The Crabtree effect is where Saccharomyces and Brettanomyces yeasts digest carbohydrates both anaerobically – producing ethanol, an antiseptic – and aerobically. Most yeasts in the presence of both sugar and oxygen prefer to only convert these to carbon dioxide and water, which is considerably more efficient but doesn't deter competitors for that sweet, sweet sugar.

Source: https://www.forbes.com/sites/quora/2017/08/03/why-isnt-bread-alcoholic/