An Efficient Bioprocess Of Succinic Acid Production By Using Fibrous Bed Bioreactor

Succinic acid, as one of most important C4block platform chemicals, has been widelyused in pharmaceutical, food, biodegradable plastics and chemical industry. Bio-basedsuccinic acid bioprocess has the advantages of low-carbon and environmental friendly, whichmeet the requirement of sustainable development. According to the report given by USEnergy Department in2011, succinic acid was regarded as one of the most promisingplatform chemicals that could be industrially produced using raw materials. One of theproblems is that due to the anaerobic fermentation process the low biomass concentration andlow cell growth rates usually limit the productivity and efficiency, resulting in that few reportsreached the productivity criteria of2.5g·L-1·h-1set by US Energy Department. In this work,Actinobacillus succinogens M2012036stored in our lab was chosen as the experimental strain.Cotton fibrous matrix was fixed on the stirring shaft as carriers to attach cells in thefermentation broth. In order to achieve high succinic acid productivity, high succinic acidconcentration and high succinic acid yield, repeated-batch, repeated-fed-batch and continuousfermentation was operated. Specific results from this work were listed as followed:First of all, cotton fibrous matrix was selected as the carriers to attach A. succinogenesduring fermentation. Effects of cell attaching time, fibrous matrix superficial area, initialglucose of fermentation medium, initial fermentation pH on succinic acid production wereinvestigated respectively. The best attaching fermentation conditions were superficial area/medium volume2:1, initial glucose concentration of50g·L-1, initial pH of7.2. Tenfermentation batches were carried out in anaerobic bottle according to the optimalfermentation condition. The result showed that average succinic acid concentration of36.5g·L-1and average succinic acid yield of0.85g·g-1were achieved within average fermentationtime of48h. After fermentation, the biomass on the fibrous matrix was determined at48.4g·L-1, which was about10times of those during free cell fermentation.Secondly, repeated-batch fermentation, repeated fed-batch fermentation and continuousfermentation were carried out in a3L fibrous bed bioreactor (FBB). Ten batches fermentationwere investigated at initial glucose concentration of50g·L-1, pH of6.0-6.5. The resultshowed that average succinic acid concentration of39.0g·L-1, average succinic acid yield of0.85g·g-1and average succinic acid productivity of3.61g·L-1·h-1were obtained during therepeated-batch fermentation. Four repeated fed-batch fermentations were operated. The resultshowed that average succinic acid concentration of88.1g·L-1, average succinic acid yield of0.87g·g-1and average succinic acid productivity of2.27g·L-1·h-1were achieved. Maximumsuccinic acid concentration was occurred during the second batch, and succinic acidconcentration was98.7g·L-1, succinic acid yield was0.87g·g-1, succinic acid productivity was2.77g·L-1·h-1. The optimal continuous succinic acid fermentation condition in FBB wasfeeding glucose concentration of80g·L-1, pH between6.0and6.5, dilution rate of0.05h-1.Continuous succinic acid fermentation in FBB was carried out440h, and no obviousfluctuation and blocking problems were observed. The final results showed that the averagesuccinic acid concentration was57.4g·L-1, the residual glucose concentration was9.3g·L-1,the productivity was2.87g·L-1·h-1and the yield was0.81g·g-1.Thirdly, fermentation parameters and metabolic flux analysis between FBB fermentationand free cell fermentation were investigated. The data exhibited that the succinic acidconcentration, productivity and yield in FBB fed-batch fermentation were1.13,1.4and1.1times of those in free cell fed-batch fermentation. Comparison in metabolic flux analysisbetween FBB and free cell fermentation, HMP pathway, NADH utilization improved about120%. Results above indicated that fed-batch fermentation in FBB can achieve high succinicacid concentration, high yield and high productivity simultaneously and that high viabilitywith high cell density may be the direct reason for improvement of succinic acid production.