Effect of addition of maltodextrin on drying kinetics and stickiness of sugar and acid-rich foods...

Journal of Food Engineering 62 (2004) 53–68

www.elsevier.com/locate/jfoodeng

Effect of addition of maltodextrin on drying kinetics and stickinessof sugar and acid-rich foods during convective drying:

experiments and modelling

B. Adhikari a, T. Howes b,*, B.R. Bhandari c, V. Troung c

a Centre for Energy and Environmental Processes, Ecole Des Mines D�Albi Carmaux, Campus Jarlard-Route de Teillet,

81013 Albi CT, Cedex 09, Franceb School of Engineering, The University of Queensland, St. Lucia 4072, Australia

c Food Science and Technology, School of Land and Food Sciences, The University of Queensland, St. Lucia QLD 4072, Australia

Received 16 September 2002; accepted 24 May 2003

Abstract

The effect of addition of maltodextrin on drying kinetics of drops containing fructose, glucose, sucrose and citric acid individually

and in mixtures was studied experimentally using single drop drying experiments and numerically by solving appropriate mass and

heat transfer equations. The numerical predictions agreed with the experimental moisture and temperature histories within 5–6%

average relative (absolute) errors and average differences of ±1 �C, respectively. The stickiness of these drops was determined using

the glass transition temperature ðTgÞ of the drops� surface layer as an indicator. The experimental stickiness histories followed the

model predictions with reasonable accuracy. A safe drying (non-sticky) regime in a spray drying environment has been proposed,

and used to estimate the optimum amount of addition of maltodextrin for successful spray drying of 120 micron diameter droplets

of fruit juices.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Drying kinetics; Sugar and acid-rich foods; Glass transition temperature; Stickiness; Safe drying regime

1. Introduction

Foods to be spray dried can be subjectively classified

into two broad groups: non-sticky and sticky. In general,

non-sticky materials can be dried using a simple dryer

design and the final products remain free flowing.

Materials such as skim milk and solutions such as

maltodextrins, gums, and proteins belong to this group.

On the other hand, sticky materials are difficult to dryunder normal spray drying conditions. Natural sugar

and acid-rich foods such as fruit and vegetable juices,

and honey belong to this group. The sticky behavior

of sugar and acid-rich materials is attributed to low

molecular weight sugars such as fructose, glucose, su-

crose and organic acids such as citric, malic and tartaric

acid which constitute more than 90% of the solids in fruit

juices and purees (Dolinsky, Maletskaya, & Snezhkin,

*Corresponding author. Tel.: +61-7-3365-4262; fax: +61-7-3365-

4199.

E-mail address: [email protected] (T. Howes).

0260-8774/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0260-8774(03)00171-7

2000). Stickiness is a major reason which has limited theuse of spray drying for sugar-rich and acid rich foods.

On the other hand the sticky problem is not en-

countered when less hydrolysed starch derivatives such

as maltodextrins are spray dried; instead, they facilitate

the spray drying process of the sugar-rich foods. Hence,

they are frequently used as drying aids (Bhandari,

Datta, & Howes, 1997b; Bhandari, Senoussi, Dumoulin,

& Lebert, 1993). The use of maltodextrins as drying aidshas been in practice since the 1970s (Brennan, Herrera,

& Jowitt, 1971; Gupta, 1978) but the systematic study

on how and why maltodextrins help to overcome

stickiness did not start for another two decades. Bhan-

dari and his co-workers (Bhandari, Datta, Crooks,

Howes, & Rigby, 1997a) developed a semi-theoretical

drying aid index based on product recovery, which was

successfully used to determine the optimum fruit juice/maltodextrin ratio in a pilot scale spray dryer. However,

questions arise such as what is the effect of addition of a

drying aid (maltodextrin) on the drying kinetics of low

molecular weight sugars and organic acids? How is the

mail to: [email protected]

Nomenclature

List of SymbolsCp specific heat capacity, J/kg �CCs concentration of solid, kg/m3

Df diameter of glass filament, m

Dw moisture diffusivity in solution, m2/s

F flux of water at the surface of drop contain-

ing solids, kg/m2 s

h�g corrected gas side heat transfer coefficient, W/

m2 �Ckf thermal conductivity of glass filament, W/

m �CK proportionality constant in Gordon–Taylor

equation, [–]

K�g corrected mass transfer coefficient, m/s

ms bone dry solid mass, kg

P particular parameter (such as moisture diffu-

sivity,. . .)R radius of drop, m

r radial distance, mT temperature, �CTg glass transition temperature, �C

t time, s

u moisture in drop (dry basis), kg water/kg

solid

x mass fraction, [–]

DHv latent heat of vaporization of water, J/kg

z, Z spatial variables in solute-fixed coordinate,

kg

w dimensionless time, [–]

Subscriptsa related to air

d related to the drop

i particular component

i–w binary mixture (such as fructose–water, su-

crose–water etc.)

mixture of mixture

n number of components0 initial value

s solid

W, w related to water or solution

54 B. Adhikari et al. / Journal of Food Engineering 62 (2004) 53–68

surface stickiness of these materials affected when the

drying aid is added? How does the drying medium

temperature affect the stickiness? Questions like these

have not been properly addressed.

