Optimization of kombucha production using organic green tea (Camellia sinensis) and specialty coffee (Coffea arabica): Production optimization, physicochemical composition, microbial viability, and sensory evaluation

Fermented drinks have long held a place in human culture, bridging the gap between simple preservation and complex flavor. At the heart of this tradition is kombucha, a beverage created when a specific community of microorganisms, known as a symbiotic culture of bacteria and yeast, transforms sweetened tea into a tangy, fizzy drink. This living culture, often called a SCOBY, works by consuming sugar and converting it into organic acids, ethanol, and carbon dioxide. The result is a beverage that is naturally carbonated and rich in bioactive compounds, which has sparked a global surge in interest for its potential health benefits, from antioxidant properties to anti-inflammatory effects. While traditional kombucha relies on tea, the modern food landscape is constantly seeking new ways to diversify flavor and function. Coffee, with its own rich profile of aromatic compounds and acids, presents a compelling candidate to mix with tea, but the challenge lies in understanding how these two distinct ingredients interact during fermentation without disrupting the delicate balance of the microbial community.

Researchers at the Federal University of Bahia set out to explore this interaction, aiming to create a new type of kombucha that blends organic green tea with specialty coffee. They did not simply guess the right amounts of each ingredient; instead, they treated the brewing process like a precise scientific experiment. They prepared seventeen different batches, each with a unique combination of green tea, coffee, and sugar. By systematically varying these amounts, they could observe how the specific mix influenced the final drink's acidity, chemical makeup, and taste. Their goal was to find the "sweet spot" where the coffee added interesting flavors and health benefits without making the drink too sour or killing the beneficial bacteria needed for fermentation.

The results revealed that the type of tea used was the most powerful driver of the fermentation process. When the researchers increased the amount of green tea, the drink became significantly more acidic, producing higher levels of both volatile and total acids. This happened because the tea provided essential nutrients that stimulated the bacteria to work more vigorously, converting more sugar into acid. In contrast, the amount of coffee and sugar played a different role, primarily influencing the drink's pH level, which determines its overall sourness. The study found that while coffee added a complex aromatic profile, it did not significantly boost the production of acids on its own. Instead, the coffee's presence seemed to interact with the tea in ways that modulated the microbial activity, creating a unique environment for the bacteria and yeast to thrive.

After identifying the most promising batches, the team analyzed them in depth to understand their nutritional value, color, and the specific compounds that gave them their aroma. They discovered that the best-tasting drinks were not necessarily the ones with the highest levels of beneficial compounds or the most intense acidity. The formulations that received the highest scores from taste testers were those that retained a moderate amount of residual sugar, resulting in a balanced flavor profile that was neither too sour nor too bitter. These successful batches, which contained higher levels of sugar and intermediate acidity, were also rich in esters and higher alcohols, chemical compounds produced by yeast that contribute fruity and malty aromas. This suggests that for consumers, the enjoyment of the drink relies heavily on a harmonious balance between sweetness and acidity, rather than just maximizing the concentration of health-promoting substances.

Conversely, the researchers found that pushing the ingredients to their extremes did not yield the best results. One formulation, which used a very high concentration of tea, produced a drink with the highest levels of phenolic compounds and acidity. While this might sound desirable for a health drink, it actually resulted in the lowest sensory acceptance. The intense sourness and a specific chemical profile dominated by ketones rather than fruity esters made the drink less appealing to the palate. Similarly, while coffee contributed unique aromatic notes like roasted and nutty scents, simply adding more coffee did not automatically improve the drink's quality. In fact, some coffee-heavy batches showed reduced activity from the acid-producing bacteria, which altered the fermentation process and preserved more of the raw coffee aromas but at the cost of the traditional kombucha character.

The study concludes that creating a successful coffee-infused kombucha requires a careful calibration of ingredients rather than simply maximizing the amount of either tea or coffee. The most viable products emerged from a balanced interaction where the sugar content was sufficient to support a robust fermentation but left enough residual sweetness to please the consumer. The researchers demonstrated that it is entirely possible to introduce coffee as a substrate for kombucha, creating a beverage with a distinct chemical and sensory profile. However, the path to a market-ready product lies in finding the precise ratio where the microbial community remains healthy and active, the acidity is pleasant, and the complex aromas of both tea and coffee merge into a cohesive, enjoyable experience. This work provides a clear roadmap for producers looking to innovate in the fermented beverage sector, showing that the key to a new flavor lies in the delicate equilibrium of the ingredients.