Application of Ohmic Heating for Green Coffee Bean Fermentation: Effect of Voltage Gradient on the Heating Performance, Energy, EC, pH, and Fermentation Index

PURPOSE: Fermentation in the form of green coffee beans has the potential to alter the flavor and even produce new flavor profiles. However, existing coffee fermentations were still performed traditionally and used ineffective fermenter equipment. In this study, ohmic heating (OH) was first applied to the green coffee bean fermentation. The impact of ohmic heating on the fermentation of green coffee beans was examined. METHODS: The experiments were performed using green coffee beans, water, and a 1% of instant dry yeast (S. cerevisiae). Three voltage gradient (VG) levels were used as follows: 4, 8, and 12 V/cm. Non-OH fermentation (0 V/cm) and coffee beans without fermentation were also evaluated as controls. The observed parameters included voltage, electrical current, electrical conductivity (EC), power, total energy consumption, coffee bean temperature, environmental temperature, pH of the fermentation medium, and the fermentation index of coffee beans. RESULTS: The results showed that OH was effective in generating heat for the green coffee bean fermentation process. The set point temperature was reached faster with increasing VG. The coffee bean temperature was successfully maintained throughout the fermentation process for all VG variations. The variation in VG significantly impacted the electrical current, power, and time needed to reach the set point temperature but did not have a significant effect on total energy consumption and EC. During fermentation, the EC of the green coffee beans with instant yeast solution increased due to an increase in electrical current. The final pH value was significantly different between OH and non-OH fermentation, with OH fermentation resulting in the highest fermentation index. CONCLUSIONS: The findings of this study indicate that OH treatment can significantly improve the fermentation process of green coffee beans indicated with lower pH and higher fermentation index.