柠檬酸-沼气双发酵耦联工艺中钠离子的影响

On the effect of the sodium ions on the citric acid fermentation in the integrated citric acid-methane fermentation process

提出了柠檬酸-沼气双发酵耦联工艺,用于处理柠檬酸发酵过程中产生的废水,柠檬酸废水经厌氧消化处理后作为配料水回用于下一批次的柠檬酸发酵,从而实现废水零排放。厌氧水中超过1 000mg/L的Na^+在回用过程中会严重抑制柠檬酸发酵,柠檬酸产量为118.5 g/L,较空白下降了15.6%。单因素试验表明,Na^+对柠檬酸发酵的抑制质量浓度为200 mg/L。通过对发酵过程中糖化酶、异麦芽糖酶及柠檬酸合成酶、异柠檬酸脱氢酶等胞内关键酶活力变化的分析,初步探讨了其影响机理。结果表明,高达1 000 mg/L的Na^+会导致黑曲霉胞内异柠檬酸脱氢酶活力升高,柠檬酸代谢活力增强。同时,培养基中糖化酶和异麦芽糖酶活力受到抑制,发酵结束时部分糊精和异麦芽糖不能被完全降解,残总糖质量浓度升高至30 g/L,黑曲霉可利用的糖浓度减小,柠檬酸合成速率和柠檬酸产量降低。加入过量糖化酶可以有效减弱高质量浓度Na^+的抑制作用,但柠檬酸产量仍然低于去离子水发酵。

The present paper intends to propose an improved integrated citric acid-methane fermentation method to solve the problem of sewage treating in citric acid industry. According to the proposed processing method,the sewage under processing has to be treated by an anaerobic digestion so as for the sewage to be able to be recycled to make a mash for the next batch of citric acid fermentation and eliminate the sewage discharge with the reduction of water resource consumption. As a matter of fact,metal ions is a kind of restrictive factor for the citric acid fermentation with their concentrations in the anaerobic digestion effluent detected by the flame atomic absorption spectrometry. And,then,we can confirm that more than 1 000 mg/L Na＋laden in the anaerobic digestion effluent would come up to be the major inhibitor in the recycling process,which can help to produce 118. 5 g/L of the citric acid tions and then tends to decrease by 15. 6% as compared with the fermentation with the tap water. Our investigation of the effect of Na＋on the citric acid fermentation has indicated that the citric acid fermentation has no effect on the concentration of Na~＋in the medium when its temperature remains below 200 mg/L. However,the increase of Na＋concentration tends to lead to the significant decrease of citric acid production. Furthermore,we have investigated the activity of the glucoamylase,isomaltase,citrate synthase,isocitrate dehydrogenase and some other endoenzymes of Aspergillus niger that contribute to the citric acid fermentation in hoping to disclose the influential mechanism of the high concentration of Na~＋and make out that the content of 1 000 mg/L Na＋in the fermentation media is likely to lead to the increase of the activity of isocitrate dehydrogenase and acceleraqtion of the ciric acid metabolism. At the same time,the activity of glucoamylase and isomaltase in the fermentation process tends to be inhibited so as to cause the incomplete degradation of dextrin and isomaltose. And,in so doing,the residual total content of sugar in the medium in the end of fermentation may tend to approach to 30 g/L,which indicates that the available sugar content comsumed for the citric acid fermenatation may get reduced with the significant decrease of the citric acid synthesis rate and citric acid production. Thus,with the excessive glucoamylase being added to the fermentation medium,the inoculation and citric acid production can be improved,though the process remains still slower than the fermentation with the tap water.