Alcohol is made through fermentation, a natural process where yeast consumes sugar and converts it into ethanol and carbon dioxide. Every alcoholic beverage on earth, from beer to bourbon, starts with this same basic reaction. The differences between drinks come down to what sugar source you begin with, whether you stop at fermentation or continue to distillation, and how you treat the liquid afterward.
How Fermentation Turns Sugar Into Alcohol
Yeast cells, most commonly a species called Saccharomyces cerevisiae, eat simple sugars like glucose and produce two waste products: ethanol (the alcohol you drink) and carbon dioxide gas (the bubbles in beer and champagne). The chemistry happens in two steps inside the yeast cell. First, glucose is broken down into a smaller molecule, which releases a burst of CO2. Then the yeast converts what remains into ethanol.
This process has a built-in ceiling. As ethanol accumulates, it becomes toxic to the very yeast producing it. At around 5% alcohol concentration, yeast growth starts to slow significantly. Most standard brewing yeast strains top out between 8% and 12% alcohol by volume before they die off or go dormant. Specially bred strains can push toward 15% or even 18%, which is why some wines reach higher alcohol levels than beer. But fermentation alone rarely produces anything above 20%. To go higher, you need distillation.
Where the Sugar Comes From
Yeast can only ferment simple sugars, so the starting ingredient determines how much prep work is needed before fermentation begins.
- Fruit (wine, cider, brandy): Grapes, apples, and other fruits already contain plenty of simple sugars. Crush them, add yeast, and fermentation can start almost immediately.
- Grain (beer, whiskey, vodka): Barley, corn, rye, and wheat store their energy as starch, which yeast cannot eat directly. The starch must first be broken down into sugar through a process called mashing.
- Sugar cane or molasses (rum): These are already rich in fermentable sugar, so they need minimal processing before yeast is added.
- Agave (tequila, mezcal): The agave plant stores energy as a complex sugar called inulin, which is released by slowly roasting the plant’s core before fermentation.
Mashing: Unlocking Sugar From Grain
For grain-based drinks, the raw ingredient needs to be coaxed into releasing its sugar. This starts with malting, where grain (usually barley) is soaked in water until it begins to sprout. Sprouting activates natural enzymes inside the grain that are capable of breaking starch into sugar. The grain is then dried in a kiln to stop it from growing further while preserving those enzymes.
Next comes mashing. The malted grain is crushed into a coarse powder called grist and mixed with hot water, typically held around 63 to 64°C (about 146°F). At this temperature, the enzymes work efficiently, converting the starch into fermentable sugars over the course of an hour or so. The resulting sweet liquid, called wort in brewing and wash in distilling, is drained off and cooled before yeast is added. The leftover grain solids are discarded or used as animal feed.
Fermented vs. Distilled Drinks
If you stop after fermentation, you have beer, wine, or cider, drinks that typically range from about 3% to 16% alcohol by volume. To make spirits like whiskey, vodka, rum, or gin, the fermented liquid must be distilled.
Distillation works because ethanol boils at a lower temperature than water. Ethanol vaporizes at about 78°C (173°F), while water boils at 100°C (212°F). By heating the fermented liquid and collecting the vapor that rises first, distillers capture a concentration of alcohol far higher than what fermentation alone can produce. That vapor is cooled back into liquid form, and the result is a spirit.
There is a physical limit, though. Ethanol and water form what chemists call an azeotrope at 96% ethanol by weight. At that concentration, the two liquids evaporate together at the same rate, so simple distillation can never produce pure ethanol. This is why even the strongest spirits are diluted back down with water before bottling.
Pot Stills vs. Column Stills
The type of still used shapes both the flavor and strength of the final spirit. The two main designs work quite differently.
Pot stills are large copper vessels that process one batch at a time. The fermented liquid is poured in, heated, and the vapor is collected through a curved neck and condensed. Once a batch is finished, the still is emptied and refilled. Pot stills produce spirits with more character and flavor because they don’t strip out as many of the aromatic compounds. They’re traditional in Scotch whisky, cognac, and many craft distilleries.
