| A self-flocculating yeast strain SPSC01, a fusant from Saccharomyces cerevisiae and Schizosaccharomyces pombe and for ethanol production, was selected as a model strain in this study, and a new instrument, the focused beam reflectance measurement analyzer (FBRM) developed by Laser-Tech (Redmond, WA, USA), was used for the on-line monitoring of the flocs.Using a simulating system composed of the floc-buffer suspensions, the total floc chord length counts per second were directly correlated with the floe biomass concentrations so that the floc biomass concentrations could be in situ detected. Furthermore, a characterization method of the floes was established by properly weighted treatments of the detected floe chord length distributions.The impact of temperature, pH, Na~+, Ca~++, Mg~++, glucose and ethanol concentrations on the flocculating of SPSC01 was investigated. It was found that the de-flocculating of SPSC01 flocs occurred when temperature increased to 70 ℃, which provides a practical method for industry to process yeast by-product. The desirable flocculating of SPSC01 could be maintained when pH was controlled at 3.5-6.0. Glucose and sucrose did not exerted negative impact on flocculating of SPSC01. Meanwhile, xylose and maltose slightly decreased the size of SPSC01 flocs, and mannose almost de-flocculated the flocs. The flocculating of SPSC01 was promoted by a low concentration of Ca~++, or by a relatively high concentration of Mg~++. The influence of Na~+ was different. Citric acid that can chelate Ca~++ decreased the size of SPSC01. The lower level of Ca~++ or ethanol was beneficial to the flocculating of SPSC01, but the inverse impact was observed when the level of Ca~++ was increased to about 10 mmol.l~-1 or the level of ethanol was increased to about 15% (v/v). Both the zymolectin hypothesis and the calcium bridge hypothesis weresupported by these experimental results.The intrinsic kinetics of yeast cell growth and ethanol fermentation of SPSC01, was investigated by means of mechanically dispersing and on-line monitoring techniques developed, through which the self-flocculating yeast cells were dispersed into micrometer scales from their original millimeter scales without any potentially negative impacts on their physiological metabolism, but effectively eliminating the intra-particle mass transfer limitations. It was found and validated that CO2 produced during ethanol fermentation exerted significant disturbances on detected signals. By applying 1/length-weighted treatment and floe chord length stimulation, these disturbances were effectively overcome. Both the substrate and product inhibitions previously reported for freely suspended yeast cell ethanol fermentations were also observed for this self-flocculating yeast strain when high gravity media was fed and high level ethanol achieved. The intrinsic kinetic expressions for yeast cell growth and ethanol fermentation were developed based on continuous culture theories and experimental work reported previously. Model parameters were directly obtained from experimental measurement and evaluated through numerical calculations.0-584-Q (l _ ￡jL)?* + 0.004 for growth 125(l 0.155+ Cs+C//l 60.7 1251 9Q8 Cv = .(1 0.427 + CS+C5 2/366.7 125 C(1 - ^L)1-72 + 0.060 for ethanol formation 125Finally, the effect of floe size on the observed bioreaction rates for substrate (glucose) uptake and product (ethanol) formation was quantitatively examined. No significant mass transfer resistance was observed for the floes with a size smaller than 100|j.m, and the mass transfer resistance started to present when the size of SPSC01 floe was beyond this...
|
PDF Full Text Request