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03062686.1974.11904161

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Plant Foods for Man
ISSN: 0306-2686 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/iijf16
Single Cell Protein
J.T. Worgan
To cite this article: J.T. Worgan (1974) Single Cell Protein, Plant Foods for Man, 1:2, 99-112, DOI:
10.1080/03062686.1974.11904161
To link to this article: http://dx.doi.org/10.1080/03062686.1974.11904161
Published online: 27 Sep 2017.
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Download by: [Australian Catholic University]
Date: 28 October 2017, At: 20:58
Plant Foods for Man 1, 99-112. Printed in Great Britain
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SINGLE CELL PROTEIN
J.T. WORGAN, MSc, PhD.
National College of Food Technology, University of Reading, England
SUMMARY
lthough Food Yeast is the only example of a micro-organism which is being produced
on a commercial scale as a source of single cell protein, there are a number of
processes currently being investigated which have reached the development stage.
Hydrocarbons or carbohydrates are the main substrates being studied although there
has been recent interest in the possibility of using methanol.
The products from these developing processes will probably have a better nutritional value as
a source of protein than food yeast. Textured food products, which will appeal to the consumer,
are being developed from bacteria and yeasts; fungal mycelium with a suitable texture may be
prepared directly from the product which is harvested from the fermenter vessel. The economics
of these processes are difficult to assess. However, if the price of meat continues to increase,
it seems probable that single cell protein products, which can partially replace meat in the diet,
will be marketed in the not too distant future.
A
Bacteria, yeasts, fungi and algae have all been investigated as potential sources of food.
Biomass containing at least 50 per cent protein can be produced by species from each of these
groups of micro-organisms and the term single cell protein (SCP) has been applied to the products.
There are a large number of species which could be investigated and the criteria for selection are
listed below:
a) Capable of rapid growth on low cost culture media
b) Yield biomass with a high protein content
c) Produce protein which is palatable and non toxic.
Although there are some micro-organisms which require complex media there are many species
which have simple growth requirements and only need a mixture of inorganic salts and an
organic source of carbon (known as substrate). It is essential that micro-organisms should be able
to utilise cheap readily available substrates if a process for produc"ing SCP is to be economical.
Aerobic processes are also essential since about 20 times as much energy is made available than
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SINGLE CELL PROTEIN
from anaerobic conditions and this energy is required for the synthesis of protein and cell biomass.
The most difficult of the above criteria to establish is the safety of the product as a food.
Optimum conditions for the growth of the micro-organism must first be established and
sufficient quantities of material produced for long term animal feeding trials. If any adverse
symptoms develop during these trials then the whole programme of work involving the culture
under test may have to be abandoned.
The environment and the culture media in which micro-organisms are grown have a
considerable influence on the cell composition and statements in the literature that certain
species have high protein contents have little meaning. In general bacteria yield biomass
containing a higher proportion of protein and in the laboratory grow more rapidly than either
fungi or yeasts. The advantage of their high protein content is partially offset by their higher
content of nucleic acids. Fungi are frequently reported to be more slow growing and to yield
biomass with lower protein contents. Where protein content and protein yield are used as
the criteria for assessing the opt\mum conditions for growth, several fungal species will grow
as rapidly and give equivalent yields of protein to those of yeasts (Delaney and Worgan, 1970;
Worgan, 1971). Examples of growth rates and protein contents of several species of fungi are
given in Table 1.
