EFFECT OF MICROWAVE ASSISTED EXTRACTION ON THE PHENOLIC CONTENT AND PROFILE OF DIFFERENT SORGHUM PHENOTYPES

Abstract

Sorghum contains diverse bioactive compounds, particularly polyphenolics, which have been associated with various health benefits. These compounds are concentrated in the bran fraction that is lost during milling. The goal of this study was to establish how microwave assisted extraction (MAE) affects the extractability and profile of polyphenolics from the bran of different sorghum phenotypes. Sorghums with different grain and plant colors were selected and the effect of microwave energy on the phenolic profile (UPLC-MSMS), and content (HPLC, UV-vis spectroscopy) was evaluated. Conventional extraction (1% HCl in MeOH/2 hr.) was used as a control. Extractable phenolic content increased 3.4-3.5X for the white pericarp sorghums, while non-tannin pigmented sorghums only increased to 1.1-1.2X versus the control. The greatest increase in extractable phenolic content from MAE treatment increased was in the red tannin sorghum phenotype (3.8X). Similar trends occurred in the antioxidant capacity of each phenotype. White and tannin phenotype antioxidant capacity increased 3.4-3.6X, while non-tannin pigmented sorghums increased 1.3-2.2X versus control treatment. Phenolic profile and structural identification paralleled the findings from the UV-vis analysis, and helped elucidate changes in phenolic content due to MAE. Phenolic acid content increased (1.6-11X) in MAE extracts in phenotypes which contained low levels of free phenolic acids, while the phenolic content in phenotypes high in free phenolic acids decreased. Free phenolic acids that were easily extracted in the conventional method, were rapidly degraded under MAE. In contrast, bound phenolic acids attached to the cell wall material were released by MAE and contributed to the significant increases in quantified phenolic acid content in MAE extracts. Flavanone aglycones seemed to be more resistant to degradation than the glycosides under MAE. Following MAE, anthocyanidins were detected in the tannin sample; this was likely due to the oxidative depolymerization of the condensed tannins under MAE conditions. MAE is potentially a useful and efficient tool for extracting polyphenolic compounds from sorghum bran. Further understanding of how phenotype plays a role in the extraction efficiency of MAE can help improve extraction efficiency of specific polyphenolic compounds.