Naturally occurring antimicrobial compounds could be applied as food preservatives to protect food quality and extend the shelf life of foods and beverages. These compounds are naturally produced and isolated from various sources, including plants, animals and microorganisms, in which they constitute part of host defense systems. Many naturally occurring compounds, such as nisin, plant essential oils, and natamycin, have been widely studied and are reported to be effective in their potential role as antimicrobial agents against spoilage and pathogenic microorganisms. Although some of these natural antimicrobials are commercially available and applied in food processing, their efficacy, consumer acceptance and regulation are not well defined. This manuscript reviews natural antimicrobial compounds with reference to their applications in food when applied individually or in combination with other hurdles. It also reviews the mechanism of action of selected natural antimicrobials, factors affecting their antimicrobial activities, and future prospects for use of natural antimicrobials in the food industry.

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Interest in chitosan, a biodegradable, nontoxic, non-antigenic, and biocompatible biopolymer isolated from shellfish, arises from the fact that chitosans are reported to exhibit numerous health-related beneficial effects, including strong antimicrobial and antioxidative activities in foods. The extraordinary interest in the chemistry and application in agriculture, horticulture, environmental science, industry, microbiology, and medicine is attested by about 17,000 citations on this subject in the Scopus database. A special need exists to develop a better understanding of the role of chitosans in ameliorating foodborne illness. To contribute to this effort, this overview surveys and interprets our present knowledge of the chemistry and antimicrobial activities of chitosan in solution, as powders, and in edible films and coating against foodborne pathogens, spoilage bacteria, and pathogenic viruses and fungi in several food categories. These include produce, fruit juices, eggs and dairy, cereal, meat, and seafood products. Also covered are antimicrobial activities of chemically modified and nanochitosans, therapeutic properties, and possible mechanisms of the antimicrobial, antioxidative, and metal chelating effects. Further research is suggested in each of these categories. The widely scattered data on the multifaceted aspects of chitosan microbiology, summarized in the text and in 10 tables and 8 representative figures, suggest that low-molecular-weight chitosans at a pH below 6.0 presents optimal conditions for achieving desirable antimicrobial and antioxidative-preservative effects in liquid and solid foods. We are very hopeful that the described findings will be a valuable record and resource for further progress to improve microbial food safety and food quality.

The heat resistance of 35 Salmonella strains was determined at 55 to 65°C. No correlation between the heat resistance and the origin of the Salmonella spp. could be established. D-values in chicken broth, using a linear regression, of an 8 Salmonella serotype cocktail were 4.87, 2.72, 1.30, and 0.41 min at 55, 58, 60 and 62°C, respectively. Using a linear model, the D-values ranged from 4.86 min at 55°C to 0.38 min at 62°C. When the 8 Salmonella serotype cocktail was heated in meat, D-values at the common test temperatures of 58 and 60°C calculated by both approaches were significantly higher (p < 0.05) than those observed in chicken broth.

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Detection of pathogens in contaminated food products by PCR can result in false-positive data due to the amplification of DNA from nonviable cells. A new method based on reverse transcription-PCR (RT-PCR) amplification of mRNA for the specific detection of viable Listeria monocytogenes was developed. The expression of three L. monocytogenes genes, iap, hly, and prfA, was examined to determine a suitable target for amplification of RT-PCR. Total RNA from L. monocytogenes was isolated, and following DNase treatment, the RNA was amplified by both RT-PCR and PCR with primers specific for the three genes. Amplicon detection was accomplished by Southern hybridization to digoxigenin-labeled gene probes. The levels of expression of these three genes differed markedly, and the results indicated that the iap gene would provide a good target for development of a specific method for detection of viable L. monocytogenes based on RT-PCR amplification. After a 1-h enrichment, the 371-bp iap-specific product was detected with a sensitivity of ca. 10 to 15 CFU/ml from pure culture. Detection of the 713-bp hly-specific amplicon was ca. 4,000 times less sensitive after 1 h, whereas detection of the 508-bp prfA product showed the lowest level of sensitivity, with detection not observed until after a 5-h enrichment period. The amplification of the iap mRNA was specific for L. monocytogenes. Overall, the assay could be completed in ca. 54 h. The use of RT-PCR amplification for the detection of viable L. monocytogenes was validated in artificially contaminated cooked ground beef. Following a 2-h enrichment incubation, the iap-specific amplification product could be detected in a cooked meat sample that was originally inoculated with ca. 3 CFU/g. These results support the usefulness of RT-PCR amplification of mRNA as a sensitive method for the specific detection of viable L. monocytogenes and indicate that this method may prove useful in the detection of this pathogen in ready-to-eat, refrigerated meat products.

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