Untargeted metabolomics for the characterization of wheat, rice and some of their coproducts applying conventional, deep eutectic solvents and in situ biotransformation

Resumen

Wheat (T. aestivum) and rice (O. sativa L.) are the most important cereals in the world, being among the top three crops. The nutritional benefits of cereals are notably related not only to their dietary fibers but also to their phytochemicals. Cereal grains and their coproducts are rich in secondary metabolites showing a wide spectrum of bioactivities. The class of major interest is the phenolic compounds (PC) due for instance to their protective effect against the development of chronic diseases and great antioxidant capacity. PC composition is strongly affected by genetic and external factors throughout the grain growth until grain processing. PC can be found in soluble, soluble-conjugated or insoluble forms linked to cell wall components and are concentrated in the grain outer layers. These outer layers also enclosed important amount of bioactive polar lipids (PL) such as phospholipids that are the backbone of cell membranes. PC extraction should be considered as a pre-treatment allowing the development of dietary or pharmaceutical formulations. PC have a moderate to low solubility in apolar media that restricts their application in oil-based products. Improving their amphiphilic character by biotransformation (e.g., esterification with acyl chain) can be a strategy to preserve their functions and enhance their release. The choice of the solvent to extract PC is crucial to maximize yield, recovery, and subsequent application. Natural deep eutectic solvents (NaDES) appear as green alternative to conventional organic solvents due to their capacity to improve the solubility, stability of some amphiphilic bioactive compounds and to their good enzymatic compatibility. The chemical complexity of these extracts can be resolved by metabolomics techniques that allow a broad and reliable identification of compounds with different physicochemical properties. In this study, the comprehensive characterization of bioactive components, such as PC but also some lipids (including PL), extracted based on different strategies from several genotypes of wheat, rice and their coproducts, were performed by ultraperformance liquid chromatography coupled to mass spectrometry en tandem (UPLC-MS-MS). Wheat metabolomic analysis revealed different profiles of PC, with 237 PC identified during grain maturation and the highest diversity and abundance in immature grains. Despite of the milling process, refined wheat flours presented an interesting phenolic profile with 43 PC identified. Undoubtedly, extracts from aleurone and wheat bran enclose a distinct set of PC (globally 44 PC). Stabilized rice brans (RB) showed high PC contents, especially for pigmented rice (89 PC in red and black RB), together with an interesting amount of unsaturated fatty acid. Lipidomics revealed the presence of bioactive lipids, 163 lipids in RB (e.g., PL and oxylipins) in a large collection of RB and highlighted putative modulations in the lipid profile in human plasma (e.g., endocannabinoids) associated with RB consumption. Due to this interesting chemical profile, RB was selected for the one-pot enzymatic biotransformation, combining PC and lipids to obtain enriched potentially esterified or transesterified extracts with enhanced protection against lipid oxidation. We could evidence that NaDES can be used as reaction medium and remain in the formulation, unlike conventional solvents. Chromatographic changes after biotransformation suggests a structural rearrangement between PC and lipids and a pro prooxidant effect of the NaDES tested was evidenced. NaDES showed a specificity to extract some PC and proved to be a sustainable media for in situ biotransformation of the PC using a lipase.