An optimization of extractive procedures for phenolic compounds applied to millet from Burkina Faso

Introduction: Millet is a staple food in the diet of Burkina Faso, contributes to reduce the incidence of gastro intestinal disases and diabetes in Burkina's children diet also thanks to its phenolic compounds. These molecules are present in cereals as free and bound forms. Aim of this work was to select the best hydrolytic methods to evaluate phenols on different batches of African millet. Methods: Acid and basic extractions were applied on different batches of millet using MeOH (1% HCl) and MeOH/H2O (NaOH 0.1M) 7:3 v/v, respectively. Different hydrolysis were tested on a reference batch of millet (MP-3): the basic ones were NaOH 0.1, 2 and 4 M, while for acid hydrolysis was selected H2SO4 0.1 M. The phenolic composition was investigated by HPLC-DAD, UHPLC, ESI-MS and MSn. Results: The results pointed out mainly the presence of C-glycosylated flavonoids as free molecules and hydroxycinnamic acids as bound phenols. Methyl ferulate was the most abundant cinnamic derivative firstly identified in millet. The effect of process variables (time, temperature and molarity) on the final yield of the basic hydrolysis, were evaluated by building a first experimental design by means of the Resonse Surface Methodology (RSM). preliminary results indicated that, in order to maximize the recovery of cinnamic compounds, higher molarity and lower time or longer extraction times (150 minutes) and lower molarity values would be required. Overall, among the different procedures, the acid extraction was the most advantageous for the recovery of bound phenols: 80% higher than the best basic hydrolysis. Conclusions: By this work different hydrolysis methods were compared for the first time to recover the phenolic fraction from millet, a cereal known as richer in phenolic compounds and suitable to be used in gluten-free products. Differently from literature to date, the acid hydrolysis resulted more efficient than the basic one to recover the whole pool of phenolic compounds.