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Single Cell protein

By-

Ananda Subramani K

1MS07BT004

M.S.Ramaiah Institute of technology

Bangalore

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Contents

Introduction 3 - 5

Production of SCP 6 - 19

Harvesting the SCP 20 - 21

Properties of SCP 22 - 25

References 26

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Introduction

SCP is the name given to a variety of microbial products that are produced by fermentation. SCP

refers to the dried microbial cells or total protein extracted from pure microbial cell culture

(monoculture Algae, bacteria, filamentous fungi, yeasts, etc), which can be used as food

supplement to humans (Food Grade) or animals (Feed grade). When properly produced, these

materials make satisfactory proteinaceous ingredients for animal feed or human food. The

production of protein from hydrocarbon wastes of the petroleum industry is the most recent

microbiological industry.

The term SCP was coined by Prof.C.L.Wilson in 1966. This term is more appropriate as most of

the microorganisms grow as single or filamentous individuals. SCP contains high protein content

(60 80% of dry cell weight), fats, carbohydrates, nucleic acids, vitamins, and minerals. It is

also rich in essential amino acids such as Lys and Met.

Yeast, fungi, bacteria, and algae are grown on hydrocarbon wastes, and cells are harvested as

sources of protein. It has been calculated that 100 lbs of yeast will produce 250 tons of proteins

in 24 hours, whereas a 1000 lbs steer will synthesize only 1 lb of protein 24 hours and this after

consuming 12 to 20 lbs of plant proteins. Similar, algae grown in ponds can produce 20 tons (dry

weight) of protein, per acre, per year. This yield is 10 to 15 times higher than soybean and 25 to

50 times higher than corn. There are both advantages and disadvantages in using microorganisms

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for animal or human consumption. Bacteria are usually high in protein (50 to 80 percent) and

have a rapid growth rate.

Single cell protein has the potential to be developed into a very large source of supplemental

protein that could be used in livestock feeding. In some regions single cell protein could become

the principal protein source that is used for domestic livestock, depending upon the population

growth and the availability of plant feed protein sources. This could develop because microbes

can be used to ferment some of the vast amounts of waste materials, such as straws; wood and

wood processing wastes; food, cannery and food processing wastes; and residues from alcohol

production or from human and animal excreta. Producing and harvesting microbial proteins is

not without costs, unfortunately. In nearly all instances where a high rate of production would be

achieved, the single cell protein will be found in rather dilute solutions, usually less than 5 %

solids. Methods available for concentrating include - filtration, precipitation, coagulation,

centrifugation, and the use of semi-permeable membranes. These de-watering methods require

equipment that is quite expensive and would not be suitable for most small-scale operations.

Removal of the amount of water necessary to stabilize the material for storage, in most instances,

is not currently economical. Single cell protein must be dried to about 10 % moisture, or

condensed and acidified to prevent spoilage from occurring, or fed shortly after being produced.

ADVANTAGES OF USING MICROORGANISMS FOR SCP PRODUCTION

y Protein synthesis is much more rapid than higher living systems.

y Microbes have short generation time.

y Easily modifiable genetically for determining the amino acid composition.

y Microbes have high protein content (7.12g protein Nitrogen/100g dry weight).

y Microbes can be grown on media containing cheap sources of C and N.

y Easy regulation of environmental factors for efficient yield.

DISADVANTAGES OF USING MICROORGANISMS FOR SCP PRODUCTION

y Bacterial cells have small size and low density, which makes harvesting from the

fermented medium difficult and costly.

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y Bacterial cells have high nucleic acid content relative to yeast and fungi This can be

detrimental to human beings, tending to increase the uric acid level in blood. This may

cause uric acid poising or gout. To decrease the nucleic acid level additional processing

step has to be introduced, and this increases the cost.

y The general public thinking is that all bacteria are harmful and produce disease. An

extensive education programme is required to remove this misconception and to make the

public accept bacterial protein.

Yeasts have as advantages their larger size (easier to harvest), lower nucleic add content, high

lysine content and ability to grow at acid pH. However the most important advantage is

familiarity and acceptability because of the long history of its use in traditional fermentations.

