| Much attention has recently been paid to the bulk bio-based chemicals, including1,3-propanediol(1,3-PD) and lactic acid, which have tremendously applied in cosmetics, foods, medicines and especially in the production of novel polymers.1,3-Propanediol and lactic acid can be used as the monomer to synthesize polytrimethylene terephthalate (PTT) and polylactic acid (PLA), respectively, which will lead to prospective market. At present, pursuiting of the single product via microbial production is limited by low utilization efficiency of raw materials and the waste of by-products. In the previous research, the by-products in bioconversion of glycerol to1,3-PD by Klebsiellia pneumonia include2,3-butanediol, lactic acid, succinic acid, acetic acid and ethanol. The yield of1,3-PD on glycerol is less than50%. In order to increase the utilization ratio of raw material and reduce the production cost, this paper is focused on improving the yield of1,3-PD and lactic acid. The strategies, such as adjusting the pH of the fermentation process with sodium carbonate, increasing partial pressure of carbon dioxide in the fermentor, adding succinic acid and changing the raw materials, were used to achieve the combined production of1,3-propanediol and lactic acid with improving the total yield.Firstly, fed-batch fermentations for the coproduction of1,3-PD and lactic acid by K. pneumoniea were carried out by adjusting the pH with sodium carbonate during fermentation, using sodium hydroxide as control. The results demonstrated that adjusting the pH with sodium carbonate was not only beneficial to promote cell growth, but also increased1,3-PD and lactic acid production. The highest optical density (OD) raised to13.6, which was25%higher than the control. The concentractions of1,3-propanediol and lactic acid were79.48g/L and41.97g/L at33h, increasing by38%and28%respectively, compared to the control. The yields of1,3-PD and lactic acid were up to41.46%and21.89%respectively. The total yield of1,3-PD and lactic acid reached63.35%increasing by4%compared to the control.Secondly, based on adjusting the pH with sodium carbonate, the metabolic regulation of the conversion of glycerol to1,3-PD and lactic acid was investigated by increasing partial pressure of carbon dioxide in the fermentor, adding succinic acid and changing the raw materials. When the partial pressure of carbon dioxide in the fermentor was adjusted to4kPa (pressure in meter), the highest OD was9.6, the concentrations of1,3-PD and lactic acid were52.81g/L and56.02g/L respectively, the yields of1,3-PD and lactic acid were34.70%and 36.85%respectively and the total yield was71.55%. When succinic acid was added, the highest OD was10.54, the concentrations of1,3-propanediol and lactic acid were52.11g/L and54.85g/L respectively, the yields of1,3-PD and lactic acid were33.83%and35.62%respectively and the total yield was69.45%. When crude glycerin was used as raw material for fermentation, the highest OD was9.61, the concentrations of1,3-PD and lactic acid were46.67g/L and56.13g/L. The yields of1,3-PD and lactic acid were29.69%and35.71%respectively and the total yield was65.40%. The results revealed that designed strategies were beneficial to improve the total yield of1,3-PD and lactic acid, although against cell growth and1,3-PD accumulation.Finally, based on the previous research obtained by salting-out extraction of1,3-PD from fermentation broths efficiently, the feasibility of salting-out extraction of lactic acid was studied to lay the foundation for the separation of the combined production of1,3-propanediol and lactic acid. Different systems composed of hydrophilic organic solvents/inorganic salts were investigated for the extraction ability of lactic acid. The results showed that the systems composed of dipotassium hydrogen phosphate/methanol and dipotassium hydrogen phosphate/ethanol appeared to be favorable. The partition coefficient and recovery of lactic acid reached up to4.01and86.0%for the system composed of25%(w/w) dipotassium hydrogen phosphate and26%(w/w) methanol, when the concentration of lactic acid is167g/L in the fermentation broth. For the system of14%(w/w) dipotassium hydrogen phosphate and30%(w/w) ethanol, the partition coefficient and recovery of lactic acid reached up to3.23and90.6%. The removal ratio of glucose, cells and proteins reached67.3%,100%and85.9%, respectively. The experimental results indicated that the salting-out extraction was also available for the separation of lactic acid, which can provided a possibility for multistep salting-out of1,3-PD and lactic acid from glycerol-based fermentation broths... |
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