An Overview of Process Parameters and Spray

drying agents involved in Spray drying of

Herbal Extracts

Sachinkumar D. Gunjal

Department of Pharmaceutics,

Dr. D.Y. Patil Institute of Pharmaceutical Sciences & Research, Pimpri, Pune – 411018, India Email- sachin13802004@gmail.com

Satish V. Shirolkar Department of Pharmaceutics,

Dr. D.Y. Patil Institute of Pharmaceutical Sciences & Research, Pimpri, Pune – 411018, India Email- satish.shirolkar@dypvp.edu.in

Abstract- Spray drying of herbal extract is important in pharmaceutical and food industries for obtaining powder with

better physical properties, and stability of active ingredients. The inlet air temperatures, Feed flow rate and spray drying

agent’s concentrations are the variables which affect percentage yield, moisture content, bulk density and hygroscopicity

of the powder obtained by spray drying. Interdependence of spray drying process variables are discussed in this article

for spray drying of herbal extracts. The stickiness of extracts, wall deposition and low percentage yield are major

problems associated with spray drying process. Various ingredients like aerosol, maltodextrin are used as spray drying

agents. Advantages and limitations of various spray drying agents used for herbal extracts are reviewed in this article.

Powder can be obtained as per required specifications by understanding effect of additives on processing parameter of

spray drying.

Keywords – Process parameters, Spray drying, Herbal, Temperature, Variables.

INTRODUCTION:

The development of spray drying equipment and techniques evolved over a period of several decades from the

1870s through the early 1900s. Spray drying comes of age during World War II, with the sudden need to reduce the

transport weight of foods and other materials1. Spray drying is a conventional method involves low cost in

comparison to other method. Spray drying is a drying method that produces droplets of liquid feed into powdered

products. The conversion involved atomization of liquid feed, undergoes heat treatment for rapid evaporation of

moisture2. The spray drying process converts liquids to powders with some but at an acceptable level of degradation

and oxidation of volatile compounds. Spray drying is based on the preparation, homogenization, atomization,

dispersion and subsequently dehydration of the solution3. The gas generally used is air or more rarely an inert gas,

particularly nitrogen gas3. It increases powder stability and resistant to microbiological and oxidative degradation.

Commercialization of spray drying process depends on parameters like process yield, end product characteristic and

production cost2. Few articles are published on the effectiveness of additives of spray drying optimization, in which

there is no clear linear relationship between additives and spray drying parameters has been identified2. The

atomizing stage must create a spray for optimum evaporation conditions in order to achieve an economic production

of the desired products. Spray-air contact is determined by the position of the atomizer in relation to the drying air

inlet. In a co-current flow design, spray evaporation is rapid and as the drying air cools, accordingly the evaporation

time is shortened. The product is not subject to heat degradation3.

1. Basic steps in spray drying process

Basic steps in spray drying are shown in Fig.1.

1.1. Concentration of herbal extract

Generally, extract is concentrated before introducing into the spray dryer. The concentrated extract has increased in

solid contents thereby reducing the amount of liquids which is evaporated in the spray dryer. In conventional large

scale spray dryer normally extract is concentrated to 50%-60% before introducing to spray dryer. However, the

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small scale laboratory spray dryer will have more diluted extract because it will be clogged easily if the feed have

high viscosity4.

Fig.1. Steps in spray drying process.

1.2. Atomization

Atomization refers to the conversion of bulk liquid into a spray or mist, often by passing the liquid through a nozzle.

The liquid which sprayed through nozzle will increase the surface area of the liquid which later will be contacted to

hot air and dried into a powder. The nozzle size may differ according to the size of spray dryer. Droplet size ranges

from 20 μm to 180 μm and it depends on the nozzle. Smaller spray dryer occupies smaller nozzles and reverse in the

industrial scale spray dryer. The aim of this stage is to create a maximum heat transferring surface between the dry

air and the liquid, in order to optimize heat and mass transfers. The choice upon the atomizer configuration depends

on the nature and viscosity of feed and desired characteristics of the dried product4.

The atomizers/sprayers are of three basic types5.

a. Pressure atomizer / nozzles

The distribution and size of the particles are related to the size of droplets formed by the aspersion process. In the

pressure atomizer, the material is pumped into the nozzle at a high pressure and it is forced to pass through a very

narrow orifice. Therefore, the use of special high pressure pumps and abrasion resistant material are essential in the

composition of the nozzle. From an energy point of view, the pressure atomizers are the most economic, however

these tend to suffer from clogging5.

Concentration of Herbal Extract

Atomization

a. Pressure atomizer / nozzles b. Pneumatic fluid / Double piston atomizer c. Centrifugal atomizer

Droplet-air contact

Droplet drying

Separation of dried particles

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b. Pneumatic fluid / Double piston atomizer

In the pneumatic fluid atomizer a lower spray pressure is applied to the input. In this case, the liquid material is

ruptured by the shearing force generated by the difference in speed between both fluids: air and product. This system

has a higher energy demand. It is used due to its versatility, and high control of the size and uniformity of droplets. It

is often used in small drying operations and when using a more viscous input material5.

c. Centrifugal atomizer

The centrifugal atomizer, or rotating disc, is basically a disc that turns at the extremity of an axis, where the liquid

material is injected and accelerates radially, pulverizing the input in the drying chamber. There are various variants

of the disc project that provide a wide range of droplets sizes, therefore these are more widely used in industrial

projects5.