Hence this paper aims to answer the above questions

in the light of results obtained from single drop drying

experiments and with the aid of predictive tools deve-

loped previously for prediction of stickiness of drops ofbinary solutions (Adhikari, Howes, Bhandari, & Tru-

ong, 2003c). Section 2 briefly presents the model used

for prediction of the stickiness history of a drop con-

taining a multicomponent mixture. Section 3 presents

the experimental and predicted drying kinetics of ter-

nary systems and also shows the stickiness history of

these drops. Section 4 presents the experimental and

predicted drying kinetics of drops containing modelsugar-rich solutions including their predicted stickiness

history. Section 5 presents the predicted stickiness

history of a drop of model sugar and acid-rich foods.

Finally, Section 6 concludes the paper.

2. Modelling of drying kinetics and stickiness of multi-

component drops

2.1. Drop drying model for solutions

The following assumptions were made to develop the

model:

• The drop is a solid (non-hollow) sphere.

• It shrinks uniformly with loss of water (ideal shrink-

age).

• It neither expands nor bursts.

• There are no temperature gradients within the drop.

• There is no internal circulation.

• Moisture transfer within the drop is by molecular dif-

fusion and species convection.• Heat transfer to the drop is solely by convection.

• The drop is pseudo-binary in composition (i.e. water

and solids).

• The heat of sorption is negligible.

2.1.1. Prediction of moisture, drop temperature and glass

transition temperature histories

The distribution of moisture, uðr; tÞ, within a drying

drop, is computed by solving the diffusion equation (1)

(Van der Lijn, Kerkhof, & Rulkens, 1972) in a solute-

fixed coordinate system. The moisture history is ob-

tained by averaging the moisture distributions. Thetemperature history is predicted by Eq. (2), which was

developed from an energy balance around the drop. The

second term in the numerator of this equation accounts

for the heat conducted through the supporting filament.

The filament is considered as an infinite fin here

(Incropera & DeWitt, 2002).

ouot

¼ o

ozDwðu; T ÞC2

s r4 ouoz

� �ð1Þ

B. Adhikari et al. / Journal of Food Engineering 62 (2004) 53–68 55

dTddt

¼4pR2½h�gðTa � TdÞ � DHvF � þ 0:5pDf

ffiffiffiffiffiffiffiffiffiffiffiffiffih�gDfkf

pðTa � TdÞ

msðuCp;w þ Cp;sÞð2Þ

Dwðu; T Þ, u, Cs and r in Eq. (1) are the moisture diffu-

sivity (m2/s), moisture (kg water/kg solid), concentration

of solid in solution (kg/m3) and radial distance (m),respectively. z is the spatial variable in a solute-fixed

coordinate system, which is defined by Eqs. (3) and (4)

below. R, F , ms, Cp;s, Cp;w, and DHv in Eq. (2) are the

drop radius (m), flux of water [kg/(m2 s)] leaving the

drop surface, mass of dry solid in the drop (kg), specific

heat capacity of solid [J/(kg �C)], specific heat capacity

of water [J/(kg �C)] and latent heat of vaporization of

water (J/kg), respectively. Similarly, h�g, Df , kf , Tf and Tdare the heat transfer coefficient [W/(m2 �C] corrected for

high flux, diameter (m) and thermal conductivity [W/

(m �C)] of the glass filament, temperature of the bulk air

(�C) and temperature of the drop (�C), respectively.

dzdr

¼ Csr2 ð3Þ

4pZ ¼ ms ¼ 4pZ R

0

Csr2 dr ð4Þ

ð4pZÞ represents the mass of the dry solid (kg) in the

drop, which is unchanging with time. The numericalsolution of Eq. (1) requires one initial and two boundary

conditions while the solution of Eq. (2) requires an ini-

tial condition only. The initial and boundary conditions

are given in Eqs. (5)–(7).

IC : t ¼ 0; 0 < z < Z; u ¼ u0 and Td ¼ Td;0 ð5ÞBC1 : t > 0; z ¼ 0; ou=oz ¼ 0 ð6Þ

BC2 : t > 0; z ¼ Z; Dwðu; T ÞC2sR

2 ouoz

¼ �F ð7Þ

u0, and Td;0 are initial moisture content and temperature

of the drop, respectively. The first boundary condition

reflects the symmetry at the centre of the drop and thesecond boundary condition states that the amount of

water leaving the drop surface equals the diffusive flux at

the surface.

2.1.2. Glass transition temperature

The glass transition temperature, Tg, of a solid–water

mixture is strongly dependent on the water concentra-

tion, that is, Tg ¼ f ðuÞ. Once the distribution of mois-ture ðuÞ within a drying drop is known through the

solution of Eq. (1), the distribution of Tg within the drop

can be determined using the Gordon–Taylor equation

(Gordon & Taylor, 1952). The Gordon–Taylor equation

can be re-written in terms of moisture ðuÞ, shown in Eq.

(8).

TgðuÞ ¼Tg;s þ KTg;wu

1þ Kuð8Þ

K is the solid–water binary constant which has to bedetermined experimentally. The numerical solution of

Eqs. (1) and (2) along with Eq. (8) has been detailed

elsewhere (Adhikari et al., 2003c).

2.1.3. Physical parameters

The determination of moisture and glass transition

temperature within the drop with the solution of Eqs. (1)

and (8), respectively require the availability of physical

parameters such as moisture diffusivity, water activity,

solid density and glass transition temperature of the

solid mixtures. Similarly, the determination of tempe-

rature history requires reliable specific heat capacityvalues. These parameters are determined as discussed

below.

