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Anaerobic fermentation of whey : acidogenesis Kisaalita, William Ssempa


Based on the initial exploratory results of single-phase (acidogenesis and methanogenesis takes place in one vessel) whey biomethanation studies, a two-phase (acidogenesis and methanogenesis takes place in two separated serial vessels) biomethanation process was found to be more suitable for dealing with the current whey utilisation and/or disposal problem. Acidogenesis was found to be less understood in comparison to methanogenesis and therefore acidogenesis became the central problem of this thesis. Given that 90% of the five-day biochemical oxygen demand in whey is due to lactose, continuous culture (Chemostat) experiments were undertaken to examine the general mechanism of lactose acidogenesis by a mixed undefined culture using ¹⁴C-labeled tracers. Also the influence of whey protein (mainly β-lactoglobulin) on the general fermentation scheme was addressed. Experimental factors included a pH range of 4.0 to 6.5, a mesophilic temperature of 35°C and a dilution rate (D) range of 0.05 to 0.65 h⁻¹. At a fixed pH level, the observed variability in the main acidogenic end products (acetate, propionate, butyrate and lactate) with respect to D were found to be a consequence of the systematic separation of the various microbial groups involved in acidogenesis. Batch incubation of a [¹⁴C(U)]-lactate tracer with chemostat effluent samples and preparative separation of the end products followed by a liquid scintillation assay of the location of the radio activity demonstrated that a microbial population lactate to other end products and hence the observed increase in lactate concentrations at high D values. Further use of [¹⁴C(U)]-butyrate and [¹⁴C(2)]-propionate revealed the predominant carbon flow routes from pyruvate to the various end products. A qualitative lactose acidogenic fermentation model was proposed, in which lactose is converted to pyruvate via the Embden-Meyerhof-Parnas pathway. Pyruvate in a parallel reaction is then converted to lactate and butyrate. In the presence of hydrogen reducing methanogens lactate is converted to acetate in a very fast reaction and not propionate as previously believed-. The implications of these findings with regard to optimising the acidogenic phase reactor are discussed. Acidogenic fermentation of protein together with lactose did not affect the carbon flow scheme. In the D range of 0.05 to 0.15 h⁻¹ low pH (pH < 5.0) was found to favour the butyrate route at the expense of the lactate route and at high pH (pH > 5.5) the lactate route was favoured at the expense of the butyrate route, the pH region of 5.0 to 5.5 being the transition range. In order to describe the microbial growth, the Monod chemostat model was chosen among the various alternatives, because of its simplicity and its physico-chemical basis. The estimated model parameters are reported.

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