Column stills, developed in the 1830s, work continuously. They consist of one or more tall cylindrical columns with stacked plates inside that create separate compartments. Steam rises through the column while the fermented liquid flows downward, and alcohol is stripped out at each plate level. Column stills are far more efficient, producing higher-proof spirits with greater consistency. They’re the standard for vodka, most American whiskeys, and large-scale gin production.
Making the Cut: Removing Harmful Compounds
Not everything that comes out of a still is safe to drink. During distillation, the vapor doesn’t emerge as pure ethanol. It arrives in stages, and distillers divide the output into four portions called “cuts.”
The first liquid to emerge is called the foreshots. This fraction contains the most volatile compounds: acetone, methanol, and other chemicals that are genuinely dangerous. Methanol vaporizes at around 64°C (148°F), well below ethanol, so it concentrates in these early vapors. The foreshots are always discarded.
Next come the heads, which contain lighter alcohols and sharp-tasting compounds like ethyl acetate. These are usually set aside. The middle portion, called the hearts, is the good stuff: clean ethanol with the desirable flavors the distiller wants. Finally, the tails arrive at the end of the run, carrying heavier compounds called fusel oils, including butanol and amyl alcohol, which taste oily and unpleasant.
Knowing where to make these cuts is one of the core skills of distilling. Column stills automate much of this separation through their internal plate design, which is one reason they produce more consistent results batch to batch.
How Aging Changes a Spirit
Many spirits go into wooden barrels after distillation, and the transformation that happens inside those barrels is dramatic. Bourbon, Scotch, rum, brandy, and tequila all owe much of their final flavor to time spent in wood.
When a barrel is toasted or charred before use, the heat breaks down the wood’s internal structures. Lignin, a structural compound in oak, degrades into flavor molecules responsible for vanilla, spice, and woody notes. Hemicellulose and cellulose break down into compounds that contribute caramel and toasty flavors. Naturally occurring sugars in the wood caramelize from the heat, producing additional aromatic compounds that serve as markers of how heavily the barrel was toasted.
Over months or years, tiny amounts of oxygen seep through the barrel’s pores. This micro-oxygenation drives a slow cascade of chemical reactions: oxidation, esterification, and polymerization. Harsh, raw flavors mellow. New aromatic compounds form. Tannins extracted from the wood add body and astringency while also stabilizing the spirit’s color. The longer the spirit ages, the more integrated and complex these flavors become, as tannins, lactones, and phenolic compounds interact to create a layered sensory profile.
Color comes entirely from the barrel. Fresh-off-the-still whiskey is clear. The amber and gold tones develop gradually as the spirit pulls pigments from the wood.
What Defines Each Type of Spirit
The same basic process of fermentation and distillation produces very different drinks depending on the starting ingredients, distillation method, and aging requirements. U.S. federal regulations define each category precisely.
Vodka is neutral spirit distilled to at least 95% alcohol by volume (190 proof), then typically filtered through charcoal to remove any remaining flavor. The goal is a spirit with no distinctive character, aroma, or taste. It’s bottled at no less than 40% alcohol (80 proof).
Whiskey is distilled from a fermented grain mash at less than 95% alcohol, specifically so that it retains flavor from the grain. Bourbon, a subcategory, must be made in the U.S. from at least 51% corn and aged in charred new oak containers. Rye whiskey follows the same rules but with at least 51% rye grain. Both must be bottled at 40% alcohol or higher.
Gin gets its defining flavor from juniper berries. It can be made by distilling grain spirit directly with juniper and other botanicals, by redistilling a neutral spirit with those aromatics, or simply by mixing neutral spirit with juniper extracts. All gin must be bottled at a minimum of 40% alcohol.
Rum starts from sugar cane or molasses. Tequila starts from blue agave. Brandy starts from fruit, usually grapes. Each follows the same fundamental path of fermentation, distillation, and (often) aging, but the raw ingredient and specific production rules give each category its identity.