The various procedure involved in the actual recovery of useful products after fermentation or any other process together constitute Downstream Processing. It is a very important step in the manufacture of different product in pharmaceutical industry (such as antibiotics, hormones, antibodies, vaccine, enzymes), Food industry etc. The product is either present in the cell, in the medium or both. The concentration of product is generally low, in either cases, and it is mixed with other molecules from which it has to be separated.
Industrial
fermentations has both upstream processing (USP) and downstream processing
(DSP) stages. USP involves all factors and processes leading to, and
including, the fermentation, and consists of three main areas.
1. First
aspect of USP deals with the producer microorganism. They include
obtaining a suitable microorganism, industrial strain improvement to enhance
productivity and yield, maintenance of strain purity, preparation of a suitable
inoculum and the development of selected strains to increase the economic
efficiency of the process.
2. Second
aspect of USP involves fermentation media, especially the selection
of suitable cost-effective carbon and energy sources, along with other
essential nutrients. Media optimization is a vital aspect of process
development to ensure maximum yield and profit.
3. The
third component of USP relates to the fermentation, which is
usually performed under controlled conditions. The ultimate aim is to optimize
the growth of the organism or the production of a target microbial product.
DSP
includes all processes following the fermentation. It aims at efficient,
safe and reproducible recovery of the target product
with biological activity, purity, etc. The target product may be
recovered by processing the cells or the spent medium depending upon whether it
is an intracellular or extracellular product. Maximum recovery yield and
minimum costs are therefore important to be ensured.
Two
critical factors to be remembered during DSP
· The
level of purity of product which is usually determined by the
specific use of the product.
· The product’s
biological activity is very important and must be retained.
Fermentation
factors affecting DSP include
· The properties
of microorganisms, particularly morphology, flocculation characteristics,
size and cell wall rigidity. These factors have major influences on the
filterability, sedimentation and homogenization efficiency.
· The presence
of fermentation by-products, media impurities and fermentation additives,
such as antifoams, may interfere with DSP steps and accompanying product
analysis.
· Choice
of fermentation substrate influences subsequent DSP. A cheap carbon and
energy source containing many impurities may provide initial cost savings, but
may require increased DSP costs. Overall cost savings may be achieved with a
more expensive but purer substrate.
· The physical
and chemical properties of the product, along with its concentration and
location, are key factors as they determine the initial separation steps
and overall purification strategy. It may be the whole cells themselves that are
the target product or an intracellular product, possibly located within an
organelle or in the form of inclusion bodies. Alternatively, the target product
may have been secreted into the periplasmic space of the producer cells or the
fermentation medium.
· Stability
of the product also influences the requirement for any pretreatment
necessary to prevent product inactivation and/or degradation.
· Adopting methods
that use existing available equipment may be more cost-effective than
introducing more efficient techniques which may need new facilities.
Steps
in DSP
DSP
can be divided into a series of distinct unit processes linked together to
achieve product purification.
The number
of steps is kept to a minimum to reduce cost. Also, even though individual
steps may obtain high yields, the overall losses of multistage purification
processes may be huge. So, lesser the treatment steps, lesser the overall loss
and cost.
The
specific unit steps chosen will be influenced by the economics of the
process, the required purity of the product, the yield attainable at each step
and safety aspects.
In
many cases, integration of fermentation and DSP is preferred to increase
productivity, decrease the number of unit operations and reduce both the
overall time and costs.
The
fermentation broth at the time of harvesting is a complex heterogeneous mixture
of cell debris, metabolic products, and unused portions of the medium. The
required product usually forms a small proportion of the broth. The product
required could be the cells themselves such as in yeast manufacture, or within
the cells (such as in streptomycin or some enzymes) or free in the medium as
with penicillin.
In a
few cases no separation takes place such as in the acetone butanol
fermentation, where the entire beer is used. In most cases, however, the
separation methods used are filtration, centrifugation, decantation, and foam
fractionation. Where the required fraction is in the cells then much of the
impurities are removed with the filtrate after the cells have been
isolated.
The
various methods used in solids removal are discussed below with the approximate
level of purification obtained in each stage is shown below. The procedure
followed within each stage depends on the material being extracted.
Step
Process |
Recovery/Purity
(%) |
1a.
Cell harvesting |
0.1-1.0
(if product is soluble) |
90-99
(if product is cell such as yeasts) |
|
Filtration |
|
Centrifugation |
|
Decantation |
|
1b.
Disruption of cells |
|
2.
