By Dominique Fink, MSc Biology
Harvest is underway and pretty soon, musts will be fermenting. Magic will be operating and a new vintage will be born. Of course, we now know, thanks to the work of Louis Pasteur, that alcoholic fermentation is the result of sugars being metabolised into alcohol by Saccharomyces-typed yeasts. However, this process is far more complex than it seems. What are the scientific bases of alcoholic fermentation? This is what we will explore in this short article.
Microorganisms, much like humans, are adaptation “machines”. Saccharomyces cerevisiae is a prime example. This yeast thrives in environments with high osmotic pressure, such as grape must rich in sugars. Moreover, Saccharomyces-type yeasts have evolved to metabolize glucose and fructose under anaerobic conditions (without oxygen). This is the foundation of alcoholic fermentation. But its most remarkable adaptation is, without a doubt, its resistance to high alcohol concentrations. Few microorganisms, aside from Brettanomyces-type yeasts and certain lactic acid bacteria, remain viable at alcohol levels exceeding 13% v/v.
What is Alcoholic Fermentation ?
The goal of all living beings is to obtain energy from nutrients. For Saccharomyces yeasts, glucose is an excellent carbon source. Usually, glucose is completely metabolised to carbon dioxide and energy under aerobic condition. This process is referred to respiration.
Glucose (C6H12O6) + 6 O2 ——> 6 CO2 + ATP and heat
Alcoholic fermentation is the result of yeast adaptation to an oxygen-poor environment. It’s a survival issue! Under anaerobic conditions, glucose degradation is not complete and there will be production of ethanol and CO2.
Glucose (C6H12O6) ——-> 2 ethanol (C5H5OH) + 2 CO2
Reality is more complex. In sugar-rich musts, enzymes involved in the respiration process are inhibited. The phenomena is referred to as catabolic repression. Therefore, with or without oxygen, if yeasts are in the presence of high sugar such as in musts, alcoholic fermentation will occur and ethanol will be produced.
The Role of Nitrogen
Sugars are not the only compounds required for effective alcoholic fermentation. Yeasts absolutely need nitrogen to ferment sugars. A nitrogen deficiency will lead to either a stuck fermentation or a sluggish one. Another consequence of low nitrogen levels is the production of undesirable sulphur compounds. On the other hand, excess nitrogen will alter both the quantity and type of aromatic molecules. As a result, the wine’s aroma will be affected. That’s why must analysis is so important.
Other Molecules Produced During Alcoholic Fermentation
While the main effect of alcoholic fermentation is the production of alcohol from sugars, other molecules are also generated.
Glycerol, which contributes to the viscosity of wine, is produced from the very beginning of alcoholic fermentation. It influences the mouthfeel on the palate and gives the wine a smoother, more velvety texture.
Esters are molecules which contributes greatly to wine aroma. Several Saccharomyces strains have been carefully selected based on their hability to produce several types of esters (floral, fruity, citrus etc…). Your yeast provider or your œnologist will glady help you select the appropriate strain.
Higher alcohols are molecules from the alcohol family and are comprised of more than 2 carbon atoms (ethanol has 2 carbons: CH3CH2OH). Higher alcohols, like esters, play a very important role in wine aroma. The most prevalent molecules are isobutanol and amyl alcohols.
To Conclude...
Alcoholic fermentation is a complex process that stems from the remarkable adaptation of Saccharomyces-type yeasts to an initially hostile environment. Certain bacteria, such as Zymomonas mobilis, also have the ability to ferment sugars into alcohol. However, ethanol concentrations above 4% v/v are generally harmful to most microorganisms. As discussed earlier in this article, must analysis is essential to ensure a complete alcoholic fermentation. The same applies to end-of-fermentation analysis. Remaining residual sugars can lead to issues such as lactic spoilage. An End-of-Fermentation analysis is quick and inexpensive, and it helps effectively manage the risk of spoilage. Contact us at info@oenoscience.com or at 514-564-2050.
