Downstream Process

Class: B.Sc.
(Hons) Botany, VI Semester

Paper: Industrial and Environmental Microbiology
Unit 3: Microbial production of industrial products
Topics: Downstream processing and uses:
Filtration
Centrifugation
Cell disruption
Solvent extraction
Precipitation and ultrafiltration
Lyophilization
Spray drying
Dr. Preeti Rawat

E-mail ID: prawat2k@gmail.com
Assistant Professor
Department of Botany
Deshbandhu College
Fermentation - Down Stream Processing
Five stages in downstream processing after Fermentation:
1. Solid-Liquid Separation
2. Release of Intracellular Products
3. Concentration
4. Purification by Chromatography and
5. Formulation.
2
Stage 1: Solid-Liquid Separation:
• The first step in product recovery is the separation of whole cells (cell biomass) and other
insoluble ingredients from the culture broth (Note: If the desired product is an intracellular
metabolite, it must be released from the cells before subjecting to solid-liquid separation).
• Some authors use the term harvesting of microbial cells for the separation of cells from the
culture medium.
• Several methods are in used for solid-liquid separation, these are:
1. Flotation
2. Flocculation
3. Filtration
4. Centrifugation
3
1. Flotation:
When a gas is introduced into the liquid broth, it forms bubbles. The cells and other solid
particles get adsorbed on gas bubbles. These bubbles rise to the foam layer which can be
collected and removed. The presence of certain substances, referred to as collector
substances, facilitates stable foam formation e.g., long chain fatty acids, amines.
2. Flocculation:
In flocculation, the cells (or cell debris) form large aggregates to settle down for easy removal.
The process of flocculation depends on the nature of cells and the ionic constituents of the
medium. Addition of flocculating agents (inorganic salt, organic polyelectrolyte, mineral
hydrocolloid) is often necessary to achieve appropriate flocculation.
3. Filtration:
Filtration is the most commonly used technique for separating the biomass and culture
filtrate. The efficiency of filtration depends on many factors— the size of the organism,
presence of other organisms, viscosity of the medium, and temperature. Several filters such as
4
depth filters, absolute filters, rotary drum vacuum filters and membrane filters are in use.
i). Depth Filters:
They are composed of a filamentous matrix such as glass wool, asbestos or filter paper. The
particles are trapped within the matrix and the fluid passes out. Filamentous fungi can be
removed by using depth filters.
ii). Absolute Filters:
These filters are with specific pore sizes that are smaller than the particles to be removed.
Bacteria from culture medium can be removed by absolute filters.
iii). Rotary Drum Vacuum Filters:
These filters are frequently used for separation of broth containing 10-40% solids (by volume)
and particles in the size of 0.5-10µm. Rotary drum vacuum filters have been successfully used
for filtration of yeast cells and filamentous fungi. The equipment is simple with low power
consumption and is easy to operate. The filtration unit consists of a rotating drum partially
immersed in a tank of broth (Fig. 20.2). As the drum rotates, it picks up the biomass which gets
deposited as a cake on the drum surface. This filter cake can be easily removed. 5
6
iv) Membrane Filters:
In this type of filtration, membranes with specific pore sizes can be used. However, clogging of
filters is a major limitation. There are two types of membrane filtrations—static filtration and
cross-flow filtration (Fig. 20.3). In cross-flow filtration, the culture broth is pumped in a
crosswise fashion across the membrane. This reduces the clogging process and hence better
than the static filtration.
7
Types of filtration processes:
There are 3 major types of filtrations based on the particle sizes and other characters (table
20.1). These are:
1. Microfiltration
2. Ultrafiltration
3. Reverse osmosis.
8
4. Centrifugation:
• The technique of centrifugation is based on the principle of density differences between
the particles to be separated and the medium. Thus, centrifugation is mostly used for
separating solid particles from liquid phase (fluid/particle separation).
• Unlike the centrifugation that is conveniently carried out in the laboratory scale, there are
certain limitations for large scale industrial centrifugation. However, in recent years,
continuous flow industrial centrifuges have been developed. There is a continuous feeding
of the slurry and collection of clarified fluid, while the solids deposited can be removed
intermittently.
• The different types of centrifuges are depicted in Fig. 20.4, and briefly described
hereunder.
9
i) Tubular bowl centrifuge (Fig. 20.4A):

