fermentation [fur-men-tey-shuh n] noun
A process in which yeast and bacteria convert carbohydrates, such as sugar, to alcohol and carbon dioxide. Fermentation is used to produce alcoholic beverages such as wine, beer and cider, and is also responsible for leavening bread and preserving sour foods such as sauerkraut, yogurt and vinegar. It results from the addition of yeast, or – as is the case with sourdough starters – without any added yeast.
The sharp, earthy taste of sourdough bread comes from its starter, which is usually just flour and water. The fermentation happens naturally with yeast and bacterium (lactobacillus) that is already present in the flour and grows over time as the starter is fed. Some starters have been in existence for many decades.
Many welcome the distinct taste of sourdough, but creating and taking care of a starter requires some attention. If the starter is not fed regularly, it will stop growing (which means the fermentation has ceased) and become unusable. Most starters need to be fed about twice a day when they are created, and even if the starter is not being created from scratch, they still need to be fed once a week. People with starters literally have to have them babysat when they go away on vacation!