The Tg, moisture diffusivity, specific heat capacity and

water activities of multi-component solid mixtures are

determined using a mass weighted mean rule. First, the

multi-component mixture is assumed to be composed of

n individual binary solid–water mixtures, where n is the

number of solid components. The moisture dependenceof these parameters for each binary solid–water mixture

is determined first. Finally, the solids are assumed to be

perfectly mixed and the other properties of the multi-

component mixture are computed as a mass weighted

mean on a water free basis as represented by Eqs. (9)

and (10).

Pmixture ¼Xn

i¼1

Pi–wxi ð9Þ

Xn

i¼1

xi ¼ 1 ð10Þ

Pmixture represents the individual parameter (e.g. Tg)for the multi-component mixture including water.Pi–w, represents the same parameter (e.g. Tg) of binary

solid–water mixtures such as: fructose–water ðTg;f–wÞ,glucose–water ðTg;g–wÞ and so on. xi is the fraction of an

individual solid component on a water free solids basis.

The correlation of these parameters for individual solid–

water binary mixture have already been reported else-

where (Adhikari et al., 2003c).

3. Drying kinetics of a ternary system

Fructose (ADM Corn Processing, USA), sucrose

(Bundaberg Sugar Company, Queensland, Australia),

glucose (Boots Health Care, Australia), maltodextrin of

dextrose equivalent (DE) 6 (Glucidex, Roquette, Freres,

France) and Citric acid (ADM Australia) were used as

samples. Single drops of sucrose +maltodextrin, fruc-

tose +maltodextrin and citric acid +maltodextrin inwater were dried at two different initial concentrations

(40% and 50% w/w) using air temperatures (63± 1 and

95± 2 �C) and 1 m/s air velocity and 2.5 ± 0.5% relative


humidity. Three different sugar (or acid)/maltodextrinratios, 4:1, 1:1 and 1:4 were tested. Tests were carried out

in a single drop drying device based on an intrusive mode

of levitation reported earlier (Adhikari et al., 2003c).

3.1. Experimental results

3.1.1. Moisture histories

The effect of addition of maltodextrin on the moisture

histories of the sucrose, fructose and citric acid solutions

at 63 �C is illustrated in Fig. 1(a)–(c), respectively. Thesefigures also include the moisture histories of sucrose,

fructose, citric acid and maltodextrin solutions as refe-

rence. These figures show that the addition of malto-

dextrin lowers the drying rate. This indicates that it is

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

)

Sucrose

S/M (4:1)

S/M (1:1)

S/M(1:4)

Maltodextrin

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

) Fructose

F/M (4:1)

F/M (1:1)

F/M (1:4)

Maltodextrin

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

) Citric acid

C/M (4:1)

C/M (1:1)

C/M (1:4)

Maltodextrin

(a)

(b)

(c)

Fig. 1. Moisture histories of drops of 40% solutions of: (a) sucrose (S)/

maltodextrin (M), (b) fructose (F)/maltodextrin (M) and (c) citric acid

(C)/maltodextrin (M) dried at 63± 1 �C, 1 m/s air velocity 2.5± 0.5%

relative humidity.

difficult for water molecules to diffuse past the largermaltodextrin molecules even when the maltodextrin

comprises only 20% of the total solids. The spread in

moisture histories is the widest in citric acid/maltodex-

trin mixtures, while it is narrowest in sucrose/malto-

dextrin ones. It is expected due to variation of the

moisture diffusivities of the respective pure components

(Adhikari, Howes, Bhandari, Yamamoto, & Truong,

2002). The moisture histories of the above mixture dropsat 95 �C had features similar to those at 63 �C, exceptbeing steeper at the early period. This is expected be-

cause larger moisture and temperature gradients are

established between the drop surface and the bulk air at

higher temperatures.

3.1.2. Morphological developments

Common morphological changes that take place

during drying at 63 �C in sucrose/maltodextrin and

fructose/maltodextrin drops at 63 �C are illustrated in

Fig. 2. The morphological changes in drops of glucose/

Fig. 2. Morphology of drops of sucrose (S)/maltodextrin (M) and

fructose (F)/maltodextrin (M) solutions at 4:1, 1:1 and 1: 4 ratio drying

at 63± 1 �C, 1 m/s air velocity and 2.5± 0.5% relative humidity. The

time given with the image indicates the time of its capture.

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Dro

p te

mpe

ratu

re (

o C

)

Sucrose

S/M (4:1)

S/M (1:1)

S/M (1:4)

Maltodextrin

Water

Air

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Dro

p te

mpe

ratu

re (

o C)

Fructose

F/M (4:1)

F/M (1:1)

F/M (1:4)

Maltodextrin

Water

Air

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Dro

p te

mpe

ratu

re (

o C)

Citric acid

C/M (4:1)

C/M (1:1)

C/M (1:4)

Maltodextrin

Water

Air

(a)

(b)

(c)

Fig. 3. Temperature histories of 40% solutions of: (a) sucrose (S)/


(C)/maltodextrin (M) drying at 63± 1 �C, 1 m/s air velocity 2.5± 0.5%

relative humidity.


maltodextrin and citric acid/maltodextrin mixtures wereclose to those of fructose/maltodextrin mixtures. Fig. 2

shows that the drop deviates from sphericity when 20%

maltodextrin is added to sucrose, while the addition of

this amount does not bring any observable deviations in

the fructose drop. Pure sucrose and fructose drops re-

main spherical at this drying condition. When the

amount of maltodextrin in sucrose or fructose drops is

increased to 50%, the drops deviate from sphericity andbecome pear-shaped (elongated) after 5 min of drying.

