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- Author or Editor: G. Kiskó x
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The effect of lactic acid production of Lactobacillus casei on the growth characteristics of Listeria monocytogenes was examined in milk. The effect of temperature (7, 13, 20 °C) and the co-ratios with Lb. casei at different cell count ratios (10:1, 100:1, 10 000:1) on the growth of L. monocytogenes were studied. At a 10 000:1 ratio induction of an early stationary phase of L. monocytogenes occurred. An exponentially decreasing correlation was observed between the logarithmic maximum population of L. monocytogenes and the initial log counts of lactobacilli. The Baranyi model was fitted to the obtained growth curves. The Baranyi equation provided good fit for the lag and exponential phase of L. monocytogenes. According to our observations, pH decrease does not seem to be the main factor of early stationary phase induction of L. monocytogenes in milk.
Inactivation and removing efficiency of disinfectants (Descosal and Domestos) was tested against Pseudomonas aeruginosa and Candida albicans and their biofilms (2 and 72 h, 3, 7 and 14 days old, respectively). The survival of microbes was studied by pour plating and impedimetry, and the attachment by epifluorescent microscopy.In the surface test, both disinfectants were effective against both test organisms with no significant difference in their efficiency. The decreasing detection times during impedimetric measurements showed higher resistance of the older Ps. aeruginosa biofilms. Both disinfectants tested were fungicidal against C. albicans and bactericidal against Ps. aeruginosa but for biofilm removal Domestos was more efficient.
Zygosaccharomyces lentus is a significant, recently discovered new osmophilic spoilage yeast, capable of growth at low temperatures. It is remarkably resistant to commonly used preservatives above the legally permitted levels. Therefore, antimicrobial effect of chitosan, cinnamic acid, carvacrol and cymene as new preservatives was tested against Z. lentus in phosphate buffered saline at pH 7.3. Z. lentus was resistant to the antimicrobial action of chitosan (0.5 and 1.0%) and cinnamic acid (0.5–2.0 mM) but was sensitive to the biocidal action of 1.5 and 2.0 mM cymene and 1.5 and 2.0 mM carvacrol in pH-neutral buffer. Further work would need to be done to check the sensitivity of this organism to these compounds at other pH values and temperatures.
The efficiency of seven disinfectants (Divomil Forte, Nobactel, Domestos, SU 392, Buraton, Descosal, Cidex) was tested against Staphylococcus aureus ATCC 6538, Escherichia coli ATCC 8739, Pseudomonas aeruginosa ATCC 9027, Candida albicans ATCC 10231, Bacillus subtilis ATCC 6633, Aspergillus niger ATCC 16404. Surface test was used in order to evaluate the efficiency of disinfectants during the everyday sanitizing practice on special industrial surfaces. Test organisms represented pathogenic, spore forming bacteria, yeast and mould. Surface test was started with minimal concentration of agents and was not increased above the maximal concentrations, which were recommended by manufacturers in order not to corrode surfaces or risk the safety of use. Test organisms were inoculated on test areas and after drying inoculated surfaces were treated with disinfectants. Seven disinfectants were tested and four were effective against every test organism. (Buraton: 1%, Descosal: 1%, Cidex: 100%, SU 392: 75%.) Two disinfectants were ineffective against Aspergillus niger (Nobactel: 2%, Domestos: 2%) and 1 against Staphylococcus aureus and Aspergillus niger (Divomil Forte: 2%).
In the industrial practice Howard mould count (Howard, 1911) is used for the estimation of mould contamination of foods. It was developed originally for the investigation of mould contamination of tomato purée. It is also used nowadays for quality control purposes for other food products as well. Recently this constitutes the basis of the acceptance of the finished products in international trade. This technique demands experts with a lot of practice and morphological proficiency. The investigation makes use of eyes and so the tiredness of the investigator can cause uncertainties.The possibility of other methods for the determination of mould contamination of tomato purée was investigated to replace the Howard method. The NIR technique — as a rapid, non-destructive, reagentless and accurate method — was anticipated as a suitable method for the mentioned purpose. Canned tomato purée had been allowed to become mouldy then the sample was blended with non-mouldy samples in different ratio, so a series of tomato purées containing known amounts of mouldy purée was prepared. Howard mould counts and ergosterol content — another mould contamination relating value — was used as reference for NIR calibration.At quantitative investigation better results were obtained using ergosterol values. The best correlation coefficient (R=0.93) and the smallest standard error of calibration (SEC=0.008 mg g−1 ergosterol) was achieved with triangular smoothing and second derivation of the spectra. At qualitative investigation Polar Qualification System (PQS) was used. Clusters between samples with low and high ergosterol levels could be separated.
