Thermal effects of pump-overs during red wine fermentation

•Thermal effects pump-overs have been evaluated at industrial scale.•Pump-overs were able to significantly decrease the juice temperature.•The overall heat transfer coefficient was used to predict the temperature decrease. The aim of this paper is to measure the thermal effect of pump-overs during winemaking. Temperature control is a key issue in red grape alcoholic fermentation. Aerated and non-aerated pump-overs are usually performed during fermentation in order to improve the extraction of flavanols, tannins and anthocyanins. They have two main effects on grape juice temperature: they reduce the temperature gradient within the juice (homogenizing effect), and they increase the efficiency of heat exchange, thus they reduce the juice temperature. The experiment compares the effect of pump-overs on fermentation temperature in various conditions. It finds a decrease ranging from 1.9±0.5°C in Test 1 to 4.6±2.7°C in Test 3. Thus, enhanced heat exchange during the pump-over results in faster juice cooling. This heat exchange is modeled mathematically. The overall heat transfer coefficient (U) has been calculated. It is based on the tank’s characteristics (i.e. the materials used and the thickness of the wall) and two convective coefficients: the first between the juice and the tank’s wall, and the second between the tank’s wall and the environment. Pump-overs increase the first coefficient, and consequently U. Therefore, the effect of a pump-over on grape juice temperature can be easily predicted if U has been calculated. Furthermore, if the overall heat transfer coefficient and the exchange surface are known, the pump-over temperature decrease depends only by the temperature difference between the juice and the environment (or cooling system). Once this temperature decrease is known, pump-overs can be used to improve the temperature control during red grape fermentation. For instance, the frequency of pump-overs can be used when the temperature control with the cooling systems is more difficult.