Table 1: Protein production by fungi
Mould Species
Fusarium semitectum
Mucor racemosus
Rhizopus oligosporus
Rhizopus oryzae
Rhizopus stolonifer
%protein
(N X 6.25)
65.0
45.2
44.5
Protein
doubling time/hr
1.6
3.0
59.6
3.9
3.6
34.8
5.0
Source: Delaney and Worgan, 1970
When the efficiency of the conversion of substrate to protein is taken into consideration, it can
be concluded, that large scale processes which use bacteria, fungi or yeast will all operate at the
same rate of growth, since it is the ability to provide oxygen on a large scale which is the main limit
on growth rate (Worgan, 197 3a). Thus the nutritional value of the protein, safety of the product as
human food, acceptability and the ability to use available sources of energy as substrates for
growth are the criteria for the selection of species of micro-organism for the production of SCP
Food Yeast (Candida uti/is) is the only example of SCP which is currently produced for use
in human food. Comparatively small quantities, grown on molasses, are used as a protein
supplement in a number of food products (Bressani, 1968). This yeast was originally selected
because of its ability to grow on a wider range of substrates than baker's or brewer's yeast
(Saccharomyces cerevisiae) and feeding tests were first made more than 50 years ago. In World
War II, about 15,000 tons per year were used to supplement supplies of human food in Germany
(Lock, Saeman and Dickerman, 1945 ). C. uti/is has therefore acquired an established reputation
as a safe food and it is primarily for this reason that it is chosen for production on an industrial
scale. Although several predictions have been made of the potential efficiency of microbiological
processes for the production of protein, ther are very few production units in operation. The
oldest established process is that for the production of baker's yeast (S. cerevisiae) which is
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SINGLE CELL PROTEIN
grown on a molasses substrate with the addition of ammonium hydroxide or sulphate as the
nitrogen source. The other species of yeast produced on a commercial scale are food yeast
(Candida utilis formerly Torula utilis) and Saccharomyces fragilis. Food yeast has the advantage
over baker's yeast that it is able to utilise pentose sugars and may therefore be grown on a wider
variety of substrates. Waste sulphite liquor (from paper pulp manufacture) is used in Russia,
Canada and the U.S.A.; wood hydrolysates are used in Russia and Czechoslovakia, while molasses
are used in Taiwan, South Africa, Cuba and the Philippines. Only one or two small units produce
S. fragilis, a yeast which can utilise lactose and is grown on cheese whey.
Since the production of food yeast is based on the process for manufacturing baker's yeast,
on which most information is available, the latter process will be described. Although S. cerevisiae
is primarily produced for its baking properties it does nevertheless contain 50 per cent protein and
is an example of the microbiological production of protein on a large scale.
Production of yeast protein on an industrial scale
In the production of baker's yeast the process is initiated by a single yeast cell which is allowed
to develop into a yeast culture in the laboratory. After incubation the whole of this culture is
used as inoculum for a larger volume of culture medium and the procedure is repeated through
several stages of increasing size, first in the laboratory and then in stainless steel vessels in the
factory. Strict aseptic conditions are maintained throughout this procedure. From the initial
stages of the process sufficient seed yeast is produced to inoculate the main fermenter vessels of
45,000- 225,000 I. capacity. Although precautions are taken to minimise infection during the
final stage of growth it is not economical to maintain completely aseptic conditions. The
fermenter vessels are vigorously aerated throughout the final stage; temperature and pH are
controlled. Higher yields of yeast are obtained by the operation of the Zulauf or differential
process in which the carbohydrate source in the form of molasses is initially provided in dilute
concentration. As the growth of yeast in the fermenter proceeds more molasses are added to
keep pace with the growth of yeast. The quantities added ensure that there is sufficient but
never a large excess of carbohydrate in the growth medium. At the end of this procedure the
yeast, which has now reached a mass of 100 tons, is harvested by centrifugation. The whole
process from single yeast cell to 100 tons of yeast biomass occupies a period of 300 hours and
involves 60 yeast generations. The average generation time is therefore five hours. The yeast cell
mass increases from 10" 10 g to 10 11 g. Based on the sugar content of the molasses the yield of
yeast is 50 per cent and of yeast protein 25 per cent (Dawson, 1952).
In the Distillers' Company plant at Dovercourt, England, the process has been improved by
replacing the final stage with a continuous system which is maintained for a period of
approximately 80 hours. The production of seed yeast up to the 4,500 I. stage follows the
procedure described above. The seed yeast is then fed into a system of eight fermenters in series
into which molasses, other nutrients and water are introduced to keep pace with the increasing
mass of yeast cells. This procedure gives the same proportionate yield of yeast from molasses.
Power input, processing time and labour requirements are all reduced (Olsen, 1960).
Carbohydrates as substrates for the production of SCP
The production of food yeast as a protein source is very similar to the process described
above. Where molasses are not available locally, other substrates are more economical and
reference has already been made to the use of wood hydrolysates and waste sulphite liquor.
