Practice question

Fed-batch fermentation begins as conventional batch but receives intermittent or continuous feed of concentrated substrate solution without culture withdrawal, progressively increasing volume and extending productive phase while keeping residual substrate concentration below inhibitory threshold. Many industrial organisms exhibit substrate inhibition and catabolite repression: high initial glucose above critical concentration in Saccharomyces cerevisiae triggers Crabtree effect diverting flux to ethanol via pyruvate decarboxylase, while in Escherichia coli acetate overflow via Pta-AckA pathway reduces recombinant protein yield and inhibits growth. By implementing exponential feeding based on mu-max, pH-stat where feed triggered by pH rise upon substrate depletion, or DO-stat where oxygen spike signals carbon exhaustion, residual glucose held at 0.1 to 1 g per L, maintaining fully oxidative metabolism and healthy respiratory quotient below 1.1. This strategy alleviates osmotic stress, prolongs logarithmic growth, achieves biomass exceeding 100 g per L dry weight and dramatically boosts product titers for baker's yeast, antibiotics and therapeutic proteins while retaining operational simplicity relative to continuous culture.