Searching for Amaranthin—A Multipotential Betacyanin from Natural Sources and In Vitro Cultures

Amaranthin is a major red-violet betacyanin of Amaranthaceae and an increasingly relevant natural pigment for food, cosmetic, nutraceutical, and biotechnological applications. This review integrates knowledge from over 100 studies, addressing amaranthin as a chemically defined betalain, distinguishing it from other scientific uses of the term, and evaluates its natural sources, analytical methods, extraction strategies, in vitro production systems, biosynthetic regulation, and biological activity. Cultivated Amaranthus species are among the richest plant sources, with total betacyanins of 46.1–199 mg/100 g fresh weight and amaranthin comprising, on average, 80.9% of the pigment fraction. Reliable identification and quantification rely on high-performance liquid chromatography coupled with a diode array detector (HPLC-DAD), liquid chromatography-tandem mass spectrometry (LC-MS/MS), and ultraviolet–visible (UV–Vis) spectrophotometry. Microwave- and ultrasound-assisted extraction can improve pigment recovery under optimized conditions, although its stability depends strongly on pH, temperature, solvent, time and storage parameters. While plant in vitro cultures, including callus, suspension, and shoot systems, have clarified biosynthetic regulation and offer controlled production platforms, engineered yeast systems have recently expanded production options, with Yarrowia lipolytica reaching 2.97 ± 0.029 g L−1 amaranthin in fed-batch fermentation. Amaranthin-rich extracts and amaranthin-type pigments show antioxidantand anti-inflammatory potential, while antimicrobial and antiviral activities have mainly been reported for mixed betacyanin fractions; direct mechanistic, bioavailability, and in vivo evidence for purified amaranthin remains limited. Standardized analytical protocols, further investigation of stable high-yield sources, physicochemical stability assessment, and structure–activity studies are identified as priorities for advancing future application-oriented research on this multipotential pigment.