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.     

Bacteriocin activity by Lactobacillus curvatus CWBI-B28 to inactivate Listeria monocytogenes in cold-smoked salmon during 4 degrees C storage

The inhibition effectiveness of a bacteriocin produced by Lactobacillus curvatus CWBI-B28 against Listeria monocytogenes was investigated in cold-smoked salmon during storage at 4 degrees C. Three bacteriocin-based strategies for the control of L. monocytogenes in foods (i.e., producing bacteriocin in situ, spraying with partially purified bacteriocin, and packaging in bacteriocin-coated plastic film), plus a newly developed method that uses cell-adsorbed bacteriocin (i.e., a suspension of producer cells on which maximum bacteriocin has been immobilized by pH adjustments), were assessed. Although all the approaches inactivated L. monocytogenes in cold-smoked salmon, various efficacy levels were observed. The behavior of L. monocytogenes was similar in samples treated with either partially purified bacteriocin or in situ bacteriocin production. In both of these cases, the counts of the pathogen declined to below the detectable limit of 0.7 log CFU/cm2 within the first week, but a approximately 0.95- and 1.3-log increase, respectively, occurred after day 14. The bioactive packaging film resulted in a slower inactivation of the pathogen but prevented any subsequent increase in the CFU throughout 22 days of storage at 4 degrees C. Application of the cell-adsorbed bacteriocin was shown to be the most effective means, as it resulted in a complete inactivation of the pathogen within 3 days, and no increase in Listeria counts occurred up to 22 days.     

Effect of salt, smoke compound, and storage temperature on the growth of Listeria monocytogenes in simulated smoked salmon

Smoked salmon can be contaminated with Listeria monocytogenes. It is important to identify the factors that are capable of controlling the growth of L. monocytogenes in smoked salmon so that control measures can be developed. The objective of this study was to determine the effect of salt, a smoke compound, storage temperature, and their interactions on L. monocytogenes in simulated smoked salmon. A six-strain mixture of L. monocytogenes (10(2) to 10(3) CFU/g) was inoculated into minced, cooked salmon containing 0 to 10% NaCl and 0 to 0.4% liquid smoke (0 to 34 ppm of phenol), and the samples were stored at temperatures from 0 to 25 degrees C. Lag-phase duration (LPD; hour), growth rate (GR; log CFU per hour), and maximum population density (MPD; log CFU per gram) of L. monocytogenes in salmon, as affected by the concentrations of salt and phenol, storage temperature, and their interactions, were analyzed. Results showed that L. monocytogenes was able to grow in salmon containing the concentrations of salt and phenol commonly found in smoked salmon at the prevailing storage temperatures. The growth of L. monocytogenes was affected significantly (P < 0.05) by salt, phenol, storage temperature, and their interactions. As expected, higher concentrations of salt or lower storage temperatures extended the LPD and reduced the GR. Higher concentrations of phenol extended the LPD of L. monocytogenes, particularly at lower storage temperatures. However, its effect on reducing the GR of L. monocytogenes was observed only at higher salt concentrations (>6%) at refrigerated and mild abuse temperatures (< 10 degrees C). The MPD, which generally reached 7 to 8 log CFU/g in salmon that supported L. monocytogenes growth, was not affected by the salt, phenol, and storage temperature. Two models were developed to describe the LPD and GR of L. monocytogenes in salmon containing 0 to 8% salt, 0 to 34 ppm of phenol, and storage temperatures of 4 to 25 degrees C. The data and models obtained from this study would be useful for estimating the behavior of L. monocytogenes in smoked salmon.     

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.     

Inhibition of Listeria monocytogenes by Carnobacterium spp. strains in a simulated cold smoked fish system stored at 4 degrees C.