Neither baker's-, brewer's- nor food yeast are capable of utilising starch substrates. In the
Symba yeast process which has been developed in Sweden to use starch wastes, two yeasts are
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SINGLE CELL PROTE IN
grown in association. Endomycopsis fibu liger wi ll grow on starch as the C source and produces
an excess of amy lase which hydrolyses starch to sugar. C. utilis, which is not able to utilise starch,
grows on the sugar produced. When both yeasts are inoculate d in to a fermenter vesse l rap id
growth can be maintained and the harvested product consists of about nine parts C. utilis to one
part E. fibuliger (Tveit, 1967).
Laboratory studies have been made using various sources of starch as substrates for the
production of fungal protein. Several species of fungi were grown in submerged culture on
medi a prepared from sweet potato, cassava, rice and corn. The protein co ntents of the
harvested mycelium were low (20-3 0 per cent). A sum mary of these experiments has been
published (Gray, 1970).
Mycelium with higher protein cont ents could probably be pro du ced by a systematic study of
the factors which influence protein synthesis. Hi gher yields of the mycelium of so me fungi are
obtained fro m starch as sub strate than from glu cose or maltose (Cochrane, 1958). A possible
explanation of this is that sugar is released slowly by starch breakdown, creating similar
co nditions to those which prevail in the Zulauf meth od for growing baker's yeast.
The by-pro du cts available from agriculture and the food industry which could be used as
substrates for th e prod uction of single cell pro tein have been reviewed (Worgan, 197 3b ). Cellulose
and he mi-cellulose (mainly polymers of uronic acids and pentose sugars) are t he most abundant
sources of replenishable carbon compounds and vast qu antities are produced and harvested
inadvertently with many food crops. In so me cases t hey exceed the quantity of the edibl e
portion of the crop 3 or 4 fo ld. Quantities of some of these by-products are listed in Table 2.
Table 2: Quantities of some carbohydrate substrates for the production of SCP
Source
Annual world output
carbohydrate - 1000 ton
Agricultural
by-products
Wh eat straw
Wheat bran
Mai ze stover
Maize cobs
Barley straw
Sugar cane bagasse
Molasses
Estimated conversion
to protein - 1000 ton
Processing
by-products
286,580
57,320
120,040
30,070
52,920
83,000
9,300
57,000
11,460
24,000
6,000
10,060
16,600
2,300
Annual USA output million
gallons
12,000
1,530
Sulphite liquor
Whey
510
400
Source: Worgan, 1973c
Alth ough these by-products are termed cellulosic materials o nly about one third may be
cellulose. Wheat, barley and oat straws for example contain less than 36 per cent cellulose.
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SINGLE CELL PROTEIN
These materials are resistant to rapid degradation by micro-organisms due to the protective effect
of lignin and the partially crystalline structure of cellulose, which prevents contact between
enzyme molecules and 't he substrate. The stages in the degradation of ligno-cellulosic materials
are illustrated in Fig. l.
Ligno-cellulosic plant tissue
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L
Lignin breakdown products:
polysaccharides, polyuronides
Native cellulose
l
l
c,
Reactive cellulose
l
Available to micro-organisms
c,
Cellobiose
l
Glucose
Fig. 1: Biological degradation of ligno-cellulose
Only the wood-rotting fungi are capable of carrying out reaction Lon the more resistant
ligno-cellulose of wood. A wider range of fungi and some bacteria are able to degrade the
ligno-cellulose of less resistant materials such as that of cereal straws and husks. Other species
of fungi and bacteria which are not able to carry out reaction L do have cellulase (C 1 ) activity.
A much large r number of species of fungi and bacteria have cellulase {Cx) activity and are able
to hydrolyse cellulose once it is in the reactive form: Experimentally neither reactions L nor
cl h ave been found to take place at the rate which compares with the growth rate of microorganisms. If the enzyme systems involved could be isolated and their optimum conditions for
activity established, huge quantities of substrates could be made readily available for the
production of SCP. Separation of the processes into two or three stages may be necessary since
the optimum conditions for microbial growth and for L, C 1 and Cx activity may differ. The
conversion of reactive cellulose to glucose in high yields has been shown to be possible with an
enzyme preparation from Tr£choderma viride (Katz and Reese, 1968).
