Presoaking with Sodium Selenite Promotes Accumulation of Polyphenols and GABA in Foxtail Millet Sprouts Under NaCl Stress

The effects of presoaking with sodium selenite (Na2SeO3) solution on the polyphenols and γ-aminobutyric acid (GABA) in foxtail millets during germination under NaCl stress condition were investigated, and the key processing parameters, including Na2SeO3 concentration, presoaking time, presoaking temperature and NaCl concentration, were optimized via response surface methodology (RSM) based on total phenolic content (TPC) and GABA content of foxtail millet sprouts. The inhibition of sprout growth caused by salt stress was alleviated by presoaking with Na2SeO3, which did not alter phenolic compositions, resulting in a significant increase in the levels of both phenolics and GABA. The optimal germination parameters were 42 mg/L Na2SeO3, 9.8 h soaking time, 30 °C soaking temperature, and 110 mmol/L NaCl. Under these conditions, the measured TPC and GABA content were 837.22 mg FAE/100 g and 281.68 mg/kg, respectively, which closely approximated the predicted values. Correspondingly, the main free phenolic compounds 3-p-coumaroylquinic acid and N-p-coumaroylserotonin increased by 2.94 and 3.34 times, respectively, and the predominant bound phenolic compounds trans-ferulic acid and trans-p-coumaric acid increased by 2.28 and 6.39 times, respectively. Meanwhile, the total and organic selenium contents of the sprouts reached 14.74 and 12.02 mg/kg dry weight, respectively. This study provides a practical technology for preparing selenium-enriched foxtail millet sprouts with enhanced phenolic compounds and GABA, which can serve as a novel functional food resource.