You start with a liquid containing some kind of sugar, add yeast, and wait. The yeast digests the sugar, giving off alcohol in the process.
The sugar in beer comes mostly from a grain called barley. With wine, the sugar usually comes from grapes.
Either way, when the yeast finishes its work, the sugar has been turned into alcohol--which stays--and carbon dioxide, which bubbles to the surface. Since wine starts out with more sugar than beer does, it ends up with more alcohol.
If you make beer or wine at home, the trick is knowing just how much sugar to start out with in order to get the alcohol content you want to end up with.
However, if the alcohol gets too strong, it'll poison the yeast, stopping the fermentation altogether.
All About The Yeast
How much alcohol the yeast can tolerate depends partly on the type of yeast. And that's another reason wine is stronger than beer: beer-yeast, with a lower tolerance for alcohol, is killed by the time the alcohol reaches about nine percent.
Wine-yeast is hardier and can withstand concentrations up to around sixteen percent. Since most beer is less than six percent alcohol and wine is generally under twelve percent, yeast usually runs out of sugar before it creates enough alcohol to poison itself.
Not Too Much Sugar
Novice home-brewers and wine-makers often try to boost the alcohol content by using a lot of sugar. This works up to a point, but every strain of yeast has a limit to how much alcohol it can produce before quitting.
Adding more sugar will make a sweeter beer or wine, but it won't make it any stronger.