Study on Extraction of Lipids and Enrichment of DHA from Oil-rich Microalgae

Abstract: Polyunsaturated fatty acids (PUFAs) are beneficial to human bodies because of distinct pharmaceutical properties. EPA and DHA, as the representative ofω-3 PUFAs, have attracted much increasing attention due to their significant functions in lifting the acuteness of brain and improving eyesight. However, the traditional source of EPA and DHA from fish oil has failed to meet people’s demand nowadays. Marine microalgae, which contains abundant lipids and polyunsaturated fatty acids, can be an ideal alternative. Compared with traditional oil crops, microalgae has the advantages of abundant lipid, short growth cycle, no field and season restrictions and easily extensive cultivation, which makes it perfect feedstock for biological fuel production. Therefore, it is an efficient method for comprehensive development and cost reduction to obtain oil feedstock for biological fuel while extracting DHA and EPA. The purpose of this work is to get lipid from microalgae by Soxhlet extraction and supercritical CO<sub>2</sub> extraction, and then isolate DHA from the crude oil to obtain DHA with high purity. And the physical properties of microalgae were also studied to lay a foundation for future study of making biodiesel from microalgae lipid.Firstly, the analytical method of microalgae lipid was studied. Gas chromatography internal standard method was founded to analyze the amounts of DHA in the product after methyl esterification.Soxhlet extraction was used to get lipids from microalgae. Effects of different polar solvents and extraction time on lipid yield and DHA content were studied. 95% ethanol was selected as the optimal solvent considering lipid yield, DHA content and safety of the solvent. When using 95% ethanol as extraction solvent under the optimal extraction time of 8h, lipid yield 44.87%, DHA content 15.73%, DHA extraction ratio 7.06% were obtained. The experiments also showed that DHA content in lipids extracted by different solvents changed little.Ultrasonic and microwave irradiation were used to discrupt the microalgae cell wall. Both of the pretreatments made obvious higher lipid yield up to 60% with short processing time and convenient operations. Under certain ultrasonic condition (80W and 15min) DHA content of 24.77% was obtained, while all the microwave treatments showed general decrease in DHA content. Therefore, ultrasonic mothod was the appropriate mothod to get DHA from microagale. However, the two tmethods were both rather favorable for getting more lipids to prepare biodiesel. Supercritical CO<sub>2</sub> extraction was used to get lipids from microalgae. The optimal extraction conditions were obtained as folllows: 2h, 35MPa, 40℃, 95% ethanol as the co-solvnet, mass ratio of material to co-solvnet 1:1. Under these conditions, lipid yield was 33.88%, DHA content was 27.49% and DHA extraction ratio was 9.31%. Although the total lipid yield was lower, supercritical CO<sub>2</sub> extraction has advantages in higher DHA purity and excellent product quality compared with Soxhlet extraction. Ultrasonic enhanced-Supercritical CO<sub>2</sub> extraction experiment showed no significant increase in either lipid yield or DHA content, but this technology can remarkably shorten the extraction time by half.Urea complexation method was used to enrich and purify DHA from microalgae lipids.Under the optimal complexation conditions of urea/ fatty acid (2:1), complexation time (8h) and complexation temperature (-10℃), DHA purity increased from 29.65% to 60.43% with an enrichment ratio of 60.64%.
Key words: Microalgae; DHA; Supercritical CO2; Extraction; Urea; Purify;

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