Impact of Processing on the Phenolic Content and Antioxidant Activity of Sorghum bicolor L. Moench

Sorghum, a cereal grain rich in nutrients, is a major source of phenolic compounds that can be altered by different processes, thereby modulating their phenolic content and antioxidant properties. Previous studies have characterised phenolic compounds from pigmented and non-pigmented varieties. However, the impact of processing via the cooking and fermentation of these varieties remains unknown. Wholegrain flour samples of Liberty (WhiteLi₁ and WhiteLi₂), Bazley (RedBa₁ and RedBa₂), Buster (RedBu₁ and RedBu₂), Shawaya black (BlackSb), and Shawaya short black 1 (BlackSs) were cooked, fermented, or both then extracted using acidified acetone. The polyphenol profiles were analysed using a UHPLC-Online ABTS and QTOF LC-MS system. The results demonstrated that combining the fermentation and cooking of the BlackSs and BlackSb varieties led to a significant increase (p < 0.05) in total phenolic content (TPC) and antioxidant activities, as determined through DPPH, FRAP, and ABTS assays. The 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging activity of WhiteLi₁, BlackSb, RedBu₂, and BlackSs increased by 46%, 32%, 25%, and 10%, respectively, post fermentation and cooking. Conversely, fermentation only or cooking generally resulted in lower phenolic content and antioxidant levels than when samples were fully processed compared to raw. Notably, most of the detected antioxidant peaks (53 phenolic compounds) were only detected in fermented and cooked black and red pericarp varieties. The phenolic compounds with the highest antioxidant activities in pigmented sorghum included 3-aminobenzoic acid, 4-acetylburtyic acid, malic acid, caffeic acid, and luteolin derivative. Furthermore, the growing location of Bellata, NSW, showed more detectable phenolic compounds following processing compared to Croppa Creek, NSW. This study demonstrates that sorghum processing releases previously inaccessible polyphenols, making them available for human consumption and potentially providing added health-promoting properties.