Laboratory batches of fresh tomato juices were treated in several experimental trials by high hydrostatic pressure alone or in combination with various concentrations of oregano, thyme or dill seed oils. Lactic acid bacteria formed the dominating component of the spoilage microbiota during post-processing storage at 15 °C causing spoilage of the untreated samples within 4 days. One tenth of a percent oregano or thyme oils at least doubled the microbiological shelf life, while their respective concentrations of 0.5% alone, or 400 MPa 5-20 min high hydrostatic pressure treatment alone resulted in microbial stability for at least two weeks. Two hundred MPa for 10 min resulted only in an approx. 3 days delay of spoilage, whereas 0.1% thyme oil increased the efficiency of this moderate UHP-treatment, resulting in a microbiologically stable product for at least 3 weeks at the storage temperature applied.
Applying antagonistic yeasts is one of the recent possibilities for controlling postharvest disease caused by blue mould (Penicillium expansum). In this work, antagonistic activity of several Kl. lactisstrains was tested against two strains of P. expansum. Three strains of Kl. lactiswere compared to three biocontrol yeasts: Metschnikowia pulcherrima, Sporobolomyces roseusand Pichia anomala. The investigations were carried out at 25 °C, 15 °C and 5 °C, applying different yeast cell densities and culture media. Statistical analysis showed no significant differences among the three Kl. lactisstrains. The inhibitory effect of the tested yeasts was different according to the applied mould strain temperature, culture medium and cell density. Application of antagonistic yeasts combined with reduced temperature enhanced the inhibitory effect. Direct relationship was observed between increasing cell density and the biocontrol efficiency of Kl. lactis. According to this work, Kl. lactisis a possible biocontrol agent.
Numerous yeasts are reported as being effective in controlling plant pathogenic moulds. By selecting new biocontrol agents, knowledge about the mode of action of mould inhibition is important. In our study, mode of action of Kluyveromyces lactis - successfully applied against Penicillium expansum in former studies - was investigated. According to the results, volatile compounds are supposed to play a role in restriction of mould growth. Antibiotic substances and killer toxins produced by the tested Kl. lactis strain were not detected.
Fruits and vegetables are increasingly consumed as a part of healthy diets. They are routinely consumed raw, without any further antimicrobial processing. The aim of our studies was to determine radiation doses improving the microbial safety of fruits without diminishing quality parameters of these produce. Effects of low dose irradiation on the microbiota, antioxidant capacity, total polyphenol contents, firmness and sensory properties of pre-cut apple, orange and banana were investigated. Dose of 2.0 kGy was able to reduce considerably the microbiological contamination of fruits, apple, orange, banana, but microorganisms surviving the irradiation are able to recover and grow during refrigerated storage. Two kGy was an acceptable radiation dose for the treatment of these products, having no significant effect on the mentioned quality parameters.
Nigella sativa L. (black cumin) is well known for its benefits in the field of traditional medicine. The aim of this study was to determine the chemical composition and investigate the antimicrobial activity of cold pressed oil (CO) and essential oil (EO) of Nigella sativa L. on food-borne pathogenic and spoilage bacteria. The microdilution method was used to determine the minimal inhibitory concentration (MIC) of Nigella sativa crude oil (CO) and essential oil (EO) against 4 Gram-positive (Bacillus cereus, Bacillus subtilis, Staphylococcus aureus, and Listeria monocytogenes) and 3 Gram-negative (Salmonella Hartford, Pseudomonas aeruginosa, and Escherichia coli) foodborne pathogenic and spoilage bacteria occurring in food products. Total fatty acid composition of CO was analysed by GLC, while the EO was analysed by GC-MS to detect its active compounds. The results showed that the major fatty acid of CO was palmitic acid (C16:0), as saturated fatty acid, however, linoleic acid (C18:2) was the main unsaturated fatty acid. The major compounds of the EO were p-cymene and thymoquinone. The inhibition on all tested bacteria of EO was 10 times higher than of CO, and the lowest concentration value was observed in case of Bacillus subtilis (0.003%). Hence, results reinforce the ambition to apply Nigella sativa oils in food as natural preservative.
