Acid adaptation increased the resistance of Escherichia coli O157:H7 in bok choy (Brassica rapa subsp. chinensis) juice to high-pressure processing.

IMPORTANCE: Based on the U.S. Food and Drug Administration regulations, E. coli O157:H7 is a pertinent pathogen in high acid juices that needs to be inactivated during the pasteurization process. The results of this study suggest that the effect of acid adaptation should be considered in the selection of HPP parameters for E. coli O157:H7 inactivation to ensure that pasteurization objectives are achieved.

Developing an LED preservation technology to minimize strawberry quality deterioration during distribution.

This study investigated the effect of LED illumination on the inactivation of Rhizopus stolonifer and Botrytis cinerea on strawberries and physicochemical properties of the strawberries. Twelve days of illumination resulted in an antifungal effect of 3.4 and 1.9 log CFU/g on R. stolonifer and B. cinerea respectively. The illumination caused no significant effect (P ≥ 0.05) on the mass, color and texture of strawberries. Furthermore, total phenolic content, trolox equivalent antioxidant capacity and anthocyanin content of the illuminated strawberries significantly increased (P < 0.05). Vitamin C content of illuminated strawberries was only significantly different (P < 0.05) from the control starting from Day 9. These results show that 405 nm LED illumination can potentially complement temperature and humidity control in preventing mold spoilage and preserving physicochemical quality of strawberries during refrigerated storage.

Influence of temperature and relative humidity on the antifungal effect of 405 nm LEDs against Botrytis cinerea and Rhizopus stolonifer and their inactivation on strawberries and tomatoes.

In recent years, photodynamic inactivation (PDI) has emerged as a promising preservation method to complement refrigeration in the fresh produce supply chain. However, due to infrastructural limitations in the supply chain, fresh produce is often exposed to environmental conditions rather than recommended storage conditions. Hence, this study aimed to investigate the influence of two important environmental variables in the fresh produce supply chain - temperature and relative humidity (RH), on the PDI of fruit spoilage molds. It also aimed to demonstrate proof-of-concept of their inactivation on fruit surfaces. In the in vitro stage, Botrytis cinerea and Rhizopus stolonifer, the two molds selected for this study, were illuminated with 405 nm LEDs on Dichloran Rose-Bengal Chloramphenicol (DRBC) agar at three levels of temperature (7, 16 and 25 °C) and relative humidity (40, 60 and 80%). Illumination under these conditions caused reductions greater than 94% in the mold populations, at all temperatures and relative humidities. Even so, a temperature of 25 °C was observed to be marginally better for the inactivation as compared to 7 and 16 °C, as it necessitated the lowest dose (6-7 kJ) for the first log reduction of both the molds. Similarly, an RH of 40% worked slightly better for the inactivation of B. cinerea, as it induced inactivation without any lag phase and required the lowest dose (8.03 kJ) for the first log reduction. When the antifungal effect was investigated on fruit surfaces, it was discovered that the illumination reduced the populations of B. cinerea and R. stolonifer on strawberries by 67% and 19%, whereas on tomatoes, the respective inactivations were 79% and 70% respectively. These results demonstrate further promise of PDI as a postharvest technology for reducing the risk of fruit spoilage. This study is also the first to demonstrate the potential of PDI to add value to supply chains where compliance to ideal storage conditions is not feasible.

Perspectives and Trends in the Application of Photodynamic Inactivation for Microbiological Food Safety.

Photodynamic inactivation is a phenomenon that has the potential to cause microbial inactivation using visible light. It works on the principle that photosensitizers within the microbial cell can be activated using specific wavelengths to trigger a series of cytotoxic reactions. In the last few years, efforts to apply this intervention technology for food safety have been on the rise. This review article offers a detailed commentary on this research. The mechanism of photodynamic inactivation has been discussed as have the factors that influence its efficacy in food. Efforts to inactivate bacteria, fungi, and viruses have been analyzed in dedicated sections and so has the application of this technology to specific product classes such as fresh produce, dry fruits, seafood, and poultry. The challenges and opportunities facing the application of this technology to food systems have been evaluated and future research directions proposed. Thus, this review will provide insights for researchers and industry personnel looking for a novel solution to combat microbial contamination and resistance.

Inactivation and changes in metabolic profile of selected foodborne bacteria by 460 nm LED illumination.

