Single cell protein:
- The dried cells of micro-organisms (Algae, Bacteria, Actinomycetes and fungi) used as food or feed are collectively called microbial protein.
- The term microbial protein was replaced by a new term, ‘Single cell protein (SCP)” during the first international conference on microbial protein held in 1967 at the Massachurette institute of technology, USA.
- SCP is the protein production from biomass from biomass originating from different microbial biomass from both unicellular and multicellular bacteria, yeast, filamentous fungi or Algae, which can be used as food or feed additives.
- If filamentous fungi are used, the term fungal protein is also applicable.
- On an average the microbial biomass contains about 45-55% protein and in some bacteria, the protein content is as high as 80%.
- The biomass also contains other essential nutrients as well and it is becoming an ideal supplement to conventional food supply.
- First industrial production of SCP dates back to first world war when baker’s yeast (S. cerevisiae) wasproduced.
- Interest in SCP peaked again during World War II, when torula yeast (Candida utilis) was produced in Germany and used in soups and sausages.
Advantages of producing microbial protein:
- Rapid succession of generation (For algae 2-6 hrs, For yeast 1-3 hrs, For bacteria 0.5-2 hrs).
- Easily modifiable genetically (E.g. for composition of amino acids).
- High protein content of 43-85% in the dry mass.
- Broad spectrum of original raw materials used for the production, which also includes waste products.
- Production in continuous culture, consistent quality, not dependent in climate in determinable amount, low land requirement and ecologically beneficial.
| Micro-organisms used | Substrate used |
| Algae | |
| Chlorella pyrenoidosa | CO2, light |
| Scenedesmus acutus | CO2, light |
| Spirulina maxima | CO2, combination gases, bicarbonate, sunlight |
| Bacteria | |
| Achromobacter delvaevate | Diesel oil in fermenter |
| Bacillus megaterium | Collagen meat packing in fermenter |
| Cellumonas spp | Bagassae |
| Methylomonas claza | Methanol |
| Pseudomonas spp | n-alkanes fuel oil |
| Actinomycetes | |
| Nocardia spp | n-alkanes |
| Thermoonospora fusca | Cellulose pulp |
| Fungi | |
| Yeast | |
| Candida lipolytica | n-alkanes |
| Candida utilis | Potato starch waste |
| Saccharomyces fragillis | Milk whey |
| Saccharomyces cerevisiae | Molasces and beer |
| Torulopsis spp | methanol |
| Molds | |
| Aspergillus niger | molasces |
| Trichoderma viridae | Starch and straw |
| Paccilomyces varioti | Sulfite waste liquor |
| Mushrooms | |
| Agaricus campestris | glucose |
| Morchella crassipes | Glucose, whey, sulfite liquor |
Single cell protein from Algae
Advantages of using algae for SCP
- Members of the genera Chlorella, Scendesma, Spirullina are generally grown in ponds on tanks.
- Use CO2 and sunlight as substrate which are without any cost.
- SCP from algae has about 60% crude protein which is generally good in amino-acid composition except for some deficiency in Sulphur containing amino-acids.
- They are suitable for animal feed on protein rich supplement.
Disadvantages of using algae for SCP
- Rich chlorophyll content which is not suitable for human use.
- Low cell density e.g. 1-2 grams dry weight per liter.
- Serious risk of contamination.
- Costly recovery method for unicellular algae.
Single cell protein from Filamentous fungi
Advantages of using filamentous fungi for SCP
- They have been used to produce SCP mainly from polysaccharide hydrolysates, e.g. starch hydrolysates, sulfite liquor, from wood pulp industries.
- Crude protein content is 50-55% but low in sulfur containing amino-acids.
- Recovery is easy by filtration.
Disadvantages of using filamentous fungi as SCP
- Slow growth rate than bacteria and yeast.
- Chances of contamination by yeast is high.
- High nucleic acid content.
- Should be evaluated for mycotoxin production.
Single cell protein from Yeast
Advantages of using yeast as SCP:
- Widely used genera are Saccharomyces, Candida and Torulopsis.
- Crude protein content is 55-60% which has good amino-acid balance except for deficiency of Sulphur containing amino-acids.
- It is usually very rich in Vitamin B.
- Used both for human food and animal feed supplemental.
- Risk of bacterial contamination is low.
- Recovery is easy by continuous centrifugation.
Disadvantages of using yeast as SCP:
- Slower growth rate than bacteria.
- High nucleic acid content
- Deficiency in sulfur containing amino acids.
Single cell Protein from Bacteria
Advantages of using bacteria as SCP
- Large number of bacterial species have been used.
- Can use a wide variety of substrate.
- Some of these have been used for production at commercial scale. E.g. Methylophilus, Methylotrophus
- It has high crude protein over 80% of good amino acid composition although in some cases a small deficiency of Sulphur containing amino acids.
Disadvantages of using bacteria as SCP:
- High nucleic acid content.
- Sterility should be maintained during the production process since pH of cultures is kept between 5-7.
- Risk of contamination by pathogenic bacteria.
- Product recovery is problematic.
- Careful evaluation for endotoxin production particularly when gram -ve bacteria are used.
Substrates used for SCP production:
- A variety of substrates ranging from inorganic carbon. E.g. CO2 (No cost for substrates) through industrial effluent and low-cost organic materials. E.g. cellulosic wastages like straw to high cost materials like starch hydrolysate are used for SCP production.