Primary isolation of the product |
1-10 |
Adsorption
and/or ion exchange |
|
Solvent
extraction |
|
Precipitation |
|
Ultracentrifugation |
|
3.
Purification |
50-80 |
Fractional
precipitation |
|
Chromatography
(adsorption, partition, ion exchange, affinity) |
|
Chemical
derivatization |
|
Decolorization |
|
4.
Final product isolation |
90-100 |
Crystallization |
|
Drying |
|
Solvent
removal |
|
DSP
for the purification of products in the soluble portion of fermented broth
The various steps
of Downstream Processing involve:
- Separation
- Cell
disruption
- Extraction
- Isolation
- Purification
- Drying
1.
SEPARATION OF PARTICLES
· It is first step
of DSP and usually involve the separation of solids substances, from the liquid
media.
· Recovery of
extracellular proteins is from the clarified medium, whereas disrupted cell
preparations are used for both intracellular proteins and those within the
periplasmic space.
· In the case of
some recombinant proteins expressed at high levels, they sometimes form
inclusion bodies that are released by cell breakage.
·
Following
cell disruption, soluble proteins are usually separated from cell debris by
- v FILTRATION– it is used for
filamentous fungi and bacteria.
- Surface filtration
- Depth filtration
- Centrifugal filtration
- Rotating drum vacuum filtration
Filtration
Conventional
filtration of liquids containing suspended solids involves depth filters
composed of porous media (cloth, glass wool or cellulose) that retain the
solids and allow the clarified liquid filtrate to pass through. As filtration proceeds
collected solids accumulate above the filter medium, resistance to filtration
increases and flow through the filter decreases. These techniques are generally
useful for harvesting filamentous fungi, but are less effective for collecting
bacteria. The two main types of conventional filtration commonly used in
industry are as follows.
1
Plate and frame filters or filter presses, which are industrial batch filtration systems. They are normally in the
form of an alternating horizontal stack of porous plates and hollow frames. The
stack is mounted in a support structure where it is held together with a hydraulic
or screw ram. Filter cloths are held in place between the plates that
contain flow channels for the feed and permeate streams. This essentially forms a series of cloth-lined
chambers into which the cell suspension is forced under pressure. Following
batch filtration, the apparatus must be dismantled to remove the collected
filter cake. These systems are used for harvesting microorganisms from
fermentations, including the preparation of blocks of baker’s yeast, the
recovery of protein precipitates and the dewatering of sewage sludge. Similar
horizontal and vertical pressure leaf filters are also available.
2
Rotary vacuum filters are simple continuous filtration systems that are used in several industrial
processes, particularly for harvesting fungal mycelium during antibiotic manufacture,
for baker’s yeast production and in dewatering sludge during waste-water
treatment. The device comprises a hollow perforated drum that supports the
filter medium. This drum slowly rotates in a continuously agitated tank
containing the suspension to be filtered. Solids accumulate on the filter medium
as liquid filtrate is drawn, under vacuum, through the filter medium into the
hollow drum to a receiving vessel. As the drum rotates, collected solids held
on the filter medium are removed by a knife that cuts/sloughs them off into a
collection vessel.
Filter media may be precoated
with a filter aid, e.g. Kieselguhr (diatomaceous earth), which can be
continuously replenished. A filter aid improves filtering efficiency by
building up a porous, permeable and rigid structure—the filter cake—that
retains solid particles and helps flow control.
The rate of filtration (flow of filtrate), is determined
by the resistance of the cake and the filtration medium. In terms of
biosafety, neither filtration system is suitable for processing toxic products,
pathogens or certain recombinant DNA microorganisms and their products.
Membrane
Filtration
Modern
methods of filtration involve absolute filters rather than depth
filters. These consist of supported membranes with specified pore sizes that
can be divided into three main categories. They
are, in decreasing order of pore size, microfiltration, ultrafiltration
and reverse osmosis membranes. The suspension to be filtered is pumped
across the membrane (cross-/tangential-flow) rather than at a right angle to
it, as occurs with conventional filtration methods. This slows fouling of the membrane
by particulate materials. Particles whose size is below the membrane ‘cut-off’
will pass through the membrane to become the ultrafiltrate or permeate,
whereas the remainder is retained as the retentate.
Microfiltration
is
used to separate particles of 10-2μm to 10 μm, including removal of microbial
cells from the fermentation medium. This method is relatively expensive due to the high cost of membranes, but
it has several advantages such as quiet operation, lower energy requirements, the
product can be easily washed, good temperature control is possible, containment
is readily achieved and no bioaerosols are produced. Consequently, it is suitable
for handling pathogens and recombinant microorganisms.
