Why does alcohol-free beer taste different?

Because alcohol is not just the "strength" of a beer - it genuinely carries a lot of flavour, body and mouthfeel. When it is removed, whether by vacuum distillation, reverse osmosis or osmotic distillation, some of the flavour compounds that dissolved into it during brewing come out along with it.

The techniques differ in how much they strip out. Vacuum distillation (the most common method) heats the beer at low pressure so it evaporates at a lower temperature than boiling would, which preserves more flavour than older, harsher methods - but some loss is still unavoidable. That is why most alcohol-free beers add something back afterwards: extra hops, malt extract or flavouring, to compensate for what the process took out.

This is also exactly why quality varies so much between brands - see our reviews to find the ones that have actually nailed it rather than assuming all alcohol-free beer tastes the same. Some breweries brew specifically for low alcohol from the start rather than stripping a full-strength beer afterwards, which tends to preserve more of the original character.