- These substrates are broadly categorized into two groups:
Fossil carbon sources (non-renewable)
- Gaseous hydrocarbon
- Liquid hydrocarbon
- Methanol
- Ethanol
i. Gaseous hydrocarbon:
- C1 to C4 gaseous hydrocarbon have been used for SCP production.
- Methane has been extensively studied and is highly desirable as it is available in high purity from natural gas.
- It is readily removed from the fermentation and support high productivity in continuous process.
- But it has heat generation, no necessitating efficient cooling.
ii. Liquid hydrocarbon:
- Saturated straight chain hydrocarbons called n-alkanes constitutes (0-30%) crude oil.
- Generally, n-alkanes having 9-18 carbon are used as SCP substrates.
- Many bacteria, actinomycetes, yeast and mold are able to use liquid hydrocarbons.
iii. Methanol:
- A very efficient chemical conversion of methane yield methanol.
- Methanol is fully water soluble and is used by many bacteria and there is little danger of explosion.
- Industrial production: Methylophilus, Methylotrophus at 35-40oC produce protein 71% called pruteen.
– It is used as milk substitute in calf feeding.
iv. Ethanol:
- Ethanol can be obtained from ethylene by catalytic addition of H2O molecule or from organic substrate by alcoholic fermentation.
- It is used as substrate for SCP production for human use.
- Bacteria, yeast and mycelium fungi utilize ethanol.
Renewable carbon source:
- CO2
- Molases
- Whey
- Cellulose hydrolysate
- Starch hydrolysate
- Industrial effluent
- Cellulosic wastage
i. CO2:
- Utilized by algae which derive the required energy from sunlight.
ii. Molases:
- It is used for alcoholic fermentation from wheat, yeast biomass obtained as byproduct.
- Traditionally, the baker’s yeast (S. cerevisiae) and Torulla yeast (Candidautilis) are produced on this substrate.
iii. Whey:
- Whey is a liquid portion of milk remaining after the curd is separated during cheese production.
- Yeast like Candida krusei in combination with Lactobacillus bulgaricus and Candida intermedia are the most widely used for commercial scale production of SCP from Whey.
iv. Cellulose hydrolysate:
- Cellulose obtained from fungi like Trichoderma viride has been used to hydrolyze cellulose and to produce glucose.
- It is cheap substrate and is best utilized for mushroom production.
v. Starch hydrolysate:
- This rather expensive substrate, is used for SCP production using Fusarium graminearum.
- The SCP has a fibrous meat-like texture and is marketed as mycoprotein.
vi. Industrial effluent:
- Effluent from many industries like breweries distillaries, confectionery industry, potato and canning industries.
- Sulfite liquor from wood pulp industries contain large amount of carbohydrates and other organic compounds which can be used for SCP production.
Basic step involved in SCP production:
- Provision of carbon source which may need physical and/or chemical pretreatment.
- Perry cellulose – Pretreated chemically by acid hydrolysis or enzymatically by using cellulase enzyme.
- Lignocellulose- alkali or acid hydrolysis or steam exposure.
- Addition of nitrogen, phosphorus and other nutrients to the carbon sources needed to support optimal growth of the selected micro-organisms.
- Prevention of contamination by maintaining sterile or hygienic condition.
- The medium component may be heated or sterilized by filtration and fermentation equipment are also sterilized.
- Inoculate the selected microorganisms in a pure state.
- Fermentation: The fermentation process requires a pure culture of chosen organism that is in the correct physiological state.
- A production fermenter is used which for multiplication of organism and drawing the culture medium in the steady state and then cell separation is done.
- Harvesting or product recovery:
- Different methods are used for product recovery depending upon type of micro-organisms used for SCP production.
- Algal mass:
- Recovered by concentration, de watering and drying.
- Sometimes, flocculant (E.g. Aluminium sulphate and calcium hydroxide can also be used.
- For Spirullina, it can float on surface water. So, it can be filtered and suspension is dried with hot air to get fine powder.
- Bacterial biomass:
- Many problems related with recovery of bacterial cell like they are very small and have cell density in the order of 10-20 gm/litre.
- Centrifugation cost is also high.
- A device has been developed for separation of Methylomonas clara from methanol containing culture medium which is based on coagulation and centrifugation.
- Yeast biomass:
- Yeast cells are small in size and can be recovered by decantation, centrifugation, by washing and dried treatment method.
- Fungal biomass: mycoprotein
- 1-2% biomass growing in a fermenter
- Purification of SCP processing for food:
- Liberation of cell proteins by destruction of ingestible cell walls.
- This can be done by chemical treatment by using acid, base, detergents, enzyme treatment, heating, freezing and thawing.
- Removal of nucleic acid, Degradation of nucleic acid produces uric acid which may accumulate to damaging level in human because human beings donot possess uricase activity.
- It is therefore necessary to reduce nucleic acids to acceptable low level especially in SCP intended for human use.
- It can be reduced by activation of endogenous RNAse by a brief heat treatment. E.g. 20 mins at 64oC reduces RNA from 10% to 1% of dry weight in case of Fusarium graminerium.
– alkaline hydrolysis
– chemical extraction
– suitable manipulation of growth and physiology of microbial cell.