DRYING AS A UNIT OPERATION IN DOWNSTREAM PROCESSING

Prof. Kehinde Taiwo

Dept of Food Sci & Tech, OAU, Ile-Ife, Nigeria0803 582 9554, kehindetaiwo3@yahoo.com 1

3rd International Conference on Bioprocess & Engineering

• Double Tree by Hilton • Baltimore, Maryland, USA • 14 & 15 Sept. 2015• organised by OMICS Group Conferences

Introduction • All foods and biomaterials need some form of

preservation to–Reduce or stop spoilage –Make them available throughout the year –Maintain desired levels of nutritional and

bioactive properties for the longest possible time span and –Produce value added products

Downstream processing • Refers to the recovery of biomolecules from

natural sources such as animal or plant tissues besides fermentation broth

• It is an essential step which determines final cost of the product in the manufacture of biomolecules e.g.– Antibiotics, vaccines, antibodies, – Hormones (e.g. Insulin and human growth hormone) – Antibodies (e.g. Infliximab and abciximab), enzymes,

and – Natural fragrance and flavor compounds

Drying • Air-drying is an ancient preservation method • Foods are exposed to a continuously flowing

stream of hot air • It involves simultaneous mass and heat

transport • Moisture availability has a great impact on the

transfer of heat to microorganisms• Consumer demand has increased for

processed products that keep more of their original characteristics

Drying Methods• This requires the development of operations

that minimize the adverse effects of processing

• There have been various advances in the drying of foods with respect to quality, rehydration, and energy minimization

• Some of the improvements and advancements made leading to the new developments in drying are discussed

Intermittent batch drying• By varying the operating conditions of a

drying process – Airflow rate – Temperature – Humidity or – Operating pressure

• It can be monitored in order to reduce the operating cost e.g. thermal input and power input

Intermittent batch drying• The objective is to obtain high

energy efficiency without subjecting the product beyond its permissible temperature limit and stress limit while maintaining high moisture removal rate

Hybrid drying techniques• May include either use of –More than one dryer for drying of a

particular product (multi-stage drying)–More than one mode of heat transfer–Various ways of heat transfer or –Multiprocessing dryers

Hybrid drying techniques• For particulate drying – Variants of fluid bed or – Fluid bed with some other techniques can be used

in series to achieve faster drying

• For liquid feedstock – Generally spray drying is followed by the fluid bed

dryer

• To reduce moisture content to an acceptable level which is not possible by spray dryer alone

Modified atmosphere drying• The presence of oxygen results in various

unwanted characteristics in dried food materials –oxidation of the drying material–destruction of its bioactive compounds –browning

• O2 can be replaced by N2 or CO2

• In addition, it increases the effective moisture diffusivities of some food products

Superheated steam drying• Superheated steam does not contain oxygen,

hence oxidative or combustion reactions are avoided

• It also eliminates the risk of fire and explosion hazard

• It allows pasteurization, sterilization and deodorization of food and bio-products

• Net energy consumption can be minimized if the exhaust (also superheated steam) can be utilized elsewhere in the plant and hence is not charged to the dryer

Impinging stream drying• In Impinging stream dryers • The intensive collision of opposed streams

create a zone that offers very high heat, mass and momentum transfers

• Hence rapid removal of moisture from surface• Other advantages - low foot prints and high

robustness due to absence of moving parts• Effective alternatives to flash dryers for

particulate materials with very high drying loads

Contact sorption drying• The contact-sorption drying can be

achieved by • 1) contacting a wet material with heated

inert particles, thereby removing the moisture as a result of heat exchange or

• 2) contacting of wet material with heated sorbent particles where the moisture is transferred from wet solids to the sorbent particles

Contact sorption drying• A typical contact-sorption drying technique

involves good mixing of wet solid particles with the sorbent particles to achieve the heat and mass transfer and then separation of these two media

• The sorbent particles are regenerated and returned back to the dryer

• The typical inert sorbent particles (also called a carrier) are molecular sieves, zeolites, activated carbon, silica gel, etc.

