What grape juice lacks is yeast

With wine, a lot of attention is paid to the grapes. The varieties, their special characteristics, ripeness, how they are planted and pruned. But without yeasts, there would never have been more than grape juice. They take care of converting fruit sugars into alcohol.
Text: Niels van Laatum | Image Niels van Laatum, The Secret Sherry Society and Unsplash

You may remember the nature films in which monkeys and elephants display drunken behaviour after eating large quantities of ripe fruit. This is because yeast cells produce alcohol (actually: ethanol) as a chemical weapon against bacteria. So from the moment fruit and grains grew on our globe, there was alcohol. Yeasts and bacteria compete with each other for the nutrients in the fruit, only yeasts are often just that little bit stronger and smarter because bacteria are less tolerant of alcohol.

It is a completely natural process that yields a modest alcohol content. The overripe banana, often referred to, only reaches a maximum of 0.04 per cent alcohol. But our apple and orange juice sometimes reaches as much as 0.7 per cent, reports Belgian wine writer Dirk Rodriguez at vino.be. Hence perhaps the advice to drink freshly squeezed juice within 24 hours?

High percentage of alcohol

With the modern fermentation techniques we humans have figured out, we realise wines and beers with a much higher alcohol content. Higher than is possible in the wild when, for example, ripe fruit falling from the tree rots.

How? By using hyper-efficient and particularly alcohol-tolerant yeasts. Thus, alcohol percentages of even 16 per cent are achieved, think Shiraz from Australia. On rare occasions, cases of even higher percentages are known, created from natural fermentation.

In Georgia, we stumbled across a wine made by Paparavi Valley from saperavi grapes with a natural alcohol content of 19 per cent. On the Portuguese Azores island of Pico, there is the exclusive, sweet Czar wine, made from vine-dried verdelho grapes. Winemaker Fortunato Gracia confided to us that his wine is not possible according to the oenology books. He even reached 20.1 per cent once, without any alcohol enrichment. 'Everyone says it can't be done, but it really does happen.' 

What are yeasts?

Yeasts are single-celled micro-organisms from the realm of fungi. Unlike bacteria, they have a cell nucleus and belong to the Eukaryotes (quickly forgotten). The similarity with bacteria is that yeasts too cannot be seen with the naked eye. Yeasts of some genera, such as Saccharomyces, have the special property of converting glucose (sugar) into alcohol and carbon dioxide. This is why Saccharomyces cerevisiae is also called 'wine yeast'.