Single Cell Protein (SCP): Sources, Examples, Uses

A fungus grown in a steel tank can produce more protein in a day than a cow does in a month, on a fraction of the land and water. That is the idea behind single-cell protein, one of the most promising answers to a growing world's demand for protein.

Single-cell protein (SCP) is the dried protein-rich biomass of microorganisms (algae, bacteria, yeast, fungi) grown on a large scale and used as food or animal feed. It is called "single-cell" protein because the microbes are harvested as individual cells rather than as a whole plant or animal. In biology and microbiology courses, SCP is the standard term for this microbial protein; the older name was simply "microbial protein."

The appeal is simple. Microbes grow fast, double in hours, use cheap or waste substrates, need little land, and can be produced in any season under controlled conditions. Developed in the mid-20th century, SCP is now used both to feed livestock and, in a few cases, people. This article covers which microbes make it, how it is produced, what it is used for, and the one nutritional catch that limits how much humans can eat.

Figure: Single cell proteins

Sources of Single Cell Protein (SCP) Production

Microorganisms are the only source of single-cell protein. Microorganisms that have the following characteristics are selected for the production of single-cell proteins;

1. The microorganisms should be able to accumulate a substantial protein content within their cellular structures.
2. Microorganisms with a fast growth rate are preferable as they allow for more efficient and scalable production processes.
3. The selected microorganisms should be non-toxic and safe for human consumption or animal feed.
4. The microorganism should tolerate a range of conditions (temperature, pH, salinity) so it stays stable in large-scale culture.
5. The microorganisms should be capable of utilizing a wide range of substrates or feedstock for growth and protein production.
6. The microorganisms should possess genetic manipulation capabilities depending on the desired protein or compound to be produced.
7. Microorganisms selected for SCP production should be flexible to large-scale cultivation and downstream processing.

Four groups of microbes are used for SCP, and each is suited to particular substrates. The substrate is often a cheap or waste material, which is a large part of SCP's appeal.

Two real products anchor this topic. Quorn, a meat substitute eaten by people, is mycoprotein made from the fungus Fusarium venenatum. Pruteen, an animal feed made by ICI in the 1970s–80s, was bacterial SCP from Methylophilus methylotrophus grown on methanol. Naming these two makes the abstract idea concrete: one fungal SCP for humans, one bacterial SCP for livestock.

Nutritional value of Single Cell Protein

Microorganisms not only contain large quantities of protein, but they also contain carbohydrates, fats, vitamins, mineral salts, as well as non-protein nitrogenous substances (NPN) such as amino acids. Therefore, the average composition of single-cell protein, according to Al-Mudhafr et al. (2019), is given in the table below;

One number is missing from most SCP composition tables but matters more than any other: nucleic acid content. Microbial cells are very rich in RNA (often 5–15% of dry weight, far higher than in plant or animal food), because fast-growing cells are packed with ribosomes. This is the source of SCP's main safety limit, explained below.

Production of Single Cell Protein

Production of single-cell protein completes within various steps that are as follows;

Figure: Production of single cell protein (SCP)

Step 1: Selection of strains:

It is a crucial step as the quality of protein depends on the type of microbe used for production. Only the microbes that have a fast growth rate and do not harm the consumers by producing toxic effects are preferable. Further, a suitable substrate required for the growth of the selected microbe should also be chosen.

Step 2: Fermentation:

The selected microbes are grown in a fermenter fitted with aeration, temperature control, and pH control, or, for algae, in open ponds and tanks. Industrial SCP is usually grown in continuous or fed-batch culture to keep the microbes multiplying at their fastest rate.

Step 3: Harvesting:

When the colonies of microbes are fully developed, they are then harvested. The bulk of cells are separated from fermenters by decantation.

Step 4: Post-harvest treatment:

After harvesting, cells are subjected to post-harvest treatment that includes; separation by centrifugation, washing, drying, etc.

Step 5: Single cell protein (SCP) processing for food:

The dried cells are further processed to remove impurities, enhance nutritional content, and enhance texture and flavor.