Acid hydrolysis of the carbohydrates to sugars has been investigated as a method for
replacing reactions L, C 1 and Cx. Due to the resistance of cellulose to hydrolysis, the sugars
which are produced in the initial stages of the reaction, are decomposed. Careful control of the
hydrolysis conditions are therefore essential to avoid considerable destruction of carbohydrates.
In practice two types of processes have been developed to minimise these losses. Concentrated
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SINGLE CELL PROTEIN
acids at normal temperatures were used in the Bergius process and dilute acids at high
temperatures (160- 180°C) in the Scholler process. The Scholler process was operated on a large
scale in Germany during World War II (Lock, Saeman and Dickerman, 1945 ). In the U.S.A. pilot
plant studies of the Madison wood sugar process, a modification of the Scholler process, have
been made (U.S. Forest Service, 1964 ). In Russia the process for the production of C. utilis on
wood hydrolysates is considered to be sufficiently efficient to justify production on a large scale
and an annual output of 900,000 tons of yeast is envisaged (Bunker, 1968). Alternate treatments
of ligno-cellulosic materials with acid, alkali or urea reagents at temperatures below 1 00°C, may
be used to prepare culture media suitable for the growth of several fungal and bacterial species
(Worgan, 1967). Actinomucor elegans, grown on a culture medium prepared from 100 goat
husk by this method, gave a yield of 12.6 g mycelial protein. Sugar cane bagasse degraded by
oxidation in alkali conditions at 149°C has been used as a C source for the growth of a
Cellulomonas sp., and pilot plant studies of this process have been made (Han, Dunlop and
Callihan, 19 71 ).
Food production systems, from which the combined harvest consists of the conventional crop,
leaf protein and single cell protein grown on leaf juices and leaf fibre, have been investigated. In
comparison with the conventional crop, yields of protein per hectare from both maize and pea
crops were increased six fold. In the combined proteins of either system the only limiting amino
acids were cystine and methionine (Worgan, 1973c).
Hydrocarbons as substrates for the production of SCP
Many species of bacteria, fungi and yeasts are capable of utilising hydrocarbons as sources of
carbon and energy for growth. Yeasts have been the main choice for processes which have
developed as far as small scale production units. The presence of potentially toxic aromatic
compounds in petroleum requires either a purification step before the hydrocarbons are used or
extraction of all residual hydrocarbons from yeast cells. Both methods are being tested on a
production scale by British Petroleum (Llewelyn, 1967; Evans, 1968; Walker, 1973). Most of
the other petroleum companies have reached the pilot plant stage in developing processes for the
production of SCP from hydrocarbons.
Hydrocarbons are not miscible with water and to obtain rapid growth the microbial cells,
water, inorganic nutrients, the hydrocarbons and oxygen must all be maintained in intimate
contact. Because of the highly reduced state of the substrate large quantities of oxygen are
required. To provide this oxygen and to ensure thorough mixing the power input needed is
therefore greater than that required for growth on carbohydrate substrates. However the yield of
protein is approximately twice that obtained from carbohydrates and on a pilot plant scale
100 ton of hydrocarbon yields 50 ton of yeast protein. A comparison between carbohydrates
and hydrocarbons as substrates is given in Table 3.
Not all of the hydrocarbons in petroleum are suitable for rapid conversion to yeast protein
in the process and the average proportion is about 2 per cent. The world production of
petroleum in 1970 was 2,300m tons and 2 per cent of this quantity used as substrate would
yield 23m tons of yeast protein per year (Walker, 1973). Only species of bacteria have been
reported to grown on methane, the main constituent of natural gas. The yield of protein will be
approximately the same as that from liquid hydrocarbons and there are no problems associated
with residual toxic residues in the product (Klas, Iandolo and Knabel, 1969). Problems do
arise in ensuring complete utilisation of the substrate and in avoiding explosive mixtures of
air and methane. For these reasons the conversion of methane to methanol as an alternative
substrate is being investigated by ICI, Esso and Nestle.
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SINGLE CELL PROTEIN
Table 3: Comparison between hydrocarbons and carbohydrates as substrates
for protein production by micro-organisms.
