospheric relative humidity [RH]) and retained antimicrobial activity against S. enterica. Populations of S. enterica deposited on SSCs containing biofilm formed by Ps. extremorientalis strain Lettuce-28, for example, decreased by > 2.5 log CFU/coupon within 24 h at 25 °C and 43% RH, while the number of cells inoculated on SSCs lacking biofilm decreased by 1.5 log CFU/coupon. Antimicrobial activities of the three antagonistic bacteria against S. enterica persisted in desiccated biofilms. This study provides insights to developing strategies to inactivate Salmonella and perhaps other foodborne pathogens on abiotic surfaces using non-pathogenic antagonistic bacteria.

Inactivation of Salmonella enterica in chicken feces on the surface of eggshells by simultaneous treatments with gaseous chlorine dioxide and mild wet heat.

The aim of this study was to investigate the lethal effects of simultaneous treatments with gaseous chlorine dioxide (ClO2) and mild wet heat (55 °C at 100% relative humidity [RH]) on Salmonella enterica in chicken feces on the surface of eggshells. Gaseous ClO2 production decreased significantly (P ≤ 0.05) as the RH (23, 43, 68, 85, and 100%) at 25 °C was increased. The lethality of gaseous ClO2 against S. enterica in feces on eggshells increased significantly (P ≤ 0.05) as RH increased. For example, when treated with gaseous ClO2 at 85 and 100% RH at 25 °C, S. enterica (5.9 log CFU/egg) was inactivated within 4 h. In contrast, at 23, 43, and 68% RH, the pathogen remained at 5.1, 5.0, and 2.8 log CFU/egg, respectively, after 6 h. Finally, when eggshells surface-contaminated with S. enterica (5.8 log CFU/egg) were treated with gaseous ClO2 (peak concentration of ClO2: 185.6 ppm) at 100% RH and 55 °C, inactivation occurred within 1 h. These results indicate that treatment of surface-contaminated shell eggs with gaseous ClO2 at elevated RH and temperature is effective in inactivating S. enterica. These observations will be useful when developing an effective sanitation program to enhance the microbiological safety of shell eggs.

Fate of mesophilic aerobic bacteria and Salmonella enterica on the surface of eggs as affected by chicken feces, storage temperature, and relative humidity.

We compared the microbiological quality of chicken eggshells obtained from a traditional wholesale market and a modern supermarket. We also determined the survival and growth characteristics of naturally occurring mesophilic aerobic bacteria (MAB) and artificially inoculated Salmonella enterica on eggshells under various environmental conditions (presence of chicken feces, temperature [4, 12, or 25 °C], and relative humidity [RH; 43 or 85%]). The populations of MAB, coliforms, and molds and yeasts on eggshells purchased from a traditional wholesale market were significantly (P ≤ 0.05) higher than those from a modern supermarket. In the second study, when we stored uninoculated eggs under various storage conditions, the population of MAB on eggshells (4.7-4.9 log CFU/egg) remained constant for 21 days, regardless of storage conditions. However, when eggshells were inoculated with S. enterica and stored under the same conditions, populations of the pathogen decreased significantly (P ≤ 0.05) under all tested conditions. Survival of S. enterica increased significantly (P ≤ 0.05) in the presence of feces, at low temperatures, and at low RH. These observations will be of value when predicting the behavior of microorganisms on eggshells and selecting storage conditions that reduce the populations of S. enterica on eggshells during distribution.

Behaviour of Aspergillus flavus and Fusarium graminearum on rice as affected by degree of milling, temperature, and relative humidity during storage.

We investigated the survival and growth patterns of Aspergillus flavus and Fusarium graminearum, as well as mycotoxin production, on Korean rice as affected by the degree of milling (rough, brown, and white rice) and storage conditions (21 °C/85% relative humidity [RH], 21 °C/97% RH, and 30 °C/85% RH). When rice was stored at 21 °C/85% RH, the population of A. flavus remained constant and aflatoxin was not produced, regardless of the degree of milling. At 21 °C/97% RH and 30 °C/85% RH, the populations of A. flavus increased significantly (P ≤ 0.05) and aflatoxins were produced. The highest population of A. flavus and highest amount of aflatoxin B1 were observed on brown rice stored at 21 °C/97% RH. For F. graminearum, when stored at 85% RH, the populations were reduced to less than a detectable level (5 CFU/g of rice) within 120 days and no deoxynivalenol (DON) was produced, regardless of the degree of milling and storage temperature. However, at 21 °C/97% RH, the population of F. graminearum increased significantly (P ≤ 0.05) and DON was produced on all types of rice. Findings from this study provide insights concerning storage conditions necessary to prevent growth and mycotoxin production by A. flavus and F. graminearum on Korean rice with different degrees of milling.

Enhanced protection of pathogenic Escherichia coli ingested by a soil nematode Caenorhabditis elegans against sanitizer treatments.

