Sugar enters the yeast
Fermentable sugars from fruit, juice or added sugar become the yeast's food source.
Alcohol & strength
Understand the strength of your wine.
A plain-English guide to fermentation, alcohol, gravity and the things that influence the finished strength of a country wine.
The basics
Where does the alcohol come from?
Alcohol is produced during fermentation. Wine yeast consumes fermentable sugars and converts much of that sugar into ethanol (the alcohol in wine) and carbon dioxide.
The fruit itself may provide some of the sugar, while additional sugar is often added to bring a recipe to its intended strength. Yeast also produces heat and many other compounds that influence aroma and flavour.
The simple version
Sugar → yeast → alcohol + carbon dioxide
As fermentation progresses, sugar is consumed and the specific gravity normally falls. Your hydrometer is therefore one of the most useful tools for understanding what is happening.
Inside the fermentation
What the yeast is doing.
Yeast cells use sugar for energy and growth. In the low-oxygen environment of fermentation, alcoholic fermentation produces ethanol and carbon dioxide.
Fermentation releases energy
The yeast breaks down sugar through a series of biochemical reactions and uses some of the released energy to grow.
Alcohol and CO₂ are produced
Ethanol and carbon dioxide are released as the main products of alcoholic fermentation.
Why fermentation stops
More alcohol isn't always better.
A fermentation can slow or finish for several different reasons. The recipe, yeast strain and conditions all matter.
Most fermentable sugar has gone
Once the yeast has consumed most of the available sugar, gravity stops falling and fermentation may be complete.
Alcohol becomes inhibitory
As alcohol rises, it becomes increasingly difficult for yeast to continue. Different strains have different alcohol tolerances.
Temperature is unsuitable
Yeast generally performs best within a suitable temperature range. Too cold can slow it dramatically; excessive heat can stress or damage the yeast.
Yeast health or nutrients are poor
Insufficient nutrients, unsuitable conditions or an already-stressed fermentation can make yeast struggle.
How strong will it be?
Think in terms of gravity and ABV.
A rough traditional rule is that a must around 22–25° Brix can produce roughly 12–15% ABV when fermentation carries through. Actual results vary with the ingredients, sugar level, yeast and final gravity.
For a more useful measurement, take an Original Gravity (OG) reading before fermentation and a stable Final Gravity (FG) reading afterwards.
Estimated ABV ≈ (OG − FG) × 131.25For example, OG 1.090 and FG 0.995 gives an estimated 12.47% ABV.
Use the hydrometer
A hydrometer measures specific gravity. Comparing readings over time tells you far more about fermentation progress than counting airlock bubbles.
Always record your readings. Good notes make it easier to understand a batch and reproduce a successful one.
Open the ABV Calculator →
Beyond alcohol
Wine is more than a percentage.
Yeast produces many compounds alongside ethanol, and these help create the character of the finished wine.
EstersOften associated with fruity aromas.
Higher alcoholsContribute to aroma, flavour and mouthfeel.
AcidsHelp shape freshness, balance and structure.
Aldehydes & other compoundsCan contribute to the overall aroma and flavour profile.
A useful habit
Measure before you change anything.
If a fermentation appears to have stopped, don't assume that it needs more sugar or yeast. Check the temperature and take a hydrometer reading first. Compare it with an earlier reading and with the recipe's expected range.
This is one of the simplest ways to avoid turning a nearly finished fermentation into an unnecessarily complicated one.
Responsible winemaking
Alcohol should always be produced and consumed responsibly and only where permitted by local laws. Follow applicable rules for home winemaking, storage and consumption.
Put it into practice