Quantity to produce 100g protein
Substrate
Energy
joules
Weight
g
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Carbohydrate
Hydrocarbon
CH 4
CaH1a
Weight of carbon
g
400
6,240
160
200
200
11,100
9,360
150
170
Yield of protein from carbohydrate assumed = 25 per cent
Yield from hydrocarbon = 50 per cent
Algae as sources of SCP
The single celled algae are the only group of micro-organisms which undergo photosynthesis
and are therefore able to utilise the energy from sunlight. The production of algae has therefore
an apparent advantage over other methods of producing microbial protein. In practice additional
inputs of energy are needed to provide the environment for efficient growth and to harvest the
algal cells. Under natural, static conditions the cell density is low and because of the small size
of the cells simple filtration methods are not applicable. Large volumes of culture have therefore
to be centrifuged to harvest the cells. Although agitation and the addition of inorganic nutrients
have some influence on cell density further growth is limited by the supply of carbon dioxide.
A large volume of air or air enriched with carbon dioxide has therefore to be introduced into the
culture. In most climates temperature control is necessary. Cell densities even after the provision
of these requirements are lower than those obtained by the growth of other micro-organisms.
Contaminating organisms are also a problem and protection of the culture may be needed. As a
result of these factors it has been concluded that algal proteins, produced by currently available
techniques, cannot compete with the cost of conventional sources of protein.
The cells of the blue green algal species Sp£rul£na maxz'ma can be separated from the culture by
simple filtration methods. This is partly because of the larger size of the cells and partly because
the cells tend to aglomerate because of their shape. Spirulina grows at a high pH value (9.5 - 10.0)
and this increases the efficiency of the absorption of carbon dioxide into the culture and reduces
the problem of contaminating organisms. These advantages improve the economics of production.
The alga was· originally discovered about ten years ago growing in the waters of Lake Chad and
for centuries has been one of the main foods of the local people. It is harvested by scooping the
mass of algal cells from the lake and allowing the water to drain off through a layer of sand. The
residue is then allowed to dry in the sun. Pilot plants for the production of Spirulina have been
established in Mexico and the South of France by the Institut Fran<;ais de Petrole. Yields of
40- 45 ton/hectare/year/containing 62 per cent protein have been reported (Clement, Giddey
and Menzi, 196 7). Pilot plants and small scale production units for the growth of other algal
species are currently being tested in Czechoslovakia, Japan, Germany and Israel.
Nutritional value of SCP
Although values for the protein content of different species are reported in the literature these
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SINGLE CELL PROTEIN
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values have little meaning since the proportion of protein in the biomass of a given species of
micro-organism varies with the environment in which it is grown. There does however appear to
be different maximum limits for bacteria, yeasts and fungi. Bacteria have the highest values and
87 per cent of the cell has been reported to be crude protein in some samples analysed (Bunker,
1963). These values include non-protein nitrogen compounds such as the nucleic acids the
proportion of which in the cell varies with growth rate. Bacteria containing such a hi gh
proportion of crude protein may contain 16- 20 per cent nucleic acid. At a growth rate (five
hour doubling time) at which baker's and food yeast are grown on a large scale the nucleic acid
content is about 10 per cent. Examples of the effect of growth rate on the nucleic acid content
are given in Table 4.
Table 4: Effect of growth rate and temperature on the protein and nucleic
·
acid contents of micro·organisms.
Micro-organism
Doubling time
Hr.
Cell composition
%protein
(N X 6.25)
% nucleic acid
Candida uti/is
4.7
1.7
55
61
10.1
11.9
Aerobacter aerogenes
4.7
1.7
0.8
62
60
56
12.J
14.2
18.9
3.5
68.9
69.0
17.7
12.3
A. aerogenes
25°C
40°C
3.5
Source: Elsworth, Miller, Whitaker, Kitching and Sayer, 1968; Tempest and Dicks, 1967.
In addition to the reduction in the reported protein contents of micro-organisms the nucleic
acid limits the use of SCP in the diet. From results of recent feeding trials on human beings it
appears that the amount of nucleic acid should be limited to 2 g per day ( Edozien, Udo, Young
and Scrimshaw, 1970). At a level of 10 per cent nucleic acid in yeast cells this would limit the
daily allowance to 20 g per day i.e. 10 g of yeast protein. Processing of the cells may reduce
the nucleic acid content. By subjecting the cells of C. utiHs to a heat shock followed by successive
periods of incubation at 50° and 60°C the nucleic acid content has been reduced to 1 per cent.
