Wine cellar in the Agronomy and Viticulture Experimentation Unit at the INRA center in Colmar. Opening of a cask used in a microvinification experiment. © INRA, MAITRE Christophe

Yeast: how wines get made

Optimizing aroma formation during fermentation

INRA scientists have developed a model for studying how wine fermentation is affected by three major factors: nitrogen concentration, lipid concentration, and temperature. The model can be used to simultaneously optimize parameter values with a view to producing several fermentation-derived aromas.

By Pascale Mollier, translated by Jessica Pearce
Updated on 12/23/2016
Published on 09/28/2016

Fermentation: a process that transforms sugar into alcohol

During yeast-mediated fermentation, sugar is converted into alcohol. Most of the time, the goal is to produce a dry wine, and thus fermentation must be allowed to proceed to completion, leaving no residual sugar. Optimizing fermentation remains a major challenge for winemakers and researchers, especially when the goal is to limit alcohol production (1). Recently, advances have been made thanks to a better understanding of how variation in nitrogen concentration and temperature affects fermentation rate and efficiency. Indeed, licensed software has been developed by the microbiologists of the Sciences for Oenology Joint Research Unit that winemakers can use to simulate how these parameters impact fermentation time.

Nitrogen concentration and temperature affect aroma profiles

The production of 2-phenylethanol, a compound with a mild rose-like odor, is optimized at a nitrogen concentration of 200 mg/l and a temperature of 28°C (in blue). © INRA
The production of 2-phenylethanol, a compound with a mild rose-like odor, is optimized at a nitrogen concentration of 200 mg/l and a temperature of 28°C (in blue) © INRA

Fermentation by yeasts creates aroma in addition to alcohol.

Using a yeast strain that produces large quantities of aromatic compounds, the microbiologists of the Sciences for Oenology Joint Research Unit have shown that aroma formation is largely influenced by nitrogen concentration and temperature, with lipid concentration playing a more limited role. This research came out of a collaboration with the company Lallemand and has led to a model that can predict formation dynamics for five aromas using initial nitrogen concentration and temperature.

Research scientist Jean-Roch Mouret explains, “This mathematical model is not quite ready for commercial use because it was parameterized using data obtained under laboratory conditions, artificial grape must, and a single yeast strain (see sidebar 2). However, we will soon be testing it using data from experiments involving natural grape must and a variety of yeast strains. Our hope is that, a few years from now, this model for aroma production will be paired with the model for ethanol production, which will allow winemakers to optimize both.”

Following aroma production in real time: a one-of-a-kind tool

Production dynamics of isoamyl alcohol, an aromatic compound. © INRA
Production dynamics of isoamyl alcohol, an aromatic compound © INRA

Mouret continues, “Our approach allows aroma formation to be followed in real time. We have shown that the production of fermentation-derived aromas is greater during secondary than primary fermentation, which is an unexpected result.” Thanks to this discovery, it is now known that it is better to fine-tune nitrogen concentration and temperature during secondary fermentation if the goal is to maximize yeast-mediated aroma formation.

 

(1) As a result of climate change, grape sugar levels are climbing. This results in wines with higher alcohol contents. INRA scientists are currently tackling this issue from several different angles: by reducing the sugar content of the grape must (i.e., carefully selecting the grape varieties used), by partially extracting ethanol from wines using membrane-based techniques, and by employing a patented yeast strain, developed by INRA and Lallemand, which uses some of the available sugar to create glycerol (see article 4 of this report).

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Microbiology and the Food Chain
Associated Centre(s):
Montpellier

References

- Stéphanie Rollero et al.2015. Combined effects of nutrients and temperature on the production of fermentative aromas by Saccharomyces cerevisiae during wine fermentation. Appl Microbiol Biotechnol. 99 : 2291–2304 DOI 10.1007/s00253-014-6210-9.

- Jean-Roch Mouret et al. 2015. Prediction of the production kinetics of the main fermentative aromas in winemaking fermentations. Biochemical Engineering Journal 103, 211–218.

the experimental model

In these experiments, INRA scientists have been using a yeast strain that produces large quantities of aromatic compounds, facilitating aroma detection and measurement. They have also been using artificial grape must, whose composition can be carefully controlled. Using gas chromatography, the fermentation-derived aromas released from the grape must are identified and quantified. A sample of the volatiles is taken automatically every hour; their concentration is a proxy for the concentration of aromatic compounds found in the wine itself.

Wines contain hundreds of aromatic compounds

Wine contains over 1,500 different substances. In addition to aromatic compounds, there are simple and complex sugars, ethanol, glycerol, various minerals, and polyphenols (anthocyanins and tannins). The aromatic compounds can be classified into three categories:

  • Compounds specific to grape type (e.g., the profile of a Muscat wine)
  • Compounds resulting from fermentation, such as higher alcohols and esters
  • Compounds resulting from aging, such as the compounds released into wines by oak barrels

There are over 100 fermentation-derived aromas. For instance, esters create an impression of fruitiness, a characteristic that is becoming more and more popular among consumers. The pathways involved in aroma synthesis are complex, and their regulatory mechanisms are poorly described. Aroma creation largely depends on the yeast strain used, but other environmental factors, such as nitrogen concentration, lipid concentration, and temperature, also have a large impact.