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Book chapter Spray drying
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CHAPTER – 4
Spray DryingM. Amdadul Haque1,Yakindra Prasad Timilsena1 and Benu Adhikari1
Abstract
Spray drying is one of the most preferred methods of converting food andpharmaceutical solutions into powder form. Very high product throughput,hygienic and highly automated process and relatively benign thermal anddehydration stress on products associated with this process has made itubiquitous in powder manufacture. Compared to freeze drying, spray dryingis significantly less expensive both in terms of operation and capital costs.In this chapter, key features of spray drying such as underlying operationalprinciple, types of spray nozzles used, the flow pattern of droplets/particlesand drying medium within the drying chamber, the particle formationprocess of individual droplet are discussed in considerable detail. Themodelling aspect of a single droplet subjected to spray drying environmenthas also been included. Finally, the most commonly consideredcharacteristics of spray dried powders including morphology, crystalline/amorphous nature and solubility are highlighted and discussed. The contentpresented in this chapter will be very useful for practising engineers,technologists, researchers as well as postgraduate and undergraduatestudents.
1 Introduction
Drying is one of the oldest technologies of food preservation. It involves removalof water from food materials to lower the free moisture content to an acceptablelevel. The removal of free moisture lowers the water activity and thus preventsthe microbial growth and other moisture mediated deterioration. It substantiallyreduces the packaging, storage and transportation costs and also enables thestorability of product at ambient conditions (Jayaraman and Das Gupta, 1992).As a consequence, several advances in various aspects of drying technologyhave been taken place and some novel and hybrid drying technologies have
Drying Technologies for Foods: Fundamentals & Applications Pp79-106© Editors, Prabhat K. Nema,Barjinder Pal Kaur,Arun S. Mujumdar, 2015New India Publishing Agency, New Delhi - 110034, India
1School of Applied Sciences, RMIT University, VIC 3001, Australia
80 Drying Technologies for Foods
been developed to improve the efficiency of drying process and to improve theproduct quality and uniformity. Drying technology includes solar drying, cabinetor tray drying, spray drying, fluidized bed drying, foam-mat drying, vacuumdrying and freeze drying and some hybrid drying technologies that combineultrasound and microwave etc.with above mentioned more established dryingmethods (Mothibe et al., 2011).The choice of an appropriate drying methoddepends on the type and characteristics of raw material and the desired attributesof the final product.
2 Spray Drying
Spray drying involves the atomization and drying (particle formation) ofaqueous or organic solutions to produce solid material in a single unit operationusing hot air or inert gas. This technology is suitable for continuous conversionof the above mentioned solutions into dry particulate form. Solutions,emulsions,suspensions, slurries and pastes can be conveniently spray dried provided thatthey are pumpable and capable of being atomized (Bankar et al., 2014).Depending on the feed composition and operating conditions, the finalparticulates can be very fine nano-sized powders (210-280 nm), fine powdersin micron size (10-50 m) or agglomerates (up to 3mm) (Gharsallaoui et al.,2007; Jafari et al., 2008). The appearance, flow property, compressibility, bulkdensity, dispersibility, solubility, nutritional value and storage stability of spraydried powders depend on the nature of the material and the spray dryingparameters. One of the most remarkable advantages of the spray drying is itscapacity to process several kinds of materials to produce fairly dried productwith pre-specified properties. Therefore, this technology is widely used inseveral industrial sectors including food, pharmaceutical, biotechnology andchemical industries. Many biological and thermally-sensitive materials suchas milk, fruit juices and pulps, herbal extracts, enzymes, essential oils, aromacompounds and various pharmaceutical drugs have been dried by this process(Filkova et al., 2006). In fruit and vegetable processing, spray drying is usuallysuitable for producing powders from the fruit and vegetable juice or concentrates(Jayasundera et al., 2010).
3 History
The spray drying technology was first successfully utilized in 1901 for theproduction of milk powder from liquid milk (Parihari, 2009). The developmentof nozzle in 1913 for atomization further advanced the spray drying technology(Masters, 1997). The technology was successfully utilized in pharmaceuticalindustries since early 1940s for the production of dry substances and variousexcipients (Ré, 2006). Spray drying has been used in food industry since 1950s
Spray Drying 81
to encapsulate flavour compounds and oils to protect them from loss due tovolatilization and oxidative degradation and thus facilitates handling and furtherprocessing. Due to continuous advances in the technology and the underpinningscience, spray drying has now been established as one of the most convenientmethods of drying of heat-sensitive biological materials such as enzymes andpharmaceutical proteins (Bowen et al., 2013; Donz et al., 2014).
4 Advantages and Disadvantages of Spray Drying
Among the various drying methods employed in food and pharmaceuticalindustries, spray drying is one of the most popular methods and is widely usedto manufacture powders of varying functional properties. The advantages anddisadvantages of this technology are summarized in the ensuing section.
4.1 Advantages
The widespread application of spray drying technology is due to its inherentadvantages. Some of the advantages of this technology are listed below:
i) Drying is instant and produces more or less uniform and spherical particlesfrom nano scale to micron scale sizes with some greater control over sizeand morphology.
ii) It can be easily scaled up. Feed rates can be varied from a few kilogramsper hour to over 100 tons per hour.
iii) The actual spray drying process is very rapid, with the major portion ofevaporation taking place in less than a few seconds. Therefore, the thermaldegradation, denaturation and loss of the nutrients are minimal. It makesthe drying of heat sensitive materials possible with maximum retentionof the active constituents.
iv) Operation is continuous and adaptable to full automatic control.
v) Various designs of the spray dryers are available to meet various productspecifications.
vi) Solutions, slurries, pastes, gels, suspensions and even melts can be spraydried.
vii) Control over the particle size, bulk density, and degree of crystallinity isachievable to a greater extent.
viii) It is reasonably economical, for example, it is 4 to 7 times economicalcompared to freeze drying (Chavez and Ledeboer, 2007).
82 Drying Technologies for Foods
ix) It also allows simultaneous and in situ mixing, coating, complexcoacervation and drying of two different feed solutions.
4.2 Disadvantages
Despite its numerous advantages,this technology comes with the followingdisadvantages.
i) Industrial spray dryers, especially with large capacity, are becomingsophisticated in terms of structure and control mechanism. Thus, theyrequire large initial capital investment.
ii) The thermal efficiency of spray drying process is also relatively poorunless the difference between inlet and outlet temperature is very large(Masters, 1997; Bankar et al., 2014).
iii) In some cases, recovery of the dried powders is difficult and the loss ofproduct occurs due to the escape of fine particles with exhaust air and/orsignificant deposition of the product on the wall of the drying chamber.For example, wall deposition is a major problem in spray drying of sugarand acid-rich products.
iv) Spray drying requires specifically trained technical manpower forsuccessful operation and maintenance.
