The Crabtree effect describes the production of ethanol by S. cerevisiae under aerobic conditions when glucose concentration exceeds a critical level (~0.1–0.2 g/L). Above this threshold, glycolytic flux exceeds mitochondrial respiratory capacity, and excess pyruvate is diverted to ethanol. This is a major problem in recombinant protein production (growth-associated by-product). Fed-batch feeding to keep glucose below the critical concentration avoids the Crabtree effect.