Comprehensive utilization of microalgae (Chlorella vulgaris) and the application of its product

Abstract

Rapid growth, fixation of carbon dioxide, and little competition with food plus some species possessing high lipid content make microalgae a suitable feedstock to produce biodiesel. However, high cost related to dehydration, lipid extraction and conversion to biodiesel hampers industrialization of biodiesel production from microalgae. The objective of this study was to develop a biodiesel production procedure directly from wet microalgae. The system was operated under atmospheric pressure without requiring any specific apparatus. Wet microalgae (Chlorella vulgaris) was pretreated by radio frequency (RF) heating to disrupt cell walls first, followed by esterification and transesterification with relatively small amount of methanol and catalyst (either HCL or NaOH) assisted by RF heating at 55 °C for only 20 min. The fatty acid methyl esters (FAME) yield reached as high as 79.5 ± 3.0%. The two major factors impacting biodiesel yield are catalyst and methanol. The SEM images visually verified the significant effect of cell disruption by the pretreatment. Then, the procedure for biodiesel production was optimized using response surface methodology (RSM). A three-variable, five-level central composite design (CCD) was employed to evaluate the effects of three key parameters, i.e., HCl to MeOH ratio (v/v), MeOH volume and RF heating time. An optimized point was successfully found. The best predicted FAME yield of 93.1% was obtained at HCl to MeOH ratio of 4.27 (v/v), MeOH volume of 28.5 mL and RF heating time of 19.2 min. Experiments carried out at the optimized point resulted in 92.7 ± 0.1% yield, which validated the reliability of the prediction model. All processing steps, including cell destruction, esterification and transesterification, were carried out under temperatures below 100 °C and atmospheric pressure. Therefore, no pressure-proof nor high-temperature apparatus was required. The procedure shows great potential for industrial application because of its high FAME yield, simple operation, low chemical consumption and short processing time. The principles of the procedure can also be applied to other microalgae with high lipid contents.

Besides lipid, there was also high content of carbohydrate in Chlorella vulgaris. In this study, the feasibility of comprehensive recovery of lipid and carbohydrate in wet microalgae Chlorella vulgaris was explored. First, four sets of enzyme combinations of α-Amylase, Amyloglucosidase and CTec2 were evaluated for hydrolysis efficiency on microalgae disrupted with radio frequency heating. Then, the most suitable combination was applied to raw microalgae and microalgae residual after biodiesel production, respectively, for saccharification. Adsorption kinetics of the optimized enzyme combination on the aforementioned three samples were determined and adsorption isotherm was analyzed by Freundlich equation. Morphology of microalgae was also investigated by scanning electron microscopy. A yield of reducing sugars in microalgae residual at 53.7% was obtained after 72 hours saccharification. The results from enzyme adsorption kinetics, isotherm and SEM images were consistent with each other. This study demonstrated that the microalgae residual after biodiesel production could be used as carbohydrate feedstock for fermentable sugar production through simple enzymatic hydrolysis.

The fermentable sugar could be used for the production of Poly (hydroxybutyrate) (PHB), which is one of the biodegradable plastic material. In this study, Poly (lactic acid)-Poly (hydroxybutyrate) (PLA−PHB) based films containing bioactive elements were developed and characterized. Seven formulations containing different contents of plasticizers (mono-caprylin glycerate (GMC) or glycerol monolaurate (GML)) were initially developed. After basic mechanical property tests, the PLA-PHB based films with 0.5% GMC or GML were selected as two formulations for further study. In the subsequent experiments, 5% cinnamaldehyde (CIN) was added into each of the two formulations selected and EVOH copolymer (widely used non-biodegradable packaging as control here) respectively. More mechanical and active properties were investigated. The results showed that PLA-PHB based films possessed some better mechanical properties than those of EVOH based film, and better active properties when applied to high lipid food simulant. In a preservative test, changes in total bacterial counts (TBC), thiobarbituric acid (TBA) and total volatile basic nitrogen (TVB-N) were carried out on salmon dices packed with films and stored at 4±1 ℃. It was only 4.65 CFU/g on day 17 for the salmon packed with PLA-PHB based film with GML as plasticizer meanwhile the TBC of salmon dices sealed in other two films reached 6.65 and 6.35 CFU/g respectively on day 15. This study showed that it could be feasible to use biodegradable active packing as an alternative to replace non-biodegradable packaging for chilled salmon.