Fermentation of wheat starch hydrolysate by Lactococcus lactis: Factors affecting product formation
Abstract: Lactic acid exists as two optical isomers, D- and L-lactic acid. For the production of biodegradable poly-lactic acid an optically pure product is desirable, but a racemate of known and constant ratio can also be used. Lactic acid can be produced either synthetically or fermentatively. Fermentative production has some advantages: by choosing a lactic acid bacterium producing only one of the isomers an optically pure product is obtained. It is also possible to use renewable resources as substrates, such as starch and cellulose. The greatest drawback is the high purification costs. There is a rich supply of wheat in Sweden, and wheat starch fractions without technically exploitable polymer qualities can be used for fermentative lactic acid production. Fermentative lactic acid production from starch comprises the following steps: pretreatment, hydrolysis, fermentation, separation, and purification. The work presented in this thesis was focused on the optimisation of the fermentation step using whole wheat flour hydrolysate as a substrate. Fermentations using various selected strains of lactic acid bacteria were compared in whole wheat flour hydrolysate. Lactococcus lactis ssp. lactis ATCC 19435 was chosen for further studies due to its production of optically pure L-lactic acid and for its high productivity in whole wheat flour not supplemented with nutrients. The kinetics of whole wheat flour fermentation, as well as that of simultaneous saccharification and fermentation of whole wheat flour, at various pH values, temperatures, starch and lactic acid concentrations using strain 19435 were elucidated. A model describing the kinetics was developed and used to compare the two process alternatives. Simultaneous saccharification and fermentation was shown to be slightly faster than when the two steps were performed separately, but nutrients in the form of amino acids had to be added to maintain high productivity at the desired flour concentration. The effects of pH and temperature on growth and product formation of strain 19435 were further investigated on glucose and maltose in semi-defined media. The results showed that a suboptimal pH and temperature may be used to enhance the yield of lactic acid over by-products and cell mass, at the cost of lower productivity.
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