This shape was retained throughout the course of their

drying. This shape allows more water to leave the drop

and acts to offset the resistance to moisture diffusion

caused by the formation of a skin on the surface. When

the amount of maltodextrin is increased to 80%, the

drop exhibits almost all of the morphological features

such as surface folds, wrinkles and deviation fromsphericity shown by a pure maltodextrin drop. However,

the presence of 20% sugars or acids seems to lessen the

intensity at which these features are exhibited compared

with pure maltodextrin drops. The surface is less rugged

and the surface troughs are shallower. Furthermore, the

surface appears to be much softer compared to the pure

maltodextrin drop, which indicates that it is easier for

moisture to diffuse out from this softer surface. Besides,the presence of 20% sugars or citric acid delays the time

at which these changes first appear compared to a pure

maltodextrin drop. The morphological features at 95 �Cwere similar to those observed at 63 �C except that the

changes were more intense and appeared earlier. Drops

having a S/M ratio of 4:1 were more elongated, at 95 �C,than at 63 �C. It was found that the surface of drops

containing sugar/maltodextrin or citric acid/maltodex-trin at the solid ratio of 1:4 was softer (more thermo-

plastic) at 95 �C than at 63 �C which meant that the

surface of the drops becomes more plastic (soft) at

higher temperatures.

3.1.3. Temperature histories

Fig. 3(a)–(c) present the temperature histories of

drops of sucrose/maltodextrin, fructose/maltodextrin

and citric acid/ maltodextrin mixture solutions at 63 �C.The temperature histories of the respective pure com-

ponents and water drops of identical size are also in-

cluded for the purpose of comparison. Thesetemperature histories have two salient features. First,

there is no constant rate period as the drop temperature

continues to increase from the onset of drying. Second,

there is the presence of profile reversal which takes place

after about 300 s. Before the profile reversal, the tem-

peratures of sugars and citric acid are the lowest, that of

the maltodextrin is the highest and that of the mixtures

remain within these two extremes. As the proportion ofmaltodextrin in the drop increases, the drop temperature

also increases. This is due to the fact that the drops with

a higher proportion of maltodextrin have a lower

moisture diffusivity and decreased water flux leaving the

drop. This lowered flux provides less evaporative cool-ing and the drop temperature increases as a conse-

quence. After the profile reversal, the temperature of the

fastest drying drop is the highest and that of the slowest

drying one is the lowest. It is due to the fact that the

solid/water ratio in the fastest drying drop is highest and

that in the slowest drying one is the lowest. The higher

solid/water ratio decreases the rate of drying and as a

consequence the drop temperature increases. In addi-tion, the lower specific heat capacity of solids also favors

the temperature rise. However, at the later stages of

drying, the spread of temperature histories of pure

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

)

S/M (4:1)

S/M (1:1)

S/M (1:4)

Model predictions

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

) F/M (4:1)

F/M (1:1)

F/M (1:4)

Model predictions

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Moi

stur

e (k

g w

ater

/kg

solid

)

C/M (4:1)

C/M (1:1)

C/M (1:4)

Model predictions

(a)

(b)

(c)

Time (s)

Fig. 4. Comparison of experimental and predicted moisture histories

for drops of 50% w/w solutions of: (a) sucrose(S)/maltodextrin (M),

(b) fructose (F)/maltodextrin (M) and (c) citric acid (C)/maltodextrin

solutions, in ratios of 4:1, 1:1 and 1:4, drying at 63± 1 �C, 1 m/s

air velocity and 2.5± 0.5% relative humidity.


components and their mixtures is not very large. Thismay be due to the fact the specific heat capacities of the

solid materials are not far apart. Temperature histories

showed similar patterns at 95 �C. However, these ther-

mograms were steeper from the very onset of drying.

The phenomenon of profile reversal was also evident.

Furthermore, temperature histories revealed that the

constant rate period was absent at 40% w/w initial solid

concentration.

3.2. Prediction of drying kinetics

The effect of addition of maltodextrin on the moisture

and temperature histories of low molecular weight sug-

ars and organic acids was predicted by numericallysolving Eqs. (1) and (2) as discussed in Section 2. The

drop was assumed to be pseudo-binary composed of

water and solids. The input parameters such as moisture

diffusivities, water activities, specific heat capacities and

solid densities for the mixtures were determined using

the mass weighted mean rule (Eqs. (9) and (10)). The

accuracy of prediction of the drying kinetics is illus-

trated using drops of 50% w/w fructose/maltodextrin,sucrose/maltodextrin and citric acid/maltodextrin at

63 ± 1 �C, 1 m/s air velocity and 2.5 ± 0.5% relative hu-

midity.

3.2.1. Moisture histories

Fig. 4(a)–(c) present the experimental and predictedmoisture histories for drops of sucrose/maltodextrin,

fructose/maltodextrin and citric acid/maltodextrin solu-

tions. The predicted moisture histories agree with the

experimental values with 5% and 9% average and max-

imum absolute relative errors, respectively. The errors in

prediction are higher in drops having a higher propor-

tion of maltodextrin. The model, in the majority of

cases, overpredicts the moisture contents. This can beattributed to the morphological developments as dis-

cussed previously. The surface area of a drop increases

and the diffusion path for the water decreases as the

drops deviate from sphericity and develop trough like

structures on the surface. The increased surface area and

the decreased diffusion path both enhance the flux of

water leaving the drop. These morphological features

are mainly associated with maltodextrin and, as the ra-tio of maltodextrin in the drop increases, these features

become more prominent. Another possible reason is

that the skin is not constant in thickness and the mois-

ture can diffuse relatively easily through (thinner) parts

of the skin. Since the model is unable to incorporate the

effects associated with morphological changes, the error

in prediction in drops having higher amount of mal-

todextrin increases. Overall, these figures suggest thatthe effect of addition of maltodextrin on moisture his-

tories of low molecular weight sugars and organic acids

can be predicted reasonably well. Similarly, at 95 �C, the

predicted moisture histories agree with the experimental

values with 6% and 10% average and maximum absolute

relative errors, respectively. Compared to the 63 �Cdrying case, the errors in prediction have increased. This

is because more intense morphological changes take

place at 95 �C than at 63 �C.

