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 the phenols and at the aroma compounds. Now we come to the very heart of how wine comes to be: the process that turns sweet grape juice into wine. Fermentation.
This is where the yeast takes over, and where the grape's sugar is converted into alcohol, carbon dioxide and a long list of by-products that shape the wine's body, mouthfeel and aroma. For the curious, this is also where the chemistry becomes tangible: we can follow the enzymatic path from a glucose molecule all the way to ethanol, and we can understand why temperature and nutrition determine whether fermentation runs cleanly or stalls.
Hvad du lærer
- How grape sugar is converted into alcohol via the yeast's metabolism
- What role the yeast plays, and how its nutritional needs govern the process
- Which by-products fermentation forms, and what they mean for the wine
- Why temperature and nitrogen are decisive, and what happens when fermentation stalls
From grape sugar to alcohol
The starting point is the grape's sugar. Glucose and fructose are the two dominant hexose sugars in the grape and make up around 15-25% of the berry. They are what the yeast uses as a substrate to produce ethanol. Ripe grapes typically sit at 20-24 °Brix, and the fermentable sugar in must is normally in the range of 120-250 g/L.
The Saccharomyces yeasts use monosaccharides such as glucose, fructose, mannose and galactose as a carbon source, and most wine yeast strains can also process disaccharides such as sucrose, maltose and melibiose. Pentoses, on the other hand, are not utilised during winemaking. In practice Saccharomyces cerevisiae is specialised in the hexoses, and that is why glucose and fructose carry the whole load.
Theoretically, one mole of glucose yields two moles of ethanol and two moles of carbon dioxide. The actual amount of CO2 released is, however, slightly less than the theoretical figure, because some CO2 is used in anaerobic carboxylation reactions along the way. It is a good reminder that fermentation is never a clean piece of arithmetic, but a living metabolism with many side roads.
The yeast's metabolism
For the sugar to become alcohol, it must first get into the yeast cell. Sugar transport happens both by passive transport (simple and facilitated diffusion) and by active transport mechanisms. Inside the cell, glycolysis is the central breakdown pathway for glucose and fructose.
Glycolysis step by step
Glycolysis is a chain of enzyme-driven steps. Hexokinase phosphorylates glucose at the 6-hydroxyl position and requires both ATP and Mg2+. Phosphoglucoisomerase converts glucose-6-phosphate into fructose-6-phosphate (also requiring Mg2+). Phosphofructokinase is the important regulating enzyme, fine-tuned by allosteric effectors such as AMP, ADP and fructose-2,6-bisphosphate, and inhibited by, among others, PEP, ATP and citrate. Aldolase then splits fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, and glyceraldehyde-3-phosphate dehydrogenase, which requires a cysteinyl-SH group in its active site, drives the process onward.
The result of glycolysis is pyruvate, and the net yield is two ATP and two NADH per glucose molecule. ATP is the yeast's energy currency, while NADH must be recirculated for the process to continue.
From pyruvate to ethanol
The alcoholic fermentation itself converts pyruvate into ethanol and carbon dioxide in two enzymatic steps: pyruvate decarboxylase and alcohol dehydrogenase. The crucial point is that NAD+ is regenerated in this last step. Without that regeneration, glycolysis would stall, because there would be no NAD+ available to oxidise new glucose. So it is not only ethanol that matters to the yeast, but also the continued possibility of drawing energy out of the sugar.
The yeast's ability to tolerate the ethanol it produces itself is linked to the membrane's fluidity, which in turn is affected by the fatty acid residues in the membrane's phospholipids. This is part of the explanation for why some yeast strains can work longer and higher up in alcohol than others.
By-products and aroma
Fermentation is never only ethanol. A range of by-products are formed along the way, and they contribute noticeably to the wine's character.
Glycerol is the most important by-product and at the same time the most abundant compound after water and ethanol in dry wine, around 10 g/L in red wine and 7 g/L in white wine. Glycerol is formed from dihydroxyacetone phosphate via the enzyme dihydroxyacetone phosphate reductase and contributes to a softer mouthfeel. Low temperature, high tartaric acid and SO2 favour the formation of glycerol. The classic fermentation studies (Neuberg's second and third fermentation) show precisely how glycerol formation can be increased when acetaldehyde is bound by sulphite and thereby cannot act as a hydrogen acceptor for NADH, or when dihydroxyacetone takes over that role under alkaline conditions.
Higher alcohols (fusel alcohols) with more than two carbon atoms include, among others, n-propanol, isobutyl alcohol, 2-methylbutanol, isoamyl alcohol and 2-phenylethanol. They are formed by deamination and decarboxylation of amino acids, and their formation depends on the yeast species. Amino acids such as isoleucine, leucine and valine are used up quickly within the first 18-38 hours after the start of fermentation, while the formation of higher alcohols continues throughout the whole process. Up to around 400 mg/L the higher alcohols improve the wine's aroma, while levels above that pull the quality down.
