Metabolomic insights into the phytochemical profile of cooked pigmented rice varieties following in vitro gastrointestinal digestion

The untargeted metabolomic profile of five pigmented whole rice varieties, namely Ermes, Orange, Nerone, Venere, and Violet, was investigated. Samples were cooked to a comparable starch gelatinization level through an electric commercial rice cooker and in vitro digested to evaluate the fate of rice compounds during in vitro starch digestion. The untargeted metabolomics profiling depicted a broad phytochemical profile of rice grains, including phenolic compounds, terpenoids, and phytosterols, among others. Interestingly, the metabolomic profile showed distinctive shaping following cooking and in vitro digestion in a variety-dependent manner. Multivariate statistics revealed that glycosylated flavonoids and hydrolysis-products of tannins (such as 1,2-digalloyl-beta-D-glucopyranose) were the most discriminant compounds of the in vitro gastrointestinal digestion. The combination of cooking and simulated digestion was found to affect, in a matrix-driven manner, the profile of amino acids and lipids derivatives, together with sterols and terpenoids. Cooked Violet rice grains showed the lowest starch hydrolysis index (i.e., 58.2) and the highest resistant starch content (i.e., 4.8 g/100 g) compared to the other samples. Non-pigmented phenolic compounds could be included among discriminant compounds. After combining cooking and enzyme hydrolysis, Violet rice distinctively showed positive average LogFC values for flavonoids and phenolic acids. Overall, water-soluble anthocyanins were the most retained class of polyphenols following the cooking process, with peonidin 3-(6”-acetyl-glucoside) showing the highest discriminant potential (VIP score = 1.64).