3.2.2. Temperature histories

The experimental and predicted temperature historiesfor drops of sucrose (S)/maltodextrin (M), fructose (F)/

maltodextrin (M) and citric acid (C)/maltodextrin (M)

are compared in Fig. 5(a)–(c), respectively. It can be

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Dro

p te

mpe

ratu

re (

o C)

S/M (4:1)

S/M (1:1)

S/M (1:4)

Model predictions

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Dro

p te

mpe

ratu

re (

o C)

F/M (4:1)

F/M (1:1)

F/M (1:4)

Model predictions

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Dro

p te

mpe

ratu

re (

o C)

C/M (4:1)

C/M (1:1)

C/M (1:4)

Model predictions

(a)

(b)

(c)

Fig. 5. Comparison of experimental and predicted temperature histo-

ries for drops of 50% w/w solutions of: (a) sucrose(S)/maltodextrin

(M), (b) fructose (F)/maltodextrin (M) and (c) citric acid (C)/malto-

dextrin drying at 63± 1 �C, 1 m/s air velocity and 2.5± 0.5% relative

humidity.

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

Moisture (kg water/kg solid)

Gla

ss tr

ansi

tion

tem

pera

ture

(T

g, o C

)

Sucrose S/M (4:1)

S/M (1:1) S/M (1:4)

Maltodextrin –– Drop temperature ( oC)

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60


Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, o C)

Fructose F/M (4:1)

F/M (1:1) F/M (1:4)

Maltodextrin –– Drop temperature ( oC)

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60


Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, oC

)

Citirc acid C/M (4:1)

C/M (1:1) C/M (1:4)

Maltodextrin

—

–– Drop temperature ( oC)

(c)

(b)

(a)

Fig. 6. Variation of Tg of surface layer of drop of 40% w/w (a) sucrose

(S)/maltodextrin (M), (b) fructose (F)/maltodextrin (M) and (c) citric

acid (C)/maltodextrin (M) solutions at ratios of 4:1, 1:1 and 1:4 with

average moisture drying at of 63± 1 �C, 1 m/s air velocity and

2.5± 0.5% relative humidity.


seen from these figures that the model predictions agree

well with the experimental values. The average differencebetween the experimental and predicted values is �0.7

�C while the maximum difference is 2.4 �C. From this

comparison, it can be concluded that the model predicts

well the effect of addition of maltodextrin on the tem-

perature histories of the low molecular weight sugars

and organic acids. Similarly, at 95 �C, the predicted

temperature histories closely follow the experimental

values and that the average difference in prediction wasless than ±0.5 �C and the maximum difference is 3 �C.This leads to the conclusion that temperature histories

are more accurately measured and predicted compared

to their corresponding moisture histories.

3.3. Prediction of surface stickiness of drying drops

The effect of addition of maltodextrin on the surface

stickiness of low molecular weight sugars and organic

acids is explained here by comparing the glass transition

temperature ðTgÞ of the surface layer and the drop

temperature. Distribution of Tg within a drop and Tgat the surface layer were determined using Eq. (8).

Predictions were made for drops of 40% w/w solutions


of fructose (F)/maltodextrin (M), glucose (G)/malto-dextrin (M), sucrose (S)/maltodextrin (M) citric acid (C)/

maltodextrin (M) mixtures at 63 and 95 �C. These pre-

dictions were validated by experimental surface sticki-

ness tests using drops of 40% w/w sucrose/maltodextrin

(Adhikari, Howes, Bhandari, & Truong, 2003a) and

fructose/maltodextrin (Adhikari, Howes, Bhandari, &

Truong, 2003b) mixtures.

The following criteria are set to decide whether adrop surface is sticky or not (Adhikari et al., 2003c). The

drop surface is sticky if its surface layer Tg is lower thanthe drop temperature ðTdÞ. The drop surface exhibits a

peak tendency to stick when its surface layer Tg reachesor just crosses the drop temperature ðTdÞ. The drop

surface becomes completely non-sticky when surface

layer Tg is exceeds the drop temperature ðTdÞ by 10 �C.Hence a safe drying regime can be defined as the regimewhere Tg of the surface layer is P Td þ 10 �C.

