以胡枝子和糠醛渣为原料发酵制备乳酸工艺研究
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摘要
相比于传统法利用淀粉类原料发酵乳酸,木质纤维原料由于其价格低廉、可再生、无污染等优点已引起了越来越多的关注。本文以蒸汽爆破胡枝子和碱洗糠醛渣为实验原料,以提高乳酸产量和减少发酵成本为目的,探索乳酸发酵的最佳工艺条件。
蒸汽爆破预处理研究结果表明,蒸汽爆破过程中主要是半纤维素降解,纤维素少许降解,对木素含量几乎没有影响；而且降解剧烈程度随爆破压力的升高而增大,在爆破压力为2.0 MPa时,胡枝子中半纤维含量仅为4.7%,相比于原料中的29.3%,半纤维素脱除率达到85%。
正交设计实验结果表明蒸汽爆破胡枝子同步糖化发酵制备乳酸的最佳工艺条件为：温度43℃,纤维素酶加入量30FPU/g底物,碳酸钙加入量3%,底物浓度6%。且各因素的影响次序是底物浓度>温度>酶加入量>碳酸钙。不同蒸汽爆破条件下胡枝子同步糖化发酵制备乳酸结果表明,2.0MPa、4min的爆破条件胡枝子发酵乳酸产量最高,达到21.7g/L,是其理论产率的64.8%,相比于未处理的胡枝子发酵的乳酸产量8.0g/L,乳酸产率提高了1.7倍。
胡枝子蒸汽爆破过程中产生的抑制物主要有甲酸、乙酸、糠醛和香草醛。研究表明甲酸严重影响糖化和发酵,当甲酸浓度为0.5g/L时,纤维素转化率和乳酸产率分别减少19%和16.4%。而乙酸对糖化没有影响,却明显抑制发酵,乳酸产率最大减少17.6%：糠醛和香草醛相比有机酸抑制作用较小,对乳酸产率几乎没有影响,主要是延长发酵周期,当糠醛浓度为2.0g/L时,乳酸生产周期被延长36h。
皂荚皂素表面活性剂对糠醛渣糖化和发酵的研究结果表明,皂素最佳添加浓度为0.16g/L。皂荚皂素能轻微增加糠醛渣的水解率(4.4%),却明显促进乳酸发酵。首先提高最终乳酸产率,最大可以提高17.7%；其次减少纤维素酶用量,最大可以减少37.5%；此外可以维持发酵液中纤维素酶活力稳定和减少发酵周期；这对减少乳酸生产成本具有极大的指导意义。成本核算表明,加入皂荚皂素后,每生成lkg乳酸可减少成本0.513元,如果发酵液中的纤维素酶能够循环回收利用,那么每生成lkg乳酸可减少成本1.7元。所以用皂荚皂素来代替部分纤维素酶在经济上是可行的。
Compared to the traditional method for lactic acid production from starchy raw materials, lignocellulosic materials are considered to be the most promising renewable resources for lactic acid production due to its low cost, abundance in nature and free pollutiom. In this study, we try to find the optimal fermentation conditions for lactic acid production from steam pretreated lespedeza stalks and furfural residues to reduce the operational cost.
The results of lespedeza stalks composition analysis showed that steam pretreatment mainly caused the hemicellulose degradation. Sometimes slight cellulose was also degraded in the presence of high temperature and pressure (2.0MPa), while lignin content was hardly affected. The degree of hemicellulose degradation was proportional to the pressure of steam explosion. After 2.0 MPa of steam explosion, the content of hemicellulose was reduced from 29.3% to 4.7% compared with the raw material, relatively reduced by 85%.
The results of orthogonal experiments showed that the optimal conditions for lactic acid production by SSF were the substrate concentration of 6%, enzyme loading of 30 FPU/g, calcium carbonate addition of 3% and temperature of 43℃. The significance of the factors which influence the lactic acid yield was in the following order:substrate concentration>temperature>enzyme loading>addition amount of calcium carbonate. Lespedeza stalks with 2.00MPa pressure pretreatment were found to have the highest lactic yield of 21.7 g/L, which accounts for 64.8% of theory yield. The lactic acid yield was enhanced by 1.7 times than that of the untreated materials.
The inhibitors generated from steam pretreatment process mainly consist of acetic acid, formic acid, furfural and vanillin. Formic acid was the most serious inhibitor in saccharification and fermentation. The cellulose conversion rate and lactic acid yield was reduced by 19% and 16.4% resperctively in the presence of 0.5g/L formic acid. Acetic acid performed no impact on saccharification, but had significant inhibition on fermentation. The lactic acid yield was reduced by up to 17.6%. Effect of furfural and vanillin on lactic acid yield was relatively weak compared to that of organic acid, and the inhibition of them mainly focused on the lag time, resulting in the greatly prolonged fermentation period (36h in the presence of 2.0g/L furfural).
The results of influence of Gleditsia saponin on saccharification and fermentation of furfural residues showed that the optimal addition concentration of saponin was 0.16 g/L. Gleditsia saponin could slightly increase the hydrolysis rate of furfural residues (4.4%) and had positive influences on fermentation. Firstly, it could improve eventually lactic acid yield, which was increased by 17.7% at most. Secondly, it could also reduce cellulase loading, which was cut down up to 37.5%. Additionally, GS was useful to maintain the remaining enzyme activity stable in fermentation broth and reduced fermentation period. It was meaningful to reduce the overall cost of lactic acid production. The limited economic evaluation showed that the lactic acid production cost could be redcued by 0.513 yuan when lkg lactic acid was produced in the presence of GS. Furthermore, if it was possible to recover all the activity in the fermentation broth, the lactic acid production could be decreased by 1.7yuan. Therefore, it was economically feasible to use GS to replace partial cellulast for lactic acid production.