Ultrafiltration
is
similar to microfiltration except that the membranes have smaller pore sizes, and are used to fractionate
solutions according to molecular weight, normally within the range
2000–500000Da. The membranes are composed of a thin membrane with pores of
specified diameter providing selectivity, lying on top of a thick, highly
porous, support structure. A membrane manufactured with an exclusion size of
100000Da, for example, should produce a retentate of proteins and other
molecules over 100000 Da and an ultrafiltrate of all molecules below 100000 Da.
Several of these ultrafiltration units can be linked together to produce a sophisticated purification system. These
methods are extensively employed for the purification of proteins, and for
separating and concentrating materials.
Ultrafiltration is also effective
in removing pyrogens (bacterial cell wall lipopolysaccharides), cell debris and
viruses from media, and for whey processing. Another variation on the
ultrafiltration system is diafiltration, where water or other liquid is
filtered to remove unwanted low molecular weight contaminants. This can be used
as an alternative to gel filtration or dialysis for removing ammonium sulphate
from a protein preparation precipitated by this salt, for changing a buffer or
in water purification.
Reverse
osmosis is
used for dewatering or concentration steps and has been employed to desalinate sea water for drinking. In
osmosis water will cross a semipermeable membrane if the concentration of
osmotically active solutes, such as salt, is higher on the opposite side of the
membrane. However, if pressure is applied to the ‘salt side’ then reverse
osmosis will occur, and water will be driven across the membrane from the salt
side. This reversal of osmosis requires a high pressure, consequently, a strong
metal casing is required to house this equipment.
- Flocculation and flotation–
It is used for small bacterial cells which are difficult to separate even
by centrifugation.
- Flocculation: It involves
the aggregation of cells which may be induced by inorganic salt, minerals,
hydrocolloids and organic polyelectrolytes.
- Floatation: If
flocculation isn’t effective then very minute gas bubbles is made by
sparging. The gas bubble adsorbs and surround the cell, raising the gas
bubbles to the surface of media in the form of foam.
· The resultant
supernatant, containing the proteins, is then processed by several different
types of methods, such as precipitation/salting out of proteins and
ultrafiltration.
· Ammonium sulphate is popular
because of its high solubility, low toxicity and low cost. The solubility of
the salt varies with temperature, so strict temperature control is required.
Reduction of protein solubility can also be achieved by adding organic
solvents, such as acetone, ethanol and isopropanol. This is performed at low
temperature causing precipitation of the proteins.
Higher-speed
centrifuges are required for the separation of smaller microorganisms, such as
bacteria, compared with yeasts. For example, relatively slow centrifugation
effectively recovers residual yeast cells remaining in beer after the bulk has
sedimented out. But, an RCF of 20000g may be required to recover
suspended bacterial cells, cell debris and protein precipitates from liquid
media. Advantages of centrifugation include the availability of fully
continuous systems that can rapidly process large volumes in small volume
centrifuges. Centrifuges are steam sterilizable, allowing aseptic processing,
and there are no consumable costs for membranes, chemicals or filter aids.
However,
the disadvantages of centrifugation are the high initial capital costs, the
noise generated during operation and the cost of electricity. Also, physical
rupture of cells may occur due to high shear and the temperature may not be
closely controllable, which can affect temperature-sensitive products. Bioaerosol
generation is a further major disadvantage, particularly when centrifuges are
used for certain recombinant DNA organisms or pathogens. Under these circumstances
the equipment must be contained
Centrifuges
can be divided into small-scale laboratory units and larger pilot- and
industrial-scale centrifuges. Laboratory batch centrifuges include, bench-top,
high-speed and ultracentrifuges, capable of applying RCFs of 5000– 500000g.
Although industrial batch centrifuges are available, for most industrial
purposes semicontinuous and continuous centrifuges are required to process the large
volumes involved.
1.
Tubular centrifuges
usually produce the highest centrifugal force of 13000–17000g. They have hollow
tubular rotor bowls providing a long flow path for the suspension, which is
pumped in at the bottom and flows up through the rotor. Particulate material is
thrown to the side of the bowl, and clarified liquid passes out at the top for
continuous collection. Since the particulate material accumulates on the inside
of the bowl, there must be periodic removal of solids for efficient working
2.
