The natural microflora of cold-smoked fish at the end of shelf-life are lactic acid bacteria (LAB). Some of these display a capacity to inhibit spoilage as well as several strains of pathogenic micro-organisms, e.g. Listeria monocytogenes which is isolated frequently from cold-smoked salmon (CSS). Eight batches of sliced vacuum-packed CSS from Norway, Scotland and Spain were collected at retail. Packs were stored at 5 degrees C and examined for chemical and microbiological characteristics, at purchase date and at expiration date. pH, water activity and salt content were similar to available data on lightly preserved fish products. There was a consistent pattern in the development of the microflora on CSS; the initial level of LAB was low on freshly produced CSS (10(2) cfu g(-1)); however, storage in vacuum packaging at refrigeration temperature was elective for LAB. At the end of the stated shelf-life these micro-organisms, represented mainly by Lactobacillus spp., attained ca.10(7) cfu g(-1) while Enterobacteriaceae counts were consistently lower (10(5) cfu g(-1)), which indicates the ability of LAB to grow and compete with few carbohydrates available and in the presence of moderate salt concentrations. L. monocytogenes was not found in any sample. Forty-one percent of LAB strains isolated exhibited inhibitory capacity against Listeria innocua, in a plate assay. A majority of the inhibitory effects were non-bacteriocinogenic, but nevertheless were very competitive cultures which may provide an additional hurdle for improved preservation by natural means.
The objective of this study was to evaluate (eO), a biological time temperature integrator (TTI) as a quality and safety indicator for ground beef packed under modified atmosphere and spiced cooked chicken slices packed under modified atmosphere. Storage trials and challenge tests were thus performed on several batches of the studied food to monitor and model the behavior of Listeria monocytogenes, Salmonella, Staphylococcus aureus and the indigenous food flora. Then, two different prototypes of the TTI (eO) were set and manufactured according to the studied products shelf lives. The TTI evolution with time at static and dynamic temperatures was monitored and modeled. Finally, exposure assessment models were set and used under several realistic storage profiles to assess the distributions of the concentration of the indigenous food flora and the distributions of the increase in the pathogens populations obtained at the end of the product shelf life or at the end point of the TTI, taking into account the TTIs batch variability. Results showed that in case of poor storage conditions, TTI can reduce the consumer exposure to altered or hazardous foods.
Consumption of packaged fresh leafy vegetables, which are convenient ready-to-eat products, has increased during the last decade. The number of foodborne outbreaks associated with these products has concurrently increased. In our study, (1) label information, (2) O2/CO2 composition, (3) bacterial quality and (4) safety of 100 fresh leafy vegetables at the retail level were studied in Finland during 2013. Bacterial quality was studied using aerobic bacteria (AB) and coliform bacteria (CB) counts, and searching for the presence of Escherichia coli, Listeria and Yersinia. The safety was studied by the presence of Salmonella, ail-positive Yersinia, stx-positive E. coli (STEC) and Listeria monocytogenes using PCR and culturing. Important label information was unavailable on several packages originating from different companies. The packaging date was missing on all packages and the date of durability on 83% of the packages. Storage temperature was declared on 62% of the packages and 73% of the packages contained information about prewashing. The batch/lot number was missing on 29% of the packages. Very low oxygen (O2) (
Portuguese chouriço de vinho is made by drying coarsely minced meat and fat that has been previously marinated with wine (usually red), salt, and garlic for 1 to 2 days at a low temperature (4 to 8 °C). This procedure may improve the microbiological safety of the product. The aim of this study was to evaluate the behavior of three pathogens in this product, Salmonella spp., Listeria monocytogenes, and Staphylococcus aureus, to establish the minimum period of drying and maturation necessary to render safe products. The pathogens were inoculated in the chouriço de vinho batter. A factorial design was used to study the following variables in the fermentation process: (i) the presence or absence of an indigenous Lactobacillus sakei starter culture; (ii) the presence or absence of fermentable carbohydrates; and (iii) the salt level (1.5 or 3%). The samples were analyzed 24 h after the preparation of the batter (at stuffing); after 7, 15, and 30 days of drying; and after 30 days of storage at 4 °C under vacuum. Under all of the conditions studied, the levels of the three pathogens decreased during the drying period. In the early stages of drying, the addition of L. sakei starter culture and/or carbohydrates resulted in lower levels of gram-positive pathogens. After 15 days of drying, populations of all pathogens decreased by ca. 2 log in all samples. At that sampling time, L. monocytogenes was undetectable in the chouriço de vinho with L. sakei starter culture and carbohydrates. The mean count of S. aureus after 15 days of drying was below 1 log CFU/g. After 30 days of drying, no pathogens were detected. The drying period could be shortened to 15 days when considering only the gram-positive pathogens studied and the use of a starter culture and carbohydrates. Due to the low infective dose of Salmonella spp., the product should be considered safe after 30 days, when this pathogen became undetectable.