The objective of this study was to investigate the effect of 460 nm light-emitting diode (LED) on the inactivation of foodborne bacteria. Additionally, the change in the endogenous metabolic profile of LED illuminated cells was analyzed to understand the bacterial response to the LED illumination. Six different species of bacteria (Bacillus cereus, Listeria monocytogenes, Staphylococcus aureus, Escherichia coli O157:H7, Pseudomonas aeruginosa and Salmonella Typhimurium) were illuminated with 460 nm LED to a maximum dose of 4080 J/cm2 at 4, 10 and 25 °C. Inactivation curves were modeled using Hom model. Metabolic profiling of the non-illuminated and illuminated cells was performed using a Liquid chromatography-mass spectrometry system. Results indicate that the 460 nm LED significantly (p < 0.05) reduced the populations of all six bacterial species. For example, the population of S. aureus reached below detection limit within 7 h. B. cereus was most resistant to photo-inactivation and exhibited about 3-log reduction in 9 h. Metabolic profiling of the illuminated cells indicated that several metabolites e.g. 11-deoxycortisol, actinonin, coformycin, tyramine, chitobiose etc. were regulated during LED illumination. These results elucidate the effectiveness of 460 nm LED against foodborne bacteria and hence, its suitability as a novel antimicrobial control method to ensure food safety.

Irradiance and Temperature Influence the Bactericidal Effect of 460-Nanometer Light-Emitting Diodes on Salmonella in Orange Juice.

Blue light-emitting diodes (LEDs) have been known to produce an antibacterial effect on various pathogenic bacteria. To extend this application to foods, blue 460-nm LEDs were evaluated for their antibacterial effect on Salmonella in orange juice. A cocktail of Salmonella enterica serovars Gaminara, Montevideo, Newport, Typhimurium, and Saintpaul was inoculated into pasteurized orange juice and illuminated with 460-nm LEDs at irradiances of 92, 147.7, and 254.7 mW/cm(2) and temperatures of 4, 12, and 20°C. Subsequently, linear, Weibull, and Gompertz models were fitted to the resultant survival curves. The color of the orange juice during illumination was also monitored. It was observed that irradiance and temperature both influenced the inactivation of Salmonella, which ranged from 2 to 5 log CFU/ml. The inactivation kinetics was best described by the Weibull model. An irradiance of 92 mW/cm(2) and temperatures of 12 and 20°C were the most bactericidal combinations, with D-values of 1,580 and 2,013 J/cm(2), respectively. Significant color changes were also observed after illumination; these changes could be minimized by choosing appropriate irradiance and temperature. These results demonstrate the potential of 460-nm LEDs for the preservation of fruit juices in the retail markets and their utility in minimizing the risk of salmonellosis.

Antibacterial effect and mechanism of high-intensity 405 ± 5 nm light emitting diode on Bacillus cereus, Listeria monocytogenes, and Staphylococcus aureus under refrigerated condition.

This study investigated the antibacterial effect of 405 ± 5 nm light emitting diode (LED) on Bacillus cereus, Listeria monocytogenes, and Staphylococcus aureus, and examined its antibacterial mechanism by determining the bacterial membrane and DNA damages. A 405 ± 5 nm LED illuminated the Gram-positive pathogens until 486 J/cm(2) at 4 °C. Weibull model was used to calculate reliable life (tR) to compare bacterial sensitivities to LED illumination. The membrane damage was determined by NaCl and LIVE/DEAD® assay, while comet assay and DNA ladder analysis were conducted to determine DNA degradation. The illumination resulted in 1.9, 2.1, and 1.0 log reductions for B. cereus, L. monocytogenes, and S. aureus at 486 J/cm(2), respectively. The comparison of tR values revealed that L. monocytogenes was identified as the most susceptible strain to LED illumination. The percentage of the bacterial sensitivity to NaCl remarkably increased in LED-illuminated cells compared to non-illuminated cells. Moreover, loss of membrane integrity was confirmed for LED-illuminated cells by LIVE/DEAD® assay, whereas no DNA breakage was indicated by comet assay and DNA ladder analysis. Thus, these findings suggest that the antibacterial effect of 405 ± 5 nm LED illumination on these pathogens might be due to physical damage to bacterial membrane rather than DNA degradation.

Kinetics of bacterial inactivation by 405nm and 520nm light emitting diodes and the role of endogenous coproporphyrin on bacterial susceptibility.

Photodynamic inactivation studies of microbial pathogens have focused on the use of an external photosensitizer or a precursor compound to eliminate bacteria. The present study investigated the inactivation kinetics of six bacterial pathogens by a 405nm light emitting diode (LED) without the addition of any external compound. The role of endogenous coproporphyrin on the bacterial susceptibility to LEDs was also examined. Pathogens were illuminated with LEDs at 25, 10 and 4°C for 9h and the inactivation curves were modeled using six different equations. Endogenous coproporphyrin was quantified using an HPLC system equipped with a fluorescence detector. At a dose of 306J/cm(2), the 405nm LED brought about 4.0, 2.1 and 1.9 log reductions in the populations of Staphylococcus aureus at 25, 10 and 4°C, respectively. At all three temperatures, the population of Bacillus cereus and Listeria monocytogenes reduced by approximately 2.3 and 1.9 log respectively. Salmonella Typhimurium and Escherichia coli O157:H7 showed moderate susceptibility to 405nm LED while Pseudomonas aeruginosa was most resistant. Of the six models tested, Hom model proved most suitable. This study demonstrated that 405nm LEDs can be useful in the inactivation of bacterial pathogens with the aid of endogenous coproporphyrin alone.

