Colorimetric Quantification of Dopamine as a Bioactive Compound in Banana Peel Extracts: A Sustainable Approach to Food Waste Valorization

Dopamine (DA) is a bioactive catecholamine highly concentrated in banana peel (BPEEL), a common agri-food waste. In this study, a rapid and sustainable colorimetric method for DA quantification in BPEEL extracts was developed, based on the formation of a purple-blue melanochrome (MN) chromophore in a Mg2+/NH4+ buffer (150 mM each, pH 9.4) in DMSO/H2O (1:1 v/v), as confirmed by UV–vis analysis. The strong absorbance at 595 nm was monitored by standard microplate spectrophotometry. The assay was applied to quantify DA equivalent (DAE) in BPEEL extracts, yielding 5000 ± 200 μg DAE/g dry matrix, consistent with UPLC-DAD analysis (5700 ± 300 μg DAE/g) used as the validation technique, with excellent linearity and reproducibility (R2 = 0.9974, avRSD % = 1.94%). The colorimetric method’s detection and quantification limits were 1.8 ± 0.4 μg/mL and 5.4 ± 1.1 μg/mL, respectively, suitable for the expected DA levels in banana waste. Operating at the microliter scale and requiring minimal sample preparation, the proposed approach is fast, low-cost, and matrix-compatible, offering a simple and cost-effective analytical tool suitable for bioactive compound screening in food waste matrices.