Disadvantages include lower growth rates, lower protein content (45 to 65 per cent), and lower

methionine content than in bacteria. Filamentous fungi have advantages .in ease of harvesting,

but have their limitations in lower growth rates, lower protein content, and acceptability. Algae

have disadvantages of having cellulosic cell walls which are not digested by human beings.

Secondly, they also concentrate heavy metals.

Single cell protein basically comprises proteins, fats carbohydrates, ash ingredients, water, and

other elements such as phosphorus and Potassium. The composition depends upon the organismand the substrate which it grows, some typical compositions which are compared with soy meal

and fish meal. If SCP is to be used successfully, there are five main criteria to be satisfied;

y The SCP must be safe to eat.

y The nutritional value dependent on the amino acid composition must be high.

y It must be acceptable to the general public.

y It must have the functionality, i.e. characteristics, which are found in common staple

foods.

y The economic viability of the SCP process is extremely complex and is yet to be

demonstrated.

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Production of SCP

Single cell proteins develop when microbes ferment waste materials (including wood, straw,

cannery and food processing wastes, residues from alcohol production, hydrocarbons, or human

and animal excreta. The problem with extracting single cell proteins from the wastes is the

dilution and cost. They are found in very low concentrations, usually less than 5%. Engineers

have developed ways to increase the concentrations including centrifugation, flotation,

precipitation, coagulation and filtration, or the use of semi-permeable membranes. The single

cell protein needs to be dehydrated to approximately 10% moisture content and/or acidified to

aid in storage and prevent spoilage.

The methods to increase the

concentrations to adequate levels,

and de-watering process require

equipment that is expensive and not

always suitable for small-scale

operations. It is economically

prudent to feed the

locally and shortly after it is

produced.

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General Schematic for Production of SCP

Some important substrates for SCP production

Sulphite waste liquor

Candida utilis has been produced as a protein supplement by fermentation of sulphite waste

liquor in Germany during both world wars. More recently a Finnish company developed a fungal

SCP production process, the Pekilo Process, to grow Paecilomyces varioti using sulphite waste

liquor.

Cellulose

Cellulose from natural sources and waste wood is an attractive starting material for SCP

production because of its abundance. The association of cellulose with lignin in wood makes it

somewhat intractable to microbial degradation. Thermal or chemical pretreatment, used in

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combination with enzymatic hydrolysis, is usually required. Systems using cellulolytic

organisms appear to have promise, but economic viability has yet to be achieved.

Whey

Whole milk whey or deproteinised whey is a carbohydrate source, which creates disposal

problems. (High BOD) Problems associated with whey for SCP production

are usually insifficient substrate, seasonal supply variations and its high water content (>90%)

which makes transport prohibitively expensive. While most organisms do not grow on lactose as

a carbon source, strains of the yeast Kluyveromyces marxianus readily grow on lactose.

Starch

The symba process was developed in Sweden to produce SCP from potato starch using two yeast

strains. Saccharomyces fibuligera produces the enzyme necessary for starch degradation

enabling co-growth ofCandida utilis. This process is known as simultaneous saccharification

and fermentation.

Glucose

Food grade glucose was the substrate chosen by RHM for production of fungal SCP using

Fusariumgraminearum. The strategy adopted was to take advantage ofmycelial fibre content toproduce a range of highadded value products including meat analogues forhuman consumption.

Higher Alkanes

The original alkane SCP fermentation process, developed by BP in France used 10-20% wax

contained in gas oil. Substrate costs were very low, however due to their crude nature, exhaustive

processing was required to recover the yeast free of a gas-oil flavour taint. Other alkane based

SCP processes were developed in Italy, Japan and Romania but many of them suffered from the

problems of potential carcinogenic residues and most of the plants have never run on full

capacity or have been closed.

Methane/Methanol

Methane was initially considered as a SCP raw material because, as a gas product, purification

problems after fermentation would be minimal. Disadvantages associated with methane-based

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processes are related to: (a) the greater oxygen requirements necessary to fully oxidise methane

compared with paraffins, (b) the low solubility of methane in water and (c) the requirement that

the fermentation plant be flame proof as methane-oxygen mixtures are highly explosive.

Methane is however easily converted to methanol which requires less oxygen, less fermenter

cooling, is highly water soluble and has minimal explosion risks.