This is a simple and a small centrifuge, commonly used in pilot plants. Tubular bowl centrifuge
can be operated at a high centrifugal speed, and can be run in both batch or continuous mode.
10
The solids are removed manually.
ii) Disc centrifuge (Fig. 20.4B):
It consists of several discs that separate the bowl into settling zones. The feed/slurry is fed
through a central tube. The clarified fluid moves upwards while the solids settle at the lower
surface.
iii) Multi-chamber centrifuge (Fig. 20.4C):
This is basically a modification of tubular bowl type of centrifuge. It consists of several
chambers connected in such a way that the feed flows in a zigzag fashion. There is a variation
in the centrifugal force in different chambers. The force is much higher in the periphery
chambers, as a result smallest particles settle down in the outermost chamber.
iv) Scroll centrifuge or decanter (Fig. 20.4D):
It is composed of a rotating horizontal bowl tapered at one end. The decanter is generally used
to concentrate fluids with high solid concentration (biomass content 5-80%). The solids are
deposited on the wall of the bowl which can be scrapped and removed from the narrow end.
11
Stage 2. Release of Intracellular Products:
• As already stated, there are several biotechnological products (vitamins, enzymes) which
are located within the cells (intracellular). Such compounds have to be first released
(maximally and in an active form) for their further processing and final isolation.
• The microorganisms or other cells can be disintegrated or disrupted by physical, chemical
or enzymatic methods. The outline of different techniques used for breakage of cells is
given in Fig. 20.5.
• The selection of a particular method depends on the nature of the cells, since there
is a wide variation in the property of cell disruption or breakage.
12
13
Cell Disruption:
1. Physical methods of cell disruption:
i) Ultra sonication:
Ultrasonic disintegration is widely employed in the laboratory. However, due to high cost, it is
not suitable for large-scale use in industries.
ii) Osmotic shock:
This method involves the suspension of cells (free from growth medium) in 20% buffered
sucrose. The cells are then transferred to water at about 4°C. Osmotic shock is used for the
release of hydrolytic enzymes and binding proteins from Gram-negative bacteria.
iii) Heat shock (thermolysis):
Breakage of cells by subjecting them to heat is relatively easy and cheap. But this technique
can be used only for a very few heat-stable intracellular products.
14
iv) High pressure homogenization:
This technique involves forcing of cell suspension at high pressure through a very narrow
orifice to come out to atmospheric pressure. This sudden release of high pressure creates a
liquid shear that can break the cells.
v) Impingement:
In this procedure, a stream of suspended cells at high velocity and pressure are forced to hit
either a stationary surface or a second stream of suspended cells (impinge literally means to
strike or hit). The cells are disrupted by the forces created at the point of contact. Micro
fluidizer is a device developed based on the principle of impingement. It has been successfully
used for breaking E. coli cells. The advantage with impingement technique is that it can be
effectively used for disrupting cells even at a low concentration.
15
vi) Grinding with glass beads:
• The cells mixed with glass beads are subjected to a very high speed in a reaction vessel.
The cells break as they are forced against the wall of the vessel by the beads. Several
factors influence the cell breakage-size and quantity of the glass beads, concentration and
age of cells, temperature and agitator speed. Under optimal conditions, one can expect a
maximal breakage of about 80% of the cells.
• A diagrammatic representation of a cell disrupter employing glass beads is shown in Fig.
20.6. It contains a cylindrical body with an inlet, outlet and a central motor-driven shaft. To
this shaft are fitted radial agitators. The cylinder is fitted with glass beads. The cell
suspension is added through the inlet and the disrupted cells come out through the outlet.
The body of the cell disrupter is kept cool while the operation is on.
16
Mechanical and non-mechanical methods:

• Among the physical methods of cell disruption described above, ultra sonication, high-
pressure homogenization, impingement and grinding with glass beads are mechanical
while osmotic shock and heat shock are non-mechanical.
• The chemical and enzymatic methods (described below) are non-mechanical in nature.
17
2. Chemical methods of cell disruption:
Treatment with alkalies, organic solvents and detergents can lyse the cells to release the
contents.
i) Alkalies:
Alkali treatment has been used for the extraction of some bacterial proteins. However, the
alkali stability of the desired product is very crucial for the success of this method e.g.,
recombinant growth hormone can be efficiently released from E. coli by treatment with
sodium hydroxide at pH 11.
ii) Organic solvents:
Several water miscible organic solvents can be used to disrupt the cells e.g., methanol,
ethanol, isopropanol, butanol. These compounds are inflammable; hence require specialised
equipment for fire safety. The organic solvent toluene is frequently used. It is believed that
toluene dissolves membrane phospholipids and creates membrane pores for release of
intracellular contents. 18
iii) Detergents:
Detergents that are ionic in nature, cationic-cetyl trimethyl ammonium bromide (CTAB) or
anionic-sodium lauryl sulfate (SLS) can denature membrane proteins and lyse the cells. Non-
ionic detergents (although less reactive than ionic ones) are also used to some extent e.g.,
Triton X-100 or Tween. The problem with the use of detergents is that they affect purification
steps, particularly the salt precipitation. This limitation can be overcome by using
ultrafiltration or ion-exchange chromatography for purification.
19
3. Enzymatic methods of cell disruption:
• Cell disruption by enzymatic methods has certain advantages i.e., lysis of cells occurs under
mild conditions in a selective manner. This is quite advantageous for product recovery.
• Lysozyme is the most frequently used enzyme and is commercially available (produced
from hen egg white). It hydrolyses β-1, 4-glycosidic bonds of the mucopeptide in bacterial
cell walls. The Gram- positive bacteria (with high content of cell wall mucopeptides) are
more susceptible for the action of lysozyme.
• For Gram-negative bacteria, lysozyme in association with EDTA can break the cells. As the
cell wall gets digested by lysozyme, the osmotic effects break the periplasmic membrane to
release the intracellular contents.
• Certain other enzymes are also used, although less frequently, for cell disruption. For the
lysis of yeast cell walls, glucanase and mannanase in combination with proteases are used.
Combination of methods: •
In order to increase the efficiency of cell disintegration in a cost-effective manner, a
20
combination of physical, chemical and enzymatic methods is employed.
Stage 3. Concentration:
• The filtrate that is free from suspended particles (cells, cell debris etc.) usually contains 80-
98% of water. The desired product is a very minor constituent. The water has to be
removed to achieve the product concentration.
• The commonly used techniques for concentrating biological products are:
1. Evaporation
2. Liquid-liquid extraction
3. Membrane filtration
4. Precipitation
5. Adsorption.
• The actual procedure adopted depends on the nature of the desired product (quality and
quantity to be retained as far as possible) and the cost factor.
21
1. Evaporation:
• Water in the broth filtrate can be removed by a simple evaporation process. The
evaporators, in general, have a heating device for supply of steam, and unit for the
separation of concentrated product and vapour, a condenser for condensing vapour,
accessories and control equipment.
• Some of the important types of evaporators in common use are briefly described:
i) Plate evaporators:
The liquid to be concentrated flows over plates. As the steam is supplied, the liquid gets
concentrated and becomes viscous.
ii) Falling film evaporators:
In this case, the liquid flows down long tubes which gets distributed as a thin film over the
heating surface. Falling film evaporators are suitable for removing water from viscous products
of fermentation. 22
iii) Forced film evaporators:

The liquid films are mechanically driven and these devices are suitable for producing dry
product concentrates.
iv) Centrifugal forced film evaporators:
These equipment evaporate the liquid very quickly (in seconds), hence suitable for
concentrating even heat-labile substances. In these evaporators, a centrifugal force is used to
pass on the liquid over heated plates or conical surfaces for instantaneous evaporation.
23
2. Liquid-Liquid Extraction:
The concentration of biological products can be achieved by transferring the desired product
(solute) from one liquid phase to another liquid phase, a phenomenon referred to as liquid-
liquid extraction. Besides concentration, this technique is also useful for partial purification of
a product.
The efficiency of extraction is dependent on the partition coefficient i.e. the relative
distribution of a substance between the two liquid phases. The process of liquid-liquid
extraction may be broadly categorized as extraction of low molecular weight products and
extraction of high molecular weight products.
• Extraction of low molecular weight products:
By using organic solvents, the lipophilic compounds can be conveniently extracted.
However, it is quite difficult to extract hydrophilic compounds. Extraction of lipophilic
products can be done by the following techniques:
24
i) Physical extraction:
The compound gets itself distributed between two liquid phases based on the physical
properties. This technique is used for extraction of non-ionising compounds.
ii) Dissociation extraction:
This technique is suitable for the extraction of ionisable compounds. Certain antibiotics can be
extracted by this procedure.
iii) Reactive extraction:
In this case, the desired product is made to react with a carrier molecule (e.g., phosphorus
compound, aliphatic amine) and extracted into organic solvent. Reactive extraction procedure
is quite useful for the extraction of certain compounds that are highly soluble in water
(aqueous phase) e.g., organic acids.
25
iv. Supercritical fluid (SCF) extraction:

This technique differs from the above procedures, since the materials used for extraction are
supercritical fluids (SCFs). SCFs are intermediates between gases and liquids and exist as fluids
above their critical temperature and pressure. Supercritical CO2, with a low critical
temperature and pressure is commonly used in the extraction. Supercritical fluid extraction is
rather expensive, hence not widely used (SCF has been used for the extraction of caffeine
from coffee beans, and pigments and flavor ingredients from biological materials).
26
• Extraction of high molecular weight compounds:
Proteins are the most predominant high molecular weight products produced in fermentation
industries. Organic solvents cannot be used for protein extraction, as they lose their biological
activities. They are extracted by using an aqueous two-phase systems or reverse micelles
formation.
i) Aqueous two-phase systems (ATPS):
They can be prepared by mixing a polymer (e.g., polyethylene glycol) and a salt solution
(ammonium sulfate) or two different polymers. Water is the main component in ATPS, but the
two phases are not miscible. Cells and other solids remain in one phase while the proteins are
transferred to other phase. The distribution of the desired product is based on its surface and
ionic character and the nature of phases. The separation takes much longer time by ATPS.
ii) Reverse miceller systems:
Reverse micelles are stable aggregates of surfactant molecules and water in organic solvents.
The proteins can be extracted from the aqueous medium by forming reverse micelles. In fact,
27
the enzymes can be extracted by this procedure without loss of biological activity.
3. Membrane Filtration:
• Membrane filtration has become a common separation technique in industrial
biotechnology. It can be conveniently used for the separation of biomolecules and
particles, and for the concentration of fluids.
• The membrane filtration technique basically involves the use of a semipermeable
membrane that selectively retains the particles/molecules that are bigger than the pore
size while the smaller molecules pass through the membrane pores.
• Membranes used in filtration are made up of polymeric materials such as polyethersulfone
and polyvinyl di-fluoride. It is rather difficult to sterilize membrane filters.
• In recent years, micro-filters and ultrafiIters composed of ceramics and steel are available.
Cleaning and sterilization of such filters are easy.
• The other types of membrane filtration techniques are described briefly.
28
• Membrane adsorbers:
They are micro- or macro porous membranes with ion exchange groups and/or affinity ligands.
Membrane adsorbers can bind to proteins and retain them. Such proteins can be eluted by
employing solutions in chromatography.
• Pervaporation:
This is a technique in which volatile products can be separated by a process of permeation
through a membrane coupled with evaporation. Pervaporation is quite useful for the
extraction, recovery and concentration of volatile products. However, this procedure has a
limitation since it cannot be used for large scale separation of volatile products due to cost
factor.
• Perstraction:
This is an advanced technique working on the principle of membrane filtration coupled with
solvent extraction. The hydrophobic compounds can be recovered/ concentrated by this
method. 29
4. Precipitation:
• Precipitation is the most commonly used technique in industry for the concentration of
macromolecules such as proteins and polysaccharides.
• Precipitation technique can also be employed for the removal of certain unwanted
byproducts e.g. nucleic acids, pigments.
• Neutral salts, organic solvents, high molecular weight polymers (ionic or non-ionic), besides
alteration in temperature and pH are used in precipitation.
• In addition to these non-specific protein precipitation reactions (i.e. the nature of the
protein is unimportant), there are some protein specific precipitations e.g., affinity
precipitation, ligand precipitation.
30
i) Neutral salts:
The most commonly used salt is ammonium sulfate, since it is highly soluble, nontoxic to
proteins and low-priced. Ammonium sulfate increases hydrophobic interactions between
protein molecules that result in their precipitation. The precipitation of proteins is dependent
on several factors such as protein concentration, pH and temperature.
ii) Organic solvents:
Ethanol, acetone and propanol are the commonly used organic solvents for protein
precipitation. They reduce the dielectric constant of the medium and enhance electrostatic
interaction between protein molecules that lead to precipitation. Since proteins are denatured
by organic solvents, the precipitation process has to be carried out below 0°C.
iii) Non-ionic polymers:
Polyethylene glycol (PEG) is a high molecular weight non-ionic polymer that can precipitate
proteins. It reduces the quantity of water available for protein solvation and precipitates
protein. PEG does not denature proteins, besides being non-toxic.
iv) Ionic polymers:
The charged polymers such as polyacrylic acid and polyethyleneimine are used. They form
complexes with oppositely charged protein molecules that causes charge neutralisation and
precipitation.
v) Increase in temperature:
The heat sensitive proteins can be precipitated by increasing the temperature.
31
vi) Change in pH:
Alterations in pH can also lead to protein precipitation.
vii) Affinity precipitation:
The affinity interaction (e.g., between antigen and antibody) is exploited for precipitation of
proteins.
Viii) Precipitation by ligands:
Ligands with specific binding sites for proteins have been successfully used for selective
precipitation.
5. Adsorption:
The biological products of fermentation can be concentrated by using solid adsorbent
particles. In the early days, activated charcoal was used as the adsorbent material. In recent
years, cellulose-based adsorbents are employed for protein concentration.
And for concentration of low molecular weight compounds (vitamins, antibiotics, peptides)
polystyrene, methacrylate and acrylate based matrices are used. The process of adsorption
can be carried out by making a bed of adsorbent column and passing the culture broth
through it. The desired product, held by the adsorbent, can be eluted. 32
Stage 4: Purification by Chromatography:
• The biological products of fermentation (proteins, pharmaceuticals, diagnostic compounds
and research materials) are very effectively purified by chromatography.