3.3.1. Prediction of surface stickiness at 63 �CFig. 6(a) and Table 1 show that an addition of 20%

maltodextrin (S/M¼ 4:1) increases the surface layer Tgcompared to pure sucrose. However, it is well below the

drop temperature within the experimental moisture

range ðu > 0:23Þ, and hence, the surface of this drop

remains sticky. The surface stickiness experiments car-

ried out separately supported this prediction. When the

proportion of maltodextrin is increased to 50% (S/M¼ 1:1), the glass transition temperature of the surface

layer exceeded the drop temperature at u ¼ 0:28. At this

point, according to the criteria set previously, the drop

surface is expected to exhibit peak stickiness. The sur-

face stickiness experiment showed that the drop surface

attained peak stickiness at u � 0:29 (at Td ¼ 58 �C).Hence, it can be stated that the addition to 50% solids of

maltodextrin overcomes the surface stickiness of sucrosedroplets if the outlet temperature of the spray dryer is

Table 1

Prediction of surface stickiness of drops containing fructose (F), glucose (G),

acid and maltodextrin drying at 63 �C, 1 m/s air velocity, 2.5± 0.5% relative

Materials u range u at Tg ¼ Td u

F/M (4:1) 1.5–0.19 – –

G/M (4:1) 1.5–0.21 – –

S/M (4:1) 1.5–0.23 – –

C/M (4:1) 1.5–0.17 – –

F/M (1:1) 1.5–0.23 – –

G/M (1:1) 1.5–0.25 – –

C/M (1:1) 1.5–0.23 – –

S/M (1:1) 1.5–0.26 0.28 –

F/M (1:4) 1.5–0.28 0.62 0

G/M (1:4) 1.5–0.3 0.64 0

S/M (1:4) 1.5–0.29 0.70 0

C/M (1:4) 1.5–0.27 0.50 0

Maltodextrin 1.5–0.3 1.02 0

Tg is the glass transition temperature of the surface layer, Td is the drop tem

maintained at 63 �C and that its moisture reaches belowu ¼ 0:28. When the (S/M) ratio is further increased (to

1:4), the Tg of the surface layer exceeds the drop tem-

perature at moisture u ¼ 0:70 and the moisture when Tgequals Td þ 10 �C is u ¼ 0:47. The experimental peak

stickiness and the subsequent state of non-adhesion were

observed at u � 0:65 (at Td ¼ 53:7 �C) and u � 0:46 (at

Td ¼ 57:1 �C) respectively.Fig. 6(b) and Table 1 show that the surface stickiness

of fructose/maltodextrin droplets are persistent. While

drying at 63 �C, even the addition of 50% maltodextrin

(F/M¼ 1:1) fails to raise the surface layer Tg to reach

drop temperature. Hence, the addition of maltodextrin

in ratios (F/M)P 1:1 will not be helpful. When the

proportion of maltodextrin is increased to F/M¼ 1:4,

the surface layer Tg exceeds the drop temperature at

u � 0:62, which is close to the moisture u ¼ 0:63 (Td ¼53:3 �C) at which peak tensile pressure (stickiness)

was observed in a surface stickiness experiment. Tg of

the surface layer of this mixture exceeds Td þ 10 �Cwhen u ¼ 0:36, the point at which the drop surface is

assumed to reach a state of non-adhesion. The state of

non-adhesion was observed, experimentally, at moisture

u � 0:3 (Td ¼ 58:2 �C), which is slightly lower than the

model prediction. Based on this observation, the as-sumption that the complete non-sticky state is obtained

at Tg ¼ Td þ 10 �C is still reasonable. This indicates that

addition of 80% maltodextrin in fructose brings about

successful spray drying if the outlet temperature of the

dryer is maintained at 63 �C.Similarly, Fig. 6(c) and Table 1 shows that the ad-

dition of maltodextrin to citric acid solutions in the

proportions C/M¼ 1:1 or above would be unable toraise the Tg of the surface layer to reach the drop tem-

perature and that the drop surface will remain com-

pletely sticky. When the ratio is increased to C/M¼ 1:4,

the Tg of the surface layer exceeds the drop temperature

sucrose (S), citric acid (C), maltodextrin (M) and their mixtures, citric

humidity

at Tg ¼ Td þ 10 Experimental observation

Sticky

–

Sticky

–

Sticky

–

–

Max sticky at Non-sticky at

u ¼ 0:29 u < 0:28

.36 u ¼ 0:63 u ¼ 0:3

.37 – –

.47 u ¼ 0:65 u ¼ 0:46

.33

.8 u ¼ 1:0 u ¼ 0:69

perature.


at u ¼ 0:50 where peak stickiness should be observed.On further drying, when u ¼ 0:33, the Tg reaches Td þ 10

�C and hence the drop is assumed to enter a safe (non-

sticky) drying regime. Table 1 shows that the Tg of the

surface layer of C/M mixtures enters the non-sticky re-

gime at lower moistures compared to the F/M and S/M

mixtures. Hence, the presence of citric acid prolongs the

stickiness of a drying drop.

3.3.2. Prediction of surface stickiness at 95 �CFig. 7(a) presents the glass transition temperature of

the surface layer of sucrose/maltodextrin drops within

the experimental moisture range. This figure shows that

-120-100-80-60-40-20

020406080

100120140

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60


Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, oC

)

Sucrose S/M (4:1)S/M (1:1) S/M (1:4)Maltodextrin — Drop temperature (

oC)

-120-100-80-60-40-20

020406080

100120140

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60


Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, o C)

Fructose F/M (4:1)F/M (1:1) F/M (1:4)

Maltodextrin — Drop temperature ( oC)

-120-100-80-60-40-20

020406080

100120140

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60


Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, o C)

Citric acid C/M (4:1)C/M (1:1) C/M (1:4)Maltodextrin — Drop temperature (

oC)

(b)

(a)

(c)

Fig. 7. Variation of Tg of surface layer of drops of 40% w/w (a) sucrose

(S)/maltodextrin (M), (b) fructose (F)/maltodextrin (M) and (c) citric

acid (C)/maltodextrin (M) at the ratio of 4:1, 1:1 and 1:4 with average

moisture, simulated at drying conditions of 95 �C, 1 m/s air velocity

and 2.5± 0.5% relative humidity.

