Why is wine about twice to three times as alcoholic as beer?
Ultimately the amount of alcohol in a fermented beverage is determined by the starting amount of sugar. In wine-making this is a matter of how much sugar is in the grapes. The grapes are pressed to make juice and the juice is fermented to make wine. Simple. The winemaker doesn’t really have much influence over the sugar content of the original juice. Grapes contain as much as 28% sugar by weight, but more usually around 24%. The grapes themselves contain everything necessary to make wine: a powdery coating of yeast, sugar, and water content. Things stand otherwise with beer.
There is no sugar in barley. Beer is made from starchy cereal grains, so the first step in beer making is mixing ground malted barley with water and converting the starches into sugars with the help of enzymes in the malt. But in this case there is something more like a choice on the part of the brewer how much sugar to start off with. About half of the weight of malt is starch that can be converted into fermentable sugars. So, to a degree, the brewer can determine the sugar concentration by deciding how much water to mix with the ground malt.
But it’s not as simple as it sounds for two reasons. First, the sugary liquid needs to be separated from the malt husks and second the process of dissolving and converting the starch dissolves other stuff as well.
The husks are dry, having been dried by the maltster to arrest germination and prevent the starch reserves being broken down and used to grow new barley plants! When mixed into the mash, the grain can absorb about its own weight in water. The first wort drained out of the mash tun can be very concentrated (a specific gravity of 1.100 or Plato: 23.7˚), about the same as wine, but because so much of the sugar extract remains adhering to the husk and otherwise caught up in the grains, it has to be washed out. Washing out the extract dilutes the overall sugar content. So to keep the sugar concentration very high, a brewer has to forgo a lot of potential sugar, at least half and maybe more (although this can be washed out separately as used as a second low abv beer). So the first reason why brewers don’t choose to make beer as strong as wine is that if they do, it means they either waste money or make a very weak beer as a complementary product. Maybe this consideration is not decisive.
In terms of a mass balance, it might go like this. 1kg of malt contains 800g of soluble extract of which roughly ⅔ is fermentble or 53% of the original mass (530g). To dissolve the extract, 2kg water is mixed with the malt. The malt absorbs half the water so only 1 litre is collected. This wort is very concentrated, about 24˚P, or a specific gravity of 1.100. So it weighs 1.1kg and contains 24% extract by weight, or 264g. But not all this extract is fermentable. It represents 264/800 equals 33% of the total soluble material and contains 50% of the available fermentable sugar.
The dissolved but unfermentable extract is mostly carbohydrates and sugars too complex to be broken down by yeast (dextrines) although significant amounts of protein are also present. Dextrines could comprise as much as 15% of the soluble extract. These are pleasant and contribute mouth feel in beer. However, a beer brewed to be as alcoholic as wine still has this extra body not present in wine and tastes cloying and syrupy.
That’s basically it. Beer can be as strong as wine, but it is wasteful to make it that way and it doesn’t taste nice when it is.