Heat pump-assisted drying• Heat pump dryers use low temperature

dehumidified air as the convective drying medium

• It incorporates a dehumidification cycle, where condensation of water allows the removal of water from the closed system of drying air circulation

• The heat pump recovers the sensible as well as latent heat by condensing moisture from the drying air

• An auxiliary heater is generally added for better control of the temperature at dryer inlet

Radio frequency drying• Dielectric heating is the use of either microwave or

radio frequency (RF) technologies to heat materials

• Microwave and RF interact with individual molecules to quickly generate heat within a product

• This is in contrast to conventional heating where heat is applied externally

• A wet product submitted to a RF field absorbs the electromagnetic energy, so that its internal temperature increases

Radio frequency drying• If sufficient amount of energy is supplied, the

water is converted into steam, which leaves the product; and gets dried

• The amount of heat generated in the product is determined by the– Frequency– Square of the applied voltage – Dimensions of the product and – The dielectric "loss factor" of the material which is

essentially a measure of the ease with which the material can be heated by this method

Microwave drying• Microwave oven has ability to heat food products

rapidly, conveniently and economically in a compact space

• The primary drawback is its inability to heat materials in a predictable and uniform manner leading to

• -hot spots that damage the item being heated • - cold spots - under heated or under processed, thereby

compromising product quality and repeatability • Microwave heating in combination with vacuum has

been used extensively for drying in pharmaceutical processing

Drying in Downstream Processing• Process industries manufacture different

products from a variety of raw materials • The raw materials are pretreated and

conversion takes place in a reactor and separation of product of interest and its purification takes place in subsequent steps

• All the steps that are prior to the reactor form “upstream processing”

• All the steps after the reactor form “downstream processing”

Drying in Downstream Processing• In all the unit operations involved in

downstream and upstream processing only physical changes occur and do not involve chemical changes

• Unit operations for separation and purification during downstream processing include:– distillation, absorption, – extraction, crystallization, – drying, mixing, – evaporation

Downstream Processing Vs Analytical Bioseparation

• Both refer to the separation or purification of biological products, but at different scales of operation and for different purposes

• Downstream processing implies manufacture of a purified product for a specific use in marketable quantities

• Analytical bioseparation refers to purification for the sole purpose of measuring a component or components of a mixture, and may deal with sample sizes as small as a single cell

Complexity Of Downstream Processing • Two factors • 1) the desired product is generally present in low

concentrations and • 2) it is present along with several impurities or

undesired components• The economics of downstream processes are

determined by the required purity of the product which in turn depends on the applications of the product.

• As a result downstream processing mostly contributes 40-90 % of total cost

Applications in Downstream Processing• Thermal drying is more expensive than mechanical

dewatering • For dehydration of the biomass after harvest – Thermal drying should be preceded by a mechanical

dewatering step such as filtration or centrifugation• Harvesting generally results in a 50 to 200-fold

concentration of biomass• The harvested biomass slurry (5–15% dry solids) must be

processed rapidly, or it can spoil within a few hours in a hot climate

• The specific postharvest processing necessary depends strongly on the desired product

Applications in Downstream Processing• Membrane processes such as microfiltration,

ultrafiltration and reverse osmosis – the recovery and concentration of microbial

cells/biomolecules –enable volume reduction of slurry/solution

before downstream processing operations (chromatography, electrophoresis, freezing or freeze-drying )

• Drying methods include spray drying, drum drying, freeze-drying and sun drying

Additives/Carriers/Transporters• Use of additives offer protection to microorganisms

during drying• The choice of an appropriate carrier is important to

increase their survival rates during dehydration and subsequent storage

• Differences exhibited are related to their water-binding capacity and prevention of intracellular and extracellular ice crystal formation

• Additive materials increase the glass transition temperature and result in a dried product with increased stability and less hygroscopicity

• The characteristics of the transporters involved in sugar uptake lead to differences in their performance

Carriers/transporters• Protein (whey protein, skim milk)• Mrs-broth-based protectants • Sugars (e.g. maltodextrin, glucose,

fructose, lactose, mannose and sucrose)• Sugar alcohols (e.g. sorbitol and inositol)• Non-reducing sugars (e.g. trehalose)• Disaccharides give better viabilities after

freeze-drying than monosaccharides

Fig 1. Schematic diagram of a spray-drying process

Spray Drying - Advantages• Used to dry thermo-sensitive bioactive

compounds and probiotics • Increases surface to volume ratio of the liquid

particles and consequently enhance the heat and mass transfer during the drying process

• Continuous operation• Short time of contact with hot air • Drying taking place at wet bulb temperature • Process larger volumes and operate at higher

energy efficiency

Spray Drying -• Allows preparation of stable and functional

powder products • Can be implemented for large scale

throughputs • Main disadvantages • High installation costs• Removal of aromatic volatiles• Prone to damaging heat sensitive components

such as enzymes and probiotic bacteria

Process conditions in spray drying

• Air inlet temperature• Feed flow rate • Feed formulation• Out let air temperature and • Nozzle pressure • Affect • Retention of activity of bioactive compounds• Survivability of microorganisms

