A REVIEW OF MICROBIAL PROTEIN PRODUCTION: PROSPECTS AND CHALLENGES

Abstract

The increase in protein malnutrition due to low quality proteins in food supply, especially in developing countries, has stimulated the search for new and alternative sources of protein, both in human foods and animal feeds. The production of microbial protein or single cell protein (SCP) is revolutionizing protein farming and is, indeed, a key step in reducing the shortage of protein supply. These nonconventional protein sources are products of biotechnological processing of agricultural, industrial and forestry wastes. Single cell proteins or microbial proteins are dehydrated microbial cell culture or purified protein derived from a microbial cell culture, such as bacteria, yeast, algae or filamentous fungi, with potential to be a source of animal or human protein supplement. This type of protein has been cultivated by culturing appropriate microbes on different substrates like starch, corn cob, whey, wheat, starch hydrolysates, hydrocarbon, alcohols, molasses and sugarcane bagasse. The manufacturing of SCP as an alternative source of protein has considerable benefits over conventional sources because of its decreased production time, lower land requirement and ability to be produced in all kind of climates. In spite of the obvious advantages of SCP viz., its nutritive value in terms of protein, vitamins and lipid content, it is accepted with some measure of uneasiness and its chances of substituting conventional protein are still slim; its major disadvantage being its high nucleic acid content and low digestibility. This paper reviews the production of single cell protein, its benefits, safety, acceptability, cost and the limitations associated with their uses, as it portends great promise as an alternative source of proteins.

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A REVIEW OF MICROBIAL PROTEIN PRODUCTION:
PROSPECTS AND CHALLENGES
E.A. Ugbogu
1
and O.C.
Ugbogu
2
1
Department of Biochemistry, Abia State University, PMB 2000, Uturu, Nigeria
2
Department of Microbiology, Federal University Wukari, PMB 1020, Wukari, Taraba State, Nigeria
*Corresponding author: amasryal@yahoo.com;
Abstract: The increase in protein malnutrition due to low quality proteins in food supply, especially in developing
countries, has stimulated the search for new and alternative sources of protein, both in human foods and
animal feeds. The production of microbial protein or single cell protein (SCP) is revolutionizing protein
farming and is, indeed, a key step in reducing the shortage of protein supply. These nonconventional protein
sources are products of biotechnological processing of agricultural, industrial and forestry wastes. Single cell
proteins or microbial proteins are dehydrated microbial cell culture or purified protein derived from a
microbial cell culture,
such as bacteria, yeast, algae or filamentous fungi, with potential to be a source of
animal or human protein supplement. This type of protein has been cultivated by culturing appropriate
microbes on different substrates like starch, corn cob, whey, wheat, starch hydrolysates, hydrocarbon,
alcohols, molasses and sugarcane bagasse. The manufacturing of SCP as an alternative source of protein has
considerable benefits over conventional sources because of its decreased production time, lower land
requirement and ability to be produced in all kind of climates. In spite of the obvious advantages of SCP viz.,
its nutritive value in terms of protein, vitamins and lipid content, it is accepted with some measure of
uneasiness and its chances of substituting conventional protein are still slim; its major disadvantage being its
high nucleic acid content and low digestibility. This paper reviews the production of single cell protein, its
benefits, safety, acceptability, cost and the limitations associated with their uses, as it portends great promise
as an alternative source of proteins.
Keywords: Microbial protein, production, prospects, challenges
Microbial protein/single cell protein
Single cell proteins or microbial proteins are dehydrated
microbial cell culture or purified protein derived from a
microbial cell culture, such as bacteria, yeast, algae or
filamentous fungi, with potential to be a source of animal
or human protein supplement. Microbial protein or single
cell protein comes to market as dehydrated, purified
microbes derived mostly from unmixed cultures that are
used as a source of protein for human and animal feed
because of their high protein concentration, low fat content
and high vitamin content especially B vitamins
(Kurbanoglu, 2001; Garcia-Garibay et al., 2003., Yalein et
al., 2008).