Tg of the surface layer of the sucrose/maltodextrin dropsfails to reach the drop temperature even though it

comprises 50% maltodextrin (S/M¼ 1:1). This means, at

this temperature, the surface of this drop/particle re-

mains sticky within the range of moistures studied

ðu > 0:12Þ. If we compare this result with its corres-

ponding 63 �C case, the drop had become almost non-

sticky at u < 0:28. This indicates that higher drying

temperatures worsen the problem of stickiness. Whenthe proportion of maltodextrin is increased to 80% Tgexceeded the drop temperature at u ¼ 0:32 where it is

expected to exhibit peak stickiness. On further drying,

surface layer Tg attains Td þ 10 �C when u ¼ 0:20 at

which point it is expected to be non-sticky. The surface

stickiness experiments showed that the peak stickiness

was observed at about u ¼ 0:57 (Td ¼ 82:6 �C) and that

the surface was rendered completely non-sticky at aboutu ¼ 0:23. Hence the prediction that the drop reaches a

non-sticky state at u ¼ 0:2 is quite reasonable. Com-

pared to the 63 �C, the drop enters the safe drying (non-

sticky) regime at a lower moisture content.

Fig. 7(b) presents the Tg of surface layer for drops offructose/maltodextrin solution (summarized in Table 2),

among drops with F/M ratios of 4:1, 1:1, and 1:4. Only

the Tg of the surface layer at a ratio F/M of 1:4 exceedsthe drop temperature. The surface Tg exceeds drop

temperature at u ¼ 0:28 and attains Td þ 10 �C at

u ¼ 0:17, which are slightly lower than moistures found

for sucrose/maltodextrin at the same ratio and air tem-

perature. The prediction that the surface of this drop

enters the non-sticky regime at u ¼ 0:17 is close to the

experimental finding that the surface of this mixture was

completely non-sticky at u ¼ 0:20 (Td ¼ 82:7 �C). Simi-larly, Fig. 7(c) shows the history of surface Tg for dropsof citric acid/maltodextrin mixture within the experi-

mental moisture range. The Tg of surface layer of C/

M¼ 1:4 can exceed the drop temperature and hence

enters the safe drying regime. However, it enters the

non-sticky regime at u ¼ 0:14 which is a lower moisture

than fructose/maltodextrin. Here too, it is evident that

among the materials studied the surface of citric acid/maltodextrin drop remains sticky for the longest time.

3.3.3. Surface stickiness of a drop in a simulated spray

drying environment

The surface stickiness history of a drop with an initial

diameter of 120 lm is further elaborated here. These

drops are subjected to two simulated sets of drying

conditions, 63 and 95 �C, with 1 m/s air velocity and

2.5% relative humidity. This size of droplet is frequently

produced in spray dryers and is able to hit the dryer

wall. All of the drops modelled contained an initial

concentration of 40% w/w solutes. The glass transitiontemperature history of the surface layer of sucrose/

maltodextrin, fructose/maltodextrin, and citric acid/

maltodextrin at 95 �C are presented in Fig. 8(a)–(c),

Table 2

Prediction of surface stickiness of drops containing fructose (F), glucose (G), sucrose (S), citric acid (C), maltodextrin (M) and their mixtures, citric

acid and maltodextrin drying at 95 �C, 1 m/s air velocity, 2.5± 0.5% relative humidity

Materials u range u at Tg ¼ Td u at Tg ¼ Td þ 10 Experimental observation

F/M (4:1) 1.5–0.09 – – Sticky

G/M (4:1) 1.5–0.1 – – –

S/M (4:1) 1.5–0.12 – – Sticky

C/M (4:1) 1.5–0.09 – – –

F/M (1:1) 1.5–0.11 – – Sticky

G/M (1:1) 1.5–0.12 – – –

S/M (1:1) 1.5–0.12 – – Sticky

C/M (1:1) 1.5–0.11 – – –

Max sticky at Non-sticky at

F/M (1:4) 1.5–0.14 0.28 0.17 u ¼ 0:54 0.2

G/M (1:4) 1.5–0.15 0.30 0.19 – –

S/M (1:4) 1.5–0.15 0.32 0.20 u ¼ 0:57 u ¼ 0:23

C/M (1:4) 1.5–0.14 0.21 0.14 –

Maltodextrin 1.5–0.16 0.77 0.5 u ¼ 0:79 u ¼ 0:53

Tg is the glass transition temperature of the surface layer, Td is the drop temperature.


respectively along with the drop temperature. The tem-

perature history of maltodextrin was chosen to represent

the temperature histories of the mixture drops, because

there is little variation in temperature histories com-

pared with the variations in Tg. The time for these drops

to reach the safe drying regime defined by the time taken

for the Tg of the surface layer to reach Td þ 10 �C, andthe total time required to reach u ¼ 0:05, are listed inTable 3. The corresponding values obtained at 63 �C are

presented in Table 4.

Fig. 8(a) and Table 3 show that it is not possible to

overcome the stickiness of sucrose using 6 50% mal-

todextrin, as a drying aid, when the outlet temperature

of the dryer is maintained at 95 �C. The Tg of a drop

containing 50% maltodextrin remains in between Td andTd þ 10 �C which is a limit for spray drying, that is, itmay or may not be possible to spray dry. Addition of

maltodextrin exceeding 50% leads to successful spray

drying at this temperature. If the outlet temperature of

the dryer is maintained at 63 �C, as shown in Table 4, a

drop with S/M (1:1) remains sticky for only 6.6% of total

drying time and is possible to spray dry. When malto-

dextrin comprises 80% of the solids, at 95 �C, the drop

enters the safe regime by about 13% of the total dryingtime and is easy to spray dry. The same drop will enter

the non-sticky regime after less than 2% of total drying

time at 63 �C. This provides a very good indication that

how the problem of stickiness can become overwhelm-

ing when dryers are maintained at higher temperatures.

