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Chapter 9
Problems with Single-Cell Protein
Abstract Problems with single-cell protein are presented in this chapter. Various
methods for removal of nucleic acids are discussed.
Keywords Single-cell protein Nucleic acid Nuclease Fungal ribonuclease
Pancreatic ribonuclease Polynucleotide phosphorylase Alkaline extraction
Problems with single-cell protein (SCP) include high concentration of nucleic acids
(Anupama 2000). About 70–80% of the total Nitrogen is represented by amino
acids whereas the remaining occur in nucleic acids. The problem which occurs from
the consumption of protein with high nucleic acid concentration (18–25/100 g
protein dry weight) is the production of high concentration of uric acid in the blood
causing health disorders such as gout and kidney stone (Nasseri et al. 2011).
Therefore, removal of nucleic acid is necessary for human food but not for animal
feed.
Intake of a diet which is high in nucleic acid content leads to the production of uric
acid resulting from degradation of uric acid. This acid accumulates in the body
because of the lack of the uricase enzyme in humans. Hence, nucleic acids in different
SCPs should be reduced to acceptable limits if they are to be used as food.
Bacterial SCP products are found to have nucleic acids as high as 16% of dry weight.
Human consumption greater than 2 g nucleic acid equivalent per day may lead to
kidney stone formation and gout (Calloway 1974). In rapidly growing microbial cells,
RNA forms the bulk of the nucleic acids (Singh 1998). The RNA content of yeast
cells is found to be dependent on the culture conditions and C/N ratios.
Several methods have been suggested for reducing nucleic acid levels in
SCP. These methods involve chemical and enzymatic treatments. Each method has
disadvantages both in terms of cost and potential nutritional concern. In 1977, the
extraction of nucleic acid by following has been proposed:
–
–
–
–
Acidified alcohol
Salt
Acid
Alkalis.
© The Author(s) 2017
P. Bajpai, Single Cell Protein Production from Lignocellulosic Biomass,
SpringerBriefs in Green Chemistry for Sustainability,
DOI 10.1007/978-981-10-5873-8_9
65
66
9 Problems with Single-Cell Protein
Alkaline extraction of microbial biomass at high temperature was also used in 1970.
This process resulted in high protein yield with low nucleic acid content. However,
this method causes the formation of potentially toxic compounds such as lysinoalanine which is an unusual amino acid involved in cross-linking of alkaline
protein. Damodaran and Kinsella (1983) reported that. Lysinoalanine reduces
digestion and induces kidneys changes in rats (Damodaran and Kinsella 1983). In
some persons, it also implicated in skin allergy consuming treated protein
(Scrimshaw and Dillen 1977).
Chemical modification of yeast nucleoproteins with anhydrides has been used
for reducing the nucleic acid levels.
Yeast contains substantial amounts of endogenous ribonuclease activity which is
used to hydrolyze yeast RNA and which causes reduction of nucleic acid level in
yeast protein. At the optimum conditions of ribonuclease activity, significant activation of endogenous protease also takes place. This results in proteolytic degradation of protein and so decreases the yield of protein.
Nuclease has been also added exogenously for reducing the nucleic acid content
of SCP. Fungal ribonuclease of Aspergillus candidus strain M16 and Pancreatic
ribonuclease (RNase A) were used as the source of exogenous nuclease for the
reduction of nucleic acid in the cells of yeast species. This resulted in significant
reduction of nucleic acid (Maul et al. 1970; Kunhi and Rao 1995).
Bacterial or pancreatic nucleases have been also examined for nucleic acid
removal from yeast cells. Hydrolysis of nucleic acid has also been performed by
using immobilized enzymes (Parajo et al. 1995).
By using the endogenous polynucleotide phosphorylase and RNase in
Brevibacterium, reduction of nucleic acids can be obtained (Yang et al. 1979). Two
derivatives of pancreatic RNase and an endonuclease of Staphylococcus aureus,
immobilized on corncobs, were used for reducing the percentage of nucleic acids in
SCP concentrates of yeasts. Nucleic acid reduced from 5–15 to 0.5% with a protein
loss of only 6% after treatment (Martinez et al. 1990).
Immobilized nucleases like benzonases on corncobs were also used to reduce the
nucleic acid content in protein concentrates. The percentage of DNA was reported
to be reduced to 3–6% and RNA to 50% with loss of protein in the process being
only 1% (Moreno et al. 1991).
An immobilized pancreatic RNase was also examined for the degradation of
yeast ribonucleic acid. The rapid reaction rates were obtainable at relatively low
temperatures. This offers a potential alternative method of purifying yeast SCP with
minimal loss of derived protein (Dale and White 1979). Methods for reduction of
nucleic acid content in SCP obtained from gas oil are also reported (Abou-Zeid
et al. 1995).
A major limiting factor in the use of SCP as food is its nucleic acid content. The
relationship between SCP nucleic acid and human feeding was reviewed by
Araujo-Neto and Ferreira-Pinto (1975) and analysis of nitrogen, cell wall, protein
and RNA content was carried out by Kellems et al. (1981). It was found that SCP
was higher in methionine and lysine than cottonseed meal. True proteins based
upon amino acids recovered in SCP samples ranged from 51.6 to 65.9% of crude
9 Problems with Single-Cell Protein
67
protein. In the digestion trials, sheep consumed the SCP diets readily and without
any digestive disturbances.
