Fermentation process turns unripe coffee beans into high-scoring specialty drinks

In the selection of specialty coffees, those that score above 80 points in blind tests are free of physical and sensory defects. Greenish-colored beans are known to give the drink an astringent taste, which is described as harsh, pungent, and dry. These beans are therefore discarded, along with broken, black, burnt, pitted, or undersized beans.

However, in a study published in the journal Food and Bioprocess Technology, researchers from the Federal University of Uberlândia (UFU) in Patos de Minas (state of Minas Gerais, Brazil) conducted a series of fermentations with ripe and unripe fruits of the Arara cultivar of Arabica coffee (Coffea arabica).

As a result, they obtained beverages comparable to and even superior to those prepared with beans from only ripe fruits, following all the protocols of the Specialty Coffee Association (SCA), an international organization that sets standards for specialty coffees.

In blind cup tests, which evaluate coffee based on its sensory attributes, professional tasters (known as Q-graders) gave beverages containing a percentage of unripe fruit beans scores above 80, which defines specialty coffee.

The researchers achieved these results using self-induced anaerobic fermentation (SIAF), in which the fruits are placed in bioreactors—200-liter polystyrene barrels that are hermetically sealed—for up to 96 hours after harvesting. No oxygen enters the bioreactors, and carbon dioxide is released through a valve.

The microorganisms naturally present in coffee fruits then carry out a series of biochemical processes that result in a distinctive coffee flavor. In some experiments, inoculants—specific microorganisms previously isolated for this purpose—were added to this type of fermentation.

“With this work, we saw that using SIAF at different fermentation times, with temperature and pH control and with or without the addition of inoculum, can not only minimize the deleterious effects of immature beans on the beverage but also make it superior, adding value to the product while still on the farm,” says Luiza Braga, first author of the study, conducted as part of her master’s degree in the Graduate Program in Food Engineering at the Faculty of Chemical Engineering (FEQ-UFU) in Patos de Minas.

“Anaerobic fermentation, carried out immediately after harvesting and before drying, is not a traditional process. However, coffee growers and experts have been seeking knowledge about the process because of the gain in flavor and aroma it brings to the drink, which can then fetch higher prices than those usually found on the market,” says Líbia Diniz Santos, a professor at FEQ-UFU and coordinator of the study.

The Arara cultivar was launched in 2012 by the Procafé Foundation after 15 years of research in search of a disease-resistant coffee adapted to the diverse climatic conditions of Brazil’s Cerrado savanna biome. The drink is valued for its citrus notes and robust body, making it attractive to both the domestic and export markets.

The authors of the study used an artificial intelligence tool developed by the research group and noted that 70% of the fruits used in the experiments, which were harvested at Fazenda Chuá in Patos de Minas, were immature.

Despite using green beans in the beverages tasted by the panelists, the authors emphasize that the other SCA criteria were strictly followed. Thus, broken and small beans were discarded during preparation. Consequently, green beans represented 13% to 30% of the total beverage.

“We believe that if there had been 70% green beans in the beverage, even if fermented, this would have been noticeable in the final product,” Santos points out.

A total of 32 treatments were tested, including different fermentation times ranging from 24 to 96 hours with and without temperature control. The combinations also included the presence or absence of inoculum, as well as submerged fermentation with 30% of the bioreactor filled with water or fermentation in a solid state without water.

The group developed an electronic device that monitors the pH and temperature and transmits the data from sensors inside the bioreactor to an external monitor. This eliminates the need to open the barrel and interfere with the experiment in order to collect information.

“When we controlled the external temperature at 27 °C, we observed that the scores were higher, even higher than those of preparations containing only ripe beans. With this, we can demonstrate that anaerobic fermentation, especially in a solid state, adds sensory attributes that elevate the coffee to the special category,” Braga explains.

The group now intends to understand which compound or compounds generated in the fermented green bean provide the sensory attributes that give the coffee its special qualities. Future work will also explore the effect of anaerobic fermentation on other coffee varieties.