Miscellaneous Food Products 209
SCP Products
The cells may be used directly as a protein source in animal feed formulations, thereby freeing
animal feed, such as corn, for human consumption, or they may be used as a protein source or food
ingredient for human food. In the case of animal feed or feed supplements, the dried cells may be used
without further processing. Whole cells of Spirulina maxima are consumed by humans in at least one
part of Africa as noted above.
For human use, the most likely products are SCP concentrates or isolates that can be further processed
into textured or functional SCP products. To produce functional protein fibers, cells are mechanically
disrupted, cell walls are removed by centrifugation, proteins are precipitated from disrupted cells, and
the resulting protein is extruded from syringelike orifices into suitable menstra such as acetate buffer,
HCIO 4 , acetic acid, and the like. The SCP fibers may now be used to form textured protein products.
Baker’s yeast protein is one product of this type approved for human food ingredient use in the United
States.
Nutrition and Safety of SCP
Chemical analyses of the microorganisms evaluated for SCP reveal that they are comparable in
amino acid content and type to plant and animal sources with the possible exception of methionine, which is lower in some SCP sources. All are relatively high in nitrogen. For example, the
approximate percentage composition of nitrogen on a dry weight basis is as follows: bacteria 12–13,
yeast 8–9, algae 8–10, and filamentous fungi 5–8.
28 In addition to proteins, microorganisms contain adequate levels of carbohydrates, lipids, and minerals and are excellent sources of B vitamins.
The fat content varies among these sources, with algal cells containing the highest levels and bacteria the lowest. On a dry weight basis, nucleic acids average 3–8% for algae, 6–12% for yeasts,
and 8–16% for bacteria.
28 B vitamins are high in all SCP sources. The digestibility of SCP in experimental animals has been found to be lower than for animal proteins such as casein. A thorough review of the chemical composition of SCP from a large variety of microorganisms has been
made.
7,64
Success has been achieved in rat-feeding studies with a variety of SCP products, but human-feeding
studies have been less successful, except in the case of certain yeast cell products. Gastrointestinal
disturbances are common complaints following the consumption of algal and bacterial SCP, and these
and other problems associated with the consumption of SCP have been reviewed elsewhere.
64 When
Gram-negative bacteria are used as SCP sources for human use, the endotoxins must be removed or
detoxified.
The high nucleic acid content of SCP leads to kidney stone formation and/or gout. The nucleic
acid content of bacterial SCP may be as high as 16%, whereas the recommended daily intake is
about 2 g. The problems are caused by an accumulation of uric acid, which is sparingly soluble in
plasma. Upon the breakdown of nucleic acids, purine and pyrimidine bases are released. Adenine and
guanine (purines) are metabolized to uric acid. Lower animals can degrade uric acid to the soluble
compound allantoin (they possess the enzyme uricase), and, consequently, the consumption of high
levels of nucleic acids does not present metabolic problems to these animals as it does to humans.
Although high nucleic acid contents presented problems in the early development and use of SCP,
these compounds can be reduced to levels below 2% by techniques such as acid precipitation, acid or
alkaline hydrolysis, or use of endogenous and bovine pancreatic RNases.
37
SCP Products
The cells may be used directly as a protein source in animal feed formulations, thereby freeing
animal feed, such as corn, for human consumption, or they may be used as a protein source or food
ingredient for human food. In the case of animal feed or feed supplements, the dried cells may be used
without further processing. Whole cells of Spirulina maxima are consumed by humans in at least one
part of Africa as noted above.
For human use, the most likely products are SCP concentrates or isolates that can be further processed
into textured or functional SCP products. To produce functional protein fibers, cells are mechanically
disrupted, cell walls are removed by centrifugation, proteins are precipitated from disrupted cells, and
the resulting protein is extruded from syringelike orifices into suitable menstra such as acetate buffer,
HCIO 4 , acetic acid, and the like. The SCP fibers may now be used to form textured protein products.
Baker’s yeast protein is one product of this type approved for human food ingredient use in the United
States.
Nutrition and Safety of SCP
Chemical analyses of the microorganisms evaluated for SCP reveal that they are comparable in
amino acid content and type to plant and animal sources with the possible exception of methionine, which is lower in some SCP sources. All are relatively high in nitrogen. For example, the
approximate percentage composition of nitrogen on a dry weight basis is as follows: bacteria 12–13,
yeast 8–9, algae 8–10, and filamentous fungi 5–8.
28 In addition to proteins, microorganisms contain adequate levels of carbohydrates, lipids, and minerals and are excellent sources of B vitamins.
The fat content varies among these sources, with algal cells containing the highest levels and bacteria the lowest. On a dry weight basis, nucleic acids average 3–8% for algae, 6–12% for yeasts,
and 8–16% for bacteria.
28 B vitamins are high in all SCP sources. The digestibility of SCP in experimental animals has been found to be lower than for animal proteins such as casein. A thorough review of the chemical composition of SCP from a large variety of microorganisms has been
made.
7,64
Success has been achieved in rat-feeding studies with a variety of SCP products, but human-feeding
studies have been less successful, except in the case of certain yeast cell products. Gastrointestinal
disturbances are common complaints following the consumption of algal and bacterial SCP, and these
and other problems associated with the consumption of SCP have been reviewed elsewhere.
64 When
Gram-negative bacteria are used as SCP sources for human use, the endotoxins must be removed or
detoxified.
The high nucleic acid content of SCP leads to kidney stone formation and/or gout. The nucleic
acid content of bacterial SCP may be as high as 16%, whereas the recommended daily intake is
about 2 g. The problems are caused by an accumulation of uric acid, which is sparingly soluble in
plasma. Upon the breakdown of nucleic acids, purine and pyrimidine bases are released. Adenine and
guanine (purines) are metabolized to uric acid. Lower animals can degrade uric acid to the soluble
compound allantoin (they possess the enzyme uricase), and, consequently, the consumption of high
levels of nucleic acids does not present metabolic problems to these animals as it does to humans.
Although high nucleic acid contents presented problems in the early development and use of SCP,
these compounds can be reduced to levels below 2% by techniques such as acid precipitation, acid or
alkaline hydrolysis, or use of endogenous and bovine pancreatic RNases.
37