The development of this source of protein production was
initiated during World War I and World War II as a
substitute for the shortage of conventional food protein
using Saccharomyces cerevisiae on molasses and Candida
utilis on sulphite liquor from waste papers (Nasseri et al.,
2011). In 1967 SCP was produced industrially on a large
scale and currently lots of varieties of microorganisms and
many substrates have been used in the production of this
type of protein. These microorganisms include; algae such,
as Chlorella spp and Spirullina spp that photosynthesise
carbon dioxide, yeasts such, as Candida utilis and Candida
lipolytica which grow on ethanol, filamentous fungi like
Chaetomium celluloliticum and Fusarium graminearum
which grow on cellulose waste and starch respectively, and
also bacteria such as Brevibacterium and Methylophilus
methylitropous that uses hydrocarbon and methanol
respectively as substrates (Nasseri et al., 2011). The fast
growth rate bacteria and very high protein value has
distinguished them as a potential source of SCP compared
to other microbes (Anupama, 2000; Tuse, 1984). These
microbes ferment large amounts of waste which serves as
their substrate and produce protein (Nasseri et al., 2011). It
is more advantageous to use fungi and bacteria in the
cultivation of SCP when grown on cheap waste materials.
Production of Single Cell Proteins
The photosynthetic and non-photosynthetic microbes are
used in the production of single cell protein grown on raw
materials such as hydrocarbon, agricultural or industrial
waste products (Litchfield, 1998).
Photosynthetic production of SCP
Algae are the major photosynthetic organisms that are
used in the production of single cell protein. They use
carbon dioxide as their substrate which is the cheapest,
most inexpensive and most abundant source of carbon for
microbial growth. Atmospheric carbon dioxide can be
supplemented by combustion of gases, carbonate and bio-
carbonate to enhance algae growth (Litchfield, 1998,
Garcia-Garibay et al., 2003). The sources of nitrogen used
for these microbes are nitrates, nitrites, ammonia or urea,
even though cyanobacteria are capable of fixing
atmospheric nitrogen. They also need phosphorus and
sulphur, which are incorporated in organic form, and small
quantities of micronutrients such as Iron and chlorine
(Litchfield, 1980).
Mass culture open systems and photo-bioreactors are used
for cultivation of algae for the production of single cell
protein. Mass culture open systems which are the
production of algae outdoor use lakes and ponds as
cultivation site; good environmental sanitation is
maintained to avoid contamination and other factors that
will lower the quality of the products. This method has
been used to produce Spirulina and Chlorella (Litchfield,
1980).
Photo-bioreactors are a closed system and either use
outdoor or indoor systems where a single species is
cultivated. Indoor system is preferable because it allows
better prevention of contaminants than outdoor culture
system leading to greater yield and high quality of the
product (Ugwu et al., 2008). This method can be used to
cultivate Chlorella, Spirulina and Scenedesmus

A Review of Microbial Protein Production: Prospects and Challenges
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April, 2016 Vol. 1 No. 1 – e-ISSN: 24085162; p-ISSN: 20485170 pp 182-185
183
Harvesting of the microbes (microbial harvest) is extensive
work especially when cultivated in large area such as lakes
or when low production of the microalgal biomass occurs.
Species like Spirulina and Coclastrium can be scooped or
removed from the top of water, but in most situations
harvest involves filtration using cloths or screens before
centrifugation. It is then sun-dried, spray-dried or drum-
dried and then packaged and sold as a nutritional
supplement in the category of functional foods (Garcia-
Garibay et al., 2003).
Non-photosynthetic microorganism for production of SCP
Non photosynthetic organisms that are used in the
production of single cell protein include bacteria and yeast.
These microbes have gained wide acceptability in the
production of single protein because of their notable use in
bread baking, milk fermentation in cheese production and
in the alcoholic beverage industry (Garcia-Garibay et al.,
2003).
Use of bacteria for SCP production
Bacteria were found to be more effective in the production
of SCP because of their short doubling time as compared
to those of yeast, but have been largely discontinued
because of their toxicity and low consumer acceptability
(Anupama, 2000). The carbon source of bacteria species
used in the production of SCP is mainly hydrocarbon,
ethanol, methanol, carbohydrate, cellulose, starch and
simple sugars. Methanol is the preferred substrate as a
carbon source for the bacteria because it is soluble in
water, non-explosive, free of impurities and easy to
remove from the microbial product (Moulin et al., 1983;
Ivarson and Morita, 1982). The bacterial organisms used
are Methylomonas methanolica, Methylophilus
methylotrophus and Pseudomonas spp (Singh, 1998;
Anupama, 2000).
Fig. 1: Production of bacteria from methanol substrate
(Litchfield, 1980)
Fig. 1 shows the production of bacteria cells using
methanol substrate. All the nutrients are sterilised and
bacterial cell produced in the airlift-type fermentor. The
cells are concentrated by flocculation to remove water.