1.3. Droplet-air contact

The important component of spray dryer is the chamber; here the sprayed droplet is contacted with the hot air and

the drying process begins. Air is heated by the heating element which situated before entering the chamber to a

predefined temperature depending upon the characteristics of the feed fluid. The thermal energy of the hot air is used

for evaporation and the cooled air pneumatically conveys the dried particles in the system. The contact time of the

hot air and the spray droplets is only a few seconds, once the drying is achieved and the air temperature of air drops

instantaneously. The nozzle increases the contact area of droplet and hot air influences in the huge heat transfer

between droplet and hot air. The hot air evaporates moisture content in the droplet and changes into powder form.

The hot air is brought in contact with the spray droplets in the following ways through the air distributor4.

1. Co-current-Air and particles move in the same direction.

2. Counter-current-air and particles move in the opposite direction.

3. Mixed flow - particles are subjected to co-current and counter-current phase.

In co-current process the liquid is sprayed in the same direction as the flow of hot air through the apparatus, hot air

inlet temperature is typically 150-220oC, evaporation occurs instantaneously and then dry powders will be exposed

to moderate temperatures (typically 50-80oC) which limits the thermal degradations. In counter- current drying, the

liquid is sprayed in the opposite direction of hot air flow for high temperature process. Thermo-sensitive products

are usually restricted to in this process. However, the main advantage of this process is considered as more

economic in term of consuming energy4.

1.4. Droplet drying

At the stage of droplets - hot air contacts between the liquid and gas phases and balances the temperature and

established the vapor partial pressure. Heat transfer is carried out from the air towards the product and thus induces

the difference in temperature. Water transfer is carried out in the opposite direction due to the vapor pressure

difference. Based on the drying theory, three successive steps can be distinguished. Just after the hot air - liquid

contact, heat transfer majorly causes the increase of droplets temperature up to a constant value. After that, the

evaporation of water droplet is carried out at a constant temperature and water vapor partial pressure. The rate of

water diffusion from the droplet core to its surface is usually considered as constant and equal to the surface

evaporation rate. Finally, when the droplet water content reaches a critical value, a dry crust is formed at the droplet

surface and the drying rate rapidly decreases with the drying front progression and becomes dependent on the water

diffusion rate through this crust. Drying is finished when the particle temperature becomes equal to that of the air.

Each product has a difference of particle-forming characteristics such as expand, contract, fracture or disintegrate.

The resulting particles may be relatively uniform hollow spheres, or porous and irregularly shaped4.

1.5. Separation of dried particles

This separation is performed by a cyclone, placed outside the dryer that reduces product loss in the atmosphere. The

dense particles are recovered at the base of the drying chamber while the finest ones pass through the cyclone to

separate from the humid air. In addition to cyclones, spray dryers are commonly equipped with the filters, called

‘‘bag houses’’ that are used to remove the finest powder, and the chemical scrubbers remove the remaining powder

or any volatile pollutants4.

2. Advantages of Spray drying of Herbal Extracts

a. The cost of spray drying process is less. It is eight times more economical than freeze drying and four times

more economic than vacuum drying2.

b. The spray drying technique produce powders of a specific particle size and moisture content, irrespective

of the dryer capacity3.

c. In comparison to liquid extract, volume and weight of powder is less. It is easy for handling packaging and

transportation4.

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d. The most common and economical way to carry out microencapsulation to retain and protect chemically

reactive or flavor compounds in herbal extract is spray drying so that it is widely used in commercial scale7.

e. Powders produced by spray drying are resistant to microbiological and oxidative degradation2. The dried

dosage forms of medicinal plants are preferred than liquid presentations because of their higher stability7.

These powders have much longer shelf life4.

f. The large surface obtained by the spraying assures pleasant conditions for the drying process9.

g. The technology is not very sensitive concerning the substance of the material, so that solutions, emulsions

and pastes can be dried with this technology9.

h. It is applicable to both heat sensitive and heat-resistant materials3. The short drying time allows the drying

of heat sensitive materials9.

i. It assures continuous operating conditions (continuous inlet and outlet in powder-form product) 9. It is easy

process which is fully automatically controlled with a quick response time3.

3. Limitations/Difficulties in Spray drying of Herbal Extracts

a. Powder production using spray drying requires high energy consumption, leading to high operational costs

due to the rapid heat exchanging process2.

b. Spray drying process gives less percentage yield. Low molecular weight ingredients present in extract are

responsible for less percentage yield2.

c. During spray drying, feed materials forms paste-like soft structure at the wall of the spray dryer. Low glass

transition temperature of feed material containing lower molecular weight ingredient causes sticking

problem. Adding additives during spray drying production can help commercial producers to overcome the

stickiness problem2.

d. Wall deposition is processing problem in the spray dryer particles that indirectly affects the quality and

quantity of the product. The degree of wall deposition is affected by several factors including operating

parameters, type and size of spray dryer and the spray dryer wall properties. The development of wall

depositions in the spray dryer deteriorates the yield of the products and hence increases the costs of

manufacturing and maintenance3.

e. Moisture content is observed in final product if process is not carried out properly. Also the nature of

powder can be hygroscopic3.

f. Sticking of powder at exit points, may lead to blockage, downtime and loss of product10.

4. Factors affecting spray drying process

Factors affecting spray drying process and its impact on spray dried powder properties are shown in fig.2.

