Harvesting Aurantiochytrium sp. SW1 using organic flocculants and characteristics of the extracted oil

Abstract

The use of microalgae as an alternative source of biofuel production has become an emerging topic of interest in recent years. However, the main challenge in the microalgae downstream process lies in the separation of microalgae from the growth media (harvesting process). This study proposed the use of different organic flocculants, namely, cationic guar gum, gum arabic and okra gum, to harvest the microalga Aurantiochytrium sp. SW1 for subsequent extraction of docosahexaenoic acid-rich oil from the microalgae. The physicochemical properties and bioactivities of the cell and extracted oil were studied. The results showed that cationic guar gum at 0.5% w/v and gum arabic at 0.1% w/v were the optimum concentrations for cell harvesting. The highest cell recovery was achieved using cationic guar gum with up to 12.8 g/L culture, which was 3.8-fold higher than that of the control method, probably due to the cationic charge of the cationic guar gum, which exhibited a greater efficiency in coagulating the negatively charged surface of microalgae. Overall, the different flocculants used during harvesting affect the extraction efficiency of lipids and other phytochemicals in the cells. Cationic guar gum significantly produced the highest (p ≤ 0.05) oil yield extracted from the culture, which was up to 25.06% w/w biomass. Interestingly, the significantly highest (p ≤ 0.05) total phenolic content (0.23 ± 0.01 mg gallic acid equivalent/mL) and carotene content (1.67 ± 0.42 mg/mL) were observed in microalgae oil flocculated with okra gum, signifying that the type of flocculant affected the determination of phytochemical content of the extracted oil. The oil obtained from okra gum flocculant also exhibited the highest 2,2-diphenyl-1-picrylhydrazyl scavenging activity (0.89 ± 0.01 mg Trolox equivalent/mL) and ferric reducing antioxidant powder (26.77 ± 1.52 mg Trolox equivalent/mL), which was attributed to the higher concentrations of phenolic compounds and carotene content. This work reveals the potential usage of okra gum as a flocculant with enhanced phytochemical contents and bioactivities.