Abstract
In this work, the esterification of free fatty acids (FFA) in Zanthoxylum bungeanum seed oil (ZSO) with ethanol was performed and the effects of ethanol-to-oil molar ratio (8:1–32:1), catalyst amount (0.3–3.5% wt. of oil), reaction temperature (30–65°C), and reaction time (20–180 min) on the esterification of FFA were investigated. The acid value could not be reduced from beyond 40 mg KOH/g to less than 2 mg KOH/g by two-step sulfuric acid-catalyzed esterification with ethanol. Our results revealed that water formation during esterification was not a crucial factor inhibiting the completion of esterification. A novel technique to reduce acid value from above 40 mg KOH/g to less than 2 mg KOH/g was developed by using a second-step esterification with methanol. During the second-step esterification with methanol, some fatty acid ethyl esters were transformed into fatty acid methyl esters, which was confirmed by 1H NMR.