We employed Caenorhabditis elegans as a model to study the effectiveness of sanitizers in killing pathogenic Escherichia coli strains ingested by free-living nematodes. Adult worms that had fed on six pathogenic E. coli strains (highly persistent in the nematode intestine) were treated with three chemical solutions. In planktonic cells, none of the H2O2 and acetic acid treatments influenced the survival of the pathogenic E. coli strains, whereas sodium hypochlorite critically decreased the viability of the strains. Importantly, the survival of the E. coli strains was dramatically increased by persistence in the C. elegans gut under 0.1% sodium hypochlorite, and several strains could survive at a concentration of 0.5%. In addition, all pathogenic E. coli strains in the C. elegans gut survived on the lettuce for 5 days even though they were washed with 0.1% sodium hypochlorite. Taken together, our results indicate that pathogenic E. coli ingested by C. elegans may be protected against washing treatment with commercial sanitizers on raw food materials.

Fate of Bacillus cereus and naturally occurring microbiota on milled rice as affected by temperature and relative humidity.

We studied the survival and growth patterns of Bacillus cereus, mesophilic aerobic bacteria (MAB), and molds and yeasts (MY) on rough and milled brown and white Korean rice stored at 12 and 21 °C and 43, 68, and 85% relative humidity (RH) for up to 24 wk. The initial populations of MAB present on rough rice, brown rice, and white rice were 7.7, 5.7, and 3.3 log CFU/g, respectively, and remained constant or decreased (P ≤ 0.05) by 0.7-1.8 log CFU/g during storage. The initial populations of B. cereus on the three types of laboratory-inoculated rice were 3.1-3.8 log CFU/g and remained constant (P > 0.05) during storage, regardless of degree of milling, storage temperature, and RH. The initial populations of MY on rough rice, brown rice, and white rice were 6.2, 4.2, and 2.1 log CFU/g, respectively. At 12 °C and 85% RH, the MY increased significantly (P ≤ 0.05) only on brown rice; however, at 21 °C and 85% RH, MY increased (P ≤ 0.05) on all types of rice during storage. These observations will be useful when assessing conditions affecting survival of B. cereus and determining environmental conditions necessary to prevent growth of potentially mycotoxigenic molds on various types of milled rice during storage.

Bacillus cereus and Bacillus thuringiensis spores in Korean rice: prevalence and toxin production as affected by production area and degree of milling.

We determined the prevalence of and toxin production by Bacillus cereus and Bacillus thuringiensis in Korean rice as affected by production area and degree of milling. Rough rice was collected from 64 farms in 22 agricultural areas and polished to produce brown and white rice. In total, rice samples were broadly contaminated with B. cereus spores, with no effect of production area. The prevalence and counts of B. cereus spores declined as milling progressed. Frequencies of hemolysin BL (HBL) production by isolates were significantly (P ≤ 0.01) reduced as milling progressed. This pattern corresponded with the presence of genes encoding the diarrheal enterotoxins. The frequency of B. cereus isolates positive for hblC, hblD, or nheB genes decreased as milling progressed. Because most B. cereus isolates from rice samples contained six enterotoxin genes, we concluded that B. cereus in rice produced in Korea is predominantly of the diarrheagenic type. The prevalence of B. thuringiensis in rice was significantly lower than that of B. cereus and not correlated with production area. All B. thuringiensis isolates were of the diarrheagenic type. This study provides information useful for predicting safety risks associated with B. cereus and B. thuringiensis in rough and processed Korean rice.

Controlled fermentation of kimchi using naturally occurring antimicrobial agents.

Kimchi is a traditional Korean fermented food. Since it ferments continuously during distribution and storage, the extension of shelf life by preventing over-acidification is a major concern in the kimchi industry. One of the most frequently attempted ways to delay fermentation is to add naturally occurring antimicrobial agents. Many researchers have investigated ways to delay over-acidification by adding minor ingredients, fruits or fruit seed extracts, extracts of medicinal herbs, culinary herbs and spices, and other miscellaneous substances to kimchi. The addition of naturally occurring antimicrobial agents may enhance the acceptability of kimchi to consumers over a longer period of time but may also have a disadvantage in that it may cause changes in sensory quality, especially if added in large amounts. To avoid undesirable sensory changes, application of hurdle technologies (i.e., multifactor preservative systems) which involve using combinations of low amounts of various naturally occurring antimicrobial agents as ingredients should be explored with the goal of controlling fermentation. If synergistic or additive antimicrobial effects can be achieved using small amounts of a combination of natural agents, changes in sensory qualities will be minimized, thereby prolonging shelf life. Research findings summarized in this review provide a basis for developing effective hurdle technologies using naturally occurring antimicrobial agents to extend shelf life of kimchi and perhaps other types of traditional fermented foods.