5 Working Principle of Spray Drying Process
During spray drying, highly dispersed liquid droplets are brought into contactwith a sufficient volume of hot air to achieve rapid evaporation of solvent sothat the solid particles can be collected. The exhaust air or inert gas laden withmoisture or other solvent is allowed to escape the drying chamber. Fig. 1represents the schematic diagram of the spray drying process. The method ofatomization and the arrangement of flow of droplets/particles and the dryingmedium within the drying chamber (co-current, counter-current or mixed mode)are important for minimizing the thermal and dehydration stresses in the product.It is always desirable that the outlet temperature is always held low and thedrying is achieved as quickly as possible (Masters, 1997). The evaporativecooling associated with the moisture transfer from droplets to drying mediumkeeps the droplet/particle well below the inlet temperature and even below theoutlet temperature in most of residence time. Therefore, the thermal degradationof even highly heat sensitive materials such as proteins can be minimized inspray drying process (Broadhead et al., 1992).
Spray Drying 83
Spray drying is a mostly convective drying process. There are four fundamentalsteps involved in spray drying (Broadhead et al., 1992; Filkova et al., 2006):(1) Atomization of a liquid feed into fine droplets, (2) Droplet-hot air contact(3) Evaporation of droplet water and (4) Recovery of the powder. In addition,the feedstock is normally concentrated prior to introduction into the spray dryer.The higher solid content in the feed reduces the amount of water (or solvent)that must be evaporated and hence improves the energy efficiency. Each ofthese steps is described in detail in the subsequent section.
5.1 Atomization
Atomization is the first step in spray drying. It involves atomizing and dispersingthe feed into millions of fine droplets to greatly increase the surface area andthereby to greatly increase the heat and mass transfer (Hede et al., 2008). Weprovided an example (Table 1) of the extent of increase in the surface areawhen the droplet size is successively decreased. The surface area of ahypothetical spherical droplet having a volume of 0.524 m3 and diameter of1m increases 10,000 times when it is broken down into uniform sphericaldroplets of 100 micron size (Table 1). The higher the energy applied to achieveatomization, the smaller will be the droplets/particles (Gharsallaoui et al., 2007).Atomization is the most critical step for achieving better operational economyand high product quality. The size distribution of the liquid droplet and the
Fig. 1: A schematic diagram of a single stage spray dryer
Air exhaust
Cyclone
84 Drying Technologies for Foods
size distribution of the dried powder depend on the type of the atomizers usedand the operating parameters such as rotational speed, pressure drop and flowrate of the liquid substance to be dried (Cheuyglintase, 2009; Masters, 1997).
Table 1: Increase in surface area when the size of droplet is progressively reduced keepingthe volume constant (Calculated values assuming no mass loss during size reduction)
Total volume Diameter Number Surface area Total surface Surfaceof droplets of droplets per droplets of droplets of droplets area ratio(m3) (m2)
0.524 1 m 1 3.14 m2 3.14 1
0.524 1 cm 1106 3.14 cm2 314.16 100
0.524 1 mm 1109 3.14 mm2 3141.59 1,000
0.524 100 m 11012 0.0314 mm2 31415.93 1,0000
0.524 1 m 11018 3.14 m2 3141592.65 1,000,000
Three types of atomizers are commercially available and used in spray dryingoperations. They are: (a) Single fluid pressure nozzle (b) Two-fluid nozzle(also known as pneumatic atomizer) (c) Rotary atomizer (also known as spinningdisc or centrifugal atomizer). Ultrasonic nozzle is also reported in literature asa novel atomizer (Bittner and Kissel, 1999; Freitas et al., 2004). The choice ofatomizer type depends on the nature (e.g. concentration and viscosity) andamount of the feed and the desired characteristics of the dried product (Masters,1997). For example, rotary atomiser is more suited for high viscosity andabrasive type of feed.
5.1.1 Single fluid pressure nozzle atomization
The pressure nozzle atomizer has two basic components: a device to createrotation of feed within the nozzle head and an orifice through which the feed isdischarged as a conical spray (Fig. 2- A). The feed enters into the nozzle underpressure and it exits the nozzle as fine droplets (Broadhead et al., 1992). Thesize of the droplets is determined by the size of the orifice and the operatingpressure. The higher pressure generally results into smaller droplet size. Thedroplets leave the atomizer at an angle of 60o to140o (Shafaee et al., 2011).Orifice size of the pressure nozzle usually varies within 0.5 to 3.0 mm. As aresult, a single nozzle is limited to somewhere in the order of 750 kg/h of feed,depending on pressure, viscosity, solid content and orifice size. Thus it becomesessential to use multiple pressure nozzles within a drying chamber in industrialspray drying operations in order to meet the desired product throughput. Theoperating pressure in industrially used pressure nozzles ranges from about1723 kPa to about 68.95 MPa. These atomizers are more commonly used in
Spray Drying 85
spray drying operations in milk, beverages, and food supplement industries.The advantages and disadvantages of pressure nozzles are given below.
Advantages
i) Powder with high bulk density can be achieved
ii) Better powder flowability
iii) Less deposits in the drying chamber
iv) Ability to produce big particles when larger orifice is used
v) When a dual feed/nozzle system is used, the drying plant can operatecontinuously for extended period of time
vi) Lower installation and operation costs
vii) Relatively high energy efficiency
Disadvantages
i) Difficult to control the spray pattern
ii) Nozzle capacity is the key limitation (multiple nozzles are required)
iii) Higher tendency to clogging
iv) Corrosion and erosion reduces their usable life
v) Difficult to use for high viscosity solutions
5.1.2 Rotary atomization
The rotary atomizer consists of a wheel and radial vanes (Fig. 2- B). The fluidfeed is introduced into the drying chamber by means of a spinning disc orwheel. When the feed is directed to the wheel periphery, the liquid feedaccelerates and acquires the peripheral speed of the wheel. The liquid isatomized into fine spherical droplets as soon as the thin film leaves off theedge of the wheel (Broadhead et al., 1992). The rotating wheel also creates anair pumping effect and affects the size distribution of droplets and also causessome degree of aeration in the product. Wheel speed is a critical factor andhigher speed at a constant wheel diameter and feed rate produces smallerdroplets (Masters, 1997). The advantages and disadvantages of rotary atomizerare given below.
