Alcoholic Fermentation

Alcoholic fermentation is an anaerobic metabolic pathway in which yeast cells break down six-carbon sugars (glucose and fructose) into two-carbon ethanol molecules and carbon dioxide gas. The process occurs through glycolysis followed by two additional enzymatic steps: pyruvate decarboxylase removes CO2 from pyruvic acid to form acetaldehyde, and alcohol dehydrogenase then reduces acetaldehyde to ethanol. While the Gay-Lussac equation describes the primary products, fermentation also generates roughly 30 secondary metabolites including glycerol (which adds body and mouthfeel), succinic acid, acetic acid, and a complex array of esters and higher alcohols that contribute to aroma. The process is exothermic, releasing approximately 23.5 kilocalories of heat per mole of glucose consumed, which is why temperature control is essential.

• Glycolysis converts glucose to pyruvic acid, which is then decarboxylated to acetaldehyde and reduced to ethanol by alcohol dehydrogenase
• Glycerol is the most abundant fermentation byproduct after ethanol and CO2, typically produced at 7 to 10 grams per liter, contributing body and a slight sweetness
• Esters (ethyl acetate, isoamyl acetate) produced during fermentation are responsible for fruity aromas like banana, apple, and pear
• The exothermic nature of fermentation means an uncontrolled red wine ferment can reach temperatures above 35 degrees Celsius, risking yeast death

The choice of yeast is one of the most consequential decisions a winemaker makes. Commercial strains of Saccharomyces cerevisiae are selected for specific attributes: predictable fermentation kinetics, tolerance to alcohol and sulfur dioxide, low production of volatile acidity, and the ability to enhance varietal character through specific ester and thiol production. Over 200 commercial wine yeast strains are available from suppliers like Lallemand and Lesaffre. Indigenous or wild yeast fermentation, by contrast, relies on the diverse microflora present on grape skins and in the winery environment. These ferments typically begin with non-Saccharomyces species such as Kloeckera, Candida, and Metschnikowia, which contribute aromatic complexity but are alcohol-sensitive, dying off as S. cerevisiae naturally takes over above 4 to 5 percent ABV.

• Commercial S. cerevisiae strains offer predictability, consistent results, and strain-specific flavor enhancement for particular grape varieties
• Indigenous yeast fermentation uses vineyard and cellar microflora, often involving 5 to 10 different yeast species in sequence
• Non-Saccharomyces yeasts (Torulaspora, Metschnikowia, Lachancea) contribute glycerol, esters, and complexity but cannot complete fermentation alone
• Co-inoculation strategies combine selected non-Saccharomyces yeasts with S. cerevisiae to balance complexity and fermentation reliability

Temperature management during fermentation is a primary tool for controlling wine style. White wines are typically fermented at cool temperatures between 12 and 16 degrees Celsius in stainless steel tanks to preserve volatile aromatic compounds, particularly thiols and esters that define varieties like Sauvignon Blanc and Riesling. Red wines are fermented warmer, between 25 and 30 degrees Celsius, to maximize extraction of anthocyanins (color) and tannins from grape skins during maceration. If temperatures exceed 35 degrees Celsius, yeast cells suffer heat stress, enzyme denaturation accelerates, and fermentation can stop entirely. Modern wineries use jacketed stainless steel tanks with glycol cooling systems to maintain precise temperatures, while more traditional operations may use open-top fermenters and rely on ambient conditions supplemented by dry ice or cold water additions.

• White wine fermentation at 12 to 16 degrees Celsius preserves delicate thiols and esters that define varietal aromatics
• Red wine fermentation at 25 to 30 degrees Celsius promotes extraction of anthocyanins and tannins during skin contact
• Temperatures above 35 degrees Celsius cause yeast stress or death, leading to stuck fermentation and off-flavors
• Jacketed stainless steel tanks with glycol cooling are the industry standard for precise temperature management

A stuck fermentation is one of winemaking's most dreaded problems: yeast activity halts before all fermentable sugar is consumed, leaving unintended residual sweetness and vulnerability to spoilage organisms. Common causes include extreme temperature fluctuations, nutrient deficiency (particularly yeast-assimilable nitrogen, or YAN), excessive alcohol toxicity above 15 percent, high initial sugar concentrations, and the presence of inhibitory substances like pesticide residues or excessive sulfur dioxide. Sluggish fermentations, where yeast activity slows dramatically without stopping completely, share similar root causes. Prevention involves measuring YAN levels before fermentation and supplementing with diammonium phosphate (DAP) or organic nutrient blends when levels fall below 150 to 200 milligrams per liter. Restarting a stuck fermentation requires careful rehydration of a fresh yeast strain with high alcohol tolerance, gradual acclimatization, and often the addition of yeast hulls to absorb toxic compounds.

• Yeast-assimilable nitrogen (YAN) below 150 mg/L is a primary risk factor for stuck fermentation; most winemakers supplement with DAP or organic nutrients
• High sugar musts (above 260 g/L) present elevated stuck fermentation risk due to osmotic stress on yeast cells
• Restarting requires a specialized restart yeast strain (high alcohol tolerance), gradual acclimatization, and yeast hull additions to absorb toxic fatty acids
• Prevention is far easier than cure: pre-fermentation nutrient analysis and staged nutrient additions are standard practice at quality wineries

The fermentation vessel profoundly shapes the final wine. Stainless steel is the global default for clean, fruit-driven whites and roses, offering precise temperature control and an inert environment. Oak barrels (typically 225-liter barriques or 500-liter puncheons) are used for barrel fermentation of premium Chardonnay, white Burgundy, and some Rhone whites, contributing toasty, vanilla, and spice notes while promoting controlled micro-oxygenation. Concrete tanks and eggs have seen a renaissance among producers seeking a middle path: the thermal mass of concrete moderates temperature swings naturally, while the curved interior of concrete eggs promotes gentle lees circulation without stirring. Open-top fermenters, whether wooden or steel, are preferred for premium red wines, allowing manual punchdowns and more aggressive extraction. Amphora and clay vessel fermentation, inspired by Georgian qvevri traditions, has gained traction among natural wine producers seeking minimal intervention and textural complexity.

• Stainless steel: inert, temperature-controlled, preserves primary fruit character; the global standard for aromatic whites
• Oak barrel fermentation: adds vanilla, toast, and spice while promoting micro-oxygenation and lees complexity; standard for premium Chardonnay
• Concrete tanks and eggs: thermal mass moderates temperature naturally; curved egg shape promotes gentle lees convection without batonnage
• Open-top fermenters: allow manual punchdowns for red wines; better extraction control than closed tanks with pump-overs