NUTRITIONAL ENHANCEMENT OF WHEAT MILLING BY-PRODUCTS: CHEMICAL CHANGES AND EVOLUTION OF MICROBIOTA DURING SOURDOUGHLIKE FERMENTATION OF BRAN

Several epidemiological studies indicate that high whole grains diets work as protective factors against chronic diseases, such as metabolic syndrome, cardiovascular diseases (CVD), and gastrointestinal cancer. These effects are likely related, at least in part, to their high content of fiber and bioactive compounds, with antioxidants and anti-carcinogenic properties, mainly present in bran and germ of cereal grains. Removal of these fractions during milling to improve shelf-life of the flour results in severe depletion of fiber and bioactive compounds. The loss of about 58% of fiber, 83% of Mg, 61% of folate and 79% of vitamin E has been shown in comparing the content of important nutrients in wholemeal flour and white flour. The aleurone layer (the outermost layer of the endosperm) has been shown to contain many of these functional compounds, but it is partially eliminated in wheat flour milling as a by-product mostly used for the animal feed. The increasing demand for functional foods and the possibility to take advantage of agro-industrial by-products have attracted great interest in using bran-enriched products. This should lead to a greater value for wheat industries, reducing their environmental impact and getting an economic return. The main reason behind the low utilization rate of wheat bran in baking industry is the gritty texture, bitter and pungent flavour and coarse mouthfeel of bread caused by the bran. However, the fermentation of cereal bran, with yeasts and lactic acid bacteria or with specific enzymes, has been shown to be an interesting pre-treatment to improve technological, sensorial and nutritional properties of bran-enriched products, as well as to degrade anti-nutritive factors, such as phytic acid, in order to increase mineral bioavailability. This study was aimed to increase the amount of bran’s bioactive compounds, such as soluble fiber, water-extractable arabinoxylans, free ferulic acid, through a fermentation process, in order to use the fermented bran as a potential functional ingredient. Wheat bran sourdoughlike fermentation processes were conducted through continuous propagation by back-slopping of fermented bran until a stable microbiota was established, reaching high counts of lactic acid bacteria and yeasts. At each refreshment step, bacterial strains were isolated, clustered, molecularly analysed by Randomly Amplified Polymorphic DNA PCR and identified at the species level by 16S rRNA gene sequencing. Leuconostoc mesenteroides, Lactobacillus brevis, Lactobacillus curvatus, Lactobacillus sakei, Lactobacillus plantarum, Pediococcus pentosaceus and Pichia fermentans were dominating the stable sourdough ecosystem. After fermentation, levels of soluble fiber increased (+ 30%), water-extractable arabinoxylans and free ferulic acid were respectively fourfold and tenfold higher than in raw bran, results probably related to endogenous and microbial enzymatic activities, while phytic acid was completely degraded. On the basis of the interesting nutritional results, some isolated stains were also characterized in order to select potential starter cultures. The lactic acid bacteria (LAB) were characterized by their bran fermentation capacity, antifungal activity, carbohydrate metabolism, exopolisaccharides production, as well as their antibiotic resistance profiles. The LAB and the yeast were also tested for their potential xylan- and phytate-degrading activities. Moreover, common probiotic properties of the bacterial strains, such as acid and bile tolerance, anti-listeria activity and adhesion to the human intestinal epithelial cells Caco-2 cells were examined. Results suggest that strains belonged to L. plantarum and P. pentosaceus species could have interesting technological applications, due to their antifungal activity and EPS production. Some of these strains also exhibited phytate degrading activity and thus they could be exploited to improve mineral bioavailability of fermented products. Moreover, L. curvatus (CE 83), L. brevis (CE 85), and P. pentosaceus (CE 65), seemed to be suitable candidates to be used as probiotics. In conclusion, the current study supports the hypothesis that fermentation process is an efficient means to increase the amount of bioactive compounds of wheat bran. The characterization of the bacteria involved in bran sourdoughlike fermentation was the first step toward selecting starter cultures, according to their metabolic and enzymatic activities, in order to conduct “tailored” bran fermentation process and improve technological and nutritional properties of bran-enriched products. The present study has shown that investigated properties of the lactic acid bacteria tested are highly strain-specific. In this sense, the study of microbial diversity represents an opportunity for advances in biotechnology and the possibility of mixing strains with different properties and activities could be an interesting approach to obtain fermented products with improved qualities.