Cocoa bean shell as a novel substrate for kombucha: fermentation dynamics and bioactive potential.

Fermented beverages have gained attention as functional foods for valorizing agro-industrial by-products rich in bioactive compounds. Cocoa bean shell (CBS), a by-product of cocoa processing, is rich in phenolics and methylxanthines. However, its behavior as a substrate in kombucha and its contribution to bioactivity remain unclear. This study aimed to develop and monitor kombucha prepared from CBS infusions (0.5-5.0% w/v) and evaluate its bioactive properties. Fermentation with SCOBY at 25 ± 1 °C for 7 days was monitored by pH, titratable acidity, soluble solids, and total phenolic content (TPC). Phenolics and methylxanthines were quantified by HPLC, and antioxidant and antimicrobial activities were assessed. Fermentation decreased pH and soluble solids, and increased titratable acidity. TPC was governed by CBS concentration and remained stable. (+)-Catechin and 4-hydroxybenzoic acid predominated, while theobromine was the main methylxanthine. The beverages showed concentration-dependent antioxidant and antimicrobial activity, partially preserved after neutralization, indicating bioactivity beyond organic acids.