English: Fermentation of sucrose by yeast. When dried yeast is hydrated in warm water in presence of sucrose (sugar), it produces acid, alcohol and other organic molecules. During the initial period of growth of the yeast, the dissolved oxygen is depleted rapidly with the evolution of carbon-dioxide (observable as bubbles). The respiration then shifts to anaerobic mode, resulting in formation of ethanol as well as organic acids. The picture shows formation of acid by yeast after 10 minutes of incubation in warm water in presence of sucrose. Bromothymol blue indicator turns yellow in presence of acid.