The fate of phenolic compounds in sorghum and millet sourdough fermentation

Sorghum and millets are important crops in the world and they are rich of phenolic compounds. Phenolic compounds have good antioxidant ability and they are regarded as the good natural protective screens for oxidative damage. However, the previous studies has only focused on the amount and antioxidant property of phenolic compounds in sorghum and millets and there were limited studies about the effect of sourdough fermentation on the phenolic compounds. Hence, it is necessary to study the fate of phenolic compounds during the sourdough fermentation. The aim of the experimental work was to study the changes of phenolic compounds, namely the soluble and bound phenolics, during lactic acid bacteria fermentation of wholegrain sorghum and millets, and to study the factors contributing to their modifications. Two millet varieties, i.e. finger miller and pearl millet (millet Jaune), and two types of sorghum, i.e. white sorghum and red sorghum were used in this study. Each experiment consisted of two samples, one unfermented and one fermented. The fermented sample was fermented by W. Confusa A16, which was isolated from a sourdough pancake in Burkina Faso and identified in University of Helsinki. During the experiment part, different extraction solvents and methods were used in order to obtain a better extraction of phenolics. Ethanol and methanol were selected as the extraction solvent for soluble phenolics and ultrasonic bath was used to assisst the extraction of insoluble-bound phenolics. And the content of phenolic compounds were determinated by Folin-Ciocalteu assay in coupled with spectrophotometer. The results showed that methanol as the extraction solvent was more efficient in extracting the soluble phenolics than ethanol. And ultrasonic bath could shorten the extraction time of bound phenols from 16h to 2.5h with a good extraction result compared with non-ultrasonic bath. In this study, the content of extracted phenolic compounds varied among different classes and subclasses. Sorghum generally contained higher content of phenolic compounds than millets. And the total phenolic compounds in red sorghum was higher than white sorghum. The changes of total phenolic compounds and insoluble-bound phenolic compounds during sourdough fermentation followed the same trend, both of which decreased after sourdough fermentation. Conversely, soluble phenolic compounds increased during sourdough fermentation both in millet and sorghum samples. Increased enzymatic activities and the increased acidic medium environment during sourdough fermentation might form new interactions between phenolics and other macromolecules or cause the structure changes of phenolics, thus lead to changes of the amount of extractable phenolic compounds. However, the results obtain from this experiment are not completely consistent with literature data, which might be related to the different cultivar of the sorghum and millet raw materials used and the experimental methods adopted .