Yeasts and bacteria have been particularly important for single-cell protein (SCP) production; they represent the most widely used microorganisms for this purpose: the former mainly for human consumption and the latter frequently used for feed production. Substrates for yeast and bacteria biomass growth include food-processing by-products as well as fossil hydrocarbons. Yeasts have been the preferred source for SCP production over bacteria, particularly for human consumption, as they are related to products, such as bread or beer; however, bacteria have higher protein content, higher yields, and faster growth rate, but they are less accepted for human food. The main drawback for the SCP consumption is its high content of nucleic acids and components of cell wall, which limits its uptake in the diet, at least so that the biomass can be processed to isolate the protein.