Growth of Listeria monocytogenes in vacuum-packed, smoked salmon, during storage at 4 degrees C.

Samples of smoked salmon of different hygienic quality were inoculated with low (6 cfu/g) and high (600 cfu/g) levels of a mixture of three strains of Listeria monocytogenes, after which they were vacuum-packed and stored at 4 degrees C for up to 5 weeks. L. monocytogenes grew well during storage in all the inoculated sample groups. Growth was, however, slightly faster in the fish with the better hygienic quality. The smoked salmon was still sensorically acceptable after 4 weeks. All three strains were found after 4 weeks in the fish with the better quality, while only two strains were recovered after the same time from the poorer quality salmon.     

The growth of Aeromonas hydrophila K144 in ground pork at 5 degrees C

The influence of NaCl, pH, atmosphere, and background microflora on the growth and/or survival of Aeromonas hydrophila K144 was studied in ground pork held at 5 degrees C. In ground pork, A hydrophila was sensitive to pH values below 6.0 in the form of either a low starting pH in the pork itself or induced by lactic acid bacteria action on added glucose. Growth of the organism is inhibited by NaCl levels of 3% (w/w) (approx. 4% brine content). A hydrophila grew in vacuum-packaged ground pork; its growth was diminished by the presence of the naturally occurring meat microflora. Except for pH values below 6.0, conditions which inhibited growth permitted survival of the organisms for extended periods. Data indicate that the growth of A. hydrophila in ground pork can be controlled by factors such as NaCl, pH, and background microflora. In general, measures designed to control other foodborne pathogens appear adequate to limit A. hydrophila.     

Growth of and toxin production by Aeromonas hydrophila and Aeromonas sobria at low temperatures

The effects of different temperatures on the growth and toxin production of Aeromonas hydrophila and Aeromonas sobria were studied. The results showed that these Aeromonas species are not only able to grow at low temperatures (e.g. at 4 and 10 degrees C) but may also produce cytotoxin, hemolysin and enterotoxin under suitable growth conditions.     

Intracellular physiological events of yeast Rhodotorula glutinis during storage at +4 degrees C

Samples of the cheese yeast Rhodotorula glutinis were analysed during storage at +4 degrees C for cultivability, viability, vitality (metabolic activity), membrane potential state, intracellular pH, and carbohydrate content. The results have allowed to describe cellular events occurring during storage. The loss of vitality came with the decrease of carbohydrate content. The fall of trehalose content under a threshold value induced the deterioration of the membrane potential. Later, when all the cells were depolarised, the intracellular pH decreased and the cultivability dropped, whereas viable cells still decreased slowly. Then, it led to an intermediate physiological state similar to the viable but non-cultivable state. Finally, the fall of viability dropped. In this work, we have defined rapid methods relevant to describe the sequence of intracellular events in the cheese yeast R. glutinis during storage, and we applied them to understand the weak vitality without fall of viability of yeast samples. These methods might allow to rapidly test yeast sample quality before use and to predict, at the moment of the harvesting, the conservation of the yeast.     

Mold growth on strawberries and cherries during storage at 25 degrees C

Mold species that grew on the surface of retailed strawberries (10 packs, 211 strawberries) and cherries (18 packs, 441 cherries) during storage at 25 degrees C were isolated and identified to evaluate the state of mold growth. Mold growth was observed on 208 (98.6%) of the 211 strawberries and 193 (43.8%) of the 441 cherries. The mold species most frequently isolated from strawberries was Botrytis cinerea, being observed in 81.0% of the strawberries examined, followed by Cladosporium and Alternaria alternata. The mold most frequently isolated from cherries was Alternaria (28.7%), followed by Penicillium, Botrytis, and Cladosporium. The frequency of cherries on which mold growth was observed varied among packs. Mold tended to grow more often in the areas of the fruits in contact with adjacent fruits.     