ICI, which manufactures bulk methanol, chose this substrate for backterial SCP production for

animal feed using the trade name Pruteen. The company designed a non-mechanical pressure

cycle fermenter which uses air for both agitation and aeration in the worlds largest single

aerobic fermenter of 3000m3 capacity. The process which produces 50-60,000 tonnes SCP

per year, using the organism Methylophilus methylotrophus, was comissioned in 1979-1980,but

has suffered from dramatic increases in methanol prices. The economic difficulties encountered

by ICI with animal feed processes lead to a joint venutre with RHM to produce Fusarium SCP in

the ICI plant.

Choice of Microorganism

The key criteria used in selecting suitable strains for SCP production should consider the

following:

y The substrates to be used as carbon energy and nitrogen source and the need for nutrient

supplementation.y High specific growth rates, productivity and yields on a given substrate. pH and

temperature tolerance.

y Aeration requirements and foaming characteristics.

y Growth morphology in the reactor.

y Safety and acceptability non pathogenic, absence of toxins.

y Ease of recovery.

y Protein, RNA and nutritional composition of the product.

y Structural properties of the final product.

In general, fungi have the capacity to degrade a wider range of complex plant materials,

particularly plant polysaccharides. They can tolerate low pH which contributes to reducing

fermenter infections. Growth of fungi as short, highly branched filaments rather than in pellets is

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essential in order to optimize growth rate. Bacteria, in general, have faster growth rates than

fungi and grow at higher temperatures, thereby reducing fermenter cooling requirements.

Bacterial and yeast fermentations are easier to aerate. In contrast to fungi, which are easily

recovered by filtration, bacteria and yeast require the use of sedimentation techniques and

centrifugation. Bacteria, in general produce a more favorable protein composition than yeast or

fungi. Protein content in bacterial can range from 60-65% whereas fungi selected for biomass

production and yeast have protein contents in the range of 33-45%. However, associated with the

higher acterial protein levels is a much higher level of nutritionally undesirable RNA content of

15-25%. Microorganisms involved in SCP production must be safe and acceptable for use in

food. Organisms should be stable genetically so that the strain with optimal biochemical and

physiological characteristics may be maintained in the process through many hundreds of

generations.

Production

Large scale fermenters are required. High biomass productivity requires high oxygen transfer

rates which promotes high respiration rates which in turn increase metabolic heat production and

the need for an efficient cooling system. In order to maximise fermentation productivity it is

essential to operate continuous fermentation processes. Different processes have adopted

different fermenter designs with respect to process requirements.

Diagrammatic Representation of Commercial Production of Barker's Yeast

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Commerical Production

The raw materials that can be used for single-cell protein manufacture include whey, sulphate

waste liquors, hydrocarbon waste from the pet petroleum industry, and the vats used to produce

alcoholic beverages. The production process involves growth of the organisms in largefermenting tanks with forced aeration for vigorous cell-growth. Manufacture process used by

British Petroleum Industry for single-cell protein from hydrocarbons is represented in figure

below

Yeast, fungi, bacteria,

and algae are grown on hydrocarbon wastes, and cells are harvested as sources of protein. It has

been calculated that 100 lbs of yeast will produce 250 tons of proteins in 24 hours, whereas a

1000 lbs steer will synthesize only 1 lb of protein 24 hours and this after consuming 12 to 20 lbs

of plant proteins. Similar, algae grown in ponds can produce 20 tons (dry weight) of protein, per

acre, per year.

Most of the work on single-cell protein production has been focused on the yeast, Candida utilis

(Torula utilis). The yeast meets most of the requirements named in the preceding paragraph. Not only

is the yeast easily grown, it also is a good food and fodder yeast. Although sterility is necessary,

purity of culture is not essential.

INDIRECT vs. DIRECT production

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The relation of single-cell protein production to the reclamation of useful nutrient elements in

waste is by way of the utilisation of sugars formed through hydrolization of cellulosic substances

in municipal waste. However, a separate hydrolysis step may be bypassed by culturing the yeast

directly on the cellulosic waste. For convenience, in this presentation, the two approaches are

respectively designated by the terms indirect and direct.

Indirect production

The production of C. utilis is an example of the indirect approach. The sequence of events in the

production is diagrammed in Figure IX-2.

