Fermentation Science

Part of Alcohol and Distillation

How yeast converts sugars into alcohol and carbon dioxide, and how to control this process reliably.

Why This Matters

Fermentation is the foundation of alcohol production. Without understanding how yeast works, what it needs, and what kills it, your attempts at making alcohol will be inconsistent at best and failures at worst. A batch that should produce strong wash may produce vinegar, or nothing at all.

The science of fermentation was not understood until the 1800s, but humans practiced it for thousands of years through trial and error. In a rebuilding scenario, you can shortcut those millennia of experimentation by understanding the basic biology. Knowing that yeast is a living organism with specific needs for temperature, nutrients, and pH lets you troubleshoot problems and optimize production systematically.

Fermentation is also the gateway to many other essential products beyond alcohol: bread, vinegar, preserved vegetables, yogurt, cheese, and soy sauce all depend on controlled microbial activity. Mastering the science for alcohol production gives you transferable skills for an entire category of food technology.

Yeast Biology

Yeast are single-celled fungi. The species most important for alcohol production is Saccharomyces cerevisiae, the same organism used in bread baking. Wild yeasts of various species live on the skins of fruits, in soil, on grain, and floating in the air.

How Yeast Makes Alcohol

Yeast consumes simple sugars (glucose and fructose) and produces ethanol and carbon dioxide as waste products. The chemical equation is:

C6H12O6 → 2 C2H5OH + 2 CO2

One molecule of glucose yields two molecules of ethanol and two molecules of carbon dioxide. In practical terms, 180 grams of sugar produces about 92 grams of ethanol (roughly 115 ml) and 88 grams of CO2.

This process is called anaerobic fermentation because yeast produces alcohol only when oxygen is limited. In the presence of abundant oxygen, yeast instead fully metabolizes sugar into carbon dioxide and water (aerobic respiration), producing no alcohol. This is why fermentation vessels are sealed or fitted with airlocks.

The Pasteur Effect

When you first add yeast to a sugary solution with air present, the yeast multiplies rapidly using aerobic respiration. Once oxygen is consumed, it switches to anaerobic fermentation and begins producing alcohol. This initial aerobic phase is actually beneficial because it builds up a large, healthy yeast population.

Yeast Life Cycle in Fermentation

1. Lag phase (0-12 hours): Yeast acclimates to the environment. Little visible activity.
2. Growth phase (12-48 hours): Yeast multiplies rapidly. Vigorous bubbling as CO2 is produced. Temperature rises.
3. Stationary phase (2-7 days): Yeast population stabilizes. Steady fermentation continues.
4. Decline phase (7-14 days): Sugar is depleted or alcohol concentration reaches toxic levels. Fermentation slows, then stops. Yeast settles to the bottom (flocculation).

Controlling Fermentation

Temperature

Temperature is the single most critical variable.

Temperature	Effect
Below 10C	Yeast dormant, no fermentation
10-15C	Very slow fermentation, clean flavors
18-25C	Ideal range for most yeast
25-30C	Fast fermentation, more off-flavors
30-35C	Stressed yeast, harsh flavors, possible stalling
Above 38C	Yeast begins to die
Above 45C	Rapid yeast death

For fuel alcohol, temperature control is less critical since flavor does not matter. Ferment in the 20-30C range for speed. For drinking spirits, aim for 18-24C for cleaner flavors.

Practical temperature management:

• In cold weather, place fermentation vessels near (not on) a heat source, or wrap with insulation.
• In hot weather, move vessels to shade, basements, or near cold water sources. Evaporative cooling (wrapping with wet cloth) can lower temperature by 5-10C.
• Bury vessels partially in the ground for natural temperature stabilization.

Sugar Concentration

Yeast needs sugar, but too much sugar is actually inhibitory. Very high sugar concentrations create osmotic pressure that dehydrates yeast cells.

Sugar concentration	Expected ABV	Fermentation behavior
5-8%	3-4%	Fast, easy, complete
10-15%	5-8%	Normal, reliable
15-20%	8-10%	Good for distilling wash
20-25%	10-13%	Slow, may need nutrients
Above 25%	Stalls	Yeast stressed, incomplete

For distilling purposes, target 15-20% sugar in your wash. This produces 8-10% ABV, which is efficient for distillation. Higher starting gravity is possible but risks stuck fermentation.

pH

Yeast prefers a mildly acidic environment, pH 4.0-5.5. Most fruit juices naturally fall in this range. Grain mashes tend to be closer to neutral (pH 5.5-6.5) and may benefit from acidification.

