| dc.description.abstract | Vanillin production through a microbial transformation of ferulic acid (FA) has recently attracted more attention since producing natural vanillin from cured vanilla pods is a costly process. However, the toxicity of FA and vanillin, utilization of FA and vanillin as carbon and energy sources, and high degradation of vanillin are the main limitations to obtaining a high titer of vanillin.
This work aims to maximize the efficiency of bioconversion of FA to vanillin using Amycolatopsis thermoflava by process optimization. Two approaches were demonstrated: (Part 1) bioconversion using commercial FA as a precursor, (Part 2) bioconversion using FA from alkaline hydrolysate of corn cobs to utilize agro-industrial waste as a cheap raw material.
Vanillin production using commercial FA (Part 1) showed that the timing of FA supplementation strongly affects vanillin formation. The addition of FA in the early stationary phase was favorable to obtaining high cell density and high yield. Studies on the effect of culture temperature showed that cultivation at 45 oC favored cell growth, while 30 oC favored high yields since it could reduce the rate of vanillin degradation. Therefore, a two-stage batch fermentation was conducted. This strategy allowed the use of higher concentrations of FA. To increase vanillin productivity, a two-stage fed-batch fermentation by controlling the temperature and feeding time of FA was implemented. This strategy significantly increased the yield and productivity of vanillin by 1.8 and 12.0 folds, respectively, compared to single-stage batch fermentation at 30 oC where FA was initially added to the broth at the 0th h. Concerning the feeding method of FA at fed-batch operation, expanding the time of FA supplementation could lower its toxic effect on cells, thereby increasing cell growth rate and shortening bioconversion time. However, it also accelerated the degradation of vanillin.
For part two, the work was started by optimizing the ultrasound-assisted extraction conditions to release FA from corn cobs using alkaline treatment. Optimal hydrolysis conditions were obtained at 0.5 M NaOH concentration and 10% solid loading for 30 min of extraction, resulting in relatively high concentrations of FA 715 mg/L and p-Coumaric acid 1025 mg/L. To maximize vanillin formation from FA, five parameters were investigated including the effects of sterilization method, nutrient limitation, initial biomass concentration, culture temperature, and reducing sugar control. The results showed that bioconversion using autoclaved hydrolysate gave poor results both for cell growth and vanillin formation. Pasteurization was more favorable however, the presence of contaminants limited the entire process. Therefore, a nutrient limitation strategy was developed. Observations on the effects of initial biomass concentration and culture temperature showed that the maximum vanillin formation was obtained at 1.5 g/L and 45 oC, respectively. To optimize vanillin production, a reducing sugar control strategy was developed. The yield and productivity of vanillin significantly increased by 29.5 and 585 folds, respectively, compared to bioconversion using autoclaved hydrolysate.
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