CELL DISRUPTION
- Disruption
of microbial cell is usually difficult because of their small size, rigid
cell walls and high osmotic pressure inside the cells. This is required in
case of recovery of intracellular products.
Disruption of cell
is generally achieved by mechanically, lysis or drying:
- Mechanical
cell disruption:
this involves the uses of shear, E.g.-, colloid mill, ball mill grinder
etc., homogenizer and ultrasound.
- Drying: it involves
the drying of cells by adding the cells into a huge amount of cold acetone
and extracted using buffer or salt solution.
- Lysis: lysis of
microbial cells may be achieved by chemical means, e.g., salt or
surfactants, osmotic shock, freezing, or Lytic enzymes, e.g., lysozyme,
etc.
Some
target products including many enzymes and recombinant proteins, form inclusion
bodies and are intracellular. Therefore, methods are required to disrupt the
microorganisms and release these products. The breaching of the cell
wall/envelope and cytoplasmic membrane can pose problems, when cells possess
strong cell walls. General problems associated with cell disruption include the
liberation of DNA, which can increase the viscosity of the suspension. This may
also affect further processing, such as pumping the suspension on to the next
unit process and flow through chromatography columns. A nucleic acid
precipitation step or the addition of DNase can help to prevent this problem.
If mechanical disruption is used then heat is invariably generated, which
denatures proteins unless appropriate cooling measures are implemented. Cell
disruption can be achieved by both mechanical and non-mechanical methods. The
disruption process is often quantified by monitoring changes in absorbance, particle
size, total protein concentration or the activity of a specific intracellular
enzyme released into the disrupted suspension.
Several
mechanical methods are available for the disruption of cells. Those based on solid
shear involve extrusion of frozen cell preparations through a narrow orifice
at high pressure. This approach has been used at the laboratory scale, but not
for large-scale operations.
Ultrasonic
disruption of
cells involves cavitation, microscopic bubbles or cavities generated by
pressure waves. It is performed by ultrasonic vibrators that produce a
high-frequency sound. However, this technique also generates heat, which can
denature thermolabile proteins. Rod-shaped bacteria are often easier to break
than cocci, and Gram-negative organisms are more easily disrupted than
Gram-positive cells. Sonication is effective on a small scale, but is not
routinely used in large-scale operations, due to problems with the transmission
of power and heat dissipation.
An
alternative to mechanical methods of cell disruption is by autolysis, osmotic
shock, rupture with ice crystals (freezing/thawing) or heat shock. Autolysis,
for example, has been used for the production of yeast extract and other yeast
products. It has the advantages of lower cost and uses the microbes’ own
enzymes, so that no foreign substances are introduced into the product.
- It
involves the recovery of a compound or a group of compounds from a mixture
or from cells into a solvent phase.
- It
usually achieved both separation of particles as well as concentration of
the product.
- The
process of extraction is frequently useful for the recovery of Antibiotics
and most of the lipophilic substances.
- Liquid-liquid extraction
- Whole broth (medium + cells)
extraction
- Aqueous multiphase extraction
Aqueous
two-phase separation
involves partitioning the protein between the two phases, depending upon its
molecular weight and charge. Commonly used systems include dextran and
polyethylene glycol (PEG), or PEG and potassium phosphate. The two phases can
then be separated by centrifugation. This method is cheap, gentle and
versatile, and can be scaled up for industrial applications, including the
purification of enzymes. Many alkaloids, antibiotics, steroids and vitamins are
recovered by liquid–liquid extraction methods using organic solvents.
Antibiotics, for example, are extracted from culture media into solvents such
as amyl acetate.
Those
solvents used should be non-toxic, selective, inexpensive, immiscible with
broth and must have a high distribution coefficient for the product, i.e. the
ratio of the product in the two phases.
Some
of the product concentration may occur during the extraction step.
- It
aims at recovery of the product in a highly purified state. Purification
is achieved by
The different
chromatographic procedures are:
- Adsorption
- Ion exchange
- Gel filtration
- Hydrophobic
- Affinity
- Partition chromatography.
Chromatographic
techniques are usually employed for higher-value products. These methods,
normally involving columns of chromatographic media (stationary phase), are
used for desalting, concentration and purification of protein preparations. In
choosing a chromatographic technique for protein products, molecular weight,
isoelectric point, hydrophobicity and biological affinity should be taken into
consideration. Each of these properties can be exploited by specific
chromatographic methods.
