Abstract
Background and Objective: The cost of substrate represents a significant percentage of the total biosynthesis cost of citric acid. There is need to continue searching for better and cheap underutilize starchy raw material for citric acid biosynthesis. Therefore, the objective of this research was to biologically synthesis citric acid in submerged fermentation of Aspergillus niger using wild Dioscorea esculenta (wild yam) tubers.
 Methodology: Submerged fermentation was conducted and the effect of Dioscorea esculenta concentrations, combined pretreatment and membrane permeability induction on citric acid biosynthesis were carried out.
 Results: Citric acid concentration increased as the concentration of the wild yam increased with maximum citric acid production of 10.5 ± 0.9 g/ l after 96 hours of fermentation. The overnight combined pretreatment with 0.3 % NaOH before pretreatment at 121oC for 20 minutes as one time combined pretreatment yielded 15. 0 ±1.2 g/l citric acid. Calcium chloride membrane induction produced 24.5 ± 1.3 g/l citric acid concentration respectively after 96 hours of fermentation.
 Conclusion: The result showed that wild Dioscorea esculenta tubers can be used as a substrate for the biosynthesis of citric acid. The biosynthesis of citric acid from Dioscorea esculenta tubers was enhanced by increased in concentration, combined pretreatment and membrane permeability induction.
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