Growth of Aeromonas hydrophila in modified-atmosphere-packed cooked meat products

Growth of Aeromonas hydrophila was investigated in modified-atmosphere-packed cooked meat products by developing predictive models. Modified brain–heart infusion (BHI) was shown to be suitable as a simulation medium for cooked meat products. Predictive models were developed for the growth parameters (maximum specific growth rate and lag phase) of A. hydrophila in modified BHI as a function of temperature, water activity and concentration of dissolved carbon dioxide. The growth of A. hydrophila was compared with the growth of the Specific Spoilage Organism (SSO) for cooked meat products Lactobacillus sake, to determine possible risk areas for A. hydrophila in modified atmosphere-packed cooked meat products. Aeromonas hydrophila was shown to multiply very rapidly at refrigerated temperatures. The developed models clearly demonstrated however that proliferation of A. hydrophila could be prevented by the use of carbon dioxide in the package atmosphere in combination with a decreased water activity (<0·985). Gas-packed cured cooked meat products will not sustain the growth of A. hydrophila when kept at refrigerated temperatures (<7°C).     

Influence of probiotic Lactobacillus acidophilus and L. helveticus on proteolysis, organic acid profiles, and ACE-inhibitory activity of cheddar cheeses ripened at 4, 8, and 12 degrees C

ABSTRACT:  The influence of adjunct bacteria on composition of cheeses, organic acid profiles, proteolysis, and ACE-inhibitory activity during ripening at 4, 8, and 12 °C for 24 wk was investigated. cheddar cheeses were made with starter lactococci (control), Lactobacillus acidophilus L10, and starter lactococci (L10), and L. acidophilus L10, L. helveticus H100, and starter lactococci (H100). The counts of L. acidophilus in L10 cheeses remained at >10⁶ colony forming units (CFU)/g after 24 wk of ripening at 4, 8, and 12 °C. Concentrations of lactic, acetic, and propionic acids of the L10 and H100 cheeses were significantly higher than those of the control cheeses after 24 wk of ripening ( P < 0.05). Proteolysis of the cheeses was improved as the ripening temperature increased. Water-soluble nitrogen, trichloroacetic acid soluble nitrogen, and phosphotungstic acid soluble nitrogen of L10 and H100 cheeses were significantly higher than those of the control cheeses ( P < 0.05). Increase in ripening temperature from 4 °C to 8 and 12 °C increased the percentage of ACE inhibition. The IC₅₀ value among cheeses ripened at 4, 8, and 12 °C, however, was not significantly different ( P > 0.05). Hence, probiotic L. acidophilus L10 can be added into cheddar cheeses to improve proteolysis and ACE-inhibitory activity.     

Changes in the proteome of Escherichia coli during growth at 15 degrees C after incubation at 2, 6 or 8 degrees C for 4 days

For better understanding of the complex behaviour of Escherichia coli at chiller temperatures, log phase E. coli grown at 15 degrees C were incubated at 8, 6, or 2 degrees C for 4 days, and were then incubated at 15 degrees C for 12 h. Cultures were sampled after incubation at the lower temperatures, and during subsequent incubation at 15 degrees C. Proteins extracted from the samples were separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). Spots of 45 previously identified proteins that were differentially expressed at 15 or < or =8 degrees C were quantified by image analysis. After incubation at 8 or 6 degrees C for 4 days cells were growing with or without formation of elongated cells (filaments), respectively, but growth did not occur at 2 degrees C. In cells incubated at 8 or 6 degrees C proteins associated with the stress response and energy generation were upregulated and proteins associated with protein synthesis were downregulated, while protein levels in cells incubated at 2 degrees C were little changed. When cells were then incubated at 15 degrees C, the levels of differentially expressed proteins in cells that had been incubated at 8 or 6 degrees C decreased or increased towards the levels found in cells growing at 15 degrees C, but some proteins were still under or over expressed after 12 h. In cells incubated at 15 degrees C after incubation at 2 degrees C, the levels of many of the proteins declined but the levels of proteins associated with protein synthesis increased. The findings indicate that the physiological states of log phase E. coli incubated at < or =2 degrees C or at higher chiller temperature are different, but that for both states incubation at an above chiller temperature for >3 generations is required before protein levels adjusted to those usual for the higher temperature. Cells in these different physiological states may respond differently to other stresses encountered during warming of chilled foods.     

