In a fed-batch process, you can adjust the medium and roduct of control conditions such as nutrients and compounds. For a fermentation process where the main nutrient inhibits production growth at high concentration, you can add more nutrient to the fermenter as the previous nutrient gets used up by the system. This is a useful tool in order to keep the nutrient concentration low enough to not inhibit growth, but to supply nutrients to the cells in order to keep them alive and working!