Preservation of smoked salmon from bacterial spoilage, and especially from Listeria monocytogenes by bacteriocin producers is a promising challenge. Over a hundred lactic acid bacteria, isolated from commercial vacuum packaged cold smoked salmon, were screened for their antagonistic activity against L. innocua. Twenty-two strains were able to produce bacteriocin-like proteinaceous substances. These strains were characterized physiologically and biochemically as Carnobacterium strains. Three different groups were determined by pulsed-field gel electrophoresis after Sma I and Apa I DNA digestion. Peptidoglycan hydrolases patterns completed the characterization of these strains. All were confirmed as being Carnobacterium piscicola. Growth and bacteriocin production of three strains of each group and two well known bacteriocin producers (C. divergens V41 and C. piscicola V1) were tested in a simulated cold smoked fish system at 4 degrees C. These strains were able to reach 10(8) cfu ml(-1) in 21 days and to produce as much bacteriocin activities in the cold smoked fish system as in the rich media. Carnobacterium divergens V41 and C. piscicola V1 were the most effective strains in co-culture experiments, inhibiting L. monocytogenes as early as day 4, whereas C. piscicola SF668 inhibiting effect was observed at day 13. The potential for using such biopreservation treatments on whole smoked salmon is discussed.     

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.     

Control of growth and survival of Listeria monocytogenes on smoked salmon by combined potassium lactate and sodium diacetate and freezing stress during refrigeration and frozen storage

In this study, we evaluated the antimicrobial effects of different levels of a potassium lactate (PL) plus sodium diacetate (SDA) mixture against the growth and survival of Listeria monocytogenes Scott A inoculated onto smoked salmon stored at 4, 10, and -20 degrees C. The effect of freezing stress on the growth kinetics of L. monocytogenes Scott A on smoked salmon at 4 and 10 degrees C was also investigated. The use of PL+SDA at all tested levels (1.5, 3.3, and 5% of a 60% commercial solution of PURASAL P Opti.Form 4) completely inhibited the growth of L. monocytogenes Scott A on smoked salmon stored at 4 degrees C during 32 days of storage. It also delayed the growth of L. monocytogenes Scott A on smoked salmon stored at 10 degrees C for up to 11 days, but a listeriostatic effect was observed only with 5% PURASAL P Opti.Form 4 at 10 degrees C after 11 days. Addition of PL+SDA at all tested levels decreased the surviving populations of L monocytogenes Scott A on smoked salmon during 10 months of frozen storage at -20 degrees C. Freezing stress significantly (P < 0.001) extended the lag time and delayed the growth of L. monocytogenes Scott A at both 4 and 10 degrees C. However, the effect of freezing stress was more significant at 4 degrees C than at 10 degrees C, indicating the importance of temperature control of smoked salmon during the retail storage period.     

Control of Listeria monocytogenes with combined antimicrobials on beef franks stored at 4 degrees C

Contamination of ready-to-eat meat products such as beef franks with Listeria monocytogenes has become a major concern for the meat processing industry and an important food safety issue. The objective of this study was to determine the effectiveness of combinations of antimicrobials as aqueous dipping solutions to control L. monocytogenes on vacuum-packaged beef franks stored at 4 degrees C for 3 weeks. Commercial beef franks were dipped for 5 min in three antimicrobial solutions: pediocin (6,000 AU), 3% sodium diacetate and 6% sodium lactate combined, and a combination of the three antimicrobials. Samples were then inoculated with 10(7) CFU/g of either four L. monocytogenes strains individually or a cocktail of the four strains, vacuum packaged, and stored at 4 degrees C for 3 weeks. Sampling was carried out at day 0 and after 2 and 3 weeks of storage. Individual strains, as well as the cocktail, exhibited different responses to the antimicrobial treatments. After 2 and 3 weeks of storage at 4 degrees C, pediocin-treated beef franks showed a less than 1-log reduction for all bacterial strains. Samples treated with the sodium diacetate-sodium lactate combination showed about a 1-log reduction after 2 weeks of storage for all strains and between a 1- and 2-log reduction after 3 weeks of storage, depending on the bacterial strain. When the three antimicrobials were combined, reductions ranged between 1 and 1.5 log units and 1.5 to 2.5 log units after 2 and 3 weeks of storage, respectively, at 4 degrees C. These results indicate that the use of combined antimicrobial solutions for dipping treatments is more effective at inhibiting L. monocytogenes than treatments using antimicrobials such as pediocin separately.     