Advantages
i) Generally greater flexibility & ease of operation. Can handle high feedrates in a single wheel
86 Drying Technologies for Foods
ii) Does not require high pressure feed system
iii) Negligible blockage problems
iv) Suitable even for abrasive materials
v) Control of droplet size can be achieved with relative ease by adjustingthe wheel speed
Disadvantages
i) Requires larger drying chamber
ii) Higher wall deposition occurs especially when sticky foods are used
iii) Higher installation and operating cost as compared to pressure nozzles
iv) Less energy efficient
v) Cannot be used in horizontal dryers
5.1.3 Two-fluid nozzle atomization
In two-fluid nozzle atomizer (Fig. 2-C), liquid feed and the compressed air (orsteam) are the two fluids which are mixed internally or externally. The energyof the compressed gas is utilized to atomize the liquid. In internal mixing, thedrying air is rotated within the nozzle and comes into the contact with the feedliquid within the nozzle. In case of external mixing, the drying air mixes withthe liquid as soon as the liquid emerges from the nozzle. The spray angle of atwo-fluid atomizer should be such that it provides a good mixing between thetwo fluids which cause the liquid jets to be disintegrated into the gas stream.Capacity to produce very fine droplets and handling highly viscous fluids arethe major advantages of two-fluid nozzle atomizer (Masters, 1968). However,the cost of operation is very high and therefore the two-fluid nozzles aregenerally confined to laboratory scale spray dryers (Broadhead et al., 1992).
Spray Drying 87
Fig. 2:Types of atomizers: A. Pressure nozzle, B. Rotary atomizer, C. Two-fluid atomizer,D. Ultrasonic atomizer
A = GEANiro (http://www.niro.com/niro/cmsdoc.nsf/webdoc/ndkw5y7gkk)B = Svendsen, G. (1996). Rotary atomizer and a method of operating it: US Patent No. 5518180 A.C = http://www.nubilosa.com/About_Nubilosa/Nozzle/hauptteil_nozzle.htmD = S. Berliner, III Consultant in Ultrasonic Processing (http://berliner-ultrasonics.org/uson-5.html)
5.1.4 Ultrasonic atomization
In recent years ultrasound energy has been used in lieu of pressure or centrifugalforce to generate droplets (Fig. 2–D). In this method, a liquid is placed on arapidly vibrating surface at ultrasonic frequencies. At sufficiently highamplitude, the liquid spreads becomes unstable and collapses resulting in theformation of very fine droplets. These nozzles are excellent for creating dropletsbelow 50 m. Their application is expected to grow over time (Ré, 2006).
88 Drying Technologies for Foods
5.2 Droplet-hot air contact
The second step of spray drying process is mixing of the atomized dropletswith the heated air (or inert gas) stream which facilitates the evaporation ofwater (or solvent) and produces dried powders. For ideal drying process, thefinal product should be sufficiently dry before contacting the chamber wall sothat almost all the final product could be collected.
The contact of the atomized droplets and the drying medium (air or inert gas)can be brought about in different ways. In co-current design (Fig. 3-A) thefeed and the drying medium move within the chamber in the same direction.This arrangement is preferred for drying the heat sensitive materials becausethe wet product comes in contact with the driest medium while the progressivelydried product comes into contact with the progressively cooled drying medium.In this method, the high rate of moisture evaporation enable the temperature ofthe dry product to be considerably lower than that of the air leaving the dryingchamber. In this arrangement, the liquid droplets come into contact with theinlet temperature of 150oC -220oC which causes instant evaporation of thewater so that the air temperature drops down to moderate temperature (typically50oC – 80oC) (Gharsallaoui et al., 2007).
In counter-current design, the atomized droplets and the drying medium enterthe drying chamber from opposite directions (Fig. 3-B). In this process, thehottest air comes in contact with the driest particles which may result inunacceptable heat damage in the product (Masters, 1997). However, the countercurrent method is superior in terms of heat utilization and energy economy.Therefore, a combination of co- and counter-current method is developed toharness the benefits of both the methods of droplet-air contact so that superiorproduct quality can be achieved (Fig. 3-C).
Spray Drying 89
5.3 Evaporation of water or solvent from droplets
When the feed droplets come into the contact with hot air, evaporation of watertakes place and the dry powder particles are produced. During evaporation,the temperature and vapour pressure gradients between the droplets and thedrying medium are established such a way that the heat transfer takes placefrom the drying medium to the droplets while the mass transfer takes placefrom the droplets to the drying medium.
Fig. 3: Schematic diagrams of the methods of contact of air with the droplets
C: Mixed-current flow
A: Co-current flowB: Counter-current flow
Injection
90 Drying Technologies for Foods
When the drying medium and droplets come in contact with each other, heattransfer causes increase of the droplet temperature until a constant value isachieved (in the vicinity of wet bulb temperature) where most of water (orsolvent) evaporates from the droplets. In this condition, the rate of diffusion ofwater (or solvent) from within the droplet to its surface remains almost constantand is equal to the surface evaporation rate. Finally, when the solid content inthe droplet/particle increases to a certain value (corresponding to criticalmoisture content) the effective moisture diffusion decreases significantly. Thedrying rate rapidly decreases with the drying front progressively moves to theinterior of the particle. The effective moisture diffusivity and hence the dryingrate becomes the function of nature and concentration of solid. When the driedparticles exit the drying chamber, their temperature is close to the dryer outlettemperature. In practice, as soon as the hot air contacts the feed droplets, thedrying takes place almost instantaneously and rapid evaporation occurs fromthe surface of each droplet. This rapid evaporation keeps the droplet cool untilthe dry state is reached. Usually, drying times are of the order of 5–100 s(Corrigan, 1995).
The nature of the product and air inlet and outlet temperatures play importantrole in the drying process. If higher air inlet temperature is used, the water (orsolvent) evaporation and formation of dry particles occur faster. If the differencebetween the inlet and outlet air temperature is larger, the temperature and vapourpressure gradients will be higher which lead to faster evaporation and particleformation. The drying rate also depends on the surface to volume ratio of theatomized droplets (Table 1). The smaller the droplet size, the higher the surfaceto volume ratio and the faster will be the drying process.
5.4 Recovery of powders
The final step in the spray drying process is product recovery. It involvesseparation of the dry product from the drying medium which is oftenaccomplished using a cyclone and a bag filter placed outside the drying chamber(Fig. 1). The ease or difficulty of separation of particles from the drying mediumdepends on the density of particles, size of the particles and their settling velocitywithin the cyclone. Larger particles are collected at the base of the dryingchamber while the smaller ones are recovered at the cyclone and the bag filter.