Process conditions• Low outlet temperature, lower residence

time, low nozzle pressure - good enzyme activity retention and survivability of microorganisms has been observed

• However, too low out let air temperature may result in higher residual moisture content leading to loss of viability and enzyme activity retention during storage

Selection of Dryers• Drying technologies have become more

diverse and complex• Dryer selection has become an

increasingly difficult task• The need to meet –Stricter quality specifications –Higher production rates –Higher energy costs and –Stringent environmental regulations

Selection of Dryers• Characteristics of different dryer types

should be recognized when selecting dryers

• Changes in operating conditions of the same dryer can affect the quality of the product

• The dryer type & right operating conditions for optimal quality and cost of thermal dehydration

Selection of dryers• Drying of products require adherence to Good

Manufacturing Practice and hygienic equipment design and operation

• Drying kinetics play a significant role• Location of the moisture (whether near

surface or distributed in the material)• Nature of moisture (free or strongly bound to

solid)• Mechanisms of moisture transfer (rate

limiting step)

Selection of dryers• Physical size of product• Conditions of drying medium (temperature,

humidity, flow rate of hot air for a convective dryer) • Pressure in dryer (low for heat-sensitive products)• Demands on product quality may not always permit

one to select the least expensive option based solely on heat and mass transfer considerations

• In the drying of non-aqueous (organic) solvent or a mixture of water (pharmaceutical products) with a solvent, care is needed to recover the solvent and to avoid potential danger of fire and explosion

Classification of dryers• Mode of operation• Heat input-type• State of material in dryer• Operating pressure• Drying medium• Drying temperature• Relative motion between drying medium

and drying solids• Number of stages

Table 1 - Classification of dryersCriterion Types

Mode of operation

BatchContinuous*

Heat input-type

Convection* Conduction Radiation Electromagnetic fields Combination of heat transfer modesIntermittent or continuous*Adiabatic or non-adiabatic

Table 1 - Classification of dryersCriterion TypesState of material in dryer

Stationary

Moving, agitated, dispersed

Operating pressure

Vacuum*Atmospheric

Drying medium (convection)

Air*Superheated steamFlue gases

Drying temperature

Below boiling temperature*Above boiling temperatureBelow freezing point

Table 1 - Classification of dryersDrying temperature Below boiling temperature*

Above boiling temperatureBelow freezing point

Relative motion betweendrying medium and drying solids

Co-currentCounter-currentMixed flow

Number of stages Single*Multi-stage Residence time Short (< 1 minute)Medium (1 – 60 minutes)Long (> 60 minutes)

* Most common in practice

Drying system includes

•Pre-drying stages •Post-drying stages

Drying system - Pre-drying stages

• E.g. - –Evaporation Mechanical dewatering

–Dilution Pelletization–Feeding Size reduction–Flaking Extrusion–Pre-conditioning of feed by solids

back-mixing with dry product

Drying system - post-drying stages

• Exhaust gas cleaning• Product collection• Partial recirculation of exhausts • Cooling of product• Coating of product • Agglomeration, etc.

• The optimal cost-effective choice of dryer will depend on these stages

Over-drying

• Increases the energy consumption • Increases drying time • Can be avoided by • Reducing the feed liquid content by less

expensive operations such as – Filtration– Centrifugation and – Evaporation

Future Potentials and Challenges• Downstream processing of biological

products has been affected by –The growth of the

biopharmaceutical industry–Drastically changing purity

expectations–Processing volume–Production flexibility to accommodate

new products

Future Potentials and Challenges• A volume-reduction step should achieve high

cell concentration, with minimal product loss or change in product quality even at large scale

• Such high cell concentrations can be achieved with appropriately sized systems and consideration of system hold-up volume

Future Potentials and Challenges

• Detailed knowledge on protein stability i.e. understanding of structural changes of biomolecules as a result of environmental influences can help in process design

• The product bioavailability challenge is more related to improving solubility which may play an important role as it may promote super saturation

• In the scale-up process control over particle size is a priority in spray drying

Conclusions• Drying of heat labile biological materials

preserves activity of enzymes/cells during storage and stabilize the bulk product until it can be formulated

• Drying becomes expensive unless the product is of high value and low volume

• Suitable drying methods need to be selected depending on the value of the product

•Thank you