4.1 Inlet Air Temperature

Temperature of the heated drying air is called ‘Inlet Air Temperature’. Temperature of the air with the solid particles

before entering the cyclone is designated as ‘outlet temperature’. Usually, an inlet temperature of 150-220 oC and an

outlet temperature of 50-80 oC is maintained5. Powder properties such as moisture content, bulk density, particle

size, hygroscopicity and morphology were affected by inlet air temperature4.

4.1.1 Effect of Inlet air temperature on moisture content

Moisture content is decreased with the increase of drying temperature, due to the faster heat transfer between the

product and drying air. At higher inlet air temperatures, there is a greater temperature gradient between the atomized

feed and drying air and it results the greatest driving force for water evaporation4.

4.1.2 Effect of Inlet air temperature on bulk density

An increase in temperature causes the reduction in bulk density. An increase in the inlet air temperature often results

in a rapid formation of dried layer on the droplet surface and particle size and it causes the skinning over or

casehardening on the droplets at the higher temperatures. This leads to the formation of vapor-impermeable films on

the droplet surface, followed by the formation of vapor bubbles and, consequently the droplet expansion, the

increase of drying air temperature generally causes the decrease in bulk, particle density and provides the greater

tendency to the particles to hollow4.

4.1.3 Effect of Inlet air temperature on particle size

The use of higher inlet air temperature leads to the production of larger particles and causes the higher swelling and

thus increases particle size6. When the inlet air temperature is low, the particle remains more smaller4.

4.1.4 Effect of Inlet air temperature on particle shape

The particles exhibited the spherical shape and in various sizes, caused by materials produced in spray drying. When

the inlet air temperature was low, the particles showed a shriveled surface, while increasing drying temperatures

resulted in a larger number of particles with smooth surface. This is associated with the different drying rates, which

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has the highest rate at higher temperatures, causing the faster water evaporation and then it led to the formation of

smooth, porous and hard crust4.

4.1.5 Effect of Inlet air temperature on hygroscopicity

The powders produced at higher inlet temperatures were more hygroscopic due to the presence of lower moisture

content in the powder4.

4.1.6 Effect of Inlet air temperature on yield

The increase of inlet temperatures has given the higher process yield and it was due to the greater efficiency of heat

and mass transfer processes occurring when higher inlet air temperatures were used. Sometimes the increase of inlet

air temperature has reduced the yield and it might be caused by melting of the powder and cohesion wall and

therefore the amount of powder production and yield was reduced4.

Fig.2. Factors affecting Spray drying process and effect of spray drying process parameters on powder properties.

4.2. Velocity of Inlet Air

Generally, the energy available for evaporation is varied according to the amount of drying air. The rate of air flow

must be at a maximum in all cases. The movement of air is decided the rate and degree of droplet evaporation by

inducing, the passage of spray through the drying zone and the concentration of product in the region of the dryer

walls and finally extent the semi-dried droplets and thus re-enter the hot areas around the air disperser. A lower

drying air flow rate causes an increase in the product halting time in drying chamber and enforces the circulatory

effects. The increased residence time led to the greater degree of moisture removal. As a result, an increase of the

drying air flow rate, decrease the residence time of the product in the drying chamber and it leads to have higher

moisture contents. In addition, inlet air flow rate affects bulk density, moisture content and sticky nature of the

Bulk density

Glass Transition

Temperature (Tg)

Particle size

Colour index

Solubility index

Percentage yield

Hygroscopicity/Moisture content

Flavor retention

Powder flow properties

Factors affecting Spray Drying Process Effect of Spray Drying Process Parameters

on Powder Properties

Inlet Air Temperature

Velocity of Inlet Air

Outlet air temperature

Atomizer speed

Feed flow rate

Type of Spray drying agent

Concentration of carrier agent

Type of solvent used for preparation of extract

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product. The higher moisture content in the powder leads to stick together and consequently leaving more

interspaces between them and it results the larger bulk volume. Therefore, the raise of air flow rate leads to an

increase of moisture content in the powder and decreased in powder bulk density. The effect of drying air flow rate

on powder solubility depends on its effect on powder moisture content, as low moisture content seemed to be

associated with the fast dissolution. The rising of air flow rate was led to the increased of powder moisture content

and decrease in powder solubility4.

4.3. Outlet air temperature

If outlet temperature is more, then the moisture content in dried powder is less and vice versa. The outlet air

temperature controls the final moisture content of the powder11.

4.4. Atomizer speed

The residual moisture content was decreased when increasing the atomizer speed. At higher atomizer speed, the

smaller droplets were produced and more moisture was evaporated resulting from an increased contact surface. The

higher atomizer speed resulted in a smaller particle size and quicker drying due to the larger surface area and

consequently it prevented the ‘‘skinning’’ over the droplets. The increased of atomizer speed was spread the liquid

into thin film layer and thus caused the smaller droplet and particle size4.

4.5. Feed flow rate

Higher flow rates imply in a shorter contact time between the feed and drying air and making the heat transfer less

efficient and thus caused the lower water evaporation. The higher feed flow rate showed a negative effect on process

yield and that was resulting the decreased heat, mass transfer and the lower process yield. In addition, when higher

feed rates were used, a dripping inside the main chamber was observed, when the mixture was passed straight to the

chamber and that was not atomized and finally resulting the lower process yield4.

4.6. Type of Spray drying agent

The addition of high molecular weight additives to the product before atomizing is widely used as an alternative way

to increase Tg of powder. The use of carrier agents such as maltodextrins, gum Arabic, waxy starch, and

microcrystalline cellulose, was influenced the properties and stability of the powder. Crystalline and amorphous

forms of the same material powder show differences in particle size, particle shape, bulk density, physicochemical

properties, chemical stability, water solubility and hygroscopicity. The common carrier agents used for fruit juices

are maltodextrins and gum Arabic4.