6 Droplet Drying and Modelling
In droplet drying process the formation of particles from a solution is mostlythe kinetic processes due to short time frame. The vapour pressure andtemperature gradients within a drying droplet and the drying medium are the
Spray Drying 91
key driving forces for the mass transfer from, and heat transfer to the droplet.The increasing research interest on spray drying kinetics is motivated by theneed for fundamental understanding of the drying phenomena both at dropletand particulate levels. During the last decades, growing attention has beengiven to theoretical and experimental studies of single droplet drying to betterunderstand the drying behaviour of droplets within the spray drying chamber.To understand the physics of the drying droplets/particles in the spray dryingprocess, the following modelling approaches are available in the literature.
i) Characteristic drying rate curve (CDRC) modelling
ii) Distributed parameter modelling
iii) Reaction engineering approach (REA) modelling
6.1. Characteristic drying rate curve (CDRC) modelling approach
The CDRC model is a semi-empirical approach which uses a small set ofsimplified equations and allows fast computation and is shown to work wellfor small particles (Keey, 1992). This model does not consider the temperatureand moisture distribution within drying droplets/particles mathematically butempirically lumps the effect of such distributions. This has subsequently beenfound to be useful for scaling up the results of drying kinetics obtained fromsingle droplets to large scale spray dryers (Langrish and Kockel, 2001). In thismodelling approach, the first drying stage is simplified by analogy withevaporation of a small pure water droplet. The falling rate period is determinedby using relative decrease in dying rate compared to at the critical moisturecontent which may or may not be linear. The critical moisture content isdetermined through separate experiments and depends on the characteristicsof the solid component and the drying conditions (temperature, relative humidityand velocity of drying medium) (Chen and Putranto, 2013).
6.2. Distributed parameter modelling approach
This model considers the moisture and temperature gradients with the droplets/particles during drying and uses set of partial and ordinary differential equations.Due to fairly low Biot Number (heat), the temperature gradient within thedroplet/particle is usually neglected. Due to large Biot Number (mass) themoisture gradient within the droplet/particle is considered. This approach ofmodelling was used to predict the moisture and temperature profiles (Lin andChen, 2006; Adhikari et al., 2003; 2005), expansion and loss of volatiles (Hechtand King, 2000) and surface stickiness (Adhikari et al., 2007) in single dropletssubjected to convective drying. However, the solution of the partial differential
92 Drying Technologies for Foods
equations used in this modelling approach is difficult due to continuouslyreceding evaporation front within the droplet in the falling drying rate period(Mezhericher et al., 2007). To be realistic, there is need of a priori informationon how and how long the droplet/diameter decreases. Nevertheless, when theeffective moisture diffusivity, desorption isotherm, thermal conductivity valuesand air-side properties are available, this modelling approach has shown topredict the experimental data with good accuracy (Adhikari et al., 2007).
6.3. REA modelling
The REA modelling approach is shown to provide good agreement betweenmodel predictions and the experimental data. This approach also does not requirea priori information on the moisture diffusivity and sorption isotherm. Thesorption isotherm related information is obtained by reverse engineering. Theordinary differential equations predicting moisture content and droplet/particletemperature histories can be conveniently solved with less computing resources.The mass transfer rate is determined as a function of vapour concentrations atthe droplet interface and in the drying medium. The vapour concentration atthe interface is related to its saturation value with the help of a fractionalitycoefficient. This fractionality coefficient is expressed by using Arrhenius-typeequation using apparent activation energy (AAE) and droplet temperature. Thismodelling approach also needs an empirical correlation between the partialvapour density at the droplet surface and the mean moisture content of thedroplet. This correlation has to be determined experimentally for each materialas a function of moisture content. The REA approach, despite its simplicity, isproven to be quite accurate in modelling drying behaviour of number ofmaterials (Putranto et al., 2011; Haque et al, 2013a). In this model, the massand heat transport equations are resolved as follows (Chen and Xie, 1997).
Mass balance:
bvsatv
vms T
RT
EAh
dt
dXm ,, )()exp( (1)
T
Ahdt
dXm
RTEsatv
bvm
s
v,
,
1
ln
(2)
Spray Drying 93
bbv
v XXfE
E
,
(3)
bbbv RHRTE ln, (4)
where, ms is the dry solid mass of the sample (kg), X is the average moisture
content on dry basis (kg kg-1), t is time (s), hm is the mass transfer coefficient
(m s-1) and A is the surface area of the droplet/particle (m2). v,b
is the watervapor concentration in the drying medium (kg m -3). Similarly, R is the universalgas constant (J mol-1K-1), T is the temperature of the droplet/particle beingdried (K) and
v, satis saturated vapor concentration. E
v is the ‘correction factor’
represents the additional difficulty to remove moisture from the material whenit is not free water or water activity is not 1. E
v,b is the equilibrium activation
energy representing the maximum reachable Ev,b
in a certain drying condition,X
e is the equilibrium moisture content (kg kg-1), RH
b is the relative humidity of
drying air and Tb is the temperature of the drying medium (oK).
Energy balance
vsbpd Hdt
dXmTThA
dt
dTmC )( (5)
where, m is the mass of the droplet/particle (kg), Cpd
is the specific heat capacityof droplet/particle (J kg-1 K-1), h is the heat transfer coefficient (Wm-2 K-1) andH
v is the latent heat of vaporization of water (J kg-1).
It is essential that the predictions made using the above equations are validatedwith the experimental data. The moisture content and temperature histories ofdroplets measured using a single droplet dryer (SDD) have been used to validatethese model predictions (Lin and Chen, 2002; Adhikari et al., 2007; Haqueet al., 2013a). A schematic diagram of SDD is shown in Fig. 4. This equipmentuses clean and dry air to dry the droplets. The flow rate of the air is controlledby using a rotameter or mass flow controller. The temperature of the air iscontrolled by using a proportional-integral-derivative (PID) controller. Thedroplets are suspended on the tip of a very thin (0.1-0.2 mm in diameter) glassfilament. Small changes in temperature and droplet/particle mass can be trackedand recorded by the thermocouples and load cell, respectively. A digital cameratogether with magnifying tube is attached to the instrument to observe andrecord changes in morphology of the drying droplet. The air temperature, droplettemperature, droplet mass and morphology can be recorded in the computer asa function of time as desired. The moisture and temperature histories of 10%
94 Drying Technologies for Foods
(w/v) -lactalbumin at drying air temperature of 80oC were measured usingthe SDD instrument. The prediction of these two profiles using REA modelling(equations 1 and 5) was carried out. As shown in Fig. 5, the REA modellingfollows the experimental data fairly accurately.
a Air supply regulator g Load cellb Silica gel column h Video camerac Molecular sieve column i Thermocoupled Flow regulator j Air exhauste Heater k Computerf Drying chamber
Fig. 4: A schematic diagram of single droplet dryer hanged with sample droplets inthe drying chamber.
Fig. 5: Experimental and predicted temperature (T) and moisture content, MC, (X) profiles of-lactalbumin (10% w/v) droplet during drying at 80oC for 600 s.