It is claimed that this heat processing does not result in a loss of protein from the cells (Maul,
Sinskey and Tannenbaum, 1970). If similar processes are applicable to other sources of SCP, the
nucleic acid content of harvested microbial biomass need not limit its use in the diet.
Mycelium of Fusarium semitectum and Rhizopus oryzae containing 60 per cent of proteir: have
been obtained by growth in submerged liquid culture. When grown at a mean protein doubling
time of 5 hours, the nucleic acid content of the mycelium of F. semitectum was 5 per cent
(Delaney and Worgan, 1970). Whether it is a general characteristic of fungi that, when produced
at the same growth rate, they contain less nucleic acid than yeasts and bacteria has still to be
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established.
Food yeast (C. utilis) is the only example of a microbial source of protein which is currently
produced for use as human food. The choice of C. utilis was made, because of its ability to
grow on a wider range of substrates than baker's yeast and was not based primarily on the
nutritional quality of the cell protein. Human feeding tri als were made in Germany during and
immediately following World War I and laid the foundation for the output of 15,000 tons per
year during World War II. The yeast produced was used as a supplement to protein food supplies
(Lock et al, 1945). C. utilis has there fore acquired an established reputation as a safe food.
The results of feeding trials show, that C. utilis cells are not nutritionally equivalent to sources
of animal protein. Values of 0.38 (Miller, 1963) and 0.236 (Mitsuda, Yasumoto and Nakamura,
1969) have been found for the net protein utilisation value (NPU). The diges tibility of the cells
is part of the reason for the low NPU values. Isolated C. utilis protein gave an NPU value of
0.453 which increased to 0.577 after supplementation with methionine (Mitsuda, Yasumoto
and Nakamura, 1969) (see Table 5). Equal parts of maize and yeast protein are complementary
Table 5: Comparison of the nutritional value of Candida uti/is cells and
separated cell protein
Protein source
C. uti/is
Biological
Value
Digestibility
0.366
0.473
0.611
0.646
0.958
0.945
Cells
Protein
Protein + Methionine
NPU
0.236
0.453
0.577
Source: Mitsuda, Yasumoto and Nakamura, 1969
in their amino acid patterns and the mixture has a bet ter nutritional value than the separate
components (Bressani, 1968). Only a few NPU values have been reported in the literature for
other species of micro-organisms. Examples of species with higher values than C. uti/is are given
in Table 6.
Table 6: NPU values of microbial sources of protein
Micro-organism
NPU
Reference
%
Candida uti/is
Escherichia coli
Boletus edulis
Aspergillus niger
Fusarium semitectum
0.38
0.51
0.649
0.584
0.60
2
3
4
(Lab fermenter sample)
1. Miller, 1963 2. Rafalski Salm, Kluszcynska and Switonick, 3. Mitrakos,
Sekeri, Drouliskos and Georgi, 1970 4. Miller, 1968
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SINGLE CELL PROTEIN
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The quantitative essential amino acid composition of the proteins of several micro-organisms
have been reported. Mosts ye as ts (Nel son, Anderson, Rhodes, Shekleton and Hall, 1958) and
fungi (Rhodes, Hall, Anderson, Nelson, Shekleton and Jackson, 1961) are limiting in the sulphur
amino acids, cystine and methionine. Although several bacterial species do not lack methionine
these same species are limiting in tryptophan (Anderson, Rhodes, Nelson, Shekleton, Barrets
and Arnold, 1958). Values reported in the literature for the amino acid composition of the
protein of micro-organisms do show considerable variation. Examples are given in Table 7 and
illustrate the need to confirm the information in the literature by feeding trials. The differences
may be due to analytical errors or to the strain, the growth media or the environment. It has
Table 7: Variations in the amino acid composition of micro-organisms effects of substrate, stra in and pH value
Micro-organism
Substrate
Amino acid content
g/100g protein
Lysine
Tryptophan
Glucose
4.4
0.7
Xylose
10.0
0.5
6.7
1.2
10.7
0.5
8210
7.9
0.3
8766
3.7
0.2
Candida
tropicalis
Candida
uti/is
Waste sulphite liquor
Reference
2
Molasses
Escherichia coli
3
% of cell mass.