4.7. Concentration of carrier agent

The concentration of the carrier agent also affected the powder properties. Low concentration of carrier agent may

obtain the stickiness powder.

4.8. Type of solvent used for preparation of extract

Closed cycle spray dryers should be used to handle flammable solvents, highly toxic products and oxygen sensitive

products to avoid atmospheric pollution and/or to establish complete recovery of the evaporated solvent. The closed

cycle system is based upon recycling and reusing the gaseous drying medium, which usually is an inert gas such as

nitrogen. Drying chambers are incorporated with a cyclone/bag filter, solvent vapour condenser, exhaust drying

medium particulate cleaning in wet scrubbers and indirect drying medium heating12.

5.1. Types of Spray drying additives

Among commercially available additives that is used, major type additives that available for spray drying

application are carbohydrates (hydrolysed starch, maltodextrin, dextran, cellulose and derived), gums (Arabic gums,

agar, carrageenan), proteins (gluten, caseins, albumins and peptides), lipid (wax, paraffin, diglycerides and peptides)

and biopolymers. Spray drying application using lipid and biopolymers are less significant2.Table-1 shows various

spray drying agents used for Herbal extract.

Table 1-Review of spray drying process for herbal extracts.

Sr.

No.

Herbal Extract Spray drying

additives used

Spray Dryer Model Inlet Air

Temperatur

e

Outlet

Air

Temperat

ure

Year of

Publicati

on

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1. Rose hip extract9 - RSZL-10 type 153oC 65 oC 1994

2. Camomile extract9 - RSZL-10 type 145 oC 66 oC 1994

3. Lime blossom extract9 - RSZL-10 type 150 oC 64 oC 1994

4. Faba bean (Vinca faba

variety Dulce miel)

seed extract14

- Mini spray dryer

(Buchi B-190

Laboratoriums-

Technik, Flawil-

Schweiz,

Switzerland)

180 oC 110 oC 1998

5. Tomato Pulp15 - Pilot scale spray

dryer (Buchi, B-

191) with cocurrent

operation

100-140 oC 60-90 oC 2003

6. Curcumin, obtained

from the rhizomes of

Curcuma longa L.,

Zingiberaceae

(turmeric)16

PVP K30 (PVP) Spray Dryer (Jay

Instru- ments and

Systems Pvt. Ltd.,

India)

60 oC 45 oC 2004

7. Mango (Mangifera

indica L.) bark

extract17

- NIRO Mobil Minor

(Niro Atomizer,

Denmark)

155-185 °C - 2009

8. Tanacetum

parthenium L.(

Feverfew)

hydroalcoholic

extract18

Colloidal silicon

dioxide

A mini spray-drier

model LM MSD 1.0

(Labmaq do Brasil

Ltda., Brazil)

100 oC 60 oC 2009

9. Concentrated orange

juice19

Maltodextrin Buchi, B-191 (Buchi

Laboratoriums-

Technik, Flawil,

Switzerland)

100-140 °C - 2010

10. Acerola Pomace

extract20

Maltodextrin,

cashew tree gum

Mini Spray Dryer

Buchi B-290 (Buchi

Labortechnik AG,

Flawil, Swit-

zerland)

170-185 °C - 2010

11. Garcinia indica

(Choisy fruits)

extract21

Maltodextrin

(DE 06, 19, 21,

and 33)

Spray dryer (model

BE1216, Bowen,

Somerville, NJ)

150 oC 100 oC 2010

12. Morinda citrifolia L.

(NoNI) fruit extract22

K-Carrageenan Lab Plant spray

drier SD-05 (pilot

scale) with co-

current spray.

90-140 oC - 2011

13. Liquorice (Glycyrrhiza

glabra) extract23

Maltodextrin laboratory scale

spray drier

(LabPlant SD- 04,

Huddersfield,

England)

110-130 oC - 2012

14. Morinda citrifolia L.

extract24

Carrageenan and

Maltodextrin

Lab Plant spray

drier SD-05 (pilot

scale) with co-

current spray.

90-140 oC - 2012

15. Apeiba tibourbou

Aubl. Extract25

Colloidal silicon

dioxide (Aerosil)

Laboratory-scale

spray dryer model

MSD 1.0 (Labmaq

do Brasil Ltda.,

100-140 oC - 2012

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Brazil)

16. Tomato (Solanum

lycopersicum L.)

pulp26

Starch, Arabic

gum and

Maltodextrin

Acmefil make spray

drier available at

Gandhigram Trust,

Gandhigram,

Dindigul.