Nisin treatments to control Listeria monocytogenes post-processing contamination on Anthotyros, a traditional Greek whey cheese, stored at 4°C in vacuum packages

Post-processing contamination and growth of Listeria monocytogenes in whey cheeses stored under refrigeration is an important safety concern. This study evaluated commercially available nisin (Nisaplin^®) as a biopreservative to control L. monocytogenes introduced post-processing on Anthotyros, a traditional Greek whey cheese, stored at 4°C in vacuum packages for up to 45 days. The whey used (pH 6.5–6.7) was from Feta cheese manufacture, and it was subjected either to natural acidification (pH 5.3, readjusted to 6.2 with 10% NaOH) prior to heating, or to direct acidification (pH 6.0–6.2) at 80°C with 10% citric acid. Nisin was added either to the whey (100 or 500 IU g⁻¹) prior to heating, or to the cheese (500 IU g⁻¹) prior to packaging, also inoculated with ca. 10⁴ cfu g⁻¹ of L. monocytogenes strain Scott A. In cheese samples without nisin, L. monocytogenes (PALCAM agar) exceeded 7 log cfu g⁻¹ after the first 10 days of storage, irrespective of the whey acidification method. All nisin treatments had an immediate lethal effect (0.7–2.2 log reduction) on L. monocytogenes populations at inoculation (day 0), which was more pronounced with 500 IU g⁻¹ added to the whey. This treatment also suppressed L. monocytogenes growth below the inoculation level for 30 and 45 days in naturally and directly acidified samples, respectively. All other treatments had weak antilisterial effects. Nisin reversed the natural spoilage flora of Anthotyros cheese from Gram-positive to Gram-negative, and this ecological alteration was far more pronounced in the most effective antilisterial treatments.     

Changes in lipids during the storage of krill (Euphausia superba Dana) at 3 degrees C

Lipid content and composition (classes), susceptibility to UV-catalysed oxidation and carotenoid content were determined in Antarctic krill (Euphausia superba Dana) stored for 72 h at 3 degrees C. Phospholipids making up 80% of krill lipids were observed to undergo the most drastic changes during storage. After 72 h of storage, their content dropped by about 20%; the largest drop was recorded in phosphatidylcholine, its content being reduced by almost half. The amount of free fatty acids increased to about 6% of lipids. No degradation was observed in triacylglycerols, diacylglycerols, and wax esters. Monoglycerides did not appear. The UV-catalysed lipid oxidation rate decreased with deteriorating freshness of krill, as evidenced by a slower oxidation reaction, much lower oxidation maximum attained by lipids from spoiled krill, slower carotenoid decomposition, slower coloration of lipids and a slower absorbance increase at 320 nm. As no significant differences were found between iodine numbers and the carotenoid content of the samples tested, differences in the oxidation rate can be explained by hyperoxide decomposition brought about by products of phosphatidylcholine break-down.     

Growth kinetics and predictive model of Aeromonas hydrophila in a broth-based system

This study developed a predictive model of Aermonas hydrophila in tryptic soy broth for any combination of temperatures (5 to 40°C), pH (6 to 8), and NaCl (0 to 5%) using a response surface model. A. hydrophila tended to grow within a pH range of 6.0 to 8.0 and could not tolerate NaCl up to 5.0%. The interaction of pH and NaCl did not affect the specific growth rates (SGR). The primary model to obtain the SGR showed a good fit (R²≥0.980). A secondary model was obtained by non-linear regression analysis and calculated as: SGR= 0.4577+0.0529X₁−0.1641X₂−0.1493X₃−0.0016X₁X₂−0.0001X₁X₃+0.0115X₂X₃0.0006X₁ ²+0.0114X₂ ²+0.0150X₃ ² (X₁=temperature, X₂=pH, X₃=NaCl). The appropriateness of the polynomial model was verified by the mean square error (0.0023), bias factor (0.922), accuracy factor (1.343), and coefficient of determination (0.937). The newly secondary model of SGR for A. hydrophila could be incorporated into the tertiary model to predict the growth of A. hydrophila.     

