Studying the drying mechanism of microalgae Chlorella vulgaris and the optimum drying temperature to preserve quality characteristics

Drying harvested microalgae from an average moisture content of 80% wet basis to a safe moisture content of 10% is challenging. Removing this high amount of water from microalgal biomass is time-consuming and is not as easy as agricultural crop dehydration. The long drying time results in large drying costs. Although drying is a suitable technique for algal-based fuel production, it has not been commercialized due to its associated challenges. This study was performed to fulfill the knowledge gap in the microalgae drying mechanism and to understand the reason for long drying times. For this purpose, the thin-layer drying of microalgae Chlorella vulgaris at the temperature range of 40 to 140°C was studied in a convective oven. The effect of drying air temperature on Chlorella elemental and chemical composition, surface color, and surface structure in the aforementioned temperature range was also analyzed. The results revealed that the dominant mechanism in Chlorella drying is diffusion, which is attributed to the collapse of microalgal cells structure with increased drying temperature. In fact, moisture is entrapped in the Chlorella cells and it takes a long time for them to reach the biomass surface and evaporate. The result was that both low and high drying temperatures have adverse effects on Chlorella surface color, structure, and carbohydrate and lipid composition. This suggests that microalgae should be dried at an optimum medium (60–80°C) temperature.