Step 6: RNA reduction (for human food):

SCP intended for people is treated to lower its nucleic acid content, usually by a brief heat step that activates enzymes to break down RNA. Without this step, the SCP cannot be safely eaten in large amounts. Feed-grade SCP for animals often skips this step, because livestock handle nucleic acids better than humans do.

The Nucleic Acid Problem: SCP's Main Limitation

The biggest obstacle to feeding SCP to humans is not taste or cost. It is the high nucleic acid (RNA) content.

Microbial cells grow fast, and fast growth needs many ribosomes, which are rich in RNA. As a result, SCP contains far more nucleic acid than any traditional food. When humans digest that RNA, the purine bases are broken down to uric acid. Humans (unlike most animals) lack the enzyme uricase, so we cannot break uric acid down further. High uric acid then crystallizes, causing gout in the joints and kidney stones.

The safe limit for adult humans is only about 2 grams of nucleic acid per day, which caps SCP intake at roughly 20 grams per day unless the RNA is removed. This is why:

• SCP for human food (like Quorn) is treated to reduce its RNA before sale.
• Much SCP is used as animal feed instead, since livestock tolerate nucleic acids better.

If you remember one limitation of SCP, remember this one. It explains the kidney-stone and gout risk, the need for RNA-reduction processing, and why SCP has found more use in feed than in food.

Applications of Single-Cell Protein

Single-cell protein has a wide range of applications in various sectors. These are as follows;

1. Protein supplement: SCP can be useful as a supplement or substitute for traditional protein-rich ingredients like meat, soy, and fish meal. SCP offers a sustainable solution to address the increasing global demand for protein, particularly in regions with scarce protein sources.
2. Health and specialty foods: Because it is high in protein and low in fat, SCP is used in meal-replacement and high-protein products. Spirulina and Chlorella are sold as nutritional supplements. Quorn mycoprotein is a widely sold meat substitute made from fungal SCP.
3. In therapeutics and natural medicines: Single-cell protein (SCP) has potential applications in therapeutics and natural cures, including the production of therapeutic proteins, drug delivery systems, natural medicine production, probiotics, nutraceuticals, and antimicrobial agents.
4. In cosmetics: Single cell protein (SCP) can also be utilized as an ingredient to produce cosmetic products.
5. Poultry and cattle feed: Single-cell protein (SCP) can be a valuable ingredient in poultry and cattle feed. It serves as a sustainable alternative to traditional protein sources like soybean meal or fish meal.

Advantages of Single Cell Protein (SCP)

Single-cell protein has several benefits that are as follows;

1. Single-cell proteins can be produced in a short duration due to the rapid succession generation of microorganisms.
2. The genetic content of microorganisms can be easily adaptable. As a result, diverse amino acid compositions can be produced.
3. The single-cell proteins are highly rich in protein content.
4. It requires the cheapest raw material as a substrate for the growth of microorganisms.
5. It can be produced at any season in a controlled environment.
6. Its production does not require a large land area and is ecologically beneficial.

Disadvantages of Single Cell Protein (SCP)

Despite having many benefits, single-cell proteins also have some disadvantages, such as;

1. In some cases, toxic secondary metabolites in single-cell proteins may be present.
2. Single-cell protein (SCP) from certain microorganisms may lack essential nutrients, such as certain amino acids or vitamins. Therefore, ensuring a well-balanced nutritional profile in single-cell protein (SCP) can be challenging.
3. Some single-cell proteins (SCP) derived from some microorganisms might have allergic effects in susceptible individuals. Therefore, it is important to thoroughly evaluate the allergenicity of single-cell proteins and consider potential risks for consumers.
4. Its high nucleic acid (RNA) content raises uric acid in humans, which can cause gout and kidney stones if too much is eaten without RNA-reduction treatment (see the nucleic acid section above).
5. Some SCP, especially from algae and yeast, has a tough cell wall that humans digest poorly, which can cause gastrointestinal upset and lowers how much of the protein the body can actually absorb.