Control of Listeria monocytogenes with combined antimicrobials after postprocess contamination and extended storage of frankfurters at 4 degrees C in vacuum packages

Contamination of ready-to-eat foods, such as frankfurters, with Listeria monocytogenes, is a major concern that needs to be addressed in order to enhance the safety of these products. The objective of this study was to determine the effectiveness of combinations of antimicrobials included in the formulation of frankfurters against L. monocytogenes inoculated (10(3) to 10(4) CFU/cm2) on their surface after peeling and before vacuum packaging. In addition, the antilisterial effect of immersing the packaged products, prepared with or without antimicrobials, in hot (75 or 80 degrees C) water for 30 to 90 s was evaluated. Samples were stored at 4 degrees C for up to 120 days and periodically analyzed for pH and for microbial growth on tryptic soy agar plus 0.6% yeast extract (TSAYE) and PALCAM agar. Sodium lactate (1.8%; 3% of a 60% commercial solution) used alone inhibited growth of L. monocytogenes for 35 to 50 days, whereas when used in combination with 0.25% sodium acetate, sodium diacetate, or glucono-delta-lactone (GDL), sodium lactate inhibited growth throughout storage (120 days). Immersing packaged frankfurters in hot water (80 degrees C, 60 s) reduced inoculated populations of L. monocytogenes by 0.4 to 0.9 log CFU/cm2 and reduced its growth by 1.1 to 1.4 log CFU/cm2 at 50 to 70 days of storage in samples containing 1.8% sodium lactate alone. However, immersion of frankfurters containing no antimicrobials in hot water (75 or 80 degrees C) did not inhibit growth of the pathogen for more than 10 to 20 days, unless one frankfurter was placed per bag and heat treated for 90 s. These results indicate that the inclusion of 1.8% sodium lactate with 0.25% sodium acetate, sodium diacetate, or GDL in cured meat formulations may control L. monocytogenes growth during refrigerated (4 degrees C) storage. Additional studies are required to evaluate the effects of these combinations at abusive temperatures of storage, as well as on additional processed meat formulations and on the sensory quality and shelf life of products.     

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.     

Growth characteristics of Listeria monocytogenes, Listeria welshimeri and Listeria innocua strains in broth cultures and a sliced bologna-type product at 4 and 7 degrees C

The growth characteristics of meat processing plant-derived field strains of Listeria monocytogenes, L. welshimeri and L. innocua were analyzed. The strains were inoculated in BHI broth cultures and incubated at 4 and 7 degrees C. Growth curves were determined by colony counting for 28 days. Significant variations were detected in the growth properties of these field-derived strains. In particular some of the L. monocytogenes strains displayed better cold stress tolerance. These discrepancies in growth behavior were more apparent in the cultures at 4 degrees C compared to 7 degrees C. Similar growth characteristics were observed for selected L. monocytogenes strains also in food challenge tests based on a sliced bologna-type product. The results stress the need for more evaluation of field strain growth characteristics and incorporation of such information in relevant predictive microbial growth models for L. monocytogenes risk assessment in naturally contaminated food products.     

Effects of pH and agitation on the growth of Listeria monocytogenes Scott A in brain heart infusion broth containing combined potassium lactate and sodium diacetate during storage at 4 or 10 degrees C