130 oC 85 oC 2013

17. Red Pitaya

(Hylocereus

polyrhizus) Peel 27

Maltodextrin Pilot-scale rotary

atomizer-type spray

dryer (Niro A/S,

GEA, Germany)

155–175

°C

75–85 °C 2013

18. Black tea (Camellia

sinensis) extract28

- Tall Type Spray

dryer (Model

No.IGO12X120,

Mfd by SM

Scientech Kolkata-

29)

180-240 oC 95 oC 2014

19. Gypsophilia

Saponaria officinalis

(Soapwort) roots

aqueous extract29

Maltodextrin Niro Automizer

Mobile Minor Unit

(Soeborg, Denmark)

110-160 oC 50-80 oC 2014

20. Jucara (Euterpe edulis

M.) pulp30

Gelatin, Gum

Arabic, and

Maltodextrin

Mini Spray-Dryer-

MSD1.0

(LabmaqdoBrasil,

Ribeirao Preto,

Brazil)

140-170 oC 85-130

oC

2014

21. Roselle (Hibiscus

sabdariffa L) seed

extract31

Maltodextrin Lab Plant SD-50

(England)

150–170

°C

-

2014

22. Guava slurry32 Maltodextrin Mini Spray Dryer 170–185

°C

- 2014

23. Psidiumguajava L.

leaves extract33

Maltodextrin,

Colloidal silicon

dioxide, Arabic

gum and 𝛽-

cyclodextrin

Benchtop spray

dryer (model SD 05,

Lab-Plant, UK)

150 oC 100-110

oC

2014

24. Grape Syrup34 Maltodextrin Pilot scale spray

dryer (Maham

Sanat, neishabour,

Mashhad, Iran)

150-170 oC - 2014

25. Tongkat Ali

(Eurycoma longifolia)

aqueous root extract35

- Laboratory scale

spray dryer Lab

Plant TM SD-06

(UK)

100-220 oC - 2015

26. Cinnamon

(Cinnamomum

zeylanicum)

infusions36

Maltodextrin Mini spray dryer

Buchi B – 290

(Flawil,

Switzerland)

140-180 oC - 2015

27. Zingiber officinale

(ginger) extract37

Arabic Gum,

Maltdextrin

Pilot plant scale

spray drier

(Armfield Inc.,

Jackson , NJ)

160oC 65 oC 2016

28. Blueberry (Vaccinium

myrtillus) Juice38

HP-β-

cyclodextrin

and Maltodextrin

Buchi Mini Spray

Dryer B – 290

140 oC 70 oC 2016

29. Mango Pulp39 Maltodextrin Laboratory-scale 150 oC - 2017

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spray dryer Buchi B

– 290

30. Aloe Vera Juice40 Maltodextrin LSD-48 mini spray

dryer (Jay

Instruments and

Systems Pvt. Ltd.,

Mumbai)

140-160°C - 2017

31. Laurel clock

(Thunbergia laurifolia

Lindl), Bamboo grass

or Ya-nang (Tiliacora

triandra (Colebr.)

Diels and Asiatic

pennywort or Bai-Bua

Bok (Centella asiatica

(L) Urb.) extract41

Maltodextrin

(DE 10)

Mini Spray dryer

(Buchi, B- 191,

Laboratory-

Techniques Ltd.,

Flawil, Switzerland)

120-160 °C 90 oC 2018

32. Lemongrass

(Cympopgoncitratus

L.) leaf extract42

maltodextrin and

gum Arabic

Lab Plant SD06

spray dryer (Keison,

Chelmsford, UK)

110-150 °C - 2018

33. Liquorice (Glycyrrhiza

glabra Linn.) roots

extract43

Aerosil Laboratory scale

Labultima UV222

spray drier

110-130 °C - 2018

34. Mulberry (Morus alba

Linn.) leaves extract in

70% ethanol44

Maltodextrin laboratory-scale

Mini Spray Dryer

Model LU-122

Advance, Labultima

60 ºC 55 ºC 2019

35. Passion Fruit

(Passiflora Edulis)

fruit juice45

Maltodextrin,

Arabic Gum,

Modified Starch,

Whey Protein

concentrate

Laboratory spray

dryer provided with

rotary atomizer,

pressure nozzle,

pneumatic nozzle

and sonic nozzle

120-150 oC - 2019

36. Palmyra Palm

(Borassus flabellifer)

Juice46

Arabic Gum,

Maltdextrin

Laboratory spray

dryer

120-150 oC 75-90 oC 2019

37. Fallopia multiflora

extract47

Maltodextrin Laboratory spray

dryer

145-175 oC 75-90 oC 2019

38. Watermelon (Citrullus

lanatus) juice48

Arabic Gum,

Maltdextrin

Laboratory spray

dryer

140-170 oC 75-90 oC 2019

39. Black Carrots (Pusa

asita) extract49

Maltodextrin

(20DE), Gum

Arabic and

Tapioca Starch

Laboratory Spray

Dryer (LU–228,

Labultima Pvt. Ltd.,

Mumbai, India)

150 oC 53 oC 2019

40. White horehound

(Marrubium vulgare

L.) extract50

Maltodextrin Pilot scale spray

dryer (APV

Anhydro AS,

Soborg, Denmark)

130 ± 5 oC 75-80 oC 2019

41. Basil (Ocimum

Basilicum L.) extract51

Maltodextrin Anhydro lab scale

spray dryer (APV

Anhydro AS,

Denmark)

120 oC 80 oC 2019

42. Anthocephalus

cadamba (Roxb.)

leaves extract52

Aerosil Laboratory scale

Labultima UV222

spray drier

110-140 oC - 2019

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5.1.1. Carbohydrates

High molecular weights carbohydrate additives help to increase the transition temperature of the spray drying

process. Maltodextrin is available in the form of white powder, made from corn starch hydrolysed by acids of

enzymes. Maltodextrin is non-sweet, neutral smell additives and used in spray drying production due to its low cost

and bulking properties. The variable nature of maltodextrin can be found through the degree of hydrolysis of starch

polymer. The significant higher level of dextrose equivalent (DE) in the hydrolysed starch has lower average

molecular weight and has low permeability to oxygen and water. Maltodextrin with dextrose equivalent between 10

and 20 have higher flavour retention. Otherwise, usage of lower DE with higher molecular weight can improve the

transition temperature of the product and it increases percentage yield and decreases caking problem. Maltodextrin

has a significant effect on the solubility of powder, however, has shown contradicting results in hygroscopicity

properties of the powder. Maltodextrin is also used in combination of other additives like Arabic gum, modified

starch and whey proteins2.

