Experimental investigation of heating pattern, energy requirement and electrical conductivity in a batch ohmic heating system for coffee fermentation

Abstract

This study investigated the effect of voltage gradient (VG) and process capacity on the heating pattern and energy requirement during temperature conditioning by ohmic heating for coffee fermentation. Three levels of VG (8, 12 and 16 V/cm) and batch capacity (50, 75 and 100 g) were taken as independent variables using a full-factorial design. The setpoint temperature was 37 °C. Experimental research revealed that temperature conditioning for coffee fermentation can be performed properly using ohmic heating. The temperature distribution in coffee beans had a high level of uniformity, and the temperature could be maintained according to the setpoint temperature with a minimum deviation value. It was found that an increase in VG and batch capacity increased the electric current, power, and heating rate. Specific power at three levels of capacity was relatively the same and increased as the VG increased. As the VG and capacity increased, efficiency tended to increase while specific energy consumption (SEC) decreased. This result demonstrated that a high-capacity coffee fermentation was recommended for low SEC and high efficiency (minimum heat loss). Electrical conductivity (EC) of coffee beans was also studied during the test where VG affected the EC of coffee beans, also final EC at 37 °C always showed significantly higher (p-value<0.05) than the initial EC at 25–27 °C for all VG variations. Furthermore, the established mathematical models were found to be fit for estimating the amount of electrical current and power requirement with R2 > 0.97. Industrial relevanceIn the coffee fermentation, to maintain the consistency of the coffee produced, a controlled and consistent process is needed by using a fermenter (fermentation vessel). Ohmic heating is a promising technology to condition the optimum temperature in the fermentation process with high efficiency and uniform heating. To develop an industrial scale ohmic type fermenter for coffee fermentation and its energy requirements, an in-depth study of the heating pattern, energy requirement and electrical conductivity is essential. The present paper contributes to the disclosure of the heat generation process in Robusta coffee beans and investigate the heating pattern, energy requirement, and electrical conductivity during ohmic heating. Several empirical models generated from this investigation could be useful for the coffee industry regarding the development and scaling up of ohmic heating-based coffee fermenters.