Grape sugar changes into alcohol during fermentation, thereby turning grape juice into wine. If grapes are under-ripe, there won’t be enough sugar to make the alcohol. Let’s start at the beginning.
Grape Sugar
A grape vine requires sunlight, heat, water and nutrients in order to undergo photosynthesis and ripen its berries. Healthy, ripe grapes contain a balanced amount of water, sugar and acid at the time of harvest.
Sugar & Growing Season
When berries form on the vines and begin to develop their pulps, they have low levels of sugar. As grapes ripen during the course of the growing season, a process known as verasion, the acidity levels fall and the sugars levels begin to rise. If you were to pick a grape in July and eat it, the tart acids would be very strong and off-putting. However, a grape from that same bunch in October would have less acid and and more sugar. Using specialized instruments and often their own intuition, grape-growers must determine the absolute best time to harvest the grapes with their ideal balance of acidity and sugar. In some cases, grapes are purposely left on the vine to produce sweet or dessert wines.
Fermentation
When grapes come into the winery, they undergo a process called fermentation, whereby yeasts convert the juice sugars into alcohol. Wild (ambient) yeasts exist naturally on grape skins, stems, and even in the winery itself. A winemaker can choose to use these naturally-present yeasts to complete the fermentation process, or add yeast to the juice (most common). The grape juice temperature must be adjusted to the right setting in order for yeast to begin and continue their job.
Over the course of several weeks, the yeasts consume the grape juice sugar and replace it with Carbon Dioxide and Ethanol (alcohol). Once the yeasts consume all the sugar, they die and are filtered out of the wine. However, if the winemaker were to remove the yeasts before all the sugars were consumed, the wine would remain sweet with lower levels of alcohol.
In some cooler regions where grapes don’t ripen enough to create the amount of sugar needed to make alcohol, winemakers are permitted to add sugar to the grape juice so that more is present for the yeasts to consume. This practice, known as chaptalisation, is outlawed in many wine-growing regions.
ABV Levels (Alcohol By Volume)
Most wines have an ABV rate between 11% and 13.9%. In some cases the rate can be higher. However, the majority of grapes do not have enough natural sugar to exceed an ABV above 15.5%.
There are several factors that impact the final alcohol level:
- Natural Sugar Levels: Some grapes naturally produce more sugar, which turns to alcohol
- Climate: Grapes in warmer climates generally produce more sugar, which turns to alcohol
- Chaptalisation: Sometimes additional sugar is added to the juice to increase the alcohol
- Fortification: Some wines include additional spirit or liquor, i.e. Port, which increases the alcohol
Cheers!
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Your article inspired me to check the alcohol content of 4 of my unopened bottles (a dark red blend, a Cabernet Sauvignon, a different red wine blend and a Chianti). Three of them were 13.5% alcohol by volume and only one was 14.5%. I’d never paid attention to this before.
A lot of the California wines have higher percentages because it’s hot and the grapes get riper.