Biochemical analysis and potential applications of aqueous and solid products generated from subcritical water extraction of microalgae Chlorella pyrenoidosa biomass

Subcritical water extraction (SWE) technology has become useful as a green extraction technology for generating various biochemical and bioactive compounds from a wide range of biomass feedstocks, including microalgae. SWE processes result in two different product phases: an aqueous phase containing solubilized or immiscible bioactive compounds from the extract, and solid-phase biomass debris containing residual or unextracted bioactive molecules in the matrix. While most SWEs of microalgal biomass focus on maximizing lipid production for biofuel application, the present work seeks to explore opportunities to identify and characterize compounds obtained from both the aqueous and solid phases. SWE of Chlorella pyrenoidosa biomass was carried out under varying conditions of temperature (170–370 °C), extraction time (1–20 min), and microalgal biomass loading (1–15%). Analysis of aqueous and solid-phase samples collected under varying SWE conditions showed a higher protein concentration of 4.13 mg mL−1 in the aqueous phase compared with carbohydrate (1.35 mg mL−1) for the optimum extraction conditions. About 17.52 mg g−1 of amino acids and 12.72 mg g−1 of organic acids were obtained from the aqueous phase with glutamic acid and lactic acid being the highest. The findings from this work would be useful in identifying and harnessing important microalgal biochemical from SWE process to develop new and improved bioproduct technologies.