Upcycling blueberry pomace and spent coffee grounds into 10-catechyl-pyranoanthocyanins via short Lactiplantibacillus plantarum fermentation

10-catechyl-pyranoanthocyanins (cPACNs) are formed by reaction between anthocyanins (ACNs) and caffeic acid (CA) or 4-vinylcatechol (4VC) providing colors with enhanced stability. cPACNs are scarce in nature and current methods to produce them are considered expensive, limiting their application as plant-derived colorants. Our study evaluated cPACN production from spent plant materials containing the precursors and nutrients needed for a short Lactiplantibacillus plantarum fermentation to accelerate production. ACNs and CAs were extracted from blueberry pomace and spent coffee grounds by different extraction techniques. Extracts were fermented at pH 6 and 37 °C for up to 120 min. After each time point, bacteria were removed and the extract was incubated at pH 2.7 and 48 °C for an additional 22 h. Phenolic changes (HPLC-PDA, HPLC-PDA-MS/MS), spectra (380–700 nm), color (CIELAB), and pH were monitored throughout the process. In all extracts, L. plantarum transformed CA into 4VC (half-lives <60 min), a preferable structure for cPACN formation. After 22 h, cPACNs formation yields were around ~40–65%, affected by extraction method and fermentation time. Color of extracts turned from a red hue (4.9°–12.4°) to red-orange (17.5°–30.5°) after cPACN formation. The major cPACN was malvidin-3-galactoside derivative. cPACN formation from spent plant materials was feasible.