The
order and choice of technique will depend on the particular product, but the
following chromatographic parameters should be considered: capacity, recovery
and resolving power (selectivity). The capacity refers to the sample size that
can be applied to the system in terms of protein concentration and volume, and
the recovery is the yield of product at each stage. Yield values should be as
high as possible otherwise the overall process will be uneconomic. Resolving
power and selectivity relate to the ability to separate two components, one
being the product and the other the impurity. This is particularly important at
the final purification stage. Care must be taken not to allow a protein to
denature during purification, so columns are usually run at 4°C. Changes of pH,
excessive dilution and addition of certain chemicals can also affect the
stability of the protein.
1.
Adsorption chromatography separates
according to the affinity of the protein, or other material, for the surfaces
of the solid matrix. Alumina (Al2O3), hydroxyapatite (Ca10(PO4)6(OH)2) or
silica (SiO2) are used for purifying non-polar molecules, whereas polystyrene based
resins are effective matrices for polar molecules. This technique involves
hydrogen bonding and/or van der Waals forces. The adsorbed proteins are usually
eluted by increasing the ionic strength, often by using a gradient of
increasing phosphate ion concentration.
2.
Affinity chromatography is a particularly
powerful and highly selective purification technique. It can often give up to
several thousand-fold purification in a singlestep. However, this
chromatographic method is expensive on an industrial scale. The technique
involves specific chemical interactions between solute molecules, such as
proteins, and an immobilized ligand (functional molecule). Ligands are
covalently linked to the matrix material, e.g. agarose. Some ligands interact
with a group of proteins, notably nicotinamide adenine dinucleotide, adenosine
monophosphate, and Procion and Cibacron dyes; other ligands are highly
specific, especially substrates, substrate analogues and antibodies.
Since
monoclonal antibodies have become more readily available, immunoaffinity
chromatography methods have been developed for the purification of various
antigens. The loading capacity of affinity columns is large, as the volume of
the sample is unimportant and high resolution can be attained. This is also a
high-speed technique and elution is achieved using specific cofactors or
substrates; alternatively, non-specific elution may be performed with salt or
pH change. For industrial-scale operations, it is often necessary to sterilize
the chromatographic media in order to comply with various regulatory
requirements. This can be problematical as some ligands are sensitive to
sterilizing agents.
3.
Gel filtration chromatography involves
separation on the basis of molecular size (molecular sieving), although
molecular shape can also influence separation performance. Consequently, it is
particularly useful for desalting protein preparations. The stationary phase
consists of porous beads composed of acrylic polymers, agarose, cellulose,
cross-linked dextran, etc., which have a defined pore size. These support
materials should be sterilizable, chemically inert, stable, highly porous and
hydrophilic, containing some ionic groups. Mechanical rigidity is particularly
important in order to maintain good flow rates. The initial choice of
stationary phase material is also a key factor, as some may interact with the
target product, e.g. carbohydrate-based matrices interact with glycoproteins.
Solute
molecules below the exclusion size of the support material pass in and out of
the beads. Molecules above the exclusion size pass only around the outside of
the beads through the interstitial spaces and the apparent volume of the column
is smaller for these larger excluded molecules. As a result, they flow faster
down the column, separating from smaller molecules and eluting first. Smaller
molecules able to enter the pores are then eluted in decreasing order of size.
Ion-exchange chromatography involves the selective adsorption of ions or
electrically charged compounds onto ion-exchange resin particles by
electrostatic forces. The matrix material is often based on cellulose
substituted with various charged groups, either cations or anions.
A
commonly used example is the anion-exchange resin diethyl aminoethyl (DEAE)
cellulose. Proteins possess positive, negative or no charge depending on their
isoelectric point (pI) and the pH of the surrounding buffer. If the pH of the
buffer is below the pI, a protein has an overall positive charge, whereas a buffer
at the pI results in the protein having no charge and pHs above its pI produce
an overall negative charge. For example, if a protein has a pI of 4.2, at pH4.2
this protein is uncharged and will not bind to either positively or negatively
charged resins. When the pH is raised to pH7.0 the protein is negatively
charged and binds to positively charged resins. In conditions below pH4.0 the
protein is positively charged and binds to negatively charged resins. If the
protein is in an anionic state able to adsorb to DEAE cellulose, for example,
any contaminants pass through the column. The product can then be desorbed as a
purified fraction by altering the ionic strength of the buffer, often by using
a gradient of increasing phosphate buffer concentration.
4.
