Substrates - A variety of substrates, ranging from inorganic carbon, e.g., CO2, through industrial effluents, e.g., confectionery effluents, whey, etc. (utilization of the substrate helps effluent disposal), and low cost organic materials, e.g., cellulosic wastes like straw, etc., to high cost materials like starch hydrolysate are used for SCP production.

These substrates can be divided into two broad groups:

(i) fossil carbon sources (nonrenewable), and

(ii) renewable carbon sources. But substrates like methanol and ethanol can be produced from both renewable and nonrenewable carbon sources.