Based on in vitro and laboratory results, the SCP produced from secondary
clarifiers of pulp mill had the potential to be a viable protein supplement for the live
stock.
To prove that nucleic acid consumption increased levels of uric acid in the body,
rats were fed with Fusarium derived SCP. Plasma and kidney uric acid concentrations showed an increase after 21-day trials in the absence of uricase. During the
trials, the uricase activity was inhibited by oxonate, a uricase inhibitor in the diet
(Winocour et al. 1978). Hence, SCP for human consumption should be free from
nucleic acid as humans lack uricase in their system. Digestibility also plays an
important role in the efficient utilization of SCP in a diet supplemented by
SCP. Protein digestibility values expressed as a percentage, range from 65 to 96%
for the various cultures tested. Protein efficiency ratio (PER) values range from 0.6
to 2.6 (Frazier and Westhoff 1990). An analysis was done for the apparent
digestibility of diets containing fishmeal, soybean meal and bacterial meal when fed
to Salmo salar. The digestibility of the diet with bacterial meal was comparable with
that of the other supplements (Storebakken et al. 1998).
SCP, being a novel product, demands considerable sanitation and purification
processes before the final product is approved for consumption as per quality
control standards. The Protein Evaluation Group of the United Nations and US
Food and Drug Administration have developed guidelines for the safety evaluation
of SCP products in domestic livestock and humans (Litchfield 1985). Rigorous
sanitation and quality control procedures should be maintained throughout the
entire process for avoiding spoilage and contamination by toxigenic and pathogenic
microorganisms when biomass for SCP is being cultivated.
References
Abou-Zeid AA, Khan JA, Abulnaja KO (1995) On methods for reduction of nucleic acid content
in single cell protein from gas oil. Biores Technol 52:21–24
Anupama Ravindra P (2000) Value-added food: single cell protein. Biotechnol Adv 18:459–479
Araujo-Neto JS, Ferreira-Pinto G (1975) Nucleic acid and single cell protein utilization in human
feeding: a review. Arch-Latinoam-Nutr 25(2):105–118
Calloway DH (1974) The place of single cell protein in man’s diet. In: Davis P (ed) Single cell
protein, Academic Press, New York, pp 129–46
Dale BE, White DH (1979) Degradation of ribonucleic acids by immobolized ribonuclease.
Biotechnol Bioeng 21(9):1639–1648
Damodaran S, Kinsella JE (1983) The use of chaotropic salts for separation of ribonucleic acids
and proteins from yeast nucleo-proteins. Biotechnol Bioeng 25:761–770
Frazier WC, Westhoff DC (1990) Food microbiology, Tata McGraw Hill Publishing Company
Limited, New Delhi, pp 398–415
Kellems RO, Aseltine MS, Church DC (1981) Evaluation of single cell protein from pulp mills:
laboratory analysis on in vivo digestibility. J Anim Sci 53(6):1601–1608
68
9 Problems with Single-Cell Protein
Kunhi AAM, Rao MRR (1995) The utility of a fungal ribonuclease for reducing the nucleic acid
content of permeabilized yeast cells. Food Biotechnol 9:13–28
Litchfield JH (1985) Bacterial biomass. In: Moo-Young M, Bull AT, Dalton H
(eds) Comprehensive biotechnology, vol III. Pergamon Press, New York, pp 463–481
Martinez MC, Sanchez-Montero JM, Sinisterra JV, Ballesteros A (1990) New insolubilized
derivatives of ribonuclease and endonuclease for elimination of nucleic acids in single cell
protein concentrates. Biotechnol Appl Biochem 12(6):643–652
Maul SB, Sinskey AJ, Tannenbaum SR (1970) New process for reducing the nucleic acid content
of yeast. Nature 228:181
Moreno JM, Sanchez-Montero JM, Ballesteros A, Sinesterra JV (1991) Hydrolysis of nucleic acids
in single cell protein concentrates using immobilized benzonases. Biotechnol Appl Biochem 31
(1):43–51
Nasseri AT, Rasoul-Amini S, Morowvat MH, Ghasemi Y (2011) Single cell protein: production
and process. Am J Food Technol 6(2):103–116
Parajo JC, Santos V, Dominguez H, Vazquez M (1995) NH4OH-based pretreatment for improving
the nutritional quality of single cell protein. Appl Biochem Biotechnol 55:133–149
Scrimshaw NS, Dillen JC (1977) Single cell protein as food and feed. In: Garattini S,
Paglialunga S, Scrimshaw NS (eds) Single cell protein-safety for animal and human feeding.
Pergamon Press, Oxford, UK, pp 171–173
Singh BD (1998) Biotechnology. Kalyani Publishers, New Delhi, pp 498–510
Storebakken T, Kvien IS, Shearer DD, Grisdale-Helland B, Helland SJ, Berge GM (1998) The
apparent digestibility of diets containing fish meal, soybean meal or bacterial meal fed to the
Atlantic salmon (Salmon salar): evaluation of different fecal collection methods. Aquaculture
169:195–210
Winocour PD, Turner MR, Taylor TG, Munday KA (1978) Platelet aggregation in rats in relation
to hyperuricaemia induced by dietary single cell protein and to protein deficiency. Thromb
Haemost 39(2):346–359
Yang HH, Thayer DW, Yang SP (1979) Reduction of endogenous nucleic acid in single cell
protein. Appl Environ Microbiol 38(1):143–147
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