Modeling the effect of temperature on survival rate of Listeria monocytogenes in yogurt

The aim of the study was to (i) evaluate the behavior of Listeria monocytogenes in a commercially produced yogurt, (ii) determine the survival/inactivation rates of L. monocytogenes during cold storage of yogurt and (iii) to generate primary and secondary mathematical models to predict the behavior of these bacteria during storage at different temperatures. The samples of yogurt were inoculated with the mixture of three L. monocytogenes strains and stored at 3, 6, 9, 12 and 15°C for 16 days. The number of listeriae was determined after 0, 1, 2, 3, 5, 7, 9, 12, 14 and 16 days of storage. From each sample a series of decimal dilutions were prepared and plated onto ALOA agar (agar for Listeria according to Ottaviani and Agosti). It was found that applied temperature and storage time significantly influenced the survival rate of listeriae (p<0.01). The number of L. monocytogenes in all the samples decreased linearly with storage time. The slowest decrease in the number of the bacteria was found in the samples stored at 6°C (D-10 value = 243.9 h), whereas the highest reduction in the number of the bacteria was observed in the samples stored at 15°C (D-10 value = 87.0 h). The number of L. monocytogenes was correlated with the pH value of the samples (p<0.01). The natural logarithm of the mean survival/inactivation rates of L. monocytogenes calculated from the primary model was fitted to two secondary models, namely linear and polynomial. Mathematical equations obtained from both secondary models can be applied as a tool for the prediction of the survival/inactivation rate of L. monocytogenes in yogurt stored under temperature range from 3 to 15°C, however, the polynomial model gave a better fit to the experimental data.

High pressure processing for food safety

Food preservation using high pressure is a promising technique in food industry as it offers numerous opportunities for developing new foods with extended shelf-life, high nutritional value and excellent organoleptic characteristics. High pressure is an alternative to thermal processing. The resistance of microorganisms to pressure varies considerably depending on the pressure range applied, temperature and treatment duration, and type of microorganism. Generally, Gram-positive bacteria are more resistant to pressure than Gram-negative bacteria, moulds and yeasts; the most resistant are bacterial spores. The nature of the food is also important, as it may contain substances which protect the microorganism from high pressure. This article presents results of our studies involving the effect of high pressure on survival of some pathogenic bacteria -- Listeria monocytogenes, Aeromonas hydrophila and Enterococcus hirae -- in artificially contaminated cooked ham, ripening hard cheese and fruit juices. The results indicate that in samples of investigated foods the number of these microorganisms decreased proportionally to the pressure used and the duration of treatment, and the effect of these two factors was statistically significant (level of probability, P

High pressure processing for food safety

Food preservation using high pressure is a promising technique in food industry as it offers numerous opportunities for developing new foods with extended shelf-life, high nutritional value and excellent organoleptic characteristics. High pressure is an alternative to thermal processing. The resistance of microorganisms to pressure varies considerably depending on the pressure range applied, temperature and treatment duration, and type of microorganism. Generally, Gram-positive bacteria are more resistant to pressure than Gram-negative bacteria, moulds and yeasts; the most resistant are bacterial spores. The nature of the food is also important, as it may contain substances which protect the microorganism from high pressure. This article presents results of our studies involving the effect of high pressure on survival of some pathogenic bacteria -- Listeria monocytogenes, Aeromonas hydrophila and Enterococcus hirae -- in artificially contaminated cooked ham, ripening hard cheese and fruit juices. The results indicate that in samples of investigated foods the number of these microorganisms decreased proportionally to the pressure used and the duration of treatment, and the effect of these two factors was statistically significant (level of probability, P <or= 0.001). Enterococcus hirae is much more resistant to high pressure treatment than L. monocytogenes and A. hydrophila. Mathematical methods were applied, for accurate prediction of the effects of high pressure on microorganisms. The usefulness of high pressure treatment for inactivation of microorganisms and shelf-life extention of meat products was also evaluated. The results obtained show that high pressure treatment extends the shelf-life of cooked pork ham and raw smoked pork loin up to 8 weeks, ensuring good micro-biological and sensory quality of the products.

Survival of Enterococcus hirae in ripened cheese subjected to ultra high pressure

The samples of a sliced, hard cheese were placed in commercially used polyamide-polyethylene bags, inoculated with E. hirae and sealed under vacuum. The samples were exposed to high hydrostatic pressure of 300, 400 and 500 MPa for 5, 10 and 15 min. The number of surviving E. hirae and Aerobic Plate Counts (APC) were determined. The bacterial counts were transformed to logarithms and D-values (time required for decimal reduction of E. hirae at given pressure) were calculated using linear regression method. It was found that numbers of E. hirae decreased with the increase of pressure and prolongation of treatment time. D-values amounted to: D300MPa - 33.67 min, D400MPa - 17.83 min and D500MPa - 16.03 min. The obtained results indicate that E. hirae is one of the most resistant vegetative bacteria to ultra high pressure treatment.

[The role of halophilic bacteria in decarboxylation of histidine in salted fish]

The purpose of this study was to determine frequency of occurrence of halotolerant and halophilic histamine-forming bacteria under laboratory conditions in salted herring stored at 20 degrees C for 2 months. The other aim was to isolate these bacteria both from herring bought in a retail shop and examine the ability of decarboxylation of histidine by these microorganisms using Karnop's method modified by the authors. The amount of histamine formed was determined by fluorometric method. The level of histamine in media containing homogenates of herring was in a range from 5 to 1950 micrograms/100 ml of a nutrient medium. All isolated bacteria belong to halotolerant and halophilic strains and produce histamine both at 8% and 20% content of NaCl in a nutrient medium. The obtained results indicate that during storage of salted herring at ambient temperature the increase of histamine content in their flesh can occur.