13 Sep Alcoholic fermentation: how is it ?

Managing alcoholic fermentation

Alcoholic fermentation is the key process in the transformation of grapes into wine: for each molecule of the sugars -glucose or fructose- two molecules of ethanol and two of carbon dioxide are produced.

C6H12O6 → 2 CH3CH2OH + 2 CO2

The protagonist of this transformation (in which many other compounds essential to the quality of the wines are also produced as by-products of the fermentation process or by other metabolic means) is the Saccharomyces cerevisiae yeast.

A fermentation is considered correct when the population of the yeasts, both those indigenous present in the must or, better yet, those inoculated from a selected culture, quickly prevail upon the undesired micro-organisms (such as bacteria, for example), having a positive commencement and proceeding correctly in the consumption of the sugars up until the point of their total depletion.

For their metabolism, yeasts require (naturally, besides sugars) nitrogenous substances (capable of assimilating both inorganic nitrogen in the form of ammonium ions and amino acids), growth factors (the vitamins and mineral salts that function as cofactors in many enzymatic activities) and survival factors (saturated and unsaturated fatty acids and sterols that form the cellular membrane guaranteeing their functionality).

The principal risk, when the lack of some of these elements provokes stress to the yeasts, is that they cease to consume sugars and the cells progressively die. In these cases, one speaks of a stunted fermentation and, in the worst cases, of arrested fermentation.

In the cellar, wine producers and oenologists are equipped with certain instruments to manage the needs of the yeasts and avoid risks: the nitrogenous nutrients (ammonium salts and derivatives of the yeast that are rich in amino acids and membrane lipids), oxygen (necessary for the synthesis of unsaturated fatty acids and sterols) and temperature (which must be maintained within a range, typical to each strain, optimal for the development of the yeast).

Control above all else

But how should these instruments be utilised? For the correct management of the fermentation and the health status of the yeasts, control is the basis of everything and needs to provide a respond to the questions: How is my fermentation proceeding? How are my yeasts?

The most common fermentation verification in the cellar is the indirect measurement of the consumption of sugars determined by the densimeter: passing from a sugar solution to a hydroalcoholic solution, the must progressively diminishes in density, with such decreasing being proportional to the reduction of sugars in the solution.

The density expressed on a different scales (Babo, Oeschle, etc.) is measured in a sample, considered representative of the mass in fermentation (assuming there is homogeneity within a vat), once or, less commonly, twice daily.

The research carried out especially in the two most active schools in the study of alcoholic fermentation management, that of Jean Marie Sablayrolles at the INRA in Montpellier in France and of Linda Bisson at the University of California, Davis, since the end of the 1990s have evidenced that the parameter best describing the characteristics of quality and the critical points of the fermentation process is the instantaneous velocity of fermentation.

The fermentation speed has a typical evolution for each phase of the growth curve for the yeast population (start-up, exponential growth, stationary and death phases). With a typical (and optimal) curvature in the initial phase, it increases until reaching a maximum peak of approximately 1/4 of the consumption of the sugars, only to then diminish whilst remaining constant before decreasing rapidly upon the complete exhaustion of the sugars.

Certain typical points on the fermentation velocity curve can provide fundamental informations for the evaluation of the fermentation quality and for its correct management: the maximum speed obtained at the end of the exponential phase, the time employed to reach such a speed and the slope of the fermentation velocity curve at the point of transition in the phase from maximum speed to the stationary phase.

The studies undertaken by Sablayrolles have also defined that the best period for the addition of nitrogenous nutrients and for the oxygenations necessary for the synthesis of the lipid compounds of the membrane ranges from the moment of maximum velocity (at around 1/4 of the alcoholic fermentation) to mid-fermentation.

When the fermentation velocity diminishes in an anomalous mode, above all after having reached its maximum peak and during the stationary phase, deviating from this typical evolution, it may be that the yeast is up against a situation of stress and is not longer able (due to the lack of nitrogenous substances or of a state of inefficiency of the cellular membrane) to assimilate and transform the sugars.

Monitoring carried out at daily intervals, such as that executed with the densimetric measurement, is often unable to detect neither the moment in which the maximum peak of velocity is achieved (and thus the best moment for technological interventions of nutrition and oxygenation) nor its level and is not even able to intervene in a timely manner in the event of a slowdown.

Dynamic Analysis of Fermentation Kinetics (ADCF – Analisi Dinamica della Fermentazione)

Following a study that commenced in 2004 in collaboration with the The Department of Agricultural and Forest Economics, Engineering, Sciences and Technologies of the University of Florence, and stemming from the necessity of measuring parameters such as the fermentation velocity and other critical points with accuracy and promptness, Parsec has developed a system suitable for the continuous monitoring of the fermentation progress, through the direct measurement of one of the products of the transformation of sugars, CO2.

The ADCF system (Dynamic Analysis of Fermentation Kinetics) provides a continuous and very precise measurement of that which is occurring in the entire fermentation mass, measuring the quantity of CO2 produced by the yeasts, without being influenced by the effect of the sampling.

To render an idea of the volumes of CO2 that should be measured, we can consider that the fermentation of a must containing 220 g/l of sugar (about 13° V/V in potential alcohol), produces about 55 litres of CO2 per litre of must. This is a very high volume, if taking into account that only 1% is kept in dissolved form within the liquid, while all the rest will be released externally.

With the volumes of gas of this type, one is led to believe that the measuring should not be difficult. Actually, this is not the case at all. The volumes of CO2 that are produced in the fermentation are not liberated at a constant velocity and a reliable system necessitates an identical precision in the detection of gas produced at each interval (from the lowest to the most elevated) and throughout the entire period in which the process should be monitored. A system able to oversee with precision only the initial phase when the volumes produced are elevated, for example, is not useful in that it does not allow for the detection of any eventual slowdowns in the final and most critical phases for the completion of the sugar consumption.

The existing ADCF system was perfected after a thorough evaluation of the sensors available on the market and the gas flow measurement systems, being reliable and accurate yet efficient only in a limited CO2 range, unable to guarantee a uniform accuracy and reliability throughout the entire interval in which it is necessary to measure the fermentation progress. ADCF rather offers a closed system applicable to any type of tank (whereby equipped with a hatch) that keep the vat in marginal overpressure, measuring even the slightest variations in pressure and temperature owing to the production of CO2.

The fermentation velocity of each tank can be monitored in real-time with ADCF. Data management via the integrated Saen5000 system not only permits the remote archiving and visualisation of the curvature, but also to communicate and interact with the fermentation parameters together with other functions managed by the system, from the temperature control to the oxygenation or remontage operations.