With respect to nutritional requirements, the sugars (glucose) satisfy the carbon needs. The other

required essential nutritional elements are nitrogen, phosphorus, and potassium, which must

come from an external source. Usually, nitrogen is added as an ammonium compound (e.g.,

ammonium sulphate); a phosphate is used for phosphorus; and a potassium sulphate or hydroxide

compound for potassium. Generally, it is not necessary to add the essential trace elements.

Principal cultural conditions are a temperature at 20 to 35C; and O2, about 1.02 kg/kg cell

mass-produced. The necessarily aerobic conditions are attained by continuously agitating the

culture. The volume of air applied to meet the oxygen demand would be a rate of about 120

millimoles O2 absorbed per L-hr (3.84 g/L-hr). The yield to be expected at such a rate is 3.66 g

yeast per L-hr.

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Under proper cultural conditions, the yield of the cell mass should be from 45% to 55% of the

sugar consumed . The production rate under continuous conditions depends upon a combination

of cell mass and hydraulic detention time (culture volume/volume feed medium/day). must be

cellulolytic, i.e., capable of breaking down cellulose molecules. Preferably, most should be

cellulolytic. A disadvantage is the inability to use submerged culture in the absence of special

adaptations.

Maximu

m cell

concentra

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tion is a function of the hydrolysate sugar concentration multiplied by the sugar conversion

efficiency of the yeast. All sequences are not occurring simultaneously and, collectively, they

constitute a single unit process. Therefore, at least some of the microorganisms

Direct production

Direct production differs from indirect production in that organisms are cultured upon

unhydrolyzed wastes. Indirect production involves two discrete steps (hydrolysis and cell

production); whereas in direct production, the two steps are neither spatially nor always

temporally discrete. Although of necessity, the steps are sequential (hydrolysis must precede

utilisation for cellular growth; both may involve the same microorganism). In other words, an

organism can degrade a cellulosic molecule and utilize the constituent sugars to synthesise

cellular mass. All sequences are not occurring simultaneously and, collectively, they constitute a

single unit process. Therefore, at least some of the microorganisms must be cellulolytic, i.e.,

capable of breaking down cellulose molecules. Preferably, most should be cellulolytic. A

disadvantage is the inability to use submerged culture in the absence of special adaptations.

Most of the experience with single-cell production from waste has been at the laboratory- and

pilot-scale levels and has been with paper and bagasse. Paper is from 40% to 80% cellulose, 20%

to 30% lignin, and 10% to 30% hemicellulose and xylosans. Bagasse is the residue remaining

after the juice has been extracted from sugar cane by milling. Inasmuch as the studies were

limited to laboratory- and pilot-scale levels, projections and estimates based on the studies must

be considered in that light.

Among the cellulolytic microorganisms that have been studied are the yeasts, C. utilis and

Myrothecium verrucaria, and the bacteria, Cellulomonas flarigena .

In a study that involved the culture of M. verrucaria on a substrate composed of ball-milled

newspaper, a yield of crude protein amounting to 1.42 g/L was obtained . A pilot-scale study

involved the application of a system such as is diagrammed in Figure IX-3 . The organism used

in the investigation was C. utilis. The bagasse was pre-treated because experience had shown that

without pre-treatment, the soluble carbohydrate content of untreated bagasse is only about 2%;

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whereas after treatment, it is almost 18%. Pre-treatment reduces the cellulose crystallinity of the

bagasse from almost 50% to only 10%. As stated earlier, pre-treatment generally takes one or a

combination of the following forms: fine milling and exposure to moderately elevated

temperature under either acid or alkaline conditions.

The bacteria C. flavigena and C. uda constituted the product in a pilot study in which the

feedstock was bagasse. The study confirmed the need to pre-treat bagasse -- specifically, alkaline

pre-treatment. Moreover, in the study, extent of conversion of feedstock to cell mass was very

modest despite a continuous fermenter efficiency of 75% and an approximate 90% solubilization

of bagasse. Supplementary nutritional needs could be supplied by fertiliser and industrial

chemicals. From 50% to 55% of the product is crude protein that has a good amino acid balance.

