Characterisation of dietary fibre, bioactive compounds and prebiotic effect of plant food by-products for their valorisation as high added-value ingredients

Resum

The activity of the agri-food industry generates a large quantity of by-products, which management cause a negative impact at economical and at environmental level. These by-products remain the chemical characteristics from the plant foods, so they are an important source of nutrients, mainly dietary fibre and bioactive compounds. Therefore, the general aim of this Doctoral Thesis has been to valorise different by-products generated in the agri-food industry, derived from berries, broccoli and orange to obtain ingredients rich in dietary fibre and with a high content of bioactive compounds ((poly)phenols, carotenoids and glucosinolates), which can be used as functional ingredients in the food industry to design and develop functional foods. Four different by-products: raspberries, mixed of berries, broccoli stalks and orange peels, were selected according to their content in bioactive compounds. First, fibre-rich fractions were extracted from the samples using an enzymatic and water-ethanol extraction processes. The fractions were nutritionally characterised and the total, soluble and insoluble dietary fibre content were also analysed and characterised by measuring the proportion of neutral sugars and uronic acids by using gas-liquid chromatography and spectrophotometry, respectively. Furthermore, the pectin, cellulose and hemicellulose proportion were theoretically calculated. The physicochemical properties were analysed to evaluate its potential physiological and technological functions. On the other hand, the content of extractable (poly)phenols (EPP), and non-extractable (poly)phenols (NEPP), as well as the glucosinolates content of broccoli stalk samples and carotenoids in the orange peel samples were determined by liquid chromatography. Total phenolic content, by the Folin-Ciocalteu method, and the total flavonoids were analysed by spectrophotometry, and ferric reducing antioxidant power (FRAP) and the oxygen radical absorbance capacity (ORAC) methods were applied to determine the antioxidant capacity of the fractions. To determine the prebiotic effect of the different fractions an in vitro digestion was performed. After that, in vitro fermentations with human faeces were carried out during 48 h, analysing short chain fatty acids (SCFAs) by gas-liquid chromatography and ammonium production as a measure of the microbial activity. Finally, the production of phenolic catabolites produced by the microbiota were analysed by liquid chromatography. The use of enzymatic and water-ethanol extraction methods allows the extraction of fibre-rich fractions, whose chemical characteristics depend on the initial by-product and the method used. All fibre fractions obtained showed antioxidant capacity ought to their content of (poly)phenols, however the content of EPP and NEPP was influenced by the extraction procedure. The extraction of the soluble fibre with ethanol, removed the EEP, while the insoluble fibre retained a higher proportion of both EEP and NEPP. In addition, due to (poly)phenols are bound to cell wall, the insoluble fibre-rich fraction showed a high content of NEPP. Moreover, glucosinolates were also found in the broccoli stalk fractions and carotenoids in the orange peel fractions. After in vitro fermentation, all fibre fractions showed a prebiotic effect leading to the production of SCFAs (acetate, propionate and butyrate), being correlated with the content of soluble fibre (hemicellulose and pectins). Parallelly, the formation of (poly)phenols catabolites produced by the microbial activity were evaluated. Urolithins formation was analysed from ellagitannins and ellagic acid in berry fibre-rich fractions; and phenolic acids, including phenylpropionic, phenylacetic and benzoic acid derivatives, from the flavanones and hydroxycinnamic acids catabolism in the fractions from orange peel. These catabolites are more bioavailable than the parent compounds, which during fermentation of the fibre-rich fractions obtained they can be absorbed in the colon, allowing their systemic distribution, and hence, may have beneficial effects on consumer health. In this sense, the fractions obtained can be used as potential ingredients for the development of functional foods with enhanced nutritional and functional properties.