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Browsing by Subject "products of metabolism"

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  • Hiippala, Kaisa (2014)
    The literature review dealed with the energy metabolism of Propionibacterium freudenreichii with a particular focus on carbon substrates and metabolic end-products. Also, carbon sources and other factors affecting the efficiency of vitamin B12 production by Propionibacterium were studied, including different food related matrices as a source of growth substrates. Cereal matrix as a possible growth substrate was discussed. The aim of the experimental part was to study the effect of different carbon sources on the B12 yield and metabolism of three P. freudenreichii strains of dairy or cereal origin. After preliminary screening, five carbon sources (lactate, glucose, lactose, fructose and inositol) and three strains were selected for the fermentation process, which consisted of anaerobic (72 h) and aerobic incubation (96 h) at 30 °C. The phosphate-buffered medium (0.1 M) contained only tryptone, yeast extract, CoCl2 and a carbon source (pH 6.8). Optical densities and pH values were measured at two time points and cell masses after the fermentation. Vitamin B12 produced by the strains was extracted from the fermentation samples and analysed using UHPLC method. Also consumed riboflavin was determined from the culture broths with UHPLC. Carbon consumption and metabolic end-products were analysed with HPLC after both stages of fermentation. Differences in fermentation and growth patterns between the strains were observed depending on the carbon source. Strain 1 and 2 produced the highest amounts of cell mass (396 and 589 mg, fw) fermenting fructose. Moreover, the carbon consumption was slower with over half of fructose remaining in the medium after the anaerobic phase. In addition, the pH value did not drop as quickly as in the other sugar media. When B12 yield was measured per gram cells, lactate was the most profitable carbon source for strains 1, 2 and 3 (100, 115 and 58 ?g/g cells, respectively). Yet, strains 1 and 2 synthesised the most vitamin B12 catabolising fructose when the yield was calculated per mL medium (958 and 731 ng/mL). In fructose medium both strains also produced less inhibitory metabolite, propionate. Strain 3 grew poorly during the fermentation, which affected the metabolite production. As a conclusion, fructose, besides lactate and glucose, appeared as a suitable carbon substrate for the B12 production by P. freudenreichii but the effect of fructose on its metabolism should be further investigated.