Investigation of exopolysaccharide production by lactic acid bacteria.
Jones, Rachael Ann
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This thesis is an investigation into the production of exopolysaccharides (EPS) produced by strains of Lactobacillus delbrueckii ssp. bulgaricus and Lactococcus lactis ssp. cremoris. These are used in the dairy industry for the production of yoghurt and fermented drinks products. For many years EPS producing lactic acid bacteria have been used by the dairy industry as a thickening agent in the production of yoghurt. However, this EPS producing trait is unstable and is readily lost which can cause an alteration in the texture of the final product. It was found that all the strains of Lb. delbrueclcii ssp. bulgaricus and Le. /aetis ssp. eremoris produced quantities of EPS that could be used for further analysis. They were found to be in the molecular weight range of 6.6 x 1 06g /mol to 1.26 x 1011 glmol and were composed of different quantities of glucose, galactose and rhamnose. Temperature, carbon source and shaking all affected the quantities of EPS produced by all strains of Le. laetis ssp. eremoris. The firmness and viscosity of fermented milks produced by strains of Lb. delbrueckii ssp. bulgaricus were higher than those produced by strains of Le. laetis ssp. cremoris indicating that firmness and viscosity are not solely related to the levels of EPS production. A 40kb plasmid was found in all strains of Le. /aetis ssp. cremoris that could potentially contain the genes for EPS production. The plasmid could not be removed using elevated temperature or with the addition of acriflavin. Fourier transform infrared spectroscopy (FTIR) showed that it was possible to differentiate different strains based on their spectra and that differences were found in the protein and EPS regions of the spectra. It was also established that the age of culture, whether the growth medium was liquid or solid and the carbon source of the growth media had an effect on the FTIR spectra produced and the ability to differentiate between strains. There is further potential to develop this technique to provide a quick and easy method of identifying strains of lactic acid bacteria and monitor their EPS producing ability.