3.2. Drum drying at different temperature with variable speed
Drum speed and surface temperature of the drum are the two major variables that affect the residual moisture content of the dried algal biomass. The residual moisture content of the microalgal slurry was found to be reduced maximum up to 41.4 ± 2.7% (wb) under the drum speed of 0.1 rpm at drum surface temperature of 70 °C (Figure 3A). By increasing the drum surface temperature to 80 °C, the residual moisture content of the microalgal slurry was reduced to < 10% (wb) under the drum speed of 0.1, 0.2, and 0.3 rpm (Figure 4A). Likewise, at 90 °C of drum surface temperature, the residual moisture content was reduced up to 4.6 ± 1.1, 5.3 ± 1.3, 6.1 ± 1.5, and 7.3 ± 2.6% (wb) at 0.1, 0.2, 0.3, and 0.4 rpm, respectively (Figure 5A). Further, at 100 °C of drum surface temperature, the residual moisture content was 3.5 ± 1.7, 3.9 ± 1.5, 4.4 ± 2.3, 6.3 ± 1.6, and 7.1 ± 2.1% (wb), respectively, for the drum speed of 0.1, 0.2, 0.3, 0.4, and 0.5 rpm (Figure 6A). From Duncan’s new multiple range test analysis, it can be concluded that there was no significant variation in the residual moisture content of the drum-dried biomass at 80 °C with drum speed of 0.1-0.3 rpm, 90 °C with drum speed of 0.1-0.4 rpm, and 100 °C with drum speed of 0.1-0.5 rpm (Table 3).
The summary of lipid yield (mg g-1 of dry biomass) and lipid recovery (%) at different drum surface temperature under varying drum speed is presented in Table 4. The maximum lipid yield was found to be 72.1 ± 5.2 mg g-1 of dry biomass with the corresponding lipid recovery of 50.7% only as compared to the bone-dried samples under the drum speed of 0.1 rpm at 70 °C of drum surface temperature (Figure 3B, C). This is well in agreement with the report of Balasubramanian, Doan, & Obbard (2013), where a significant reduction in lipid extraction efficiency was evident from the biomass ofNannochloropsis sp. with higher moisture content. The possible reason could be that the hydrophilic outer layer of the microalgal cell wall was surrounded by water molecules that resist the non-polar solvent penetrating inside the cell and hinders the extraction of lipids (Balasubramanian et al., 2013). At the drum surface temperature of 80 °C, the lipid yield was estimated to be ranged between 130.6-131.3 mg g-1 of dry biomass at the drum speed of 0.1-0.3 rpm, and the maximum lipid yield of 131.3 ± 3.1 mg g-1 of dry biomass with lipid recovery of 92.4% was obtained at 0.3 rpm (Figure 4B, C). Similarly, the lipid yield was found to be in the range of 130.2-132.2 mg g-1 of dry biomass with 0.1-0.3 rpm at the drum surface temperature of 90 °C. The maximum lipid yield of 132.2 ± 3.4 mg g-1 dry biomass was obtained under the drum speed of 0.4 rpm at 90 °C with the corresponding lipid recovery of 93% as compared to the bone-dried samples (Figure 5B, C). Likewise, at 100 °C of drum surface temperature, the maximum lipid yield was found to be in the range of 127.5-133.3 mg g-1 of dry biomass between 0.1-0.5 rpm; the maximum lipid yield was estimated to be 133.3 ± 5.4 mg g-1 of dry biomass under 0.5 rpm with the corresponding lipid recovery of 93.8% at drum surface temperature of 100 °C (Figure 6B, C). From Duncan’s new multiple range test analysis, no significant variation was observed in the lipid yield of the drum-dried biomass at 80 °C from 0.1-0.3 rpm, 90 °C from 0.1-0.4 rpm, and 100 °C from 0.1-0.5 rpm (Table 4). From this observation, drum surface temperature of 80, 90, and 100 °C with the drum speed of 0.1-0.3, 0.1-0.4, and 0.1-0.5 rpm, respectively, could be selected for maximum lipid recovery from the algal biomass. Zepka, Jacob-Lopes, Goldbeck, & Queiroz (2008) reported that the constituents of biomass such as lipid, carbohydrate, and protein contents could alter significantly based on the drying temperature. A study revealed that Chlorella vulgaris biomass dried at 60 °C in an oven resulted in a slight reduction of lipid content, whereas at 80 °C or much higher temperature, the lipid content was considerably decreased (Widjaja, Chien, & Ju, 2009). The study cited the possible explanation for the low triacylglycerol yield as oxidation of fatty acid upon exposure to the higher temperature. Contrary to this, the present study showed that drying the microalgal slurry from 80-100 °C in a drum dryer has insignificant impact on lipid recovery. The probable explanation could be the short duration of exposure of the wet microalgal slurry to a high temperature that reduces the risk of damage to the dried-biomass and its constituents.