Purification of docosahexaenoic acid by selective esterification of fatty acids from tuna oil with Rhizopus delemar lipase

To purify docosahexaenoic acid (DHA), we attempted the selective esterification of fatty acids originating from tuna oil with lipases. Tuna oil was hydrolyzed in NaOH-ethanol solution, and the resulting fatty acid mixture [DHA, 23.2%; named tuna-free fatty acid (FFA)] was used as a starting material. Rhizopus delemar which acted lightly on DHA, was a suitable catalyst for the selective esterification of tuna-FFA, and lauryl alcohol was the best substrate. The reaction proceeded most effectively when a mixture of 2.4 g lauryl alcohol/tuna-FFA (2:1, mol/mol), 0.6 g water, and 600 U Rhizopus lipase was incubated at 30 degrees C for 20 h with stirring at 500 rpm. Under these conditions 72% of tuna-FFA was esterified, and 84% of DHA was recovered in the unesterified fatty acid fraction. The DHA content in the fatty acid fraction rose from 23 to 73% with this reaction. To further elevate the DHA content, the unesterified fatty acids were extracted, and then esterified again under the same conditions. By this repeated esterification, DHA was purified to 89% with a recovery of 71% of its initial content.