The objective of this study was to compare the effects of pH on the growth kinetics of Listeria monocytogenes Scott A in static and agitated broths stored at 4 and 10 degrees C with and without a combination of 1.85% potassium lactate (PL) and 0.13% sodium diacetate (SDA) (3.3% of a 60% commercial solution, PURASAL P Opti.Form 4). The pH of brain heart infusion broth without (control) or with 1.85% PL + 0.13% SDA was adjusted to 5.5, 6.0, 6.5, and 7.5. L. monocytogenes Scott A was inoculated (at 10(2) CFU/ml) into pH-adjusted broth, which was stored at 4 or 10 degrees C with or without agitation. At pH 5.5, a listeriostatic effect was observed for the broth containing 1.85% PL + 0.13% SDA at 4 and 10 degrees C both with and without agitation. At pH 6.0, 1.85% PL + 0.13% SDA fully controlled the growth of L. monocytogenes Scott A in static broth at 4 degrees C for up to 20 days and significantly slowed the growth of the pathogen in agitated broth. At 10 degrees C, the growth of L. monocytogenes Scott A was significantly reduced by 1.85% PL + 0.13% SDA in agitated and unagitated broths. At pH 6.5, 1.85% PL + 0.13% SDA significantly suppressed the growth of L. monocytogenes Scott A at both 4 degrees C (P < 0.001) and 10 degrees C (P < 0.01). At pH 7.5, 1.85% PL + 0.13% SDA had a limited effect on the growth of L. monocytogenes Scott A in broth stored at 4 and 10 degrees C. At 4 degrees C, agitation decreased the lag time and increased the growth rate of L. monocytogenes Scott A at all tested pHs. A similar but less obvious trend was observed for broths stored at 10 degrees C. These results indicate that lactate-diacetate combinations effectively acted with low pH and temperature to inhibit the growth of L. monocytogenes Scott A.     

Thermal resistance of Listeria monocytogenes in inoculated and naturally contaminated raw milk

Raw whole milk inoculated with 10(5) CFU/ml of Listeria monocytogenes was thermally processed at 60-72 degrees C for a minimum holding time of 16.2 s with survival being observed at temperatures up to 67.5 degrees C. In addition, milk naturally contaminated with L. monocytogenes serotype 1 (around 10(4) CFU/ml) was pooled for 2 to 2.5 days and then run through an HTST pasteurizer at temperatures ranging from 60-78 degrees C. Viable L. monocytogenes were detected in the temperature range of 60-66 degrees C. No viable Listeria were detected after treatment at temperatures of 69 degrees C and above in any of five trials. Efficacy of pasteurization and widespread use of processing conditions well above the minimum HTST guidelines ensure the absence of Listeria in pasteurized milk products. However, survival of Listeria at sub-pasteurization temperatures (60-67.5 degrees C) is of concern with regard to heat-treated or raw-milk cheeses.     

Inhibition of Listeria monocytogenes using nisin with grape seed extract on turkey frankfurters stored at 4 and 10 degrees C

Recontamination of cooked ready-to-eat (RTE) chicken and beef products with Listeria monocytogenes has been a major safety concern. Natural antimicrobials in combinations can be an alternative approach for controlling L. monocytogenes. Therefore, the objectives of this study were to evaluate the inhibitory activities against L. monocytogenes of nisin (6,400 IU/ ml), grape seed extract (GSE; 1%), and the combination of nisin and GSE both in tryptic soy broth with 0.6% yeast extract (TSBYE) and on the surface of full-fat turkey frankfurters. TSBYE was incubated at 37 degrees C for 72 h and turkey frankfurters at 4 or 10'C for 28 days. Inocula were 6.7 or 5 log CFU per ml or g for TSBYE or frankfurters, respectively. After 72 h in TSBYE, nisin alone did not show any inhibitory activity against L. monocytogenes. The combination of nisin and GSE gave the greatest inhibitory activity in both TSBYE and on turkey frankfurters with reductions of L. monocytogenes populations to undetectable levels after 15 h and 21 days, respectively. This combination of two natural antimicrobials has the potential to control the growth and recontamination of L. monocytogenes on RTE meat products.     

Control of natural microbial flora and Listeria monocytogenes in vacuum-packaged trout at 4 and 10 degrees C using irradiation

