All of the above methods either require the availability of expensive equipment or the substantial investment of time. In reality, the most often used method simply monitors the optical density of the sample. The absorbance of the sample measured in a spectrophotometer is correlated to either the dry weight or the number of cells per volume.
Biomass concentration is one of the most critically needed measurements in fermentation studies. It is also one of the most difficult and unreliable ones. For example, all the above dry/wet weight methods and all the automated counting equipment fail completely if the broth contains other insoluble particulate matter, which is often the case in a practical fermentor. Similarly, the optical density measurement only has limited usefulness if the fermentation broth is not clear. In addition, these methods cannot distinguish the viable cells from the dead ones. On the other hand, the standard plate count can detect viable cells among other particulate matters. However, the method requires elaborate preparations, and it takes 24-48 hours for the cells to be incubated and counted; the cost of Petri dishes and media can also be prohibitive. Consequently, the direct plate count is useless in feedback control of a fermentation process; it is mainly used industrially to countercheck other measurements, especially the optical density.
In this experiment, the cell density of a given sample will be measured with the following five methods: wet weight, dry weight, optical density, direct cell counting with a chamber, and successive dilutions followed by plating.
Statistically, the most reliable results are given by plates with between 30 and 300 colonies. Only about two significant figures can be obtained from this method. The accuracy can be improved if multiple plates can be prepared. This, however, is rarely done due to the cost.