• Chromatography is basically an analytical technique dealing with the separation of closely
related compounds from a mixture. Chromatography usually consists of a stationary phase
and mobile phase.
• The stationary phase is the porous solid matrix packed in a column (equilibrated with a
suitable solvent) on to which the mixture of compounds to be separated is loaded. The
compounds are eluted by a mobile phase.
• A single mobile phase may be used continuously or it may be changed appropriately to
facilitate the release of desired compounds. The eluate from the column can be monitored
continuously (e.g. protein elution can be monitored by ultraviolet adsorption at 280 nm),
and collected in fractions of definite volumes.
33
The different types of chromatography techniques used for separation (mainly proteins) along
with the principles are given in Table 20.2. A large number of matrices are commercially
available for purification of proteins e.g., agarose, cellulose, polyacrylamide, porous silica,
cross- linked dextran, polystyrene. Some of the important features of selected
chromatographic techniques are briefly described.
34
i) Gel-filtration chromatography:
This is also referred to as size-exclusion chromatography. In this technique, the separation of
molecules is based on the size, shape and molecular weight. The sponge-like gel beads with
pores serve as molecular sieves for separation of smaller and bigger molecules. A solution
mixture containing molecules of different sizes (e.g. different proteins) is applied to the
column and eluted.
The smaller molecules enter the gel beads through their pores and get trapped. On the other
hand, the larger molecules cannot pass through the pores and therefore come out first with
the mobile liquid (Fig. 20.7). At the industrial scale, gel-filtration is particularly useful to
remove salts and low molecular weight compounds from high molecular weight products.
35
ii) Ion-exchange chromatography:
It involves the separation of molecules based on their surface charges. Ion-exchangers are of
two types (cation- exchangers which have negatively charged groups like carboxymethyl and
sulfonate, and anion- exchangers with positively charged groups like diethylaminoethyl (DEAE).
The most commonly used cation-exchangers are Dowex HCR and Amberlite IR, the anion-
exchangers are Dowex SAR and Amberlite IRA.
In ion-exchange chromatography, the pH of the medium is very crucial, since the net charge
varies with pH. In other words, the pH determines the effective charge on both the target
molecule and the ion-exchanger. The ionic bound molecules can be eluted from the matrix by
changing the pH of the eluant or by increasing the concentration of salt solution. Ion-exchange
chromatography is useful for the purification of antibiotics, besides the purification of
proteins.
36
iii) Affinity chromatography:
This is an elegant method for the purification of proteins from a complex mixture. Affinity
chromatography is based on an interaction of a protein with an immobilized ligand. The ligand
can be a specific antibody, substrate, substrate analogue or an inhibitor. The immobilized
ligand on a solid matrix can be effectively used to fish out complementary structures.
In Table 20.3, some examples of ligands used for the purification of proteins are given. The
protein bound to the ligand can be eluted by reducing their interaction. This can be achieved
by changing the pH of the buffer, altering the ionic strength or by using another free ligand
molecule. The fresh ligand used has to be removed in the subsequent steps.
37
Stage 5. Formulation:
• Formulation broadly refers to the maintenance of activity and stability of a
biotechnological products during storage and distribution.
• The formulation of low molecular weight products (solvents, organic acids) can be achieved
by concentrating them with removal of most of the water. For certain small molecules,
(antibiotics, citric acid), formulation can be done by crystallization by adding salts.
• Proteins are highly susceptible for loss of biological activity; hence their formulation
requires special care. Certain stabilizing additives are added to prolong the shelf life of
protein.
• The stabilizers of protein formulation include sugars (sucrose, lactose), salts (sodium
chloride, ammonium sulfate), polymers (polyethylene glycol) and polyhydric alcohols
(glycerol).
• Proteins may be formulated in the form of solutions, suspensions or dry powders.
38
1. Drying:
Drying is an essential component of product formulation. It basically involves the transfer of
heat to a wet product for removal of moisture. Most of the biological products of fermentation
are sensitive to heat, and therefore require gentle drying methods. Based on the method of
heat transfer, drying devices may be categorized as contact, convection, radiation dryers.
These three types of dryers are commercially available.
2. Spray drying:
Spray drying is used for drying large volumes of liquids. In spray drying, small droplets of liquid
containing the product are passed through a nozzle directing it over a stream of hot gas. The
water evaporates and the solid particles are left behind.
3. Freeze-drying/ Lyophilization
Freeze-drying or lyophilization is the most preferred method for drying and formulation of a
wide-range of products—pharmaceuticals, foodstuffs, diagnostics, bacteria, viruses. This is
mainly because freeze-drying usually does not cause loss of biological activity of the desired
39
product.
Lyophilization is based on the principle of sublimation of a liquid from a frozen state. In the
actual technique, the liquid containing the product is frozen and then dried in a freeze-dryer
under vacuum. The vacuum can now be released and the product containing vials can be
sealed e.g., penicillin can be freeze dried directly in ampules.
Source: http://www.biologydiscussion.com/biotechnology/downstream-processing/stages-in-
downstream-processing-5-stages/10160
40

Downstream Process

Uploaded by

Copyright:

Available Formats

Downstream Process

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Downstream Process

Uploaded by

Copyright:

Available Formats

Class: B.Sc.

(Hons) Botany, VI Semester

Dr. Preeti Rawat

Five stages in downstream processing after Fermentation:

2. Release of Intracellular Products

4. Purification by Chromatography and

i) Tubular bowl centrifuge (Fig. 20.4A):

Mechanical and non-mechanical methods:

iii) Forced film evaporators:

iv. Supercritical fluid (SCF) extraction:

You might also like