The cells are then centrifuged; spray dried, grind and the
product packaged.
Use of yeast for SCP production
The most frequent and commonly used microorganism in
single cell protein is yeasts, such as Saccharomyces
cerevisiae, Kluyveromyces maxianus and Candida utilis
because of their good acceptability by the consumers. C.
utilis grows on sulphite liquor substrate, while K.
maxianus grows on milk whey substrate. These substrates
are cheap carbon and energy source, produced from waste
product in the paper industry and they contain large
amount of organic carbon compounds (Batt and Sinskey,
1987).
Fig 2: Production of yeast from carbohydrate substrate
(Litchfield, 1980)
Fig. 2 shows the production of yeast from carbohydrate.
All the nutrients used are processed under a sterilized
condition and yeast cell produced in sparged fermentor.
The yeast cells produced are centrifuged, filtered, spray
dried or drum dried and products packaged.
Removal of Nucleic acid content and cell wall of SCP
The production of single cell protein from bacteria for
human consumption requires, reducing the nucleic acid
content of the cell and removal of the cell wall. The reason
for reducing nucleic acid content is because high content
of the nucleic acid can lead to formation of uric acid which
causes gout whereas the cell wall has very poor
digestibility in humans.
Nucleic acid removal in SCP
This involves both the use of chemicals; such as alcohol,
acid, alkali, and enzymatic methods which includes the use
of nucleases and endogenous ribonucleases which
hydrolyses the ribonucleic acid and reduces nucleic acid
levels (Kunhi and Rao, 1995).
Removal of cell wall
Mechanical and non mechanical methods are used in
removal of the cell wall. The mechanical methods involves
the wet milling, sonification, high pressure
homogenization, decomposition crushing and grinding
(Middleberg, 1995), while non mechanical methods are the
use of chemical treatment using acid, base or detergents.
Enzymatic methods such as autolysis and physical
treatment such as freeze-thaw, heating and drying are used
(Baldwin and Ribinson, 1994; Benaiges et al., 1989).
These processes result in production of large amounts of
single cell protein from the microbial cell and organelles
(Nasseri et al., 2011).
Table 1: Comparison of SCP production from different
microorganisms
Sources: Singh (1998)

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April, 2016 Vol. 1 No. 1 – e-ISSN: 24085162; p-ISSN: 20485170 pp 182-185
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Table1 shows how SCP can be produced from different
microorganisms using the same parameters. It also
indicates the risk of contamination, in algae and bacteria;
deficient of sulphur containing amino acids in bacteria and
yeast, and requirement of removal of nucleic acid content
from bacteria, yeast and fungi.
Nutritional value of single cell protein
It has been observed that single cell protein contains a very
high protein content that compared favourably with
conventional protein sources, like soybean and fish meal.
Bacteria single cell protein contents range from 50-100%,
yeast from 45-70% and algae from 45-75%, these protein
sources are higher in protein content as compared with
soybean 40%. They are also very acceptable when
compared with vegetable protein contents (Garcia-Garibay
et al., 2003). Generally single cell proteins from all
sources are very limited in sulphur-containing amino acids
such as methionine and cysteine but when supplemented
with these amino acids their protein quality accelerates and
get close to that of casein. The vitamin contents of single
cell protein such as thiamine, riboflavin, pyridoxine, folic
acid, niacin and carotenes are higher in some microbial
proteins compared to some vegetable foodstuffs (Frazier
and Westhoff, 1990).
Table 2: Comparison of composition of SCP from
algae, fungi and bacteria
NA- Not available; Source: Anupama (2000)
Table 2 shows percentage weight composition of SCP
proteins produced from algae, fungi and bacteria. Bacteria
have the highest true protein content followed by fungi and
algae. There is reasonable percentage content of
Chlorophyll, fiber and ash in algae but these components
are not present both fungi and bacteria.
Advantages of Single Cell Protein
Single cell proteins are used in food industries such as
baking bread, preparation of cookies, noodles, meat
product and baby meal as a protein supplements (Garcia-
Garibay et al., 2003). For the following advantages Listed
below;
i) It has very high protein contents, vitamins especially
B-complex vitamins (yeast), amino acids and low fat
content.
ii) They can be modified genetically to produce amino
acid of specific interest.
iii) There are no constraints in the production as they can
be produce throughout the year since growth is
independent of climatic and seasonal changes.
iv) They use waste materials (El-Nanwwi and El-Kder,
1996), as their substrate in producing this protein
thereby helping to reduce pollution by recycling
waste materials.
v) They grow faster producing large amounts of SCP in
small portions of land within short period of time
(Anupama, 2000; Litchfield, 1980).
