Welcome to the fifth part of the series Terroir & the chemistry of wine. We have looked at soil and climate, at acids and pH, at phenols and at the aroma compounds. Now we step into the engine room itself: fermentation, where the sugar of the grape becomes alcohol, and where a large part of the wine's character is actually created.
This is the central step between must and wine. If you understand what happens biochemically, you also understand why temperature, nutrition and the yeast's own limits matter so much for the finished glass. We stay with the chemistry and its consequences in the taste, and we go into depth, because this is an expert part.
Hvad du lærer
- How grape sugar is converted into alcohol through the yeast's metabolism
- What role the yeast plays, and how its metabolism works step by step
- Which by-products fermentation creates, and what they mean for aroma and mouthfeel
- How temperature and nutrition steer the process, and why a fermentation can stall
From grape sugar to alcohol
Ripe grapes typically contain 20 to 24° Brix, and the two most important sugars are the hexoses glucose and fructose, which together make up around 15 to 25 percent. These are what the yeast lives on. Wine yeast from Saccharomyces uses monosaccharides such as glucose and fructose efficiently, while most wine yeast strains can also take disaccharides such as sucrose and maltose. Pentoses, on the other hand, are not metabolised during wine fermentation, and in practice it is the hexoses that carry alcohol production.
The conversion itself follows a well-described path. The sugar is transported into the yeast cell via both passive diffusion and active transport mechanisms. Inside the cell, glucose and fructose are broken down through glycolysis, which is the main route for sugar catabolism in wine fermentation. The result of glycolysis is pyruvate, and the accounting per molecule of glucose comes to a net yield of two ATP and two NADH.
Theoretically, one mole of glucose gives two moles of ethanol and two moles of carbon dioxide. In practice, slightly less CO2 is released than the theoretical value, because some CO2 is used in anaerobic carboxylation reactions. Under normal oenological conditions, the actual ethanol yield lies around 91 to 94 percent of the theoretical stoichiometric value (0.510 grams of ethanol per gram of sugar metabolised).
The yeast's metabolism
Glycolysis is a chain of enzyme-controlled steps, and each of them tells us something about how sensitive the system is. Hexokinase phosphorylates glucose at the 6-hydroxyl position and requires both ATP and Mg2+. Phosphoglucoisomerase converts glucose-6-phosphate into fructose-6-phosphate, also with Mg2+ as a prerequisite. Phosphofructokinase is the central regulating enzyme, modulated by allosteric effectors such as AMP, ADP and fructose-2,6-bisphosphate and inhibited by, among others, ATP and citrate. Aldolase then cleaves fructose-1,6-bisphosphate into two triose phosphates, and glyceraldehyde-3-phosphate dehydrogenase, which requires a cysteinyl-SH group in its active site, carries the reaction onward.
The decisive step from pyruvate to ethanol happens in two enzyme steps. Pyruvate decarboxylase removes CO2 and forms acetaldehyde, and alcohol dehydrogenase reduces acetaldehyde to ethanol. In this last step NAD+ is regenerated, and that is the whole point: without that regeneration, glycolysis would stall, because it itself consumes NAD+. The yeast thus produces ethanol not for our sake, but in order to be able to continue its energy metabolism under oxygen-poor conditions.
The yeast's ability to tolerate the alcohol it itself creates is linked to the fluidity of the cell membrane, which in turn depends on the fatty acid residues in the membrane's phospholipids. This is one of the reasons that oxygen access early in fermentation matters: with sufficient oxygen, the yeast can build a more robust membrane before the alcohol rises.
By-products and aroma
Fermentation produces far from only ethanol. Glycerol is the most important by-product and the most widespread compound after water and ethanol in dry wine, typically around 10 g/l in red wine and 7 g/l in white wine. It is formed from dihydroxyacetone phosphate via a reductase enzyme and contributes to the wine's body and soft mouthfeel. Low temperature, high tartaric acid content and SO2 promote glycerol formation.
The higher alcohols, also called fusel alcohols, are another important group. They include, among others, n-propanol, isobutyl alcohol, 2-methylbutanol, isoamyl alcohol and 2-phenylethanol, and they are formed by deamination and decarboxylation of amino acids. Amino acids such as isoleucine, leucine and valine disappear quickly within the first 18 to 38 hours of fermentation, while the formation of higher alcohols continues throughout the entire process. In concentrations up to around 400 mg/l they lift the aroma, but above that level they pull the taste in an unfortunate direction.