Spray Drying 95
7 Modelling of Spray Dryer
Although spray drying technology is widely used in industry, the understandingof droplet trajectory, modelling of mass, heat and momentum transfer processeswithin a spray dryer is still limited to the researchers and dryer manufacturers(Patel and Chen, 2007). Fast drying process, presence of millions of dropletsat any time and complex flow pattern of air drying medium within the chambermake it very difficult to track or observe the spray drying process. However,the recent development of computational fluid dynamics (CFD) modelling toolsand increased computing power has made it possible to simulate the spraydrying process (Dalmaz et al., 2007; Huang et al., 2004; Fletcher et al., 2006).CFD simulations make it possible to simulate events and conditions within thedrying chamber which is not possible by traditional computational spread sheets(Langrish et al., 2004; Woo et al., 2013). CFD modelling tool is able to takeinto account the actual air flow pattern especially at the air inlet region, is ableto accommodate the internal swirling and also can identify the areas wherelarge number of particles are deposited (Woo et al., 2013). Equations (6) to(10) are the main equations used in CFD software (Norton and Sun, 2006).
Conservation of mass
+ = 0 (6)
Conservation of momentum
( )+ = − + + + (7)
Conservation of energy
( )+ − λ = (8)
= 1− − (9)
= (10)
where, µ is the velocity component (m s-1), t is the time (s), x is the dimensionof Cartesian coordinate (m), ρ is the density (kg m-3), p is the pressure (Pa), δis the Kroneckor delta, µ is the dynamic viscosity (kg m-1 s-1), g is theacceleration due to gravity (m s-2), C
a is the specific heat capacity (W kg-1 K-1),
T is the temperature (K), is the thermal conductivity (W m-1 K-1), ST is the
heat sink or source (W m-3), is the thermal expansion coefficient (K-1), R is
96 Drying Technologies for Foods
the gas constant (J kmol-1 K-1), Wa is the molecular weight of air (kg kmol-1).
The subscripts i, j are the Cartesian coordinate indexes, ref stands for referenceand a stands for air.
8 Application of Spray Drying
Spray drying is very commonly and widely used in the dairy industry. The rawmilk first goes through various unit operations such as pasteurization,microfiltration and evaporation depending on requirement to increase the milksolid content. This concentrated milk is spray dried to convert into the powder.Milk protein concentrates, whey protein isolates and concentrates are producedthrough spray drying.
Spray drying is also used to produce fruits and vegetables powders such asmango powder, tomato powder and pineapple powder. In addition spray dryingis also used to produce plant protein powders such as peanut protein concentrate(Yu et al., 2007) and lentil protein isolate (Joshi et al., 2011).
Due to high throughput, hygienic and closed loop operation and ability to dryheat sensitive products, spray drying is being used to microencapsulate thefunctionally important but unstable compounds. In this process a wall or coatis formed around the particle which successfully protects the unstable product(Yep et al., 2002). The antioxidant capability of flavonoids can also be betterprotected by spray drying based encapsulation (Sansone et al., 2011). The film/skin or coat forming materials such as gums and proteins are used to coat theunstable core (Krishnan et al., 2005; Bylaite et al., 2001). Spray drying is alsoused to produce dry capsules of omega-3 fatty acids. For example, Eratte et al.(2014) first produced liquid microcapsules of omega-3 fatty acid throughcomplex coacervation and finally used spray drying to convert these liquidcapsules into powder. Fig. 6 presents a schematic diagram of encapsulatedparticles where the unstable core is coated by wall material. Spray drying isalso used to microencapsulate food colours (Jime´ nez-Aguilar et al., 2011)and food flavours (Couto et al., 2012).
Fig. 6: Schematic diagram of encapsulated core in shell material produced through spray drying.
Solution of wallmaterial
Core
High speed stirrer
Spray drying
Encapsulated particles
Core
Walmaterial
Spray Drying 97
9 Spray Drying and The Quality of Products
The environment around the atomized droplets normally remains more of lesssaturated during atomization. Although the inlet air enters the chamber at veryhigh temperature, the drop temperature remains close to the wet bulbtemperature due to evaporative cooling. As the drying progresses, the increasedsolid content reduces the diffusion of water from the droplet surface and thedroplet temperature starts increasing. Towards the end of drying, the temperatureof the particles reaches close to the outlet air temperature. It is suggested thatdue to recirculation of some particles their residence time is longer. It wasobserved that about 2-10 min was required to recover the all dried powdersfrom the drying chamber (Jeantet et al., 2008; Gianfrancesco, 2009; Schmitzet al., 2011). The injurious effect of spray drying on the bioactive compoundsdepends on the sensitivity of the biomolecules to heat, composition of feedand spray drying conditions. In a slow convective drying process requiringlonger time (e.g. single droplet drying), it was found that the hydrophobicgroup of the whey protein -lactoglobulin is most sensitive to denaturationcompared to α-lactalbumin and bovine serum albumin (Haque et al., 2014a).The secondary structural features (-helix, -sheet, -turn and random coil)of whey protein isolate (WPI) were altered significantly when spray dryingwas carried out at 180o/80oC (inlet/outlet) (Table 2). Anandharamakrishnan etal. (2007) conducted series of experiments applying different drying conditionsand feed concentrations to observe the effect of spray drying on WPI. Theyreported that the denaturation of WPI increased by many factors such as highoutlet temperature, high retention time and high solid content in feed. Moreover,the extent of damage to proteins also depends on the structural configuration.For example, the proteins which have more hydrophobic residues (such asvaline and isoleucine) and contain more disulfide bonds (such as bovine serumalbumin) are generally more stable during drying (Abdul-Fattah et al., 2007;Haque et al., 2013b).
Table 2: The composition (%) of secondary structural elements of control and spray driedwhey protein isolate (WPI).
Sample Secondary structural elements (%)
-sheet Random coil -helix -turn
Control WPI 43.0 0.00 22.8 34.2
Spray dried (180/80oC) WPI 38.6 20.7 21.0 19.7
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10 Prevention of Damage of Heat Sensitive Materials
It is suggested that the thermal, interfacial and dehydration stresses are the mainstresses experienced by droplets during convective drying (Maa and Hsu,1997;Haque et al., 2014b). Measures should be put in place to minimise these stressesto protect the biomolecules from drying induced damage. The process based andmaterial based approaches are normally introduced to mitigate the effect of drying.In the process based approach,mild drying conditions are used to minimise thedamage due to drying (Anandharamakrishnan et al., 2007). In the material basedapproach protectant solids such as sugars (e.g. lactose, trehalose, sucrose) andsurfactants are incorporated in the feed before drying. The sugars protect thebiomolecules by converting themselves into glassy solids and occupying thespace vacated by the evaporating solvent (Bellavia et al., 2011). The surfactantspreferentially cover the air-droplet interface and minimise the interface relatedstresses (Maa et al., 1998).
11 Physical Characteristics of Spray Dried Powders
The physical characteristics of spray dried powders such as particle shape andsize, bulk density and crystalline/amorphous nature have important impact onproduct quality. These physical characteristics regulate the ease or difficultyof handling, transportation and reconstitution of the powder. A good degree ofcontrol over these powder characteristics can be achieved by adjusting theformulation of feed and operating conditions of spray dryer. Some importantphysical characteristics of spray dried powder are discussed in the ensuingsections.
