Fed-batch culture is, in the broadest sense, defined as an operational technique in biotechnological processes where one or more nutrients (substrates) are fed (supplied) to the bioreactor during cultivation and in which the product(s) remain in the bioreactor until the end of the run. An alternative description of the method is that of a culture in which base medium supports initial cell culture and a feed medium is added to prevent nutrient depletion. It is also a type of semi-batch culture. In some cases, all the nutrients are fed into the bioreactor. The advantage of the fed-batch culture is that one can control concentration of fed-substrate in the culture liquid at arbitrarily desired levels (in many cases, at low levels). Generally speaking, fed-batch culture is superior to conventional batch culture when controlling concentrations of a nutrient (or nutrients) affects the yield or productivity of the desired metabolite. The fermentation process can be carried out in batch, fed-batch, or continuous modes of operations. While all the materials are fed once to the reactor in the batch process, in the fed-batch and continuous processes they are fed continuously with a small amount of the media. Therefore, the sugars as well as the inhibitors are in high concentrations at the beginning of the fermentation in a batch reactor, while their concentrations can be kept low in the fed-batch and continuous modes. A combination of these facts, as well as the ability of converting some of the inhibitors at their low concentrations by the fermenting microorganisms, results in the success of the fed-batch and fermentation, while the batch fermentations of the same hydrolyzates might fail. If the hydrolyzate is severely inhibiting, the cells will not grow in batch cultivation and no fermentation will occur. However, if the culture is slightly toxicin situ detoxification can partly occur, but might not be enough to complete the fermentation. Chung and Lee tried to ferment acid hydrolyzates by continuous cultivation. Although they succeeded in the fermentation, they observed 90% decrease in the viable cell number after three residence times. Reducing the cell viability results in lowering the dilution rate, and consequently, the productivity of the cultivation is reduced and eventually washout of the cells occurs. Although the cells can be retained in the reactor by, for example, filtration, the cells have difficulties with toxic substrates and lose their viability. In such cases, fed-batch cultivation could be an alternative.
