Microbial biomass is produced commercially as single-cell protein (SCP) for human food or animal feed and as viable yeast cells for the baking industry. The industrial production of bakers' yeast started in the early 1900s and yeast biomass was used as human food in Germany during World War I. The development of large-scale processes for the production of microbial biomass as a source of commercial protein began in earnest in the late 1960s. Several of the processes investigated did not come to fruition owing to political and economic problems, but the establishment of the ICI Pruteen process for the production of bacterial SCP for animal feed was a milestone in the development of the fermentation industry. This process used continuous culture on a large scale (1500 m3). The economics of the production of SCP as animal feed were marginal, which eventually led to the discontinuation of the Pruteen process. The technical expertise gained from the Pruteen process assisted ICI in collaborating with Rank Hovis McDougall on a process for the production of fungal biomass for human food. A continuous fermentation process for the production of "Fusarium venenatum" biomass (marketed as Quorn) was developed using a 40-m3 air-lift fermenter.