The objective was to study the distribution of metabolic products during MMC fermentation in continuous mode and statistical optimization of key process parameters for enhanced productivity and yield of butyric acid and 1,3 propanediol.
The enrichment and adaptation of MMCs proved to be a winning strategy to efficiently convert complex substrates, such as crude glycerol derived from animal fat (with no pretreatment). The adaptation allowed the bacteria to grow in CSTR on high feed concentration, performing a high glycerol consumption rate. 1.3 PDO turned out to be the dominant metabolite during steady state, followed by butyrate. The statistical optimization (Inscribed Central Composite Design) allowed to maximize productivity of PDO and butyric acid, with a 5-fold increase compared to steady state results in standard fermentation conditions prior to optimization. The results showed a model with a complex interaction between the key factors (pH, HRT and glycerol concentration), which implied a very careful choice of operating parameters. The model further showed the need for a fine tuning of HRT in combination with the other parameters, in order to maximize productivities and avoid cells wash out. To avoid this problem, preliminary tests and kinetic characterization of the consortium turned out to be fundamental to choose the proper experimental range.