Alcoholic Fermentation

Alcoholic fermentation is the transformation of grape sugars into ethanol, carbon dioxide and secondary products. Put simply, the yeast attack the sugar and convert it into alcohol and carbon dioxide. The fermentable sugars are glucose and fructose, and Brix, the usual basis for estimating potential ethanol, is not equal to them. Measuring glucose and fructose gives a more accurate estimate: potential ethanol in % v/v can be estimated as (glucose g/L + fructose g/L) divided by 16.83. Actual yield varies, since Saccharomyces cerevisiae strains may vary in ethanol yields, some non-Saccharomyces yeasts consume some sugar but do not produce ethanol, and higher fermentation temperatures can lead to slightly lower ethanol. Fermentation also feeds on itself: a higher fermentation temperature increases the rate of fermentation, which raises the temperature further in a positive feedback loop.

• Some aroma compounds are released during fermentation from precursors that exist in the grapes
• Fruity esters form from reactions between certain acids and alcohols as fermentation progresses, giving aromas akin to apple, pineapple and banana commonly found in young wines
• The fermentation vessel, temperature and type of yeast all have a role in determining which aromatics are enhanced or subdued during fermentation
• After alcoholic fermentation, malolactic fermentation converts tart-tasting malic acid to softer lactic acid

Choice of yeast strain is a key decision for winemakers, since different strains perform differently and produce different aroma and flavour compounds. Strains of Saccharomyces cerevisiae have traditionally been used for fermenting grape juice, and yeast breeding research has added new hybrid strains, such as AWRI2526. Fermentation can take place spontaneously, by means of yeasts naturally present on the grape skins, in the must and in the cellar, or through inoculation of the must or crushed grapes with selected yeasts, before or during fermentation. It is normal nowadays to add a yeast culture to the must to start the fermentation, while naturally occurring wild yeasts are found in the bloom, the greyish, waxy layer on the outside of the grape skin. In uninoculated ferments, non-Saccharomyces yeasts dominate the early stages, whereas Saccharomyces yeast conducts the majority of the alcoholic fermentation. S. cerevisiae is present in no more than tiny quantities on grape skins and rarely plays much of a role in getting fermentation underway, but once active it outgrows rival strains through the production of alcohol and heat.

• A main risk of uninoculated fermentation is high acetic acid or ethyl acetate forming early, which Hansenula and Kloeckera species can produce when no SO2 is present
• Kloeckera can significantly deplete nitrogen and thiamine, so supplementing uninoculated fermentations with nitrogen and vitamins is recommended
• Yeast vary in their nitrogen requirements, from low to medium to high demand, so it is important to know the must's YAN before fermentation
• Effective rehydration of active dry wine yeast is paramount in obtaining a healthy and viable yeast inoculum

Control of the fermentation temperature, as a function of the type of wine desired, is one of the ways to influence fermentation. Most white wines are fermented at a low temperature between 15 and 20 degrees to retain freshness, and it takes about two weeks for all the sugar to be converted, leaving a dry white wine. Red wine ferments in contact with the skins, from which colour and tannin are extracted, and most red wines are fermented relatively warm, at 25 to 32 degrees, to assist the extraction of colour. The maximum extraction of colour occurs before that of tannins, and maximum colour is obtained earlier at higher temperatures. Heat carries risk: extreme temperatures above 35°C, or constant increasing and lowering of fermentation temperature, can begin to affect yeast health and viability and increase the risk of a stuck fermentation.

