A Multivariate Approach to Understand Kombucha Production: The Role of Spent Tea Leaves Addition During the Fermentation Process

Abstract

The global increase in tea consumption has led to significant amounts of tea waste, particularly spent tea leaves (STLs), which are commonly discarded despite retaining a significant quantity of fiber, proteins, and functional compounds. This study aimed to explore the reuse of STLs to produce a sustainable variant of kombucha, a beverage with health-promoting properties obtained through sweetened tea fermentation. Two kombucha formulations were prepared: a traditional one using fresh tea and an experimental formulation with the addition of post-brewing STLs directly during fermentation. The final goal was to understand the impact of STLs on fermentation dynamics, chemical characteristics, and volatile organic compounds (VOCs) profile. Microbiological analyses included total aerobic mesophilic bacteria, lactic acid bacteria, yeasts, molds, and acetic acid bacteria count, while chemical parameters such as pH, color, and °Brix were measured, alongside spectrophotometric quantification for bioactive compounds. VOCs were identified through gas chromatography-mass spectrometry (HSPME-GC/MS). The results showed that STLs effectively supported microbial fermentation, and fermentation started earlier compared to the conventional process. Distinct differences were observed in VOCs profiles especially for terpenes content, suggesting that the STLs directly influence aroma development. This research highlights the potential of STLs as a valuable ingredient in kombucha production and demonstrates how the fermentation dynamics could be modified by valorizing discarded tea residues.