Selective extraction of triacylglycerols from a corn/sorghum-based bioethanol production side-stream to purify high-melting point waxes

Abstract: Increasing energy demands and growing environmental concerns have led to ever-growing green and sustainable energy needs. Therefore, alternative energy source, i.e., bioethanol production, is growing worldwide. Specifically, sorghum as a starch feedstock has been increasingly used due to its high drought tolerance. Grain sorghum-to-ethanol production generates a lipid-rich slurry side-stream (i.e., stillage) that is discarded as either waste or used as part of animal feed. However, this side-stream is rich in high melting point waxes along with other high-value lipids and phytochemicals. Even though there have been some efforts to extract/purify waxes from this side-stream, the separation and purification of waxes are very challenging due to (i) the presence of high amounts of lipids with similar solubilities, (ii) clogging issues in membrane-filtrations, and (iii) use of toxic organic solvents, resulting in underutilization of this side-stream. Therefore, the objective of this study was to develop a novel, green process based on supercritical carbon dioxide (SC-CO₂) to selectively extract and fractionate lipids present in the stillage (corn/sorghum mixture feed to the fermenter) to purify high-melting point waxes.

In this study, triacylglycerols were selectively extracted via neat SC-CO₂, while high-melting point waxes were concentrated. During extraction, each factor (pressure, temperature, time, flow rate) was screened for the crude yield via a definitive screening design (α=0.05). Subsequently, a response surface optimization was employed to determine the pressure (8-40 MPa), temperature (35-75), and time (2-6 h) for the maximum lipid extraction. Approximately 70% (w/w stillage) of crude yield was obtained at 40 MPa and 55 °C, where pressure was identified as the key factor. The extracts and waxes were analyzed for their melting point and composition.

The expected outcomes are (i) sustainable valorization of an agro-industrial side product, (ii) high-purity waxes, and (iii) a food-grade lipid source and fractionation method for food and cosmetic industries.