While batch culture has cell concentration, medium concentration, pH, products, viscosity, and other properties changing, continuous cultivation achieves steady state or shows small excursions from it. Relating cause and effect during experimentation is far easier for continuous fermentation. This great advantage for research is not a convincing argument for commercial processing where there must be cost advantages for continuous culture to be selected over batch.
Some assumptions greatly oversimplify the cost analysis.
When the composition of the medium is the same for batch and continuous operation, this cost cancels out. However, it may require a great deal of research on continuous cultivation to reach high titers of product. A product such as penicillin that is extracted from the broth with an immiscible solvent is extracted back into water at a concentration to match subsequent purification steps. This means that it matters little in terms of purification whether the fermentation is batch or continuous. If the solvent to broth ratio is fixed, the solvent cost is also unaffected by continuous operation.