Acids and other compounds
Succinic acid is the primary carboxylic acid that the yeast forms during fermentation, in concentrations up to around 2.0 g/l, and amino acids as well as glutamate increase its formation. Methanol, on the other hand, does not stem directly from the yeast's metabolism, but from the action of pectinesterase on pectin in the must. The more pectin, the more methanol, and it occurs only in trace concentrations in wine. Finally, the yeast forms mannoproteins during fermentation. They make up about 32 percent of the wine's total polysaccharides and act as natural inhibitors of the crystallisation of potassium hydrogen tartrate, which matters for the wine's stability.
Temperature, nutrition and control
Fermentation proceeds in three phases: a lag phase, in which the must becomes saturated with CO2, an exponential phase with rapid CO2 production up to the maximum yeast population, and a stationary phase, in which activity falls but the cells remain viable. Alcohol production begins just before the exponential growth ends.
Temperature is one of the strongest levers. Lower fermentation temperatures give slower growth and slower ethanol production, but actually result in higher final cell mass and higher final alcohol content compared with warmer processes. The yield falls as the temperature rises, but is not appreciably affected by the amount of yeast added. At the same time, fermentation generates heat in direct relation to the sugar metabolism, and it is precisely that heat development that is decisive when dimensioning cooling during vinification.
Nutrition is the second great lever, and here nitrogen is central. Nitrogen limitation lowers the activity of the glucose transporters and thereby the overall glycolytic rate. Ammoniacal nitrogen also functions as an allosteric effector for phosphofructokinase and pyruvate kinase. The yeast's growth follows Monod kinetics in relation to the nitrogen content, and ethanol acts as a non-competitive inhibitor that lowers the maximum growth rate, while the substrate affinity is unaffected. Fermentation takes place under acidic conditions (pH 3.0 to 3.6) on a mixed substrate of glucose and fructose in the range 120 to 250 g/l, and bisulfite (50 to 150 ppm) is typically added as an antioxidant and antiseptic agent.
When fermentation stalls
A fermentation is called stuck when it ceases before all the available sugar has been converted. The primary cause is nutrient deficiency, and especially nitrogen deficiency, because the yeast cannot build a sufficient population to complete the fermentation. Vineyards with nitrogen-poor fruit therefore give too small yeast populations.
Other factors come into play: ion imbalances, substrate inhibition, ethanol toxicity, pesticide residues, pH shifts and temperature fluctuations. The yeast thrives best between 18 and 25°C, at pH 3 to 4 and with oxygen access at the start. The consequences of a stuck fermentation are real, for residual sugar creates biological instability: it opens the way for bacterial growth, rising volatile acidity, possible restart after bottling and sediment. A volatile acidity above 0.04 percent expressed as acetic acid is regarded as undesirable.
Kort fortalt
- Glucose and fructose are broken down through glycolysis into pyruvate, which is converted into ethanol and CO2, while NAD+ is regenerated.
- Theoretically, one mole of glucose gives two moles of ethanol; in practice the yield lies at 91 to 94 percent of the stoichiometric maximum.
- Glycerol is the most important by-product, and higher alcohols lift the aroma up to around 400 mg/l.
- Lower temperature and sufficient nitrogen give a more complete and controlled process.
- Nitrogen deficiency is the primary cause of stuck fermentation and the biological instability that follows from it.
Ofte stillede spørgsmål
Why does the yeast produce alcohol at all?
Because it needs to regenerate NAD+ in order to be able to continue its energy metabolism under oxygen-poor conditions. When acetaldehyde is reduced to ethanol, NAD+ is recovered, so glycolysis can keep running. The alcohol is thus a result of the yeast's own energy economy.
Why does a cooler fermentation often give a higher final alcohol?
Lower temperature lowers the rate, but gives higher final cell mass and higher final ethanol content, because the yeast retains its viability longer and manages to metabolise more sugar before it is inhibited.
Klar til næste skridt?
When the alcoholic fermentation is finished, the story is rarely over. For many wines a second microbial process follows, one that softens the acidity and changes the aroma. We look closer at that in the next part, Malolactic fermentation and microbiology.
In the meantime, you are welcome to look in at the cellar's selection and feel for how glycerol, alcohol and acidity play together in a glass. And remember that the best combination is always the wine you yourself like, with the food you yourself feel like.