11.1 Surface morphology
Spray drying normally produces spherical particle. However, this shape canchange depending on the nature of the feed, concentration of solid in the feedand drying conditions. Depending on the formulation of feed and dryer operatingparameters, spray dried particles can be hollow, inflated, distorted, shrivelledand could have folded surface. The scanning electron microscope (SEM)micrographs shown in Fig. 7 presents the particle morphology of spray driedMilk protein concentrate (MPC) (15%, w/w) at 180oC/90oC (inlet/outlet). Ascan be seen in the figure, the overall appearance of the particles is spherical.However, numerous wrinkles/surface folds and blowholes are visible on theparticle surface. This implies that there was formation of thick skin around thesurface of the protein-rich particles and due to internal vaporization andexpansion the particles become hollow. Due to development of resistive skinon the particle surface the internal water vapour of the particle could not freely
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diffuse, which yielded outward pressure on the skin. This pressure resultedinto formation of folds and blowholes on the surface.
Fig. 7: Scanning electron microscopic (SEM) micrograph of milk protein concentrate powderspray dried at 180oC inlet and 90oC outlet temperature.
11.2 Bulk and particle density
Increased concentration of solids in feed increases the powder particle density(Walton and Mumford, 1999). Higher drying temperature causes inflation orballooning of the particles. Particularly, this is common to skin or crust formingmaterials. This phenomenon decreases the bulk density of the powder. Theother factors such as feed flow rate and residual moisture content also affectthe bulk density. The feed viscosity influences the manner and speed ofatomization and residence time of particle and ultimately affects the bulk andparticle densities of the produced powder. It was observed that higher flowrate of the atomizing compressed air and the higher pressure across the nozzleproduced smaller particle size and resulted into higher bulk density of tomatopowder (Goula and Adamopoulos, 2005). If higher amount of air is incorporatedduring the rotary atomization, both the particle and bulk density of the resultantpowder are expected to decrease.
100 Drying Technologies for Foods
11.3 Crystalline/amorphous nature
Stickiness of powder surface especially in sugar and acid-rich foods addsdifficulty in the spray drying due to deposition of powders on the dryer wall(Adhikari et al., 2003). Normally spray drying produces amorphous powders(Chiou et al., 2008). These authors suggested that partial or completecrystallisation of the powder can reduce the powder stickiness. On the otherhand, Jayasundera et al. (2010) reported that the deposition of sucrose on thedryer wall was so severe that no powder was recovered in the absence of dryingaids. They also found that addition of only 0.5% (w/w) protein (Sodiumcaseinate) produced essentially amorphous sucrose powder and at recoveryrate of >82%. The incorporation of surface-active proteins in sugar-rich foodsgreatly minimises the stickiness problem. Hence, it can be argued that thecrystallinity in spray dried powders depends not only on nature of the materialsbut also on the drying conditions, feed concentration and residual moisturecontent of the powder. The characteristics of the sugars and carrier materialsused in spray drying of mango concentrate was found to affect the crystallinebehaviour of the resultant powder remarkably (Cano-Chauca et al., 2005). Theseauthors suggested that mixing of 12% (w/w) cellulose and waxy starchindividually or together produced crystalline mango powder. Interestingly, useof same amount of maltodextrin as a drying aid produced amorphous mangopowder.
11.4 Solubility of spray dried powders
Solubility is one of the key indicators of quality of spray dried powders. Easeof reconstitution makes the product attractive for use in households or asingredient in food formulations. The amorphous powders have better solubility;however, they are less stable during processing and storage (Chiou et al., 2008).The concentration and characteristics of carriers or drying aids used duringspray drying, for example gum Arabic and cellulose, also affect the solubilityof the powders (Yousefi et al., 2011). These authors spray dried pomegranatejuice using different carriers at different concentrations and showed that thejuice mixed with maltodextrin and gum Arabic yielded amorphous powderand hence resulted into better solubility. On the other hand, the use of waxystarch as carrier produced partially crystalline powder with less solubility.Besides, outlet temperatures of >80oC and high feed concentration (>20%)negatively affected the solubility of whey protein powder due to aggregationof proteins (Anandharamakrishnan et al., 2008). A higher residual moisturecontent in powders was also found to take longer time to dissolve tomato powderin water (Goula and Adamopoulos, 2005).
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Concluding Remarks
In this chapter the fundamentals of spray drying technology and its applicationin producing food powders have been discussed in considerable detail. Severalimportant physical characteristics of the spray dried powders such as bulk/particle density, crystalline/amorphous nature and solubility have beenoverviewed. This chapter also briefly discussed about the effect of spray dryingstresses on the structure and denaturation of bio macromolecules, especiallyproteins.The material and process based methods to minimise the effect ofthese stresses have also been discussed. The modelling approaches of massand heat transfer in a single droplet level have been overviewed briefly. Althoughthe spray drying technology has been used in producing food powders for morethan a half century, many aspects of this technology are still poorly understood.It is essential to understand and model the mass, heat and momentum transferin the spray drying process incorporating as many droplets/particles as possiblein the simulation. The development of more user friendly CFD simulationtogether with the development of robust and validated mass, heat andmomentum transport models (e.g, distributed parameter) would broaden theapplication of this technology to highly sensitive food materials such as proteinsand antioxidants.
References
Abdul-Fattah, A.M., Kalonia, D.S. and Pikal, M.J. (2007). The challenge of drying methodselection for protein pharmaceuticals: product quality implications. Journal ofPharmaceutical Sciences, 96: 1886-1916.
Adhikari, B., Howes, T., Bhandari, B.R. and Troung, V. (2003). Surface stickiness of drops ofcarbohydrate and organic acid solutions during convective drying: Experiments andmodeling. Drying Technology, 21 (5): 839–873.
Adhikari, B., Howes, T., Lecomte, D. and Bhandari, B.R. (2005). A glass transition temperatureapproach for the prediction of the surface stickiness of a drying droplet during spraydrying. Powder Technology, 149: 168-179.
Adhikari, B., Howes, T. and Bhandari, B.R. (2007). Use of solute fixed coordinate system andmethod of lines for prediction of drying kinetics and surface stickiness of single dropletduring convective drying. Chemical Engineering and Processing, 46: 405-419.
Anandharamakrishnan, C., Rielly, C.D. and Stapley, A.G.F. (2008).Loss of solubility of-lactalbumin and -lactoglobulin during the spray drying of whey proteins. LWT-FoodScience and Technology, 41: 270-277.
Anandharamakrishnan, C., Rielly, C.D. and Stapleya, A.G. F. (2007). Effects of process variableson the denaturation of whey proteins during spray drying. Drying Technology, 25(5):799-807.