High-performance liquid chromatography (HPLC) was originally
developed for the separation of organic molecules in non-aqueous solvents, but
is now used for proteins in aqueous solution. This method uses densely packed
columns containing very small rigid particles, 5–50µm diameter, of silica or a
cross-linked polymer. Consequently, high pressures are required. The method is
fast and gives high resolution of solute molecules. Equipment for use in
large-scale operations is now available.
5.
Hydrophobic chromatography relies on
hydrophobic interaction between hydrophobic regions or domains of a solute
protein and hydrophobic functional groups of the support particles. These
supports are often agarose substituted with octyl or phenyl groups. Elution
from the column is usually achieved by altering the ionic strength, changing
the pH or increasing the concentration of chaotropic ions, e.g. thiocyanate.
This technique provides good resolution and, like ion-exchange chromatography,
has a very high capacity as it is not limited by sample volume.
6.
Metal chelate chromatography utilizes a matrix
with attached metal ions, e.g. agarose containing calcium, copper or magnesium
ions. The protein to be purified must have an affinity for this ion and binds
to it by forming coordination complexes with groups such as the imidazole of histidine
residues. Bound proteins are theneluted using solutions of free metal-binding
ligands, e.g. amino acids.
5.
DIALYSIS & ELECTRODIALYSIS
These
membrane separation techniques are primarily used for the removal of low
molecular weight solutes and inorganic ions from a solution. The membranes
involved are size selective with specific molecular weight cut-offs. Low
molecular weight solutes move across the membrane by osmosis from a region of
high concentration to one of low concentration. Electrodialysis methods
separate charged molecules from a solution by the application of a direct
electrical current carried by mobile counter-ions. Membranes used contain
ionexchange groups and have a fixed charge; e.g. positively charged membranes
allow the passage of anions and repel cations. They are essentially
ion-exchange resins in sheet form and have also been used for the desalination
of water.
6.
DISTILLATION
Distillation
is used to recover fuel alcohol, acetone and other solvents from fermentation
media, and for the preparation of potable spirits. Batch distillation in pot
stills is used for the production of some whiskies, but with ethanol continuous
distillation yields a product with 96.5% (v/v) ethanol concentration. This
azeotropic mixture is the highest concentration that can be achieved from
aqueous ethanol.
Some
continuous stills may be in the form of four or five separate columns, but the
Coffey-type still comprises just two columns, the ‘rectifier’ and ‘analyser’,
each containing a stack of 30–32 perforated plates. Incoming fermentation broth
is heated, as it passes down a coiled pipe within the rectifier column, by the
ascending hot vapour produced by the analyser column. The now hot broth is
released into a trough at the top of the analyser column and as it falls down
the column it is heated by steam. Hot vapours generated are then conveyed from
the top of the analyser column to the bottom of the rectifier column. As it
passes upwards it is condensed on the coils carrying incoming broth. There is a
temperature gradient in the rectifier column and each volatile compound
condenses at its appropriate level, from where the fraction is collected.
7.
Finishing Steps
Product
crystallization may be achieved by evaporation, low-temperature treatment or
the addition of a chemical reactive with the solute. The product’s solubility
can be reduced by adding solvents, salts, polymers (e.g. nonionic PEG) and
polyelectrolytes, or by altering the pH.
- Drying
is the most important step in downstream processing which makes the
product suitable for handling and storage.
The most frequent
approaches of drying are:
- Vacuum drying
- Spray drying
- Freeze drying.
· Rotary drum driers remove water by
heat conduction. A thin film of solution is applied to the steam heated surface
of the drum, which is scraped with a knife to recover the dried product.
· In vacuum tray
driers the material to be dried is placed on heated shelves within a chamber to
which a vacuum is applied. This allows lower temperatures to be used due to the
lower boiling point of water at reduced pressure. The method is suitable for small
batches of expensive materials, such as some pharmaceuticals.
· Spray drying involves
atomization and spraying of product solution into a heated chamber, and
resultant dried particles are separated from gases using cyclones. Pneumatic
conveyor driers use hot air that suspends and transports particles.
·
Freeze-drying (lyophilization) is often used
where the final products are live cells, as in starter culture preparations, or
for thermolabile products. This is especially useful for some enzymes, vaccines
and other pharmaceuticals, where retention of biological activity is of major importance.
In this method, frozen solutions of antibiotics, enzymes or microbial cell
suspensions are prepared and the water is removed by sublimation under vacuum,
directly from solid to vapour state. This method eliminates thermal and osmotic
damage.