Safety and acceptability of single cell protein
Single cell protein for human consumption or animal feed
must be free from all kinds of pathogens, toxins,
contaminants from heavy metals or other metal
compounds, hydrocarbons and free from the risk of
causing food allergies or cancer. It has been observed that
most foreign proteins are not suitable for human
consumption because of their ability to cause allergens,
gastro-enteric disturbance, diarrhoea and vomiting
(Anupama, 2000). The high content of nucleic acid in
bacterial cells can also cause urinary disease such as
kidney stone formation or gout and should be reduced to
the minimal level before consumption. Therefore from this
viewpoint it is very essential to use toxicological studies to
evaluate the safety of any produced single cell protein
before marketing the products (Litchfield, 1980). Spirulina
and Chlorella however, are widely acceptable and sold for
human consumption and are produced in countries like
Japan, Israel, Thailand and United States (Trehan, 1993).
The acceptability is very low especially the bacteria SCP
because of a generally thinking of the masses that bacteria
are harmful and can cause diseases.
Economics and market of single cell proteins
Microbial proteins or single cell proteins are developed
and produced with the sole aim of reducing world hunger
and protein malnutrition (Khan and Dahot, 2010). For this
reason it must compete with conventional protein sources
with the factors such as cost of production including;
energy used in production, investment, operational unit
cost safety of using waste materials as the source of its
substrate, acceptability by the populace and its market
price (Garcia-Garibay et al., 2003). The major market for
single cell protein is its use for animal feeds. Although it
competes poorly with soya protein as soya is 50% lower in
price as compared with the price for SCP. It is anticipated
that with efficient, improved fermentation and down-
stream processes there will be reduction in the elevated
price of SCP to minimal level (Litchfield, 1991).
Disadvantages of Single Cell Protein/Limitations
Bacteria, Algae, Fungi and Yeast
Bacteria
It has been observed experimentally that bacterial SCP has
elevated nucleic acid content (Anupama, 2000). The high
content of the nucleic acid can lead to formation of uric
acid when metabolised which can accumulate in the body
due to a lack of Uricase in humans, the enzyme used to
metabolise it. This can lead to severe diseases such as
gout.
There are also very high risks of contamination of the
product during production with heavy metals, or methanol
which can lead to disorders in body function and may also
cause cancer.
Bacterial toxins can be hazardous to humans when
consumed, causing fever, lesions, vomiting and may also
lead to paralysis.
Bacterial proteins are also very poor in sulphur containing
amino acids, methionine and cysteine which are essential
amino acids in human body.
Bacterial sizes are very small and they also have low
density, this makes there harvest expensive (Anupama,
2000).

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April, 2016 Vol. 1 No. 1 – e-ISSN: 24085162; p-ISSN: 20485170 pp 182-185
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Algae
It has very low density.
It contains cell wall which has very poor digestibility in
humans.
There is high risk of contamination during production
(Garcia-Garibay, 2003).
Fungi and Yeast
They have the potential of containing mycotoxins and
neurotoxin which are capable of causing allergy, and
adverse nervous reactions.
They have very low content of methionine and cysteine.
There is tendency of having high contaminants.
They also have high nucleic acid content (Anupama,
2000).
Conclusion
Single cell protein, or microbial protein, is a potential
source of protein for human food as it gives promising fate
as an alternative source of protein. The use of
microorganisms in the cultivation of this protein have
many advantages over the conventional protein including,
their short doubling time, easy cultivation, utilization of
many cheap/widely available substrates as energy sources,
small land mass for propagation as well as being able to
adapt to climatic changes. Although SCP can be used as a
source of protein, it is not without challenges which
currently prevent it from competing with conventional
proteins. These challenges include its high nucleic acid
content when produced with bacteria, possibility of
causing diseases, poor digestibility, and high cost of
production due to substrate cost, utilities, capital loads and
product-specific variables. These problems can be
minimised by improving appropriate fed- batch
fermentation and developing cheaper down-stream
processes, using genetically engineered microorganisms
with improved substrate utilization. Also the use of
efficient toxicological tests will enhance and improve its
acceptability, making its price affordable by the consumer
when compared with conventional proteins.
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