Sucrose, glucose and fructose are not suitable with high temperature of spray drying which can cause formation of

caramel. Non-hydrophobic properties of starch have high viscosity value. Modified starch has found their way into

commercial spray drying production, where modified starches have high retention of volatiles and stability.

However, maltodextrin is commonly added with modified starches to improve against oxidation during storage of

product. Cellulose has been used as complementary additives in spray drying to produce partial crystalline surface,

reduce stickiness and caking problem2.

5.1.2. Proteins

Gelatin, casein, whey protein concentration are used for multiple feed products. These additives show ability to

combine with a different type of feed products through their molecular chain diversity and functional properties.

Protein additives have rapid film-forming properties, large molecular weight and expanded functional properties

such as high solubility and viscosity. Proteins can be used to improve spray drying of high sugar content powder.

Small concentration of protein is capable of forming film properties around the surface spray-dried particles,

prompting a thin protein-rich film to resist particle-to-particle cohesive and particle-to-wall stickiness2.

High-protein concentration powders have substantially different structure and stability compared to low-protein

concentration powders, considering the type of protein-additives and concentration of feed-additives. Whey powder

has the capability of retaining flavoring compound. Protein additives are commonly used in spray drying for milk

protein and gelatins. Spray drying efficiency and preservation of milk fat yield is greater without the use of

additives. New studies have found that vegetable proteins additives have additional functional advantages like

biocompatibility, biodegradability, emulsifying and foaming capacity2.

5.1.3. Gums

Acacia gum (Gum Arabic) consists of a combination of complex carbohydrates and protein (D-glucuronic acid, L-

rhamnose, D-galatose and L-arabinose in the proportion of 4:2:2:1). The protein component of the acacia gum

improves the emulsification properties. It is used for its surface activity and film forming activity. Acacia gum has a

produces stable emulsion in wide range of pH, so it can be used for lipid-based products. Product obtained using

acacia gum does not have crystalline configuration and bulk density is less. Arabic gum is associated with high cost,

limited availability and impurities due to prone to variability in supply and quality2.

5.2. Factors affecting Selection of additive/s in Spray Drying Process While selecting an additive for spray drying, physiochemical properties of the additive, molecular weight, glass

transition temperature, concentration of additives are to be considered. The type of feed material used together with

added additives has a significant impact on the properties of extract obtained2.

5.2.1. Molecular weight of additives

The molecular weight of additives represents the molecular size of the additives which plays important role in spray

drying process. Shorter chain molecules of additives have low transition temperature than longer chain additives.

Maltodextrin (DE 36) with a molecular weight of 500 has transition temperature of 100°C and maltodextrin (DE 5)

has a significantly higher transition temperature of 188°C2.

Increasing transition temperature also contributes to powder stability and reduce caking and stickiness problem.

Gum acacia has higher molecular weight in comparison to maltodextrin. As the transition temperature increase with

the increase in molecular weight, the addition of gum acacia has a higher Tg compared to the addition of

maltodextrin. The larger molecular weight of maltodextrin has a direct relationship with faster rehydration and

reabsorption of powdered particles, due to the higher surface area over volume ratio exposed to moisture2.

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5.2.2. Concentration of additives

High concentration additives used in feed material produce a difference in the yield percentage, transition

temperature, hygroscopicity and other physicochemical properties. Multiple published papers have shown that

increase of maltodextrin concentration has resulted in an increase of yield product percentage. The yield product

percentage can be supported by the fact that surplus level of maltodextrin increases the total solid and reduced the

level of total water for evaporation.

Higher concentration of maltodextrin added produces powder with lower moisture content, but the concentration of

maltodextrin added does not affect its rehydration ability to absorb moisture. The concentration of protein additives

has inconsistent results towards physiochemical properties of spray dried powder. Addition of protein additives at

certain concentration increase yield production but doubling the concentration yield insignificant results. The ratio

of protein-carbohydrate additives has proven contradicting results in term crystallization process2.

5.2.3. Combination of additives

Maltodextrin is also used in combination of other additives like Arabic gum, modified starch and whey proteins2.

5.2.4. Feed Material Properties

Liquid properties relevant to spray drying are: solids content, density, surface tension and viscosity. High

concentration of solutes in the liquid is desirable to increase dryer thermal efficiency. Liquid components should be

thermally stable to withstand thermal treatment in the dryer13. Type of feed materials varies in properties like sugar

and fat content, viscosity, and transition temperature. Understanding of spray drying of various feed material on its

wide range of processing parameter and quality parameter of the powder attained requires through knowledge.

Powder recovery greater than 50% is stated to be an efficient spray drying system. The challenge of feed selection is

also hindered by the complexity of spray drying operating parameters on the physiochemical properties of the

powder. There is lack of viable and reliable results showing a comparison of performance between the commercially

available additives so, spray drying requires trial-and-error method as it involves a many factors1. The feeding can

either be in solution, suspension, emulsion or paste form3.

6. Effect of Spray Drying Parameters on Powder Properties

The selection of additives is heavily supported by understanding the properties of both spray drying feed and the

additives used, as different additives have different physical and chemical structure. Gum acacia is a highly ramified

structure that contains shorter chains and hydrophilic groups, while protein additives have many different functional

advantages such as biocompatibility, biodegradability, emulsifying and foaming capacity, water and fat absorption,

gelation and film-forming properties2.