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Some Common processes for SCP production -

SCP from N. Alkanes

In the late 1950's, British Petroleum (BP) became interested in the growth of a micro-organism

in C12-C20 alkanes. This constitutes the wax fraction of gas oils for treating. Some crude oils

contain up to 15% in wax, and these waxes must be removed since they make oil more viscous,

precipitate out at low temperatures, block tubes etc.

BP uses two yeasts, Candidor lipolytica and C. tropicals and built a 16,000 tons/year plant in

Cap Lavera, France, and a 4,000 tons/year plant in England. The product produced was called

"TOPRINA". In the UK the product "TOPRINA G" was a purer product while the one in France

was not separated from alkanes.

Both processes employed NH3 as N-source and Mg ions to increase yields. No other carbon

source was used. For 12 years TOPRINA was tested for toxicity and carcinogenecity and was

marketed as a replacement for fish meal in high protein feeds and as a replacement for skimmed

milk powder in milk replacers.

There were no signs at all for toxicity or carcinogenicity. In spite of this, people were concerned

that aromatic hydrocarbons may be carried over to SCP. The main opposition came from Japan,

where environmental groups and university professors condemned SCP as dangerous, and the

matter became political. In 1972 a specialised committee decided that SCP was only for animal

feeding but later, Japan was the first country to ban petrochemical protein. Meanwhile BP and an

Italian company constructed a 100,000 tn/year plant in Sardinia. Following the Japanese attack

on SCP, there has been great concern about and opposition to the use of SCP from environmental

groups in government. The Italian government ordered further studies which showed that there

was no hazard or carcinogenesis due to SCP. Pigs fed on 30% TOPRINA in their diets showed

less n-paraffins in their fat tissue than those fed on pasture. Based on this evidence the Italian

government agreed to the use of TOPRINA in limited amounts and only for export.

In 1977 Italy stopped the SCP production for alkanes altogether due to the increase in oil prices.

The price of soya was more competitive. Now there is no factory which produces any

petrochemical protein.

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SCP from Methane

Methane is cheap, abundant and without the toxicity problems of alkanes. It is a constituent of

North Sea Gas and is also produced during anaerobic digestion. Methane contains the most

highly reduced form of carbon and consequently gives high cell yields relative to the amount of

gas consumed. The general Methylomonas and Methylococcus have been recognised as utilising

methane as a carbon source. The species which has been extensively studied isMethylomonas

methanica. Nitrates or ammonium salts can serve as N-source.

Perhaps the most important work in this field was carried out by Shell in England. The process

involves methane oxidation by stable mixed cultures. These were

1. a methane utilising G(-) rod;

2. a Hyphomicrobium;

3. two g(-) rods; Acinetobacterand Flavobacterium

This mixed culture was one of the best examples of symbiosis. The process began in 1970 in a

300 e pilot plant at Sittingbourne, UK. In 1974 Shell announced plans for a construction of a

larger pilot-plant in the same area and a development program in Amsterdam with a goal of

producing 100,000 tn/year. In spring 1976, Shell stopped commercialisation and its development

plans were indefinitely postponed. This decision was based on 3 factors:

1. the low price of soybeans & maize;

2. the potential of many countries for expanding existing protein sources;

3. the difficulty in applying Shell's sophisticated process in underdeveloped countries.

SCP from Methanol ICI process

The technology of SCP from methanol has been well studied and the most advanced process

belongs to ICI. The fermentation was carried out in a big airlift fermentor with the bacterium.

Methylophilus methylotropha. This organism was selected among other methanol utilises after

screening tests for pathogenicity and toxicity. As a nitrogen source ammonia was used. The

product was named "PRUTEEN". Pruteen contained 72% crude protein and was marketed for

feed as a source of energy, vitamins and minerals as well as a highly balanced protein source.

The methionine and lysine content of Pruteen compared very favourably with white fish meal.

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ICI has commissioned a 60,000 tn/year plant utilising the single largest fermentor in the world (2

x 10,000,000 l).

Unfortunately Pruteen now cannot compete with soya and fish meal. ICI hopes to be able to sell

their technology, because they have given up the idea of making money out of Pruteen. So today

Pruteen although a major engineering success is not economical to run.