If fermentation is sluggish and temperature and sugar are correct, pH may be the problem. Add a small amount of citrus juice, vinegar, or tartaric acid (from grape lees) to lower pH. Wood ash solution raises pH if needed.

Nutrients

Sugar alone is not enough. Yeast also needs nitrogen, phosphorus, and trace minerals to build proteins and cell structures. Fruit musts and grain mashes naturally contain these nutrients. Pure sugar solutions (from table sugar, honey, or refined syrups) are nutrient-poor and may stall.

Nutrient supplements for sugar washes:

• A handful of crushed grain or flour per 20 liters
• A tablespoon of tomato paste (rich in minerals and amino acids)
• Crushed fruit (a few apples or berries per batch)
• A pinch of Epsom salt (magnesium sulfate) if available

Sourcing Yeast

Cultivated Yeast

If you have access to commercial yeast (bread yeast, wine yeast, or brewer’s yeast), use it. Even old dried yeast packets may still contain viable cells. Rehydrate in lukewarm (30-35C) water with a pinch of sugar for 15 minutes before pitching.

Propagating yeast: You can maintain a yeast culture indefinitely. After fermentation, save the sediment (lees) from the bottom of the vessel. Store in a sealed jar in a cool place. Use a tablespoon of this slurry to start your next batch. Feed the culture periodically with a small amount of sugar water to keep it alive between batches.

Wild Yeast

When cultivated yeast is unavailable, wild yeast from the environment can be captured:

• Fruit skins: Grapes, plums, and apples carry abundant wild yeast on their skins (the pale, waxy bloom visible on dark-skinned fruits). Add unwashed fruit to your sugar solution.
• Grain: Unmalted grain harbors various yeasts and bacteria. Add a handful of crushed grain to initiate fermentation.
• Airborne capture: Leave a jar of sugar water open to the air in a warm place for 24-48 hours. Airborne yeast will colonize it. Cover with cloth to exclude insects while allowing air exchange.

Wild Yeast Risks

Wild yeast produces less predictable results than cultivated strains. Some wild yeasts produce unpleasant flavors or low alcohol tolerance. If a wild-fermented batch smells rotten (not just sour or funky), discard it and try again. Over time, you can select for good wild strains by saving lees from successful batches.

Monitoring Fermentation

Signs of Active Fermentation

• Bubbling: CO2 exits through the airlock (or bubbles up through the liquid if no airlock is used). Active fermentation produces 1-3 bubbles per second.
• Foam: A layer of foam (kraeusen) forms on the surface during the most active phase.
• Temperature: The vessel feels warm to the touch; fermentation is exothermic.
• Smell: A yeasty, bread-like, slightly fruity aroma indicates healthy fermentation.

Signs of Completed Fermentation

• Bubbling stops or slows to less than one bubble per minute
• Liquid clears as yeast settles to the bottom
• Taste is dry (not sweet) and alcoholic
• No change in density over 2-3 days

Signs of Problems

• Vinegar smell: Acetobacter bacteria have colonized the batch. The vessel was not sealed properly, allowing too much oxygen. The batch is becoming vinegar. You can let it finish as vinegar (useful in its own right) but it will not make good alcohol.
• Rotten egg smell (H2S): Yeast is stressed, usually from too-high temperature or nutrient deficiency. Lower temperature and add nutrients. The smell often dissipates during distillation.
• No activity at all: Yeast is dead or absent. Common causes: water was too hot when yeast was added, pH is too extreme, sugar concentration is too high. Re-pitch with fresh yeast after correcting conditions.
• Mold on surface: White or colored mold growing on the liquid surface. Skim it off carefully; the liquid below may still be usable. Improve sanitation and sealing for future batches.

Maximizing Alcohol Yield

To get the most alcohol from your raw materials:

1. Convert all starches to sugar before adding yeast. Yeast cannot digest starch directly. Malting (sprouting grain) or cooking with a malt addition converts starch to fermentable sugar.

2. Pitch adequate yeast. A thin, weak yeast pitch takes days to get going, during which bacteria can gain a foothold. Use at least a tablespoon of active yeast slurry per 20 liters of wash.

3. Maintain temperature. Temperatures above 30C stress yeast and cause it to produce more fusel alcohols and less ethanol per unit of sugar consumed.

4. Provide nutrients. Nitrogen-deficient washes stall before all sugar is consumed. Add a nitrogen source if using pure sugar.

5. Be patient. Allow fermentation to complete fully (7-14 days for most washes). Distilling a partially fermented wash wastes the remaining sugar.

6. Practice sanitation. Clean all equipment with boiling water before use. Competing bacteria consume sugar without producing useful alcohol and may create off-flavors or toxic byproducts.