6.1. Bulk density

There is no clear relationship between additives capabilities and bulk density. Additives addition into feed solution

has a different effect on the bulk density. Addition of maltodextrin shows additive effect that changes the bulk

density of powdered particles. Skin forming nature of additives increases the volume of air trapped in the particle

and decreases the bulk density of powders2.

6.2. Glass transition temperature (Tg)

Glass transition temperature (Tg) is a property of an amorphous material, where it defined as the temperature of

product amorphous system interchanging between a glassy and a rubbery state. During spray drying the particles

will undergo the transition from a glassy crystalize temperature to rubbery state matter depending on their water

activity level and molecular weight. Similarly, transition temperature (Tg) is highly associated with changes in

various physical properties such as boiling and melting point and appearances. Increasing temperature above Tg of a

material increase rate of deteriorative and stickiness. Spray drying of extract containing of low-molecular weight

molecules cause a stickiness problem during spray drying. To overcome low glass transition temperature of feed

material, adding additives have shown to increase the molecular weight of molecules, therefore increasing feed’s

transition temperature. By this application, the extension of shelf life of the powder can be extended. High stability

of powder is associated with high transition temperature and the risk of caking, and crystallization can be reduced2.

6.3. Particle size

The size of particle formed during spray drying is strongly related to feed viscosity, as higher liquid viscosity, the

larger the droplets formed during atomization produces larger particles obtained during spray drying. The addition

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of additives, especially an increase of maltodextrin concentration has proven to increase the particle size of spray

dried powder2.

The lack of research on additives is towards the microscopic point of view, there is substantial evidence showing

how the additives reacted on different powders products2.

6.4. Colour index

The physical appearance of the powders especially the color index depends on the type of feed material and

concentration of additives used. As the function of additives used in spray drying may increase the droplet size of

atomized feed as viscosity increases, there are no reports stating that additives improved the physical appearance of

end powder products. The increase in concentration of additives dilutes the colour of the feed solution and the end

product significantly as shows colour similar to additive. Overall, there are insufficient studies done on the effect of

additives on the powder colour index, as major studies focused on yield production, moisture content and other

parameters2.

6.5. Solubility index

Solubility is an important key factor for evaluating wettability and dispersibility of powder in aqueous solution. The

solubility index of spray dried powder is affected by the raw materials, and additives used. The solubility index also

depends on the properties of the powder (moisture content and size of particles). During spray drying, crust

formation occurred during rapid heat exchange environment in the chamber, where the least soluble substance

started to precipitate and forming crust at the droplet surface. Therefore, the formed crust is mainly constituted of

maltodextrin that is highly soluble in nature. If hard surface layer is formed over the powder particle, preventing the

diffusion of water molecules, then the wettability and solubility of the particle are reduced2.

6.6. Percentage yield

In the spray drying process, an increase of additive concentration increases percentage yield. The addition of

additives increases the total soluble solids content, resulted in an increase in efficiency yield. Sometimes, the

addition of additives in a spray drying operation led to a less efficient system, where high residual moisture content

found in powder products. Water molecules have difficulty in escaping from maltodextrin due to their large size and

film coating abilities. Furthermore, the increase of additives would produce a higher viscosity feed which reduces

the product yield of powder2.

6.7. Hygroscopicity/ Moisture content

In the spray drying process, an increase of additive concentration decreases moisture content in powder obtained and

it increases hygroscopicity of powder. The powder is more hygroscopic, if it contains less quantity of moisture2.

6.8. Flavor retention

The losses of both hydrophilic and hydrophobic flavors during the spray drying were reported in each step of

process. The losses of flavors were occurred first during the atomization step. The flavors and water are evaporated

with the hot inlet air at initial period of drying. The loss of flavors especially the high volatile flavors (with low

boiling point temperature) was expected in this period with the temperature of droplet increase. In the second step

with the constant drying periods, the formation of crust was occurred. The crust was the selective membrane allows

the higher diffusion rate of water than the flavors resulted the higher remaining of flavor in the droplet. However,

the high solubility flavor as well as hydrophilic flavor was lost in the higher amount than in the hydrophobic flavor.

The dissolved flavors were loss during the drying as the water molecule. After the solid droplets were formed, the

loss of flavors was expected to occur with the change of powder morphology. However, the losses in this step were

less amount comparing to the first and second steps7.

6.9. Powder flow properties

The powder obtained by spray drying show generally show poor flow properties because of the small size of its

particles. The small size results in problems related to the powder’s transport on an industrial scale, where uniform

mass flow out of bins and other vessels is required. In the case of dry plant extracts, the interactions between the

powder particles are often so strong that they do not pour at all. This phenomenon may also generate problems

related to obtaining proper mass uniformity in solid dosage forms. In addition, the tablet manufacturing process

requires the application of high pressure, which in turn results in tablets that are too hard and give prolonged

disintegration times53.

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7. Sticking problem associated with Spray drying

When spray drying technology is used to manufacture products, powder handling difficulties can arise for some

products due to adhesion of powder to equipment surfaces. This is generally considered to result from the formation

of liquid bridges containing amorphous sugar or other dissolved material conferring high surface tension, or perhaps

lipid material in some cases, between particles and equipment surfaces10.

7.1. Factors affecting Sticking

Factors affecting sticking are shown in Fig.3.

