Alcoholic proof
Very few microorganisms can live in alcoholic solutions. The main three are yeast, Brettanomyces, and Acetobacter. In what is essentially disinfection, yeast keeps multiplying as long as there is sugar to "eat", gradually increasing the alcoholic content of the solution and killing off all other microorganisms, and eventually themselves. There are "fortified" wines with a higher alcohol concentration than that because stronger alcohol has been mixed with them.