Effect of moisture content on immobilized lipase-catalyzed triacylglycerol hydrolysis under supercritical carbon dioxide flow in a tubular fixed-bed reactor

Surplus fats and oils were reacted with several lipases under supercritical fluid conditions for the purpose of obtaining value-added products. Lipases, however, require sufficient moisture content to act as effective biocatalysts. An immobilized lipase from Candida antarctica was chosen to examine the rate of enzyme moisture loss under laboratory ambient conditions and also under supercritical fluid conditions. A more important aspect was to determine the effect of lipase moisture content on the hydrolysis of triacylglycerols under the same supercritical fluid conditions. Under ambient conditions at constant air flow, the immobilized lipase lost water at the rate of 4 to 5%/h, from 48.3% moisture to a final moisture content of 0.2%. Water is known not to be very soluble in supercritical carbon dioxide (SC-CO2); Nevertheless, under supercritical fluid conditions of 60 degrees C, 4000 psi, acid carbon dioxide flow rates of 0.5 or 1 L/min measured as expanded gas, the enzyme moisture loss was approximately 2 to 6%/h. To determine the effect of moisture loss on enzymatic hydrolysis, lipase beds in a tubular reactor with moisture contents of 1.5 to 23.5% were reacted with tripalmitin under supercritical conditions. A lipase with an initial moisture content of 1.5% gave little evidence of hydrolysis whereas those containing 5.4 to 23.5% moisture content resulted in products that contained only palmitic acid and unreacted tripalmitin. Thus, optimal parameters for continuous lipase hydrolysis of tripalmitin require: enough enzyme moisture to compensate for complete substrate hydrolysis; sufficient enzyme moisture for losses due to water solubility in SC-CO2; temperature and pressure sufficient to solubilize the tripalmitin; high carbon dioxide total flow to solubilize all the tripalmitin; and a relatively large enzyme bed volume to increase the solubilized substrate contact time with the enzyme.