Alcoholic fermentation, a key stage in winemaking, is likely to change considerably in the future:
(1) winemakers are increasingly of the opinion that tools for controlling fermentation according to the type of desired wine and the characteristics of the must are becoming and will remain absolutely essential;
(2) the use of sensors for real time monitoring of fermentation is becoming increasingly feasible, even at the industrial scale.
Several systems have been designed for monitoring the transformation of sugar into alcohol and CO2. One major challenge for the future, in which we work through CAFÉ, is to act directly on wine characteristics by on-line measuring ‘quality markers’. We have developed an experimental device to on-line measure the production of volatile compounds in the off-gas of fermentations run at the pilot scale (100 L tanks).
16 carbon compounds considered as markers of organoleptic and/or metabolic interest and corresponding to the main higher alcohols, esters and aldehydes are measured every hour by gas chromatography. The high frequency measurement enables us to access to a very precise description of their synthesis.
Example of GC analysis
The composition of the fermenting must is also off-line measured to study the relationship with the gas composition and to consider the whole synthesis of the marker molecules. All these data are of major interest to improve our understanding of yeast metabolism, but also to elaborate new control strategies with, for example, a maximization of the production of the molecules of interest and a minimization of the losses in the off-gas, by acting on fermentation parameters such as temperature, nitrogen or oxygen additions. In the near future, we will also on-line measure the main volatile sulphur compounds in order to take into account some main markers of defaults.
Another challenge, for the development of optimized control strategies is the modeling, and therefore the prediction of the synthesis of these marker molecules. Such modeling is very ambitious because the involved metabolic pathways represent minor parts of the yeast metabolism. A first necessary step is the development of a model representing the main physiological phenomena observed during the fermentation in order to later extend it with flavour-markers equations. Another difficulty is to consider continuous changes in the metabolism throughout the fermentation process. A multi stage bioreactor which mimics a batch fermentation with different steady states has been developed to test control and optimization strategies.
Continuous multistage bioreactor
Industrial developments are expected by the end of the program. Indeed, a company is developing a prototype for the control of fermentation kinetics that will be incremented with results from the CAFÉ program.