Browsing by Author "Aleksieva P."

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    Examination of Humicola lutea immobilized in sol-gel matrices: Effective source of α-galactosidase
    (2008-01-01) Spasova D.; Aleksieva P.; Nacheva L.; Kabaivanova L.; Chernev G.; Samuneva B.
    α-Galactosidase production by the fungus Humicola lutea 120-5 immobilized in a hybrid sol-gel matrix, consisting of tetraethylorthosilicate (TEOS) as a precursor and a mixture of polyethyleneglycol (PEG) and polyvinylalcohol (PVA), was investigated under semicontinuous shake flask cultivation and compared to the enzyme secretion by free cells. The influence of the carrier weight on the α-galactosidase biosynthesis in repeated batch experiments was followed. Best results were obtained with 2 g of the sol-gel particles per culture flask using 144-h runs. The growth behaviour of the immobilized mycelium during both the growth and productive phases was observed by scanning electron microscopy. The presence of abundant mycelial growth of intact hyphae correlated with a 2-fold higher enzyme activity compared to free cells. The obtained biocatalyst retained a high level of enzyme titer exceeding the activity of free cells during four cycles of operation (24 days). This result is confirmed by the micrographs showing the retained viability of the growing vegetative cells due to the protective role of the carrier. © 2008 Verlag der Zeitschrift für Naturforschung.
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    Immobilization in nanomatrices of humicola lutea mycelium for alpha-galactosidase biosynthesis in Laboratoryair-lift bioreactor
    (2010-05-01) Djambaski P.; Aleksieva P.; Spasova D.; Chernev G.; Nacheva L.
    The sol-gel synthesis of both hybrid nanomatrices containing tetraethylortosilicate (TEOS) as an inorganic precursor and lactic acid, or sepharose as an organic component was made. Crystal as well as surface morphology of the hybrids were investigated using different methods: X-ray diffraction, infrared spectra, BET-analysis and atomic force microscopy (AFM). The obtained nanomatrices were applicated for immobilization of the α-galactosidase producing fungal strain Humicola lutea 120-5. The semicontinuous cultivation was carried out in laboratory air-lift bioreactor. Maximal level of enzyme activity (1050-1130 U/l) that was reached in the third to fourth fermentation cycle using TEOS+40% lactic acid was higher than that obtained in the samples with TEOS+20% sepharose (660-770 U/l). The correlation between enzyme productivity and fungal development in the pore structure of the carriers was examined using scanning electron microscopy observation.
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    Sol-gel nanomaterials with algal heteropolysaccharide for immobilization of microbial cells, producing a-galactosidase and nitrilase
    (2009-01-01) Djambaski P.; Aleksieva P.; Emanuilova E.; Chernev G.; Spasova D.; Nacheva L.; Kabaivanova L.; Salvado I.M.M.; Samuneva B.
    The main purpose of the present work is the sol-gel synthesis and structure of the hybrid nanomaterials as matrices for two types of cells, producing hydrolytic enzymes. The effect of different percent of algal polysaccharide included in them on the hydrolytic activity of fungal and bacterial cells was investigated. The hybrid sol-gel nanomaterials were synthesized from tetraethylortosilicate (TEOS) as a silicon precursor and heteropolysaccharide (AHPS) from the red microalga Dixonella grisea as an organic part. The structure of these matrices was investigated using different methods: FT-IR, XRD, BET-Analysis, EDS, SEM and AFM. The sol-gel hybrids were used for the immobilization of fungal (Humicola lutea) and bacterial (Bacillus sp.) cells, producing α-galactosidase and nitrilase, respectively. It was established the effect of the quantity of the heteropolysaccharide in the matrices on the activity of these hydrolytic enzymes. Using 20% AHPS in the hybrid nanomaterials the α-galactosidase yield exceeded over two-fold the enzyme titre of the free cells in the third cycle of repeated batch shake flask cultivation. These results correlated with a dense growth of immobilized mycelium observed with scanning electron microscopy (SEM). The increase of the percentage of organic part in the sol-gel matrix up to 20% led to an increase in the nitrilase activity. The addition of 40% AHPS did not significantly affect the decrease of the nitrile biodegradation. © 2009 Taylor and Francis Group, LLC.