DRYING AS A UNIT OPERATION IN DOWNSTREAM PROCESSING

DRYING AS A UNIT OPERATION IN DOWNSTREAM PROCESSING Prof. Kehinde Taiwo Dept of Food Sci & Tech, OAU, Ile-Ife, Nigeria 0803 582 9554, kehindetaiwo 3@yahoo. com 1

rd 3 • • 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 • O 2 can be replaced by N 2 or CO 2 • 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, – extraction, – drying, – evaporation absorption, crystallization, mixing,

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 waterbinding 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 dryers Criterion Mode of operation Types Batch Continuous* Heat inputtype Convection* Conduction Radiation Electromagnetic fields Combination of heat transfer modes Intermittent or continuous* Adiabatic or non-adiabatic

Table 1 - Classification of dryers Criterion State of material in dryer Operating pressure Drying medium (convection) Drying temperature Types Stationary Moving, agitated, dispersed Vacuum* Atmospheric Air* Superheated steam Flue gases Below boiling temperature* Above boiling temperature Below freezing point

Table 1 - Classification of dryers Drying temperature Below boiling temperature* Above boiling temperature Below freezing point Relative motion between Co-current drying medium and drying solids Counter-current Mixed 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 backmixing 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

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