Filamentous moulds are the main spoilage microorganisms, responsible for significant economic losses and several healthy risks in human food chain. The lactic acid bacteria (LAB), especially lactobacilli could be a natural antagonist of these dangerous organisms. In Bulgaria, a very limited data exists on the antifungal activity of LAB microbiota of fermented dairy products. In the present study, four active strains were isolated from traditional fermented curd/yogurt-like product "katak", produced in Bulgaria from centuries. The new isolates KR3, KR4, KR51 and KR53 were identified by API 50 CH biochemical test and different molecular methods (species-specific PCR, RAPD-PCR and 16S rDNA sequence analysis) as Lactobacillus brevis. According to our knowledge, this is the first data on the molecular characterization of the Lactobacillus microbiota of "katak". A broad spectrum of antifungal activity of the four L. brevis KR strains against test-cultures representatives of carcinogenic, toxigenic, deteriorative and allergenic fungi from the genera Aspergillus, Fusarium, Penicillium and Trichoderma was estimated. Strains L. brevis KR3, KR4 and KR51 completely suppress the growth of Penicillium claviforme, Aspergillus awamori and Aspergillus niger. With regard to Aspergillus flavus and Trichoderma viride, a lower and strain-specific inhibitory activity was observed. The antifungal activity of our new L. brevis isolates seems to be a promising advantage of these four strains, suggesting their potential applications in different food technologies as bio-preservative agents against moulds.

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In order for the beneficial effects of sourdough application in breadmaking to take place a proper selection of lactic acid bacteria species and strains, an appropriate technology and effective control of the purity and activity of the selected cultures. Four symbiotic starters for sourdough for the production of bread were developed and probated in a production laboratory using the selected strains Lactobacillus brevis LBRZ7, L. buchneri LBRZ6, L. plantarum X2, L. paracasei RN5, L. sanfranciscensis R and L. fermentum LBRH10 and the probiotic strain Propionibacterium freudenreichii ssp. shermanii NBIMCC 327. The starter sourdoughs that include Propionibacterium freudenreichii ssp. shermanii NBIMCC 327 had greater antimicrobial activity against saprophytic microorganisms: Bacillus subtilis, B. mesentericus, Aspergillus niger, Penicillium sp. and Rhizopus sp., but none of them inhibited the growth of bakery yeasts Saccharomyces cerevisiae. It was established that in order to prevent bacterial spoilage 10% of the selected starter sourdoughs had to be added in the breadmaking process, while for prevention of mold spoilage the necessary amount of starter sourdough had to be between 15% and 20%.The application of the developed starters for the production of wheat bread guarantees longer shelf life and no adverse alterations in the features of the final bread.

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In recent years interest in the probiotic lactobacilli has been stimulated by the use of these bacteria in products that are claimed to confer health benefits on the consumer. The probiotic effects are usually strain-specific, meaning that a correct identification is important to link the strain to the specific health effect. Taxonomical characterization of probiotic strains only in phenotypic and phisyological characteristics is often with low level of discrimination, probably due to their co-evolution in the same ecological niches. Thus, the nucleotide base techniques provide an accurate basis for phylogenetic analysis and identification. With this aim a probiotic strain Lactobacillus spp. 50P1 was studied. Trough the physiological characterization with API 50 CH the strain 50P1, was determinate as Lactobacillus helveticus, with low probability-77,5%. Three PCR-based methods, species-specific PCR, of 16S rRNA gene sequencing, and restriction enzyme of 16S rDNA ARDRA analysis were used for reliable taxonomic characterization of probiotic strain Lactobacillus sp. 50P1. The sequence obtained from the strain was compared to those of Lactobacillus species held in GenBank and the belonging of the strain 50P1 to the species Lactobacillus helveticus was confirmed.

The applicability of two lactic acid bacterial strains with probiotic potential and bioprotective properties as additions in the starter culture in yogurt fermentation was examined. The studied strains, Lactobacillus delbrueckii subsp. bulgaricus KZM 2-11-3 and Lactiplantibacillus plantarum KC 5-12, inhibited the growth of Kluyveromyces lactis, Kluyveromyces marxianus, and Saccharomyces cerevisiae. The strain L. delbrueckii subsp. bulgaricus KZM 2-11-3 directly inhibited Escherichia coli. The important characteristics for the quality of the yogurt product, such as physicochemical parameters during fermentation and storage, rheological characteristics, and sensory changes during the storage of samples were determined. The yogurt samples with the strains did not differ in most parameters from the control yogurt with the commercial starter. The added strains showed stable viability in the yogurt samples during storage. The yogurt sample with L. delbrueckii subsp. bulgaricus KZM 2-11-3 and the sample with both strains based on the total evaluation were very similar to the control yogurt with the commercial starter. Using these strains as probiotic supplements to enrich the starter cultures in yogurt production will contribute to developing new products with benefits to human health.

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