Rheological, textural and microstructural features of probiotic whey cheeses

Whey cheeses have been manufactured with probiotic bacteria - viz. Bifidobacterium animalis Bo and Lactobacillus casei LAFTI^®L26, from combinations of bovine whey and milk, following protein denaturation at 90 °C; they were subsequently inoculated (at 10%) with those strains, and homogenized afterwards; additives such as salt and sugar were then incorporated; and the resulting solid matrices were stored at 7° C for up to 21 d. Oscillatory measurements and instrumental texture profile analyses were performed, and sensory analyses were carried out by a trained panel. Microstructural features were in addition ascertained by scanning electron microscopy.L. casei exhibited a higher acidifying activity than B. animalis, which produced distinct textures; higher firmness and viscoelasticity were indeed found in matrices inoculated with the former. Incorporation of sugar and L. casei favoured consumer acceptability, relative to plain matrices. Microstructural differences were detected between matrices at different times of storage and formulated with distinct additives.     

Survival of Escherichia coli O157:H7 in ground-beef patties during storage at 2, -2, 15 and then -2 degrees C, and -20 degrees C

The survival of Escherichia coli O157:H7 and of a nonpathogenic control strain of E. coli was monitored in raw ground beef that was stored at 2 degrees C for 4 weeks, -2 degrees C for 4 weeks, 15 degrees C for 4 h and then -2 degrees C for 4 weeks, and -20 degrees C. Irradiated ground beef was inoculated with one E. coli control strain or with a four-strain cocktail of E. coli O157:H7 (ca. 10(5) CFU/g), formed into patties (30 to 45 g), and stored at the appropriate temperature. The numbers of the E. coli control strain decreased by 1.4 log 10 CFU/g, and pathogen numbers declined 1.9 log 10 CFU/g when patties were stored for 4 weeks at 20 degrees C. When patties were stored at -2 degrees C for 4 weeks, the numbers of the E. coli control strain and the serotype O157:H7 strains decreased 2.8 and 1.5 log 10 CFU/g, respectively. Patties stored at 15 degrees C for 4 h prior to storage at -2 degrees C for 4 weeks resulted in 1.6 and 2.7 log 10-CFU/g reduction in the numbers of E. coli and E. coli O157:H7, respectively. Storage of retail ground beef at 15 degrees C for 4 h (tempering) did not result in increased numbers of colony forming units per gram, as determined with violet red bile, MRS lactobacilli, and plate-count agars. Frozen storage (-20 degrees C) of ground-beef patties that had been inoculated with a single strain of E. coli resulted in approximately a 1 to 2 log 10-CFU/g reduction in the numbers of the control strain and individual serotype O157:H7 strains after 1 year. There was no significant difference between the survival of the control strain and the O157:H7 strains, nor was there a difference between O157:H7 strains. These data demonstrate that tempering of ground-beef patties prior to low-temperature storage accelerated the decline in the numbers of E. coli O157:H7.     

Metabolite production of yeasts on a strawberry-agar during storage at 7 degrees C in air and low oxygen atmosphere

Changes of different quality factors of strawberries have been described until now from a physiological point of view. Possible effects on quality caused by yeast proliferation have not been described. To elucidate the metabolic activity of yeasts (i.e. Debaryomyces melissophilus, Rhodotorula glutinis, Cryptococcus laurentii), isolated from strawberries, they were inoculated on a simulation medium of strawberries (strawberry-agar). Their activity was measured by analysing and correlating microbiological counts, metabolite concentration in the headspace as well as in the medium, and sugar consumption. The isolated yeasts from strawberries could grow on the strawberry-agar, both under air conditions and modified atmosphere (MA) conditions. The maximum count for the yeasts reached 7.5-8.5 log cfu cm(-2) (air conditions) and 5.6-6.4 log cfu cm(-2) (MA conditions). Production or consumption of a number of compounds could be detected when microbial counts reached levels between 4.7 log cfu cm(-2) and 8.5 log cfu cm(-2) depending on species and atmospheric conditions. A range of volatile organic compounds, produced by the yeasts, was detected: acetone, ethyl acetate, ethanol, isopropyl acetate, ethyl butyrate, 1-propanol, 2-methyl-1-propanol, 1-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 1-hexanol and hexyl acetate. These compounds are able to influence the sensory properties of strawberries. A simultaneous decrease in sugar concentrations (sucrose, glucose, fructose) was observed. When ethanol reached high concentrations, ethyl acetate and ethyl butyrate were produced. This production can be attributed to a detoxification of ethanol by yeasts. The fermentative metabolism of yeasts during aerobic conditions could be explained by the Crabtree effect. As the detected volatile organic compounds produced by yeasts are also found in fresh strawberries, it can be concluded that these compounds are produced both by microbiological and physiological processes.     

