In order to study how a fermenting grape must is cooled by water and ethanol natural evaporation, it is necessary to analyze the balances between the water + sugar + ethanol solution (and products assimilated to these three components) and the atmosphere which is superposed. This analysis, pursued according to the RAOULT and the DALTON laws, enables to express the cooling flux as a function of the sum of the partial pressures of water and alcohol vapour. A simple equation is deduced from literature data to express these partial pressures for the conditions of a fermentation. This equation, transposed into the cooling flux expression, simplifies it. It is no longer necessary to determine previously the molar fraction of the components, the activity coefficients, or the vapour pressure of each component.
Presented in an integral form (usable by a computer) in order to determine the energy amount exchanged during a fermenting phase, it emphasizes the influence of must temperature.
If this temperature is constant, and for any phase of the fermentation, the value of the cooling energy is approximately :
- proportional to the sugars degraded in the studied phase,
- a function of the temperature, expressed as an exponential form,
- a function of the alcohol average quantity of the must during this phase.
The obtained values are not negligible :
- the exchanged power can be, in absolute value, superior by 5 % to the heating power produced by the red wine fermentation reaction.
- The quantity of exchanged energy can be, in absolute value, over 4 % of the reaction heat, for a whole red wine fermentation.
These values, expressed for a fermentation temperature of 30° C are approximately double of the values obtained with a while wine fermentation at 18° C, with a similar initial sugar quantity.
At a practical level, and without taking into account their interest in the equipment calculation field, these expressions highlight very well the factors influencing the aroma disappearance during the fermentation, influence increased by the aromas volatility : temperature and primary sugar content.
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