Enhancing the antibacterial effect of 461 and 521 nm light emitting diodes on selected foodborne pathogens in trypticase soy broth by acidic and alkaline pH conditions.

Light emitting diodes (LEDs) with their antibacterial effect present a novel method for food preservation. This effect may be influenced by environmental conditions such as the pH of the food contaminated by the pathogen. Thus, it is necessary to investigate the influence of pH on the antibacterial effect of LEDs before their application to real food matrices. Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes in trypticase soy broth were illuminated using 10-W 461 (22.1 mW/cm(2)) and 521 nm (16 mW/cm(2)) LEDs at pH values of 4.5, 6.0, 7.3, 8.0 and 9.5 for 7.5 h at 15 °C. Using the 461 nm LEDs, the populations of E. coli O157:H7 decreased by 2.1 ± 0.02, 1.2 ± 0.08 and 4.1 ± 0.42 log CFU/ml at pH 4.5, 7.3 and 9.5 respectively, after a dosage of 596.7 J/cm(2). For L. monocytogenes, approximately a 5.8 ± 0.03 log reduction was observed after 238.7 J/cm(2) at pH 4.5 using the 461 nm LEDs, while the bacterial concentration was reduced by 1.8 ± 0.01 log at pH 9.5 after 596.7 J/cm(2). Bacterial inactivation using the 521 nm LEDs showed similar trends to the 461 nm LEDs at both acidic and alkaline pH conditions but with lower (1-2 log CFU/ml) reductions after 432 J/cm(2). Lower D-values were observed for L. monocytogenes when exposed to LEDs at acidic pH values, while the sensitivity of E. coli O157:H7 and S. Typhimurium to LED was markedly increased at an alkaline pH. Regardless of the pH at which the cultures were illuminated, the percentage of sublethal injury increased with the treatment time. These results highlight the enhanced antibacterial effect of the 461 nm LED under acidic and alkaline pH conditions, proving its potential to preserve foods as well as to have synergistic effect with acidic and alkaline antimicrobials.

Antibacterial effect of light emitting diodes of visible wavelengths on selected foodborne pathogens at different illumination temperatures.

The antibacterial effect of light emitting diodes (LEDs) in the visible region (461, 521 and 642 nm) of the electromagnetic spectrum was investigated on Escherichia coli O157:H7, Salmonella typhimurium, Listeria monocytogenes and Staphylococcus aureus. The irradiances of the 461, 521 and 642 nm LEDs were 22.1, 16 and 25.4 mW/cm², respectively. Bacterial cultures suspended in tryptic soy broth were illuminated by 10-watt LEDs at a distance of 4.5 cm for 7.5h at 20, 15 and 10 °C. Regardless of the bacterial strains, bacterial inactivation was observed with the range of 4.6-5.2 logCFU/ml at 10 and 15 °C after illumination with the 461 nm LED, while illumination with the 521 nm LED resulted in only 1.0-2.0 log reductions after 7.5h. On the other hand, no antibacterial effect was observed using the 642 nm LED treatment. The photodynamic inactivation by 461 and 521 nm LEDs was found to be greater at the set temperatures of 10 and 15 °C than at 20 °C. The D-values for the four bacterial strains at 10 and 15 °C after the illumination of 461 nm LED ranged from 1.29 to 1.74 h, indicating that there was no significant difference in the susceptibility of the bacterial strains to the LED illumination between 10 and 15 °C, except for L. monocytogenes. Regardless of the illumination temperature, sublethal injury was observed in all bacterial strains during illumination with the 461 and the 521 nm LED and the percentage of injured cells increased as the treatment time increased. Thus, the results show that the antibacterial effect of the LEDs was highly dependent on the wavelength and the illumination temperature. This study suggests the potential of 461 and 521 nm LEDs in combination with chilling to be used as a novel food preservation technology.

Prevalence of Salmonella and Vibrio spp. in seafood products sold in Singapore.

Foodborne hazards in seafood have only recently received increased attention in Singapore since the illness outbreak in 2009 that was associated with consumption of Indian rojak (a traditional salad of fruits, vegetables, and seafood). The microbiological quality of seafood must be evaluated for assurance of food safety. The aim of this study was to evaluate the microbiological quality and to determine the prevalence of Salmonella and Vibrio spp. in seafood sold in Singapore. A total of 116 samples (41 prawn, 44 shellfish, and 31 fishball samples) were collected from major supermarkets and wet markets in Singapore. The mesophilic and psychrotrophic bacterial counts for prawn, shellfish, and fishballs were 2 to 7 log CFU/g. One Salmonella Lexington strain was isolated from a thawed-frozen shellfish product and two Vibrio parahaemolyticus strains were isolated from commercial fishball and shrimp meat products. Thus, seafood sold in Singapore has the potential to be contaminated with Vibrio spp. and Salmonella, and proper handling at food service establishments is required to ensure food safety. Effective control measures also are needed to prevent cross-contamination during postharvest seafood processing.