Fig. 8(b) and Table 3 show that for drops of fructose/

maltodextrin mixture comprising 80% maltodextrin the

time for the drop to enter the non-sticky regime is 16%of total time at 95 �C, while it is less than 2% at 63 �C.Fig. 8(c) and Tables 3 and 4 show that surface of a drop

containing a citric acid/maltodextrin mixture remains

sticky for the longest time both at 95 and 63 �C.

4. Drying kinetics of model sugar-rich foods/maltodextrin

mixture solutions

In order to explain how the addition of maltodextrin

helps to overcome the stickiness of sugar-rich foods

a model mixture consisting of fructose, glucose, and

sucrose, in equal proportions was prepared. Different

proportions of maltodextrin then added to this mixture.Single drops of these model mixtures were dried at two

air temperatures, 63 ± 1 �C and 95± 2 �C, 1 m/s air ve-

locity and 2.5 ± 0.5% relative humidity.

4.1. Drying kinetics

4.1.1. Moisture history

Fig. 9(a) presents the moisture histories while drying

at 63± 1 �C. It shows that the rate of drying decreases asthe amount of maltodextrin increases which is expected

due to the low moisture diffusivity of maltodextrin. The

experimental moisture histories are close to their corre-

sponding predicted values. The average and maximum

absolute relative errors in prediction are less than 4.5%

and 9%, respectively. The majority of the experimental

moisture values are lower than the predicted ones. This

is due to the fact that the model is unable to incorporatethe morphological changes that take place during dry-

ing. The addition of maltodextrin introduces visible

surface wrinkles/folds and the drop deviates from

sphericity, which increases the rate of moisture loss re-

sulting in under prediction of the moisture history

points. The pattern of moisture histories at 95 �C was

similar to those described at 63 �C. The predicted values

agreed with the experimental values within average andmaximum absolute errors of 4.6% and 10%, respectively.

The effect of morphological development was also no-

ticeable.

-120-100-80-60-40-20

020406080

100120140160180

0 1 2 3 4 5 6 7 8 9 10

Time (s)

Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, o C)

Sucrose S/M (4:1) S/M (1:1)

S/M (1:4) Maltodextrin — Drop temperature ( oC)

-120-100-80-60-40-20

020406080

100120140160180

0 1 2 3 4 5 6

Time (s)

Gla

ss tr

ansi

tion

tem

pera

ture

(T

g, o

C)

Fructose F/M(4:1)F/M (1:1) F/M (1:4)

Maltodextrin — Drop temperature ( C)

-120

-70

-20

30

80

130

180

0 1 2 3 4 5 6

Time (s)

Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, o C)

Citric acid C/M (4:1)

C/M (1:1) C/M (1:4)

Maltodextrin — Drop temperature ( o

o

C)

(c)

(b)

(a)

Fig. 8. Glass transition (surface) temperature ðTgÞ and drop tempera-

ture histories of a initially 120 lm diameter drops of (a) sucrose (S)/


(C)/maltodextrin (M) at ratio of 4:1, 1:1 and 1:4 solutions (40% w/w

solutes initially) simulated at 95 �C, 1 m/s air velocity and 2.5± 0.5%

relative humidity.


4.1.2. Temperature history

Fig. 9(b) presents the experimental versus predictedtemperatures histories at 63 ± 1 �C. The average diffe-

rence between experimental and predicted drop tempe-

ratures is less than ±1 �C. The temperature histories at

95 �C also exhibit a similar pattern to those observed at

63 �C. The three plots converge after 300 s.

4.1.3. Morphological changes

Morphological changes observed during drying of

these drops, are presented in Fig. 10. These morpho-

logical features, in many ways, were similar to thosedescribed in Section 3 for sugar/maltodextrin and acid/

maltodextrin mixture drops. However, the drops con-

taining 20% maltodextrin ((F+G+S)/M¼ 4:1) in solids

remain more spherical than the corresponding sucrose/

maltodextrin drops. Furthermore, the surface of the

drops with (F+G+S)/M at ratios of 1:1 and 1:4 are

smoother, more thermoplastic (softer to touch) and less

rugged compared to the drops of pure maltodextrin orsucrose/maltodextrin drops with the same S/M ratios.

The softer skin on the drop surface allows easier diffu-

sion of water out from the drop; as a consequence, the

drop dries faster. On the other hand, the (more) thermo-

plastic skin retains surface stickiness for longer times.

4.2. Prediction of surface stickiness of drops during spray

drying

A drop of the sugar solution having a 120 lm initial

diameter was subjected to two simulated sets of dryingconditions, 63 and 95 �C, with 1 m/s air velocity and

2.5 ± 0.5% relative humidity. The time required for the

Tg of the surface layer to attain Td þ 10 �C was moni-

tored along with the time required to complete the

drying, that is, time to reach u ¼ 0:05. As discussed in

Section 3.3, when the glass transition temperature ðTgÞof the surface layer reaches or exceeds Td þ 10 �C, thesurface of the drop is assumed to become completelynon-sticky. At this point the drop enters the safe drying

regime. Similarly, the ratio tNS=ttotalðwÞ is an