Effect of High Hydrostatic Pressure on Aeromonas hydrophila AH 191 Growth in Milk

Abstract: Exposure to high pressure is an efficient method of bacterial inactivation that is particularly important for reducing the microbial load present in foods. In this study, we examined the high pressure inactivation of Aeromonas hydrophila AH 191, a virulent strain that produces aerolysin, a cytotoxic, enterotoxic, and hemolytic toxin. High pressure treatment (250 MPa for 30 min at 25 °C in 0.1 M PBS, pH 7.4) of A. hydrophila grown in milk reduced bacterial viability by at least 9 orders of magnitude. Under these conditions, the enterotoxic, hemolytic, and cytotoxic activities of A. hydrophila culture supernatants were unaltered. These results indicate the need for caution in the use of high pressure for food processing since although truly toxigenic bacteria may be inactivated, their toxins may not be, thus posing a risk to human health. At higher pressure (350 MPa) the inactivation of bacteria was much more effective. Scanning electron microscopy showed a significant decrease in the number of bacteria after higher pressurization (350 MPa for 1 h) and transmission electron microscopy showed irregular shaped bacteria, suggestive of important cell wall and membrane damage, and cytoplasm condensation. Practical Application: High pressure inactivates Aeromonas hydrophila efficiently but is enhanced when combined with moderate temperature (40 °C). The biological activities of toxins from this bacterium are unaltered under these conditions.     

Comparison of biogenic amine profile in cheeses manufactured from fresh and stored (4 degrees C, 48 hours) raw goat's milk

In this study, the evolution of microbial counts, biogenic amine contents, and related parameters (pH, moisture, and proteolysis) in goat cheese made from fresh raw milk or raw milk stored for 48 h at 4 degrees C was examined. In both cases the milk was nonpasteurized. This study was designed to evaluate the effect of milk quality on the profile of biogenic amines in relation to the evolution of the microbial population during cheese making. Cheese made from raw milk stored for 48 h at 4 degrees C showed the highest microbial counts and biogenic amine levels. The storage of milk under refrigeration caused significant increases in the levels of some microbial and biogenic amines during ripening, but not initially. Tyramine was the main biogenic amine in the two cheeses tested, followed by cadaverine. However, the main differences in amine contents between batches were found for putrescine, histamine, and beta-phenylethylamine, whose levels were more than twofold higher in samples from raw milk refrigerated for 48 h than in samples from fresh milk.     

Survival of Escherichia coli O157:H7 in Galotyri cheese stored at 4 and 12 degrees C

Post-process contamination of fresh acid-curd cheeses with Escherichia coli O157:H7 may pose a risk considering the low infectious dose and the ability of the pathogen to survive in acidic foods. To evaluate its survival in Galotyri, a traditional Greek acid-curd cheese, portions (0.5 kg) of two commercial fresh products, one artisan (pH 3.9+/-0.1) and the other industrial (pH 3.7+/-0.1), were inoculated with approximately 3.0 or 6.5 log cfu g(-1) of a five-strain cocktail of E. coli O157:H7, including rifampicin-resistant derivatives of the strains ATCC 43895 and ATCC 51657, and stored aerobically at 4 and 12 degrees C. Survival was monitored for 28 days by plating cheese samples on tryptic soy agar with 100 mg l(-1) rifampicin (TSA+Rif), SMAC and Fluorocult E. coli O157:H7 agar media. The pathogen declined much faster (P<0.05) in the industrial as compared to the artisan cheeses at both temperatures. Thus, while E. coli O157:H7 became undetectable by culture enrichment after 14 days at 4 degrees C in industrial samples, irrespective of the inoculation level, populations of 1.4-1.9 and 4.2-5.1 log cfu g(-1) survived after 28 days in the corresponding artisan cheeses with the low and high inocula, respectively. Survival was longer and greater (P<0.05) on TSA+Rif than on SMAC and Fluorocult, indicating the presence of acid-injured cells. Interestingly, survival of E. coli O157:H7 after 14-28 days in cheeses was better at 12 degrees C than at 4 degrees C, probably due to yeasts which grew on the surface of temperature-abused cheeses. The large difference in the pathogen's inactivation between the industrial and artisan cheeses at 4 degrees C could not be associated with major differences in pH or type/concentration of organic acids, suggesting another anti-E. coli O157:H7 activity by the industrial starter. The high survival of the pathogen in artisan Galotyri under conditions simulating commercial storage should be of concern.     

