The Completion of Esterification of Free Fatty Acids inZanthoxylum BungeanumSeed Oil with Ethanol

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.