Bankar, S.K., Chaudhari, A.V., Mahale, N.B. and Chaudhari, S.R. (2014). A review onorodispersible tablets prepared using spray dried sustained release microparticles. Journalof Advanced Drug Delivery, 1(2): 82-95.
102 Drying Technologies for Foods
Bellavia, G., Giuffrida, S., Cottone, G., Cupane, A. and Cordone, L. (2011). Protein thermaldenaturation and matrix glass transition in different protein-trehalose-water systems. TheJournal of Physical Chemistry B, 115: 6340-6346.
Bittner, B. and Kissel, T. (1999). Ultrasonic atomization for spray drying: a versatile techniquefor the preparation of protein loaded biodegradable microspheres. Journal ofMicroencapsulation, 16(3): 325-341.
Bowen, M., Turok, R. and Maa, Y.F. (2013). Spray drying of monoclonal antibodies: investigatingpowder-based biologic drug substance bulk storage. Drying Technology, 31(13-14): 1441-1450.
Broadhead, J, Edmond Rouan, S. K. and Rhodes, C.T. (1992). The spray drying of pharmaceuticals.Drug Development and Industrial Pharmacy, 18(11-12): 1169-1206.
Bylaite, E., Venskutonis, P. R. and Mapdbieriene, R. (2001). Properties of caraway (CarumcarviL.) essential oil encapsulated into milk proteinbased matrices. European Food Researchand Technology, 212: 661–670.
Cano-Chauca, M., Stringheta, P.C., Ramos, A.M. and Cal-Vidal, J. (2005).Effect of the carrierson the microstructure of mango powder obtained by spray drying and its functionalcharacterization. Innovative Food Science and Emerging Technologies, 6: 420 – 428.
Chavez, B.E. and Ledeboer, A.M. (2007). Drying of probiotics: optimization of formulation andprocess to enhance storage survival. Drying Technology, 25 (7-8): 1193–1201.
Chen, X. D. and Putranto, A. (2013). Modelling Drying Processes: A Reaction EngineeringApproach. Cambridge University Press, New York, USA, pp-20.
Chen, X.D. and Xie, G.Z. (1997). Fingerprints of the drying behaviour of particulate or thin layerfood materials established using a reaction engineering model. Transactions of the Instituteof Chemical Engineers part C: Food and Bio-product processing, 75: 213-222.
Cheuyglintase, K. (2009). Spray drying of fruit juice with vegetable fibre as a carrier. Universityof Canterbury, Christchurch, New Zealand,URI: http://hdl.handle.net/10092/3101.
Chiou, D., Langrish, T.A.G. and Braham, R. (2008). Partial crystallization behavior during spraydrying:simulations and experiments. Drying Technology, 26: 27–38.
Corrigan, O. I. (1995). Thermal analysis of spray dried products. Thermochimicaacta, 248: 245-258.
Couto, R.O., Conceicao, E.C., Chaul, L.T., Oliveira, E.M.S., Martins, F.S., Bara, M.T.F., Rezende,K.R., Alves, S.F. and Paula, J.R. (2012). Spray-dried rosemary extracts: Physicochemicaland antioxidant properties. Food Chemistry 131: 99–105.
Dalmaz, N., Ozbelge, H.O., Eraslan, A.N. and Uludag, Y. (2007).Heat and mass transfermechanisms in drying of a suspension droplet: a new computational model. DryingTechnology, 25(2): 391-400.
Donz, E., Boiron, P. and Courthaudon, J.L.(2014). Characterization of industrial dried wheyemulsions at different stages of spray-drying.Journal of Food Engineering, 126:190-197.
Eratte, D., Wang, B., Dowling, K., Barrow, C.J., Adhikari, B. P. (2014). Complex coacervationwith whey protein isolate and gum arabic for the microencapsulation of omega-3 richtuna oil.Food & Function. DOI:10.1039/C4FO00296B
Filkova, I., Huang, L.X. and Mujumdar, A.S. (2006). Industrial spray drying systems. In A. S.Mujumdar (Ed.), Handbook of Industrial Drying, 3rd ed. CRC Press, pp. 215-256
Fletcher, D.F., Guo, B., Harvie, D.J.E., Langrish, T.A.G., Nijdam, J.J. and Williams, J. (2006).What is important in the simulation of spray dryer performance and how do current CFDmodels perform? Applied Mathematical Modelling, 30(11): 1281–1292.
Spray Drying 103
Freitas, S., Merkle, H.P. and Gander, B. (2004). Ultrasonic atomisation into reduced pressureatmosphere—envisaging aseptic spray-drying for microencapsulation. Journal ofControlled Release, 95(2): 185-195.
Gharsallaoui, A., Roudaut, G., Chambin, O., Voilley, A. and Saurel, R. (2007). Applications ofspray-drying in microencapsulation of food ingredients: An overview. Food ResearchInternational, 40(9): 1107-1121.
Gianfrancesco, A. (2009). Spray drying engineering: Particle stickiness in relation withagglomeration. Dissertation; AgroParisTech; Massy; 2009.
Goula, A.M. and Adamopoulos, K.G. (2005). Spray drying of tomato pulp in dehumidified air:II. The effect on powder properties. Journal of Food Engineering, 66: 35–42.
Haque, M. A., Aldred, P., Chen, J., Barrow, C. and Adhikari, B. (2014a). Drying and denaturationcharacteristics of -lactalbumin, -lactoglobulin and bovine serum albumin in a convectivedrying process. Journal of Agricultural and Food Chemistry, 62 (20): 4695–4706.
Haque, M.A. and Adhikari, B. (2014b). Drying and denaturation of proteins in spray dryingprocess.In A. S. Mujumdar (Ed.), Handbook of Industrial Drying, 4th Ed.CRC Press,Taylor and Francis group, NW, USA, pp 971-984.
Haque, M. A., Putranto, A., Aldred, P., Chen, J. and Adhikari, B. (2013a). Drying and denaturationkinetics of whey protein isolate (WPI) during convective air drying process. DryingTechnology, 31: 1532-1544.
Haque, M.A., Aldred. P., Chen, J., Barrow, C.J. and Adhikari, B. (2013b). Comparative study ofdenaturation of whey protein isolate (WPI) in convective air drying and isothermal heattreatment processes. Food Chemistry, 141: 702-711.
Hede, P.D., Bach, P. and Jensen, A. D. (2008). Two-fluid spray atomisation and pneumatic nozzlesfor fluid bed coating/agglomeration purposes: A review. Chemical Engineering Science,63(14): 3821-3842.
Huang, L., Kumar, K. and Mujumdar, A.S. (2004). Simulation of a spray dryer fitted with arotary disk atomizer using a three-dimensional computional fluid dynamic model.DryingTechnology, 22(6): 1489-1515.