Pseudomonas
aeruginosa
Glucose
Xylose
Hexadecane
pH 7
pH 8
9.7
4.4
0.3
0.5
4
2.9
6.4
0.4
0.4
1. Lipinsky and Litchfield, 1970 2. Peppler, 1970 3. Yamada, Takahashi, Kawabata, Okada and
Onihara 4 Anderson eta/, 1958.
been found that the proportions of some amino acids in microbial cells can be increased by the
addition of precursers. The lysine content of yeast for example is increased by the inclusion of
2 oxoa-dipic acid in the growth medium Qensen and Shu, 1961}. Similar attempts to increase ~he
methionine content of yeasts were not effective (Chiao and Peterson, 1953). That methionine is
almost the only limiting amino acid in F. sem£tectum has been determined by chemical analysis
and by feeding tests. When supplemented with methionine F. sem£tectum mycelium has a
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SINGLE CELL PROTEIN
biological value of 0.95 (Palmer and Smith, 1971), see Table 8.
Table 8: Nutritional Value of the Protein of Fusarium semitectum
F. semitectum
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sample
Methionine content
of protein
g/100g
NPU
2.1
1.7
0.65
0.52
0.66
0.76
Lab Fermenter 1
Pilot plant 2
+ 0.9% Methionine
+ 1.8% Methionine
Sources:
1
Mi ller, 1968
2
Digestibility
0.80
Palmer and Smith, 1971
Thus there are several micro-organisms which are known to be better nutritional sources of
protein than food yeast. Many other species and strains remain to be investigated. From a study
of factors which determine the amino acid composition of microbial biomass, methods for
improving the nutritional value might be developed.
Toxity of SCP
Food yeast is one of the few examples of a micro-organism specifically produced as a food. Many
other harmless micro-organisms occur in foods and are consumed in significant quantities. Cheese,
·
oriental foods such as miso and tempeh and numerous types of fermen t ed liquors all contain
bacteria, fungi or yeasts. More than 2,000 edible species of the larger fungi (mushrooms) have
been reported in the literature and it is feasible that the mycelium of some of them could be
produced on an industrial scale (Worgan, 1968). There are therefore numerous species which do
not appear to have any toxic effects.
Although some bacteria and fungi are known to produce toxins they form a relatively small
proportion of the total number o f species. Micro-organisms which produce acute toxic effects
can be eliminated by biochemical tests and by animal feeding trials. Very little is known about
the effects of the remaining species if they were to b e regularly consumed in the diet. The
difficulty of establishing the safety of n ew sources o f food is one of the main obstacles to the
possibility of their use in the immediate future. Neither the genera nor even the species can be
used as an infallible guide to the selection of a micro-organism. Amanita phalloides is one of the
most toxic species of fungi: Amanita fulva is a popular edibl e species. Only certain strains of the
species Aspergillus flavus produce a~otoxin.
Palatability and processing of SCP
The biomass from potential industrial processes for producing microbial foods should be regarded
as raw material corresponding to a harvested crop. Some established foods contain toxins which
are destroyed by processing. Soya is an example, and eggs contain a toxin which is inhibited by a
short period of cooking. Simple processing methods such as mild heat treatment are known to
destroy some of the toxins which may occur in micro-organisms. More extensive processing
techniques for the ex traction of proteins may also remove toxins from the cell mass.
Bacteria, yeasts and algae grow as individual microscopic cells which have no cohesive
properties and therefore no texture. When harvested and dried they form powders which can be
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added in small quantities as supplements to other foods. Protein isolates from bacteria (Heden,
Molin, Olsson and Rupprecht, 1971) and Food yeast (Huang and Rha, 1971) have been spun into
fibres and food products similar to textured soy protein will probably be developed in the future.
The hyphae of fungi matt together with a texture similar to moist chicken meat, a food which is
acceptable in most parts of the world. An acceptable food from fungal mycelium can therefore
be produced with a minimum of processing after the material has been harvested.
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