SCP from Ethanol

Ethanol although expensive as a substrate has been used for SCP. The process comes from the

Amoco Company in the US utilising a food grade yeast: "Torula". The product is sold by the

name "TORUTEIN" and government clearances have been obtained to market Torutein in

Canada and Sweden. The yeast is about 52% protein and due to its relatively low Methioninelevel has a PER of about 1.7. The PER of wheat from 1.1 to 2.0. Torutein is being marketed as a

flavour enhancer of high nutritional value, and a replacement for meat, milk and egg protein.

However it is not very successful in the United States since soya which is plentiful and cheap can

serve as an alternative or substitute to meat and egg diets.

RHM Mycoprotein process

This is a development of Ranks Hovis McDougall and is the only mycoprotein (except edible

mushrooms) that has been cleared for human consumption. It uses a Fusarium graminearum

growing in molasse, or glucose. The medium contains NH3 for nitrogen source and pH control.

The product is heat treated for RNA reduction. The mycelium is separated by vacuum filtration,

and can be technologically treated to match food texture. In the UK it is marketed as pies and is

considered a success since having less fat than meat, it can be sold at a premium price.

SCP from Lignocellulose

The lignocellulosic wastes, mainly from agriculture, constitute the most abundant substrate for

SCP which is also renewable. The world annual production of straw for example reaches 600

million tons every year. In Greece the straw from wheat and rye, the two most important cereals,

is an estimated 1.5 million tons per year.

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For the utilisation of lignocellulose, a pre-treatment is usually necessary. Many pre-treatment

methods have been reported which vary from alkali or acid treatment, steam explotion or even x-

ray radiation.

To the present time the only economical utilisation of lignocellulosic wastes is in mushroom

production. Besides our well know cultivated mushroom Agaricus bisporus there are many

important ones which contain lignocellulolytic enzymes and are cultivated for food mainly in

Asia and Africa. Some are of great economic significance and are cultivated on an industrial

scale. Examples of important ones include the following species: Volvariella sp., Lentinus

edodes and Pleurotus sp.

Bel Fromageries process:Kl

uyveromyces marxianus from whey

Whey which contains about 5% lactose, 0.8% protein and 0.2-0.6% lactic acid, is used as a

substrate. Biomass production requires an aerobic fermentation whereas aeration is minimal for

ethanol production. For feed grade biomass, the entire fermentation minerals and lactic acid may

be recovered. For preparation of food grade material, cells are harvested by centrifugation,

washed and dried.Cell yield is 0.45-0.55 g/g based on lactose consumed.

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An Overview of SCP production

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Harvesting THE SCP

Harvesting usually is done in two main stages: a concentration stage and a concentrate

processing stage.

First-stage concentration

This stage results in the formation of a concentrate that has a sludge-like consistency and is in

need of further processing. The need for the concentration step arises from the relatively lowconcentration of cells and large volumes of material that must be processed. The sludge

(concentrate) is dewatered and dried. The concentration step is beset with many and grave

difficulties due to the microscopic size and the physical characteristics of the cells, as well as

their modest monetary value. The several technologies available for accomplishing the

concentration step can be grouped into the categories of screening, filtration, settling

(sedimentation), and centrifugation.

Screening and filtration

Screening and filtration are discussed under a single heading because they share a common

characteristic: separation of particles (cells) depends upon the difference between the size of the

particles and that of the openings (screen) or pores (filter medium). The problem is that the

screen or filter medium becomes clogged before a workable cake can be accumulated.

Settling

Their small size, low specific gravity, density, and low settling velocity render concentration by

sedimentation impractical. The settling velocity of yeast cells is approximately 1.1 x 10-5

cm/sec.

Significant advances in settling tank design and operation may enhance settling to a point at

which it becomes a feasible option. Another approach to settling or a modification is to induce

floc formation and thereby promote settling to a level at which it might be practical. Floc

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formation can be induced by altering the surface charge of yeast cells such that they agglomerate

into floc particles. Surface charge can be altered by introducing a polymer flocculant (either

anionic or cationic) into the suspension. Alteration can also be accomplished by passing the

suspension through an ion exchange column.

Centrifugation

Centrifugation is an effective concentration method. Unfortunately, it is expensive in terms of

equipment and power, and requires skilled personnel. A high-velocity rotor is necessary because

of the microscopic size and low specific gravity and density of the cells and viscosity of the

medium. A putative advantage is that the two separation stages can be accomplished in a single

operation.

