This highly nutritive product has often failed in the past for lack of proper research.
The cultivation of single-cell microorganisms, be it microbes or yeasts, for producing a variety of compounds (medicinal or nutritional) is commonplace in modern society.
After the removal of the useful compounds, say a vitamin or a drug, the remaining culture is disposed as biomass. It is most often offered at competitive prices to the animal feed industry and it is valued for its protein and/or fat concentration. Availability is rather sporadic and local, and those who sell and use such biomass waste do not want to make a big deal about it.
Now, there are startup companies, which often use public money destined to protect the environment, that produce single-cell microorganisms exclusively for the purpose of feeding them to animals. The end products are frequently heavily advertised as an ideal source of protein, especially for young animals. Some such products even come with a premium on their price.
Of course, there is nothing wrong about single-cell microorganisms being fed to animals. In fact, single-cell protein (SCP) use is nothing new. It has been attempted many times in the past, and it has always failed the scrutiny of science, because feeding it preceded careful research and development, while the opposite is the correct way to go. In other words, unless you know very well what you do with these microorganisms, you might end up with something you do not want, especially if you do not know what to look for.
All this is not to dissuade possible users from feeding SCP to their animals, but rather to encourage producers to ensure their product is safe, wholesome and does not cause long-term problems. It would be advisable to look up older references to review similar attempts so as to avoid making the same mistakes. If that is done, then SCP can offer a good solution to using inexpensive resources to produce a highly valuable product – just as cows do in their rumen.