The effect of gamma irradiation on the natural microflora of whole salted vacuum-packaged trout at 4 and 10 degrees C was studied. In addition, the effectiveness of gamma irradiation in controlling Listeria monocytogenes inoculated into trout was investigated. Irradiation at doses of 0.5 and 2 kGy affected populations of bacteria, namely, Pseudomonas spp., Brochothrix thermosfacta, lactic acid bacteria, H2S-producing bacteria typical of Shewanella putrefaciens, and Enterobacteriaceae, at both 4 and 10 degrees C. This effect was more pronounced at the higher dose (2 kGy) and the lower temperature (4 degrees C). Pseudomonads, H2S-producing bacteria typical of S. putrefaciens, and Enterobacteriaceae showed higher sensitivity to gamma irradiation than did the rest of the microbial species. Sensory evaluation did not show a good correlation with bacterial populations. On the basis of sensory odor scores, a shelf life of 28 days (2 kGy, 4 degrees C) was obtained for salted vacuum-packaged freshwater trout, compared with a shelf life of 7 days for the unirradiated sample. Under the same conditions, the growth of L. monocytogenes inoculated into the samples was suppressed by 2 log cycles after irradiation (2 kGy) and storage for up to 18 days at 4 degrees C.     

Effect of acid adaptation on growth during storage at 10 degrees C and resistance to simulated gastric fluid of Listeria monocytogenes inoculated onto bologna formulated with or without antimicrobials

The fate of acid-adapted and nonadapted Listeria monocytogenes inoculated onto bologna slices (formulated with or without antimicrobials) was examined during storage and after exposure to in vitro gastric challenge. Bologna slices formulated with no antimicrobials (control), 3% sodium lactate (SL), or 1.8% SL plus 0.25% sodium diacetate (SD) were inoculated (2 log CFU/cm2) with a 10-strain composite of acid-adapted or nonadapted L. monocytogenes strains. Growth or survival of the two inocula on bologna was evaluated during vacuum-packaged storage (10 degrees C) for up to 36 days. Survival of previously acid-adapted or nonadapted L. monocytogenes on stored bologna exposed to simulated gastric fluid (adjusted to pH 1.0 with HCl) for 20, 40, and 60 min also was determined. As expected, inclusion of antimicrobials in the product formulation inhibited growth of L. monocytogenes during storage of vacuum-packaged bologna compared with growth on control samples. Acid adaptation of L. monocytogenes prior to product inoculation did not affect subsequent survival or growth on bologna or resistance to simulated gastric fluid (P > 0.05). Survival of L. monocytogenes exposed to simulated gastric fluid during storage increased with product age, growth phase of the cells, and possibly age of the cells, particularly for control samples (no antimicrobials), in which the pathogen grew uninhibited to approximately 6 log CFU/cm2 by day 8 of storage. Inhibition of L. monocytogenes growth on product formulated with antimicrobials was associated with only sporadic and small numbers of survivors following exposure of these samples to simulated gastric fluid, especially in samples stored longer. However, cell numbers in these treatment groups before the gastric challenge did not exceed 3.8 log CFU/cm2. Inhibition of growth on product with antimicrobials precluded detection of survivors resistant to the effects of simulated gastric fluid.     

Combined effects of packaging atmosphere and lactic acid on growth and survival of Listeria monocytogenes in crayfish tail meat 4 degrees C.

The effect of lactic acid on growth and survival of Listeria monocytogenes in crayfish tail meat stored under refrigeration and various gas environments was investigated. Frozen crayfish tail meat was thawed overnight, autoclaved, cooled, and inoculated with approximately 4 log colony-forming units (CFU) of a mixed-strain (Scott A and F5027) L. monocytogenes culture per gram of meat. Inoculated samples were blended with 0, 0.5, 1.0, 1.5, or 2.0% lactic acid and packaged under air, vacuum, or modified atmosphere (74.8% CO2, 10.4% O2, and 14.8% N2) and stored at 4 degrees C for 20 days. Results demonstrated that modified atmosphere packaging inhibited the growth of L. monocytogenes more than air and vacuum packaging at 0 and 1% lactic acid. Microbial counts declined steadily in crayfish tail meat treated with 2% lactic acid, with no differences among the packaging atmospheres. The lag phase was extended by 8 days in samples treated with 1% lactic acid and modified atmosphere compared to that in air or vacuum packaging. Overall, the combination of lactic acid and modified atmosphere had the greatest potential to prevent growth of L. monocytogeines.     

