The move from development in the laboratory or research pilot plant to development in the main plant involves very considerable changes in methods and ideas. These arise mainly from the size of the equipment, but also from the very different approach that is necessary in commercial operations. In a way, it can be said that the laboratory worker is usually free to think and to experiment and to regard the fermentation process as flexible. Some experiments work and are followed up, others do not do so well and are forgotten. As the work moves to the main plant, the assumption is that the process is now clearly defined, and the objective is to bring it into operation with a minimum of risk, without embarking on a series of experiments. In any case, in large-scale work, the idea of discarding a batch is unthinkable on account of the expense. Thus the introduction of a new process to the plant requires a programme that is well thought out and very reliable, and fully backed up by laboratory work.

In this connection, the main role of the pilot plant is to check laboratory processes as well as possible to establish their suitability for plant work. The larger pilot-plant fermenters may be used for this (1000–50001), as well as for producing kilogram quantities of new substances for test.

The role of the main plant is commercial, i.e. to produce quantities of material for sale at a profit. The firm's profitability, and the availability of cash resources for research and development, depend upon the success of the main plant.