• Yeast in general do not function well at temperatures above 32°C for reds or below 15°C for whites
• Pumping the fermenting liquid through a heat exchanger and distributing the cooled liquid over the cap is the most effective way of cooling a ferment
• Pumping over, rack-and-return and, to a lesser degree, plunging help dissipate heat but also introduce oxygen, which may stimulate yeast and increase the fermentation rate
• In one study, higher fermentation temperature increased the rate but not the final concentration of skin-derived phenolics, while both rose for seed-derived phenolics

A stuck fermentation is a slow, sluggish fermentation that stalls out with a few Brix remaining. Hot, dry growing seasons can produce fruit with higher Brix content, leaving yeast to metabolize more sugar with fewer nutrients in a high-alcohol environment. Causes include improper yeast hydration, temperature management, microbial competition and residual pesticides, with low nutrients and high alcohol content two of the most common. Nitrogen is a critical grape nutrient that affects the rate and completion of fermentation, fermentation bouquet and style of wine, and the minimum YAN for a low-risk fermentation is approximately 150 mg/L for whites and 100 mg/L for reds. Monitoring Baume or Brix and temperature twice daily and plotting a fermentation curve is recommended, so a slowdown shows well before the ferment has stopped.

• Fermentation inhibitors include high sulfite, high Baume or Brix, agrochemical residues and chlorine from yeast hydration water; some yeast don't cope well in ethanol above 15%, and some cannot tolerate pH of 3.0 or less, especially when sulfite is present
• A general recommendation is 120 to 220 mg/L YAN for a 21 Brix must, with nutrient additions early and at one-third fermentation
• DAP efficiently raises YAN but contains no micronutrients, so pairing it with a complex yeast nutrient is advised; large DAP additions can increase the risk of ester taint (ethyl acetate)
• To restart a stuck ferment, a healthy population of a rescue yeast, typically a vigorous fermenting yeast, is built up and the stuck wine added to it in a stepwise manner; yeast hulls may reduce inhibitory substances

Most fermentations now take place in stainless steel vats, although some of the smarter white wines are fermented in small oak barrels. Concrete tanks are commonly used in wineries, and Michel Chapoutier designed the concrete egg with the help of Nomblot, a French company that has been making concrete vats since the 1920s. The egg shape gives a continuous flow to the wine as it ferments, and the thick walls keep temperature very stable during fermentation, avoiding the need for artificial refrigeration. In Georgia, the qvevri, a large egg-shaped earthenware vessel buried almost entirely underground, is used for the fermentation, maturation and storage of wine, and the surrounding earth acts as a natural thermal buffer during fermentation.

• Concrete eggs come in two sizes, 6hl and 16hl
• In a qvevri, released carbon dioxide lifts the grape skins to form a cap, and solids settle into the narrow base as fermentation slows
• Like oak, fired clay permits limited oxygen exchange but contributes virtually no wood-derived aromas or flavours
• The fermentation vessel, along with temperature and type of yeast, has a role in determining which aromatics are enhanced or subdued during fermentation

Primary sources (the OIV, the Australian Wine Research Institute, WSET and Oregon State University Extension): https://www.oiv.int/standards/international-code-of-oenological-practices/part-ii-oenological-treatments-and-practices/musts/alcoholic-fermentation, https://www.awri.com.au/industry_support/winemaking_resources/wine_fermentation/yan/, https://www.awri.com.au/information_services/ebulletin/2013/03/27/stuck-fermentations/, https://www.awri.com.au/industry_support/winemaking_resources/winemaking-practices/winemaking-treatment-yeast-choice/, https://www.awri.com.au/industry_support/winemaking_resources/winemaking-practices/fermentation-temperature/, https://www.wsetglobal.com/knowledge-centre/blog/2022/january/13/why-doesn-t-wine-taste-like-grapes, https://extension.oregonstate.edu/catalog/em-9619-yeast-nutrients-stuck-fermentations, https://extension.oregonstate.edu/crop-production/wine-grapes/preparing-harvest-grape-chemistry-pre-fermentation-adjustments, https://www.wsetglobal.com/knowledge-centre/blog/2026/qvevri-the-ancient-vessel-at-the-heart-of-georgian-winemaking. Secondary: https://www.decanter.com/wine-news/opinion/jefford-on-monday/jefford-on-monday-yeast-call-me-dad-12363/, https://www.decanter.com/learn/winemaking-the-facts-2-54064/7/, https://www.decanter.com/learn/winemaking-the-facts-2-54064/6/, https://www.decanter.com/learn/concrete-eggs-winery-ask-decanter-316358/. Last verified 2026-10-04