Jafari, S. M., Assadpoor, E., Bhandari, B. and He, Y. (2008).Nano-particle encapsulation of fishoil by spray drying.Food Research International, 41(2): 172-183.
Jayaraman, K.S. and Das Gupta, D.K. (1992). Dehydration of fruits and vegetables-recentdevelopments in principles and techniques.Drying Technology, 10(1): 1-50.
Jayasundera, M., Adhikari, B. P., Adhikari, R. and Aldred, P. (2010).The effect of food-gradelow-molecular-weight surfactants and sodium caseinate on spray drying of sugar-richfoods.Food Biophysics, 5:128–137
Jeantet, R., Ducept, F., Dolivet, A., Mejean, S. and Schuck, P. (2008). Residence time distribution:A tool to improve spray-drying control. Dairy Science & Technology, 88(1): 31–43.
Jime´ nez-Aguilar, D.M., Ortega-Regules, A.E., Lozada-Ramý´rez, J.D., Pe´rez-Pe´rez, M.C.I.,Vernon-Carter, E.J. and Welti-Chanes, J. (2011). Color and chemical stability of spray-dried blueberry extract using mesquite gum as wall material. Journal of Food Compositionand Analysis, 24: 889–894.
Joshi, M., Adhikari, B., Aldred, P., Panozzo, J.F. and Kasapis, S. (2011). Physicochemical andfunctional properties of lentil protein isolates prepared by different drying methods. FoodChemistry, 129: 1513–1522.
Keey, R.B. (1992). Drying of loose and particulate materials. Hemisphere Publishing: New York.
Krishnan, S., Kshirsagar, A. C. and Singhal, R. S. (2005).The use of gum arabic and modifiedstarch in the microencapsulation of a food flavoring agent. Carbohydrate Polymers, 62:309–315.
104 Drying Technologies for Foods
Langrish, T.A.G., Williams, J. and Fletcher, D.F. (2004). Simulation of the effects of inlet swirlon gas flow patterns in a pilot-scale spray dryer. Chemical Engineering Research andDesign, 82: 821-833.
Langrish, T.A.G., and Kockel, T.K. (2001).The assessment of a characteristic drying curve formilk powder for use in computational fluid dynamics modelling. Chemical EngineeringJournal, 84: 69-74.
Lin, S.X.Q. and Chen, X.D. (2002). Improving the glass-filament method for accuratemeasurement of drying kinetics of liquid droplets.Trans IChemE, 80(4): 401-410.
Lin, S.X.Q. and Chen, X.D. (2006). A model for drying of an aqueous lactose droplet using thereaction engineering approach.Drying Technology,24: 1329–1334.
Maa, Y.F. and Hsu, C.C. (1997).Protein denaturation by combined effect of shear and air-liquidinterface.Biotechnology and Bioengineering 54: 503-512.
Maa, Y.F., Nguyen, P.T. and Hsu, S.W. (1998).Spray-drying of air–liquid interface sensitiverecombinant human growth hormone.Journal of Pharmaceutical Sciences, 87(2):152-159.
Masters, K. (1997). Spray Dryers. In C. G. J. Baker (Ed.), Industrial drying of foods. London,UK: Blackie Academic and Professional.
Masters, Keith. (1968). Spray Drying—The Unit Operation Today. Industrial & EngineeringChemistry, 60(10), 53-63.
Mezhericher, M., Levy and A., Borde, I. (2007). Theoretical drying model of single dropletscontaining insoluble or dissolved solids.Drying Technology, 25: 1025-1032.
Mothibe, K. J., Zhang, M., Nsor-atindana, J. and Wang, Y.C. (2011). Use of ultrasound pretreatmentin drying of fruits: drying rates, quality attributes, and shelf life extension, DryingTechnology, 29(14): 1611-1621.
Norton, T. and Sun, D. (2006). Computational fluid dynamics (CFD) - an effective and efficientdesign and analysis tool for the food industry: A review. Trends in Food Science &Technology, 17: 600-620.
Parihari, A. (2009). Performance study of spray dryer using various salt solutions.Master Thesis,National Institute of Technology, Rourkela.
Patel, K.C. and Chen, X.D. (2007). Production of spherical and uniform-sized particles using alaboratory ink-jet spray dryer. Asia-Pacific Journal of Chemical Engineering, 2: 415–430.
Putranto, A., Chen, X.D., Xiao, Z. and Webley, P.A. (2011). Simple, accurate and robust modellingof various systems of drying foods and biomaterials: A demonstration of the feasibility ofthe reaction engineering approach (REA). Drying Technology, 29: 1519-1528.
Ré, Maria-Inês. (2006). Formulating drug delivery systems by spray drying. Drying Technology,24(4): 433-446.
Sansone, F., Picerno, P., Mencherini, T., Villecco, F., D’Ursi, A.M., Aquino, R.P. and Lauro,M.R. (2011). Flavonoid microparticles by spray-drying: Influence of enhancers of thedissolution rate on properties and stability. Journal of Food Engineering, 103: 188–196.
Schmitz, I., Gianfrancesco, A., Kulozik, A., U. and Foerst, P. (2011).Kinetics of Lysine Loss inan Infant Formula Model System at Conditions Applicable to Spray Drying. DryingTechnology, 29: 1876–1883.
Shafaee, M., Banitabaei, S. A., Esfahanian, V. and Ashjaee, M. (2011). An investigation on effectof geometrical parameters on spray cone angle and droplet size distribution of a two-fluidatomizer. Journal of Mechanical Science and Technology, 25(12): 3047-3052.
Spray Drying 105
Walton, D.E. and Mumford, C.J. (1999). Spray Dried Products- Characterization of ParticleMorphology. Transactions of the Institute of Chemical Engineers part A, 77: 21-38.
Woo, M.W. and Bhandari, B. (2013). Spray drying for food powder production. In B. Bhandari(Ed.),Handbook of food powders.Woodhead Publishing Limited, Cambridge, UK,pp 29-56.
Yep, Y.L., Li, D., Mann, N.J., Bode, O. and Sinclair, A.J. (2002). Bread enriched withmicroencapsulated tuna oil increases plasma docosahexaenoic acid and total omega-3fatty acids in humans.Asia Pacific Journal of Clinical Nutrition, 11(4): 285-291.
Yousefi, S., Emam-Djomeh, Z. and Mousavi, S.M. (2011). Effect of carrier type and spray dryingon the physicochemical properties of powdered and reconstituted pomegranate juice(PunicaGranatum L.). Journal of Food Science and Technology, 48(6): 677–684.
Yu, J., Ahmedna and M. Goktepe, I. (2007). Peanut protein concentrate: Production and functionalproperties as affected by processing. Food Chemistry, 103: 121–129.
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