Effect of salt and smoke on the microbiological quality of cold-smoked salmon during storage at 5 degrees C as estimated by the factorial design method

The simultaneous effect of salt and smoke on the natural flora of cold-smoked salmon was studied during 5 weeks of vacuum storage at 5 degrees C. The quadratic polynomial, as a function of factors, was used to express total viable count (TVC), total lactic acid bacteria, lactobacilli numerated on Rogosa agar, H2S-producing bacteria, and yeasts at different sampling times. TVC and total lactic acid bacteria were mainly inhibited by the salt concentration (5% wt/wt) in the meat and to a lesser extent by the phenol content. Inhibition was linearly proportional to salt and smoke content (the higher the concentration, the greater the inhibition). No synergistic effect on inhibition was observed between the two factors. In our working conditions, the TVC French standard (<10(6) CFU g(-1)) was maintained during 4 weeks of storage at 5 degrees C, with a minimum concentration of 2.4% (wt/wt) of salt in meat and smoking treatment corresponding to 0.6 mg 100 g(-1) of phenol. When the salt level was higher than 3%, the TVC standard was maintained, regardless of phenol level. A negative interaction between the two factors was found for H2S-producing bacteria and a positive one for yeasts.     

Survival of Listeria monocytogenes in vanilla-flavored soy and dairy products stored at 8 degrees C

The survival of Listeria monocytogenes V37 in vanilla-flavored yogurt (low-fat and nonfat) and soy milk (low-fat and Plus) stored at 8 degrees C for 31 days was investigated. Commercial samples of yogurt and soy milk were used. These samples were inoculated with either 10(4) or 10(7) CFU of L. monocytogenes per ml. Sampling was carried out every 3 to 4 days initially and was then carried out weekly, for a total storage time of 31 days. Each time a sample was collected, the pH of the sample was measured. After 31 days, low-fat plain, low-fat vanilla, and nonfat plain yogurt samples inoculated with 10(4) CFU/ml showed 2.5-log reductions in viable cell populations, and nonfat vanilla yogurt showed a 3.5-log reduction. For yogurt inoculated with 10(7) CFU/ml, reductions of 2.5 log CFU/ml were observed for plain low-fat and nonfat yogurts, and reductions of 5 log CFU/ml were observed for vanilla-flavored low-fat and nonfat yogurts. In vanilla-flavored and plain low-fat and Plus soy milk samples, cell counts increased from 10(4) and 10(7) CFU/ml to 10(9) CFU/ml at 7 and 3 days of storage, respectively, at 8 degrees C. Coagulation in soy milk samples was observed when the cell population reached 10(9) CFU/ml. In soy milk, the L. monocytogenes population did not change for up to 31 days. Vanillin had an inhibitory effect on L. monocytogenes in yogurt but not in soy milk.     

Effects of high pressure, subzero temperature, and pH on survival of Listeria monocytogenes in buffer and smoked salmon

High pressure processing is a novel food preservation technology, applied for over 15 years in the food industry to inactivate spoilage and pathogenic microorganisms. Many studies have shown the differential resistance of bacterial cells to high pressure. Listeria monocytogenes is a bacterium able to grow at refrigerated temperature and to survive for a long time in minimally processed foods such as raw smoked fish. The freezing process does not cause significant decline of L. monocytogenes. The phase diagram of water under pressure permits a pressure treatment under subzero temperature, without the disadvantages of freezing for food quality. The aim of this study was to estimate if combined effects of pressure and subzero temperature could increase the destruction of L. monocytogenes in buffer and in smoked salmon. We investigated effects of high pressure processing (100, 150, and 200 MPa) combined with subzero temperatures (-10, -14, and -18 degrees C) and pH (7.0 and 4.5). Results showed that the most effective high-pressure treatment to inactivate L. monocytogenes was 200 MPa, -18 degrees C, and pH 4.5. The relevance of pressure holding time and the synergistic effect of pressure coupled with the subzero temperature to inactivate bacteria are highlighted. Modifications of physical properties (color and texture) were a lightening of color and an increase of toughness, which might be accepted by consumers, since safety is increased.