| Summary: | The aim of the present study was the full characterization, quantification, and determination of the digestive stability and bioaccessibility of individual betalain and phenolic compounds of <i>Opuntia stricta, var. Dillenii</i> fresh fruits (peel, pulp, and whole fruit) and of the products of the industrialization to obtain jam (raw pressed juice (product used for jam formulation), by-product (bagasse), and frozen whole fruit (starting material for jam production)). <i>Opuntia stricta var. Dillenii</i> fruits and products profile showed 60 betalain and phenolic compounds that were identified and quantified by HPLC-DAD-ESI/MS and HPLC-DAD-MS/QTOF, being 25 phenolic acids (including isomers and derivatives), 12 flavonoids (including glycosides), 3 ellagic acids (including glycosides and derivative), and 20 betanins (including degradation compounds). In vitro gastrointestinal digestion was performed by INFOGEST<sup>®</sup> protocol. Fruit pulp showed the greater content of total betalains (444.77 mg/100 g f.w.), and jam only showed very low amounts of two betanin degradation compounds, Cyclo-dopa-5-O-β-glucoside (and its isomer) (0.63 mg/100 f.w.), and two Phyllocactin derivatives (1.04 mg/100 g f.w.). Meanwhile, fruit peel was the richer tissue in total phenolic acids (273.42 mg/100 g f.w.), mainly in piscidic acid content and total flavonoids (7.39 mg/100 g f.w.), isorhamnetin glucoxyl-rhamnosyl-pentoside (IG2) being the most abundant of these compounds. The stability of betalains and phenolic compounds during <i>in vitro</i> gastrointestinal digestion is reported in the present study. In <i>Opuntia stricta var. Dillenii</i> pulp (the edible fraction of the fresh fruit), the betanin bioaccessibility was only 22.9%, and the flavonoid bioaccessibility ranged from 53.7% to 30.6%, depending on the compound. In non-edible samples, such as peel sample (PE), the betanin bioaccessibility was 42.5% and the greater bioaccessibility in flavonoids was observed for quercetin glycoside (QG1) 53.7%, the fruit peel being the most interesting material to obtain antioxidant extracts, attending to its composition on antioxidant compounds and their bioaccessibilities.
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