Introduction:
Single-cell protein (SCP), often called microbial protein, is a type of crude or refined protein obtained from filamentous fungi, algae, yeasts, molds, microbes, or bacteria. It is edible and can used directly either as animal feed or human food supplements, that usually contain above 40% of crude protein on dry weight bases.
Due to protein shortages in Third World countries, large-scale growth of SCP led to the development of technology for both livestock and human consumption.
The production of SCP involves the utilization of fungi, algae, yeast, and bacteria. However, the microorganisms used for SCP production must be non-pathogenic, good nutritional value, easily and cheaply produced on scale, carcinogen and toxin free, easily accessible, fast growing, easy to isolate, separate, and purification.
E.g.: Chlorella sp. Spirulina maxima, saccharomyces cerevisiae, Paecilomyces Varioti, Methylophilus methylotrophus, Aspergillus niger, Brevibacterium sp., are few important microorganisms used for SCP production.
Substrate of SCP:
A variety of substrate ranging from inorganic carbon, e.g., Co2 (no cost for substrate), 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 can be used for SCP production. Moreover, the substrate can be divided into the nonrenewable and non-renewable carbon sources.
| Substrate | Microorganisms |
| Fossil carbon sources (Non-renewable carbon sources) | |
| 1. Liquid hydrocarbons 2. Gaseous hydrocarbons 3. Methanol (produced by chemical conversion of CH4, etc.) 4. Ethanol | Candida lipolytica Pseudomonas methanica, Methylomonas methanica Methylococcus capsulatus Pseudomonas methanitrificans Methylophilus methylotrophus Pseudomonas stutzeri Candida boidinii Torulopsis glabrata Candida utilis |
| Renewable carbon sources | |
| 1. Carbon dioxide (C02) 2. Molasses 3. Whey 4. Starch hydrolysates 5. Industrial effluents  6. Cellulosic wastes e.g., Sawdust, straw | Spirulina, Chlorella Saccharomyces cerevisiae Kluyveromyces fragilis, Lactobacillus Bulgaricus, Candida Intermedia  Fusarium graminearum  Candida utilis  Chaetomium cellulolyticum |
Production of SCP:
The process of SCP production from any microorganism or substrate should follow the following steps:
Selection of Microorganisms and Substrate Preparation
The selection of microorganism and its suitable substrate will be selected for the production of SCP. The substrate will be prepared or it is physically or chemical pre-treatment prior to the process.
Waste Materials, industrial Gases (Methanol, ethanol, etc.), agricultural Wastes and other lignocellulosic materials are generally used as substrates.
Fermentation
There are various types of fermenter or bioreactors such as batch culture, continuous culture, and fed-Batch Fermentation. Addition, to the carbon source, of course nitrogen, phosphorus and other nutrients are also needed to support optimal growth of the selected microorganism in the fermenter.
Sterile or hygienic conditions are maintained in order to prevent contamination. Generally, heating and filtration are employed for maintaining aseptic conditions. Moreover, the selected microorganism should be inoculated in a pure state.
Generally, the process of SCP production is extremely aerobic, unless algae is used. Therefore, adequate aeration must be provided by cooling is necessary when the heat is generated during the process.
Harvesting Biomass, Post-harvest treatment and post-harvest treatment
The microbial mass is recovered from the medium and processed for enhancing its usefulness and/or stability.
The various approaches such as filtration, centrifugation, flocculation, and flotation are the general approaches for the recovery of the biomass. In addition, it is very important to reduce water as much as possible to reduce drying costs.
Nutritional and Safety evaluations:
- The SCP chemical composition must have been characterized in terms of protein, amino acid, nucleic acid, vitamins, lipid, etc. contents.
- Analysis of substrate residues and toxic substances such as mycotoxins, heavy metals, polycyclic hydrocarbons, etc. must be done.
- Physical properties such as particle size, texture, colour, density, etc., properties should be determined.
- Microbial description, e.g., species, strain, should be provided and information on contamination may be also given.
- The different species or the various substrate will have various nutritional value so the proper analysis of nutritional value in the specific species should be included.
- Yeast SCPs are increasingly used in the synthesis of aquaculture feeds. Probiotic yeast strains increase larval survival by colonizing fish larvae’s guts.
- It is necessary to test for any potentially hazardous or carcinogenic chemicals. These molecules may have been present in the substrate, generated by the organism, or created during SCP processing.
Advantages of SCP:
- The SCP is high in high-quality protein but low in fat, which is desired.
- It can be produced all the year round and is not dependent on the climate (except for the process of algae).
- The microbes are very fast growing and produce large quantities of SCP relatively very small area of land.
- They use low-cost substrate, and in some cases such substrate, which are being wasted and causing pollution to the environment.
- When the substrate used for the SCP process is a source of pollution, SCP production helps reduce pollution.
- Some SCPs are very good source of vitamins, particularly vitamin-B complex.
- Strains with high biomass yields and a desired amnio acid composition can be easily selected or generated through genetic engineering.
Disadvantages of SCP:
- Some microorganisms are capable of producing toxins and after consumption of such toxin some complications such as indigestion, allergic, palatability may observe.
- Microorganisms have naturally high levels of nucleic acid due to fermentation conditions that promote rapid growth and high protein content. However, digestion of nucleic acids by humans and animals results to the synthesis of purine compounds, which can cause gout-like symptoms or kidney stones.