Second stage - concentrate (sludge) processing

Treatment consists of dewatering and drying. Flash drying is a good approach. It is rapid and is

amenable to mass production and is successfully used in food and feedstuff preparation.

Moreover, it removes threats to human and animal health posed by chance pathogens. Other

options include pressure filtration and vacuum drying, such as is used in sewage sludge

conditioning.

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Properties of SCP

One of the main advantages of SCP compared to other types of protein is the small doubling time

of cells (td) as shown in Table 1.

Table 1

Mass doubling time (S)

Due to this property, the productivity of protein production form micro-organisms is greater than

that of traditional proteins (Table 2).

Table 2

Efficiency of protein production of several protein sources in 24 hours (16

)

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It is assumed that the growth occurs without any restriction. Other advantages of SCP over

conventional protein sources are:

a. it is independent of land and climate;

b. it works on a continuous basis;

c. it can be genetically controlled;

d. it causes less pollution.

There are five factors that impair the usefulness of SCP:

a. non digestible cell wall (mainly algae);

b. high nucleic acid content;

c. unacceptable coloration (mainly with algae);

d. disagreeable flavour (part in algae and yeasts);

e. cells should be killed before consumption.

Thus SCP is treated with various methods in order to:

1. kill the cells;

2. improve the digestibility;

3. reduce the nucleic acid content.

Nutritional Value of SCP

For the assessment of the nutritional value of SCP, factors such as nutrient composition, amino

acid profile, vitamin and nucleic acid content as well as palatability, allergies and gastrointestinal

effects should be taken into consideration . Also long term feeding trials should be undertaken

for toxicological effects and carcinogenesis.

Table 3 shows the average cell composition of the major groups of micro-organisms.

Table 3

Average composition of the main groups of micro-organisms (% dry weight)

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Bacterial protein is similar to fish protein, yeast's protein resembles soya and the fungi protein is

somewhat lower than the yeast's. Of course microbiological proteins are deficient in the sulphur

amino acids cysteine and methionine and require supplementation, while they exhibit better

levels of lysine (Table 4).

Table 4

Essential amino acid content of the cell protein in comparison with several reference proteins

(grams of amino acid per 100 grams of protein)

The vitamins of micro-organisms are primarily of the B type, B12 occurs mostly in bacteria,

while vitamin A is usually found in algae. Table 5 shows the vitamin content of various food

micro-organisms.

Table 5

Vitamin content of various food micro-organisms (mg/100 g dry weight)

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Other nutritional parameters which evaluate the quality of a given SCP are:

- the digestibility (D)

- the biological value (BV)

- the protein efficiency ratio (PER)

- the net protein utilisation (NPU)

With microbial cells it is important to note that digestibility is low especially with algae cells

because of indigestible cell walls.

SCP evaluation and future prospects

The development of SCP was really the beginning of biotechnology. Prior to this the industrial

fermentation was mainly focused on antibiotics and other products which did not have to

compete. This was not the case with SCP which had to compete with similar products in the

market. The development was brought up by the oil companies rather than the food companies,

because they could take the risk of a highly costly product out with no real expected profit. They

also had all the high technology required.

The efforts tried so far by adding dry SCP as a supplement to diets in order to solve the problems

of the hungry in the Third World Countries, certainly have not given the expected results. Every

new food which appears in the market should have not only high nutritive quality, but also

satisfactory organoleptic supplementary element.

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Today in most countries where market forces operate. SCP cannot compete with soya, alfalfa or

fish meal (19). Mushroom production from lignocellulosics seems to be one economical and

promising use for SCP. For future success of SCP, first, food technology problems have to be

solved in order to make it similar to familiar foods and second, the production should compare

favourably with other protein sources.

References

www.plosmedicine.com

Biochemical Engineering, Aiba, S., A.E. Humphrey, and N.F. Mills,

Academic Press, Inc., New York, New York, USA, 1965.

Conversion of Organic Solid Wastes into Yeast: An Economic

Evaluation, Meller, F.H, prepared for Bureau of Solid Waste Management

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by IONICS, Inc., under contract PH86-87-204, U.S. H.E.W. (presently, U.S.

Environmental Protection Agency), 1969.

www.springerlink.com

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