How to Remember

SCP = Single-Cell Protein, from four groups: A-B-Y-F. Algae, Bacteria, Yeast, Fungi. Four kinds of microbe, all grown as single cells and dried for protein.

Two famous products, two audiences: Quorn for us, Pruteen for them. Quorn (fungal, Fusarium) is human food. Pruteen (bacterial, Methylophilus) was animal feed. One fungus for people, one bacterium for livestock.

The catch is RNA, and the result is gout. Fast-growing microbes are packed with RNA. Humans turn RNA into uric acid, cannot break it down further, and get gout and kidney stones. High RNA is the single biggest limit on eating SCP.

Why microbes and not cows: fast, cheap, small. Microbes grow fast (hours, not months), eat cheap or waste substrates, and need little land. That is the whole case for SCP in one line.

Key exam facts

Note on protein numbers: bacteria have the highest protein content, but yeast and fungi are more often used for human food because they are easier to grow, harvest, and accept.

Where Students Get Confused

SCP is the biomass, not a single purified protein. "Single-cell protein" means the whole dried microbial cells, eaten for their protein. It is not one isolated protein molecule. The "single-cell" refers to the microbes, not to one protein.

The main problem with SCP is RNA, not toxicity. Students often name toxins first. The real, universal limitation is the high nucleic acid content and the gout and kidney-stone risk it creates. Toxins are a risk only with certain organisms or substrates.

Why humans get gout from SCP but many animals do not. Humans lack the enzyme uricase and cannot break uric acid down further, so it builds up. Many animals have uricase and handle the same nucleic acid load, which is why SCP is used more freely in animal feed.

Highest protein does not mean most used. Bacteria have the highest protein content, but yeast and fungi dominate real products because they are easier to grow, harvest, and make palatable.

SCP is not the same as mycoprotein. Mycoprotein (like Quorn) is one type of SCP, made specifically from fungi. All mycoprotein is SCP, but not all SCP is mycoprotein.

Ringkasan dalam Bahasa Indonesia: protein sel tunggal (SCP)

Protein sel tunggal (single-cell protein, SCP) adalah biomassa mikroorganisme kaya protein yang dikeringkan dan digunakan sebagai pangan atau pakan ternak.

Contoh organisme yang digunakan sebagai sumber SCP:

• Alga: Chlorella, Spirulina, Scenedesmus (tumbuh dengan sinar matahari dan karbon dioksida).
• Bakteri: Methylophilus methylotrophus, Cellulomonas, Pseudomonas (substrat: metanol, selulosa, limbah).
• Ragi (yeast): Candida utilis, Saccharomyces cerevisiae (substrat: molase, whey).
• Jamur berfilamen: Fusarium venenatum (bahan dasar produk Quorn), Aspergillus niger.

Keterbatasan utama SCP adalah kandungan asam nukleat (RNA) yang tinggi. Pada manusia, RNA diubah menjadi asam urat yang dapat menyebabkan asam urat (gout) dan batu ginjal, sehingga SCP untuk manusia harus diproses untuk menurunkan kadar RNA.

References

1. Al-Mudhafr, A.W.H. and Al-Garawy, A.M.A. (2019). Microbiological sources and nutritional value of single cell protein (SCP). Journal of Nutrition and Food Processing, 2(2). https://doi.org/10.31579/2637-8914/013
2. Dubey, R.C. and Maheshwari, D.K. (2014). A Textbook of Biotechnology, 5th ed. S. Chand & Company. (Single-cell protein chapter.)
3. Ritala, A., Häkkinen, S.T., Toivari, M. and Wiebe, M.G. (2017). Single cell protein: state-of-the-art, industrial landscape and patents 2001–2016. Frontiers in Microbiology, 8, 2009. https://doi.org/10.3389/fmicb.2017.02009
4. Madigan, M.T., Bender, K.S., Buckley, D.H., et al. (2021). Brock Biology of Microorganisms, 16th ed. Pearson. (Industrial microbiology and microbial biomass sections.)