Minimally processed spinach has been recently associated with outbreaks of foodborne illnesses. This study investigated the effect of commercial minimal processing of spinach on the coliform and Escherichia coli counts and the prevalence of E. coli O157:H7, Salmonella, Shigella spp., and Listeria monocytogenes on two types of spinach before and after minimal processing. A total of 1,356 spinach samples (baby spinach, n = 574; savoy spinach, n = 782) were collected daily in two processing plants over a period of 14 months. Raw spinach originated from nine farms in the United States and three farms in Canada. Overall, the proportion of samples positive for coliforms increased from 53% before minimal processing to 79% after minimal processing (P 0.1) was observed. E. coli O157:H7 and Shigella spp. were not isolated from any of the samples. Salmonella and L. monocytogenes were isolated from 0.4 and 0.7% of samples, respectively. Results demonstrate that commercial minimal processing of spinach based on monitored chlorine washing and drying may not decrease microbial load on spinach leaves as expected. Further research is needed to identify the most appropriate measures to control food safety risk under commercial minimal processing of fresh vegetables.
Thermal inactivation of a mixture of five strains of Listeria monocytogenes, four strains of Escherichia coli O157:H7 and eight serotypes of Salmonella were compared with that of indigenous microflora in 75% lean ground beef.
Inoculated meat was packaged in bags that were completely immersed in a circulating water bath and held at 55, 57.5 and 60 degrees C for predetermined lengths of time. The surviving cell population was enumerated by spiral plating heat-treated samples onto tryptic soya agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. D-values, determined by linear regression, in beef were 77.49, 21.9, and 10.66 min at 55, 57.5, and 60 degrees C, respectively, for indigenous microflora (z = 5.81 degrees C). When either of the three pathogens were heated in beef, their D-values calculated were significantly lower (P
Microbiological and chemical changes were determined during the smoking and drying of salmon strips processed at 29 to 31 degrees C for 4 days at a facility in Alaska in 1993. During the process, Staphylococcus aureus populations increased to more than 10(5) CFU/g after 2 to 3 days of processing. Subsequent laboratory studies showed that a pellicle (dried skinlike surface) formed rapidly on the strips when there was rapid air circulation in the smokehouse and that bacteria embedded in or under the pellicle were able to grow even when heavy smoke deposition occurred. Under these conditions, an inoculum of 26 CFU/g of S. aureus increased to 10(5) CFU/g after 3 days of processing. Elimination of preprocess drying and reduction in air flow during smoking resulted in smoke deposition before pellicle formation and enabled the product to reach levels of water-phase salt and water activity that inhibit the growth of S. aureus and Listeria monocytogenes. In 1994, these modifications were then applied during processing at an Alaskan facility, and S. aureus could not be detected in the finished product. L. monocytogenes was detected in the raw product area, on the processing tables, and on the raw salmon strips, but it was not detected in the finished product when the smoke was applied before pellicle formation.