Fig.3. Factors affecting Sticking

7.1.1. Material of wall for drying chamber

The properties of the wall of the dryer in which deposition occurs also play a significant role in the mechanism of

product deposition. Recent experiments showed that nylon exhibited a lesser deposition rate due to the non-sticky

nature of the nylon compared to stainless steel. A teflon surface with less surface energy has a less deposition flux

compared to a stainless steel surface that has a higher surface energy. A higher wall temperature also increases the

deposition flux3.

7.1.2. Geometries or spray dryer types

The chamber geometry directly changes the air flow pattern and affect the behavior and flow pattern of the particle

within the dryer. Several researchers have studied possible chamber geometries such as pure conical, lantern and

hour-glass geometry, horizontal configuration and parabolic geometry as the spray drying chamber. It should be

noted that not only the cylinder geometry but the duration of the particle residence in the drying chamber as well as

overall chamber volume can impact drying performance3.

7.1.3. Swirling flow patterns

Swirling flow patterns in a spray dryer can be of two types: inlet vane induced swirls and atomizer-induced swirls.

Experimental measurements and observations have been undertaken for inlet-induced swirls which highlighted the

unsteady and oscillatory behavior of such flows. Experimental data indicated that the wall deposition was minimized

for no swirl, but that evaporation was still adequate3.

7.1.4. Spray Dryer Wall Temperature

Particles deposit on the wall by sticking to it, which is due to the sticky particle which occurs above the glass

transition temperature, Tg. Spray drying produces dry amorphous powders that are thermo- plastic due to heating or

exposure to high humidity, resulting in water sorption and thermal plasticization of the particle surfaces. At

temperatures above Tg, amorphous structures are in a rubbery state where the polymer molecules become softer and

more flexible because of greater molecular mobility. The temperature of the surface of the product such as

amorphous sugar should not reach more than 10–20oC above Tg during spray drying to avoid substantial product

Factors

affecting

Sticking in

Spray Drying

Process

Swirling flow

patterns

Spray Dryer Wall

Temperature

Geometries or spray

dryer types

Effect of Feed flow

rate

Effect of spray drying

agents

Nature of Feed

component Material of wall for

drying chamber

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stickiness, especially in low molecular weight carbohydrates. This is due to the greater molecular mobility of the

amorphous components in a highly viscous flow between the particle surfaces, making the powder more cohesive.

At temperatures below Tg, the amorphous parts of the materials are in a glassy state where the polymer molecules

have no segmental motion but vibrate slightly. When the wall temperature is below the sticky point temperature,

there is less wall deposition3.

7.1.5. Effect of Feed flow rate

When the feed flow rate increases, larger droplets are formed and the evaporation rate is lower. This is caused by the

larger amounts of water introduced into the dryer. Increase in feed flow rate result in higher residual moisture

content. There was a corresponding visual increase in deposits on the wall of the drying chamber. The deposition in

this case is high because of the higher feed flow rate. The rate of adhesion of the first layer was different compared

to that of other layers when particles cohere to the adhered particles. At a constant air inlet temperature, increasing

the feed flow rate increased the wall deposition. When more feed was atomized into the chamber, the residence time

of the particles was shorter and the drying time was reduced, resulting in wetter particles. In this condition, the

particles were more cohesive which caused the deposition rate to increase and the yield to decrease3.

7.1.6. Effect of spray drying agents

The effect of additive as a spray drying agent is more significant in reducing the deposition fluxes. The additives

added to the sugar-rich feed, increased its molecular weight and hence its glass transition temperature Tg, which

reduced the particle stickiness and wall deposition in the spray drying3.

7.1.7. Nature of Feed component

Wall deposition is affected by the residence time and distribution of the particles. An important factor in

determining residence times with high wall deposition rates was the time taken by the particles to slide down the

conical wall of a spray dryer. The sticking of particles to the walls and to each other, and the sliding of wall deposits

are therefore important issues. The presence of lactose and protein on the surface could have made the particle

surface more rigid due to the high glass transition point. On the other hand, another possibility is that the presence of

lactose and protein could have made the particle surface more hydrophilic. Presence of a high proportion of protein

on the surface, however, led to significant reduction in the adhesion rate at the cone of the spray dryer. The rate of

adhesion of the first layer might be different compared to that of other layers when particles cohere to the adhered

particles3.

CONCLUSION:

Medicinal plants are used by 80% of the world population as primary health care and the phytomedicine market is

growing exponentially. Currently, the production of phytopharmaceuticals with proper efficacy, safety and

consistent quality constitutes a relevant challenge. The spray drying technology can be used to produce dried

extracts from medicinal plant liquid extracts8.

Published reports that signified the effect of additives on the properties of the powder, it is shown that different

powder has shown different characteristic under the influence of different additives. However, a similar trait has

appeared that additives improve product yield through the manipulation of transition temperature. Added to that,

usage of combination additives proved positive results towards better yield, solubility and bulk density.

Optimization of spray drying requires an evaluation of both spray drier parameter and feed formulation, as the

modulating of spray drying must be controlled to avoid low yield, moisture content and sticking problem. Effective

additives can optimize the performance of spray drying parameters. The efficiency of spray drying parameters such

as inlet temperature, feed flow rate, outlet temperature and nozzle pressure can be enhanced with the use of

additives. Use of additives is cost reducing and high productivity through the functionality of additives of

manipulating the transition temperature, total soluble solids and viscosity of the solution.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Vol XIII Issue VII 2020

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