Influência da temperatura e da diferença de escalas na produção de cafés fermentados

Abstract

The definitions for the control variables related to the use of different scales in the coffee fermentation process are still poorly studied to ensure quality in the production of specialty coffees. This process is directly related to the behavior of temperature in the fermentation medium, which defines the conditions for reproducibility and homogeneity throughout the bioreactors, whether on a pilot or industrial scale. This research aimed to evaluate the induced fermentation of Arabica coffee (Catiguá MG2 cultivar) in solid state for 60 hours, in bioreactors with different volumes (200, 1,000, and 14,000 L) and with different construction materials (steel and high-density polyethylene), without external temperature control. To this end, the concentration of some chemical compounds in the fruits (sucrose, glucose, fructose, glycerol, ethanol, and citric, malic, succinic, lactic, acetic, and propionic acids), the properties of the green beans (total color variation, yellowing index, breaking strength), and the sensory quality of the roasted beans for the samples fermented in each reactor were determined, in order to verify the feasibility of scaling up. In addition, two-dimensional mapping and empirical modeling techniques (R² ≥ 0.795) were used to evaluate the distribution of temperature over time and the horizontal and vertical positions of each bioreactor, an important factor for determining the uniformity of sugar consumption and the production of alcohols and organic acids for each scale. The temperature throughout fermentation influenced the kinetics of substrate consumption and metabolite production. Undesirable production of propionic acid, high concentrations of acetic acid, and a higher yellowing index were observed in reactors that reached temperatures above 27°C, at capacities of 1,000 and 14,000 L. In these coffees, the sensory descriptors sour and fermented were observed with high relative frequency, so these samples were not classified as specialty coffees. Fermentations in pilot-scale bioreactors (200 and 1,000 L) that had similar temperature profiles showed homogeneity and repeatability for most of the chemical compounds and physical properties measured. In industrial-scale fermentations (14,000 L), temperature and metabolite concentration gradients were observed throughout the bioreactor. The empirical models obtained reflected these behaviors, mapping the temperature distribution and most of the chemical compounds at the end of fermentation for the scales under study. From this, it was evident that fermentation can contribute to improving the quality of specialty coffee production, but greater attention need to be paid to controlling the operating temperature range, as well as the material used in the design and construction of the bioreactor for the proper processing of coffee as a fermented food, since there was greater variation in the responses evaluated across industrial-scale positions (14,000 L) than for the pilot-scale bioreactors (200 and 1,000 L).