Australian Food Safety Centre of Excellence, Tasmanian Institute of Agricultural Research, School of Agricultural Science, University of Tasmania, Private Bag 54, Hobart 7001, Tasmania, Australia. email@example.com
Two commercially available organic acid salts, potassium lactate (PURASAL HiPure P) and a potassium lactate-sodium diacetate blend (PURASAL Opti. Form PD 4), were assessed as potential inhibitors of Listeria monocytogenes growth in modified atmosphere packaged (MAP) sliced ham in challenge studies. The influence of the initial inoculation level of L. monocytogenes (10(1) or 10(3) CFU g(-1)) and storage temperature (4 or 8 degrees C) was also examined. The addition of either organic acid salt to MAP sliced ham strongly inhibited the growth of L. monocytogenes during the normal shelf life of the product under ideal refrigeration conditions (4 degrees C) and even under abusive temperature conditions (i.e., 8 degrees C). During the challenge studies and in the absence of either organic acid salt, L. monocytogenes numbers increased by 1000-fold after 20 days at 8 degrees C and 10-fold after 42 days at 4 degrees C. Both organic acid salt treatments were found to be listeriostatic rather than listericidal. The addition of either organic acid salt to the MAP ham also reduced the growth of indigenous microflora, i.e., aerobic microflora and lactic acid bacteria. The influence of these compounds on the risk of listeriosis in relation to product shelf life is discussed.
Consumption of minimally-processed, or fresh-cut, fruit and vegetables has rapidly increased in recent years, but there have also been several reported outbreaks associated with the consumption of these products. Sodium hypochlorite is currently the most widespread disinfectant used by fresh-cut industries. Neutral electrolyzed water (NEW) is a novel disinfection system that could represent an alternative to sodium hypochlorite. The aim of the study was to determine whether NEW could replace sodium hypochlorite in the fresh-cut produce industry. The effects of NEW, applied in different concentrations, at different treatment temperatures and for different times, in the reduction of the foodborne pathogens Salmonella, Listeria monocytogenes and Escherichia coli O157:H7 and against the spoilage bacterium Erwinia carotovora were tested in lettuce. Lettuce was artificially inoculated by dipping it in a suspension of the studied pathogens at 10(8), 10(7) or 10(5) cfu ml(-1), depending on the assay. The NEW treatment was always compared with washing with deionized water and with a standard hypochlorite treatment. The effect of inoculum size was also studied. Finally, the effect of NEW on the indigenous microbiota of different packaged fresh-cut products was also determined. The bactericidal activity of diluted NEW (containing approximately 50 ppm of free chlorine, pH 8.60) against E. coli O157:H7, Salmonella, L. innocua and E. carotovora on lettuce was similar to that of chlorinated water (120 ppm of free chlorine) with reductions of 1-2 log units. There were generally no significant differences when treating lettuce with NEW for 1 and 3 min. Neither inoculation dose (10(7) or 10(5) cfu ml(-1)) influenced the bacterial reduction achieved. Treating fresh-cut lettuce, carrot, endive, corn salad and 'Four seasons' salad with NEW 1:5 (containing about 50 ppm of free chlorine) was equally effective as applying chlorinated water at 120 ppm. Microbial reduction depended on the vegetable tested: NEW and sodium hypochlorite treatments were more effective on carrot and endive than on iceberg lettuce, 'Four seasons' salad and corn salad. The reductions of indigenous microbiota were smaller than those obtained with the artificially inoculated bacteria tested (0.5-1.2 log reduction). NEW seems to be a promising disinfection method as it would allow to reduce the amount of free chlorine used for the disinfection of fresh-cut produce by the food industry, as the same microbial reduction as sodium hypochlorite is obtained. This would constitute a safer, 'in situ', and easier to handle way of ensuring food safety.
Expression of virulence-related genes in Listeria monocytogenes incubated on cheese was assessed by real-time quantitative polymerase chain reaction. The objective of the study was to investigate the impact of sodium chloride concentration in cheese on transcription of virulence genes and, thereby, virulence potential of L. monocytogenes. The expression studies were performed with L. monocytogenes strains characterized by different tolerance to salt stress. Strains ATCC(®) 51779 and DSMZ 15675 were incubated on the Danish hard-cheese type Samsoe, with low (