## 2 The NMR-Based Metabolomics in Food Science and the Foodomics Approach
### The NMR-Based Foodomics Approach for Food Bio-Waste or By-Products Analysis
To guarantee their bioactivity, prebiotics have to (i) be resistant to gastric acidity; (ii) avoid absorption in the gastrointestinal tract; (iii) not be subjected to enzymatic activity; (iv) be fermented by intestinal microflora to produce short-chain fatty acids, and (v) be a selective substrate for the growth of intestinal bacteria associated to human health and well-being [95; 96]. The main prebiotic agents consist of nondigestible carbohydrates, such as polysaccharides (POS), oligosaccharides (OS), and soluble dietary fibers (mainly inulin) [96]. Prebiotic foods are largely requested on the market; thus, food industries are working on the development of sustainable bioprocesses that can guarantee OS from a waste of different sources [97; 98; 99]. An important resource of OS comes from lignocellulosic agricultural waste disposals. Apart from bioethanol generation, several studies demonstrated the conversion of lignocellulosic biomass into oligosaccharides, through both microbial fermentation and enzymatic hydrolysis, by microorganisms or enzymes [100; 101]. [PERSON] and [PERSON] [100], through enzymatic hydrolysis, produced Mannooligosaccharides (MOS) from agricultural waste and their structural aspects were assessed using both 1H and 13C NMR spectroscopy. MOS respect all the important criteria of being a prebiotic as they are (i) gastrointestinal resistant, (ii) an active substrate promoting the growth of healthy bacteria, and (iii) able to confer antineoplastic properties without any cytotoxicity to normal cells [100]. The capability of prebiotics to enhance the growth of healthy bacteria can be exploited for the production of important physiological metabolites. An example is given by recent research from [PERSON], et al. [102]. The study investigated the effect of different commercial prebiotics on enhancing natural gamma-aminobutyric acid (GABA) production in cultured yogurt. In addition, the metabolomics profile of the fermentation-derived biomolecules in yogurt has been obtained via NMR-based metabolomics. This approach was useful for comparing the major metabolite profile of freeze-dried GABA-rich yogurt (GY) and standard freeze-dried yogurt (SY). A total of 16 and 13 compounds were detected in GY and SY, respectively, and this difference may be due to the strain-specific metabolic activities of GABA [102]. According to pillar 1, described in the introduction, the choice of extraction methods to obtain any kind of BC is fundamental. Ethanol is an example of a green and sustainable bio-solvent as it is completely biodegradable and obtained by the fermentation of sugar-rich materials [103; 104; 105]. Also, dimethyl ether (DME), having a high cetane number and favorable carbon/oxygen ratio, emerged in the spotlight as a cleaner, environmentally friendly, and high-efficiency ignition fuel [106]. Thus, green solvents are among the preferred ones for extraction processes. For example, [PERSON], et al. [107], compared several ethanol-based solutions to find out the best one for the extraction of primary metabolites and inulin from \"Carciofo di Paestum\" (_C. cardunculus_ subsp. _scolymus_) PGI heads. For the first time, a comprehensive investigation of green extracts was performed by LC-MS and NMR analysis to highlight the occurrence of both specialized and primary metabolites, respectively. Among all the extracts, hydroalcoholic, infusion, and decoction extracts were the most interesting, showing higher peaks for flavonoids, quinic acid derivatives, and inulin than MeOH extract [107]. Inulin is a polymer found widely distributed in nature as a plant storage carbohydrate with a high value in human nutrition. It has prebiotic proprieties for its resistance to digestion in the small intestine, it can be fermented by colonic microflora, and it can stimulate the proliferation of commensal bacteria [107]. Hence, artichokes are a source of prebiotic dietary fibers but also a source of metabolites with antioxidant activity associated with their high phenolic content [108].