Succinic acid is the primary carboxylic acid the yeast forms, in concentrations up to 2.0 g/L, and amino acids as well as glutamate increase its production. Methanol, by contrast, does not come from the yeast itself, but from the action of pectin esterase on the pectin in the must. The higher the pectin content, the more methanol, and the level is also affected by the fermentation organism, the raw material and the temperature.
Temperature, nutrition and control
Fermentation runs in three phases: a lag phase (where the must is saturated with CO2), an exponential phase (rapid CO2 production up to the maximum yeast population) and a stationary phase (declining activity, but preserved viability). Ethanol production typically begins just before exponential growth ceases.
The role of temperature
Temperature is one of the strongest levers the winemaker has. A lower fermentation temperature reduces both growth and the rate of ethanol production, but in return gives a higher final cell mass and a higher final ethanol concentration compared with warmer fermentation. The yield falls as the temperature rises. At the same time, heat development during fermentation is directly linked to the conversion of sugar and is the critical parameter when sizing cooling. That is why temperature control is not merely a stylistic decision, but also a question of keeping the process safe.
Ethanol's yield coefficient normally lies at 91-94% of the theoretical stoichiometric value (0.510) under normal oenological conditions. This tells us how efficiently the sugar actually becomes alcohol, and how much goes to biomass and maintenance.
Nitrogen and nutrition
Nitrogen is the most critical nutrient. Nitrogen limitation reduces the activity of the glucose transporters and thereby lowers the overall glycolytic rate. Ammoniacal nitrogen also acts as an allosteric effector on both phosphofructokinase and pyruvate kinase. The yeast needs nitrogen for protein synthesis, and a vineyard with nitrogen-poor fruit can give too small a yeast population to complete fermentation.
Ethanol itself acts as a non-competitive inhibitor of yeast growth: it affects the maximum growth rate, but not the yeast's affinity for the substrate. The yeast's viability falls almost linearly with rising ethanol concentration. Optimal fermentation typically happens at 18-25 °C, at pH 3-4, with sufficient nutrition, initial oxygen and the absence of toxic substances.
When fermentation stalls
A stuck fermentation occurs when the process halts before all available sugar has been converted into alcohol and CO2. It leaves residual sugar behind, and that is problematic: residual sugar creates biological instability that enables bacterial growth, increased volatile acidity, possible restart of fermentation after bottling, sediment and bubble formation in the bottle.
The primary cause is nutrient limitation, particularly nitrogen deficiency, which slows the yeast's development and metabolism. Other factors include ion imbalances, substrate inhibition, ethanol toxicity, pesticide residues, pH changes and temperature fluctuations. The yeast strains' own self-inhibiting and cross-inhibiting effects also play a part, just as L-ornithine formed by lactic acid bacteria can inhibit amino acid transport in the yeast and thereby delay fermentation. For dry yeast, viability also falls with incorrect storage, high SO2 or must temperatures below 15 °C or above 30 °C.
Kort fortalt
- Glucose and fructose are converted via glycolysis into pyruvate and onward into ethanol and CO2, while NAD+ is regenerated to keep the process going.
- The yeast has to take up the sugar, and its metabolism is governed by enzymes such as hexokinase, phosphofructokinase and alcohol dehydrogenase.
- Glycerol is the most important by-product, while higher alcohols, succinic acid and methanol contribute in different ways to quality and character.
- Temperature and nitrogen are the strongest levers: lower temperature gives higher final alcohol and cell mass, while nitrogen deficiency is the main cause of stuck fermentations.
Ofte stillede spørgsmål
Why is nitrogen so decisive for a clean fermentation?
The yeast uses nitrogen for protein synthesis, and ammoniacal nitrogen acts directly as a regulator of central glycolytic enzymes. If nitrogen is lacking, the activity of the glucose transporters falls and with it the fermentation rate, and the population can become too small to reach the finish. That is why nitrogen deficiency is the most frequent cause of stuck and sluggish fermentations.
What does the fermentation temperature mean for the finished wine?
A lower temperature slows the rate, but gives a higher final cell mass and a higher final ethanol concentration, and at the same time favours the formation of glycerol. Warmer fermentation goes faster, but gives a lower yield and develops more heat, which has to be managed with cooling.
Klar til næste skridt?
When the alcoholic fermentation is over, the wine is far from finished. For many wines a second microbiological transformation now begins, where bacteria shape acid and aroma anew. We look more closely at that in the next part, Malolactic fermentation and microbiology, where we follow the conversion of malic acid into lactic acid and dive into the microorganisms that live side by side with the yeast.
If you fancy tasting the difference between a cool, slow fermentation and a warmer style, then let your curiosity guide you around the range. And remember that the best combination is always the wine you like with the food you like.