Determination of 5-log reduction times for food pathogens in acidified cucumbers during storage at 10 and 25 degrees C

Outbreaks of acid-resistant foodborne pathogens in acid foods with pH values below 4.0, including apple cider and orange juice, have raised concerns about the safety of acidified vegetable products. For acidified vegetable products with pH values between 3.3 and 4.6, previous research has demonstrated that thermal treatments are needed to achieve a 5-log reduction in the numbers of Escherichia coli O157:H7, Listeria monocytogenes, or Salmonella enterica. For some acidified vegetable products with a pH of 3.3 or below, heat processing can result in unacceptable product quality. The purpose of this study was to determine the holding times needed to achieve a 5-log reduction in E. coli O157:H7, L. monocytogenes, and S. enterica strains in acidified vegetable products with acetic acid as the primary acidulant, a pH of 3.3 or below, and a minimum equilibrated temperature of 10 degrees C. We found E. coli O157:H7 to be the most acid-resistant microorganism for the conditions tested, with a predicted time to achieve a 5-log reduction in cell numbers at 10 degrees C of 5.7 days, compared with 2.1 days (51 h) for Salmonella or 0.5 days (11.2 h) for Listeria. At 25 degrees C, the E. coli O157:H7 population achieved a 5-log reduction in 1.4 days (34.3 h).     

Prevalence and growth of Listeria on naturally contaminated smoked salmon over 28 days of storage at 4 degrees C

Only limited data are available on the growth characteristics of Listeria in naturally contaminated ready-to-eat foods. To evaluate Listeria contamination patterns and growth in smoked salmon, 72 smoked salmon product samples from two processing plants were tested for Listeria spp. and L. monocytogenes. Samples were divided into four approximately equal portions: one portion was tested on receipt, and the other three were vacuum sealed and stored at 4 degrees C for 7, 14, and 28 days. Listeria testing was performed using both an enrichment procedure and direct plating to enumerate Listeria in samples that contained >2 to 10 CFU/g. Five samples were positive for Listeria spp., including one sample that was positive for L. monocytogenes. Most samples yielded only sporadic positive results among the portions tested on days 0, 7, 14, and 28. Only one sample contained Listeria spp. in numbers above the detection limit for enumeration. For this sample, the portions tested on days 7 and 28 contained 46 and 52 CFU/g, respectively, whereas the portion tested on day 14 was negative. Overall, our data indicate that there is considerable heterogeneity in Listeria spp. distribution within a single positive smoked fish sample. Even with refrigerated storage for 28 days, none of the naturally contaminated samples reached Listeria spp. numbers >100 CFU/g, which indicates that Listeria growth was limited within a 4-week storage period. However, because of the apparent heterogeneity of Listeria distribution within samples, the interpretation of growth data collected on naturally contaminated samples is difficult.     

Survival of Helicobacter pylori in ready-to-eat foods at 4 degrees C

The survival of Helicobacter pylori (NCTC 11638) in various semiprocessed and fresh, ready-to-eat foods, and one raw chicken was studied at 4 degrees C and under aerobic conditions by experimentally inoculating these with 10(4) CFU. Cells were concentrated by two centrifugation cycles followed by plating onto selective blood agar medium made from Wilkins-Chalgren agar supplemented with 5% whole horse blood, and 30 mg/l colistin methanesulfonate, 100 mg/l cycloheximide, 30 mg/l nalidixic acid, 30 mg/l trimethoprim, and 10 mg/l vancomycin. H. pylori was recovered from spiked pasteurized milk and tofu samples up to 5 days and from spiked leaf lettuce and raw chicken up to 2 days. H. pylori could not be recovered from yogurt after any length of storage time. H. pylori is unlikely to grow in foods; however, it may survive in low acid-high moisture environments under refrigeration and pose a possible risk for transmission of infection via foods.