Valorization of millet processing waste: Superheated steam and ultrasound-assisted extraction, encapsulation, and evaluation of bioaccessibility assessment

Abstract

Millet husks are nutrient-rich byproducts of milling and remain underutilized despite their potential as a source of phytochemicals. This study investigates the use of superheated steam (SHS) pretreatment combined with ultrasound-assisted extraction (US) to enhance the recovery of phenolic compounds, flavonoids, and antioxidants from barnyard, kodo, and finger millet husks, while evaluating the encapsulation of these compounds for potential use in functional foods. Husks were SHS-treated at 120, 150–180 °C for 1, 5, 10 and 15 min, followed by methanol-based conventional or US extraction. Total phenolic content (TPC), antioxidant activity (AOA), flavonoid levels, and antinutritional factors (tannins, saponins, phytin) were analysed. Optimal extracts were encapsulated in pectin-alginate matrices and assessed for bioaccessibility and stability. SHS-US synergistically enhanced extraction efficiency, with 150 °C/15 min yielding maximal phytochemical recovery. Finger millet husks exhibited the highest TPC (243.1 vs. 114.2 mg gallic acid equivalent (GAE)/100 g dry weight (DW) control) and AOA (65.4 % vs. 14.2 % control), while kodo millets showed a fivefold TPC increase (390.9 vs. 73.4 mg GAE/100 g DW). Barnyard husks achieved moderate gains (TPC: 74.3 vs. 41.8 mg GAE/100 g DW). Antinutritional compounds fluctuated but remained within safe thresholds. Encapsulation efficiency peaked for finger millet (59.1 %), with in-vitro bioaccessibility highest in barnyard (54.8 %) and lowest in kodo (20.1 %). The SHS-US strategy effectively valorizes millet husks, enabling sustainable extraction of bioactives for nutraceuticals with encapsulation ensuring stable nutrient delivery in food matrices, aligning with waste-to-wealth initiatives.