蒸汽爆破预处理研究结果表明,蒸汽爆破过程中主要是半纤维素降解,纤维素少许降解,对木素含量几乎没有影响；而且降解剧烈程度随爆破压力的升高而增大,在爆破压力为2.0 MPa时,胡枝子中半纤维含量仅为4.7%,相比于原料中的29.3%,半纤维素脱除率达到85%。
正交设计实验结果表明蒸汽爆破胡枝子同步糖化发酵制备乳酸的最佳工艺条件为：温度43℃,纤维素酶加入量30FPU/g底物,碳酸钙加入量3%,底物浓度6%。且各因素的影响次序是底物浓度>温度>酶加入量>碳酸钙。不同蒸汽爆破条件下胡枝子同步糖化发酵制备乳酸结果表明,2.0MPa、4min的爆破条件胡枝子发酵乳酸产量最高,达到21.7g/L,是其理论产率的64.8%,相比于未处理的胡枝子发酵的乳酸产量8.0g/L,乳酸产率提高了1.7倍。
胡枝子蒸汽爆破过程中产生的抑制物主要有甲酸、乙酸、糠醛和香草醛。研究表明甲酸严重影响糖化和发酵,当甲酸浓度为0.5g/L时,纤维素转化率和乳酸产率分别减少19%和16.4%。而乙酸对糖化没有影响,却明显抑制发酵,乳酸产率最大减少17.6%：糠醛和香草醛相比有机酸抑制作用较小,对乳酸产率几乎没有影响,主要是延长发酵周期,当糠醛浓度为2.0g/L时,乳酸生产周期被延长36h。
皂荚皂素表面活性剂对糠醛渣糖化和发酵的研究结果表明,皂素最佳添加浓度为0.16g/L。皂荚皂素能轻微增加糠醛渣的水解率(4.4%),却明显促进乳酸发酵。首先提高最终乳酸产率,最大可以提高17.7%；其次减少纤维素酶用量,最大可以减少37.5%；此外可以维持发酵液中纤维素酶活力稳定和减少发酵周期；这对减少乳酸生产成本具有极大的指导意义。成本核算表明,加入皂荚皂素后,每生成lkg乳酸可减少成本0.513元,如果发酵液中的纤维素酶能够循环回收利用,那么每生成lkg乳酸可减少成本1.7元。所以用皂荚皂素来代替部分纤维素酶在经济上是可行的。
Compared to the traditional method for lactic acid production from starchy raw materials, lignocellulosic materials are considered to be the most promising renewable resources for lactic acid production due to its low cost, abundance in nature and free pollutiom. In this study, we try to find the optimal fermentation conditions for lactic acid production from steam pretreated lespedeza stalks and furfural residues to reduce the operational cost.
The results of lespedeza stalks composition analysis showed that steam pretreatment mainly caused the hemicellulose degradation. Sometimes slight cellulose was also degraded in the presence of high temperature and pressure (2.0MPa), while lignin content was hardly affected. The degree of hemicellulose degradation was proportional to the pressure of steam explosion. After 2.0 MPa of steam explosion, the content of hemicellulose was reduced from 29.3% to 4.7% compared with the raw material, relatively reduced by 85%.
The results of orthogonal experiments showed that the optimal conditions for lactic acid production by SSF were the substrate concentration of 6%, enzyme loading of 30 FPU/g, calcium carbonate addition of 3% and temperature of 43℃. The significance of the factors which influence the lactic acid yield was in the following order:substrate concentration>temperature>enzyme loading>addition amount of calcium carbonate. Lespedeza stalks with 2.00MPa pressure pretreatment were found to have the highest lactic yield of 21.7 g/L, which accounts for 64.8% of theory yield. The lactic acid yield was enhanced by 1.7 times than that of the untreated materials.
The inhibitors generated from steam pretreatment process mainly consist of acetic acid, formic acid, furfural and vanillin. Formic acid was the most serious inhibitor in saccharification and fermentation. The cellulose conversion rate and lactic acid yield was reduced by 19% and 16.4% resperctively in the presence of 0.5g/L formic acid. Acetic acid performed no impact on saccharification, but had significant inhibition on fermentation. The lactic acid yield was reduced by up to 17.6%. Effect of furfural and vanillin on lactic acid yield was relatively weak compared to that of organic acid, and the inhibition of them mainly focused on the lag time, resulting in the greatly prolonged fermentation period (36h in the presence of 2.0g/L furfural).
The results of influence of Gleditsia saponin on saccharification and fermentation of furfural residues showed that the optimal addition concentration of saponin was 0.16 g/L. Gleditsia saponin could slightly increase the hydrolysis rate of furfural residues (4.4%) and had positive influences on fermentation. Firstly, it could improve eventually lactic acid yield, which was increased by 17.7% at most. Secondly, it could also reduce cellulase loading, which was cut down up to 37.5%. Additionally, GS was useful to maintain the remaining enzyme activity stable in fermentation broth and reduced fermentation period. It was meaningful to reduce the overall cost of lactic acid production. The limited economic evaluation showed that the lactic acid production cost could be redcued by 0.513 yuan when lkg lactic acid was produced in the presence of GS. Furthermore, if it was possible to recover all the activity in the fermentation broth, the lactic acid production could be decreased by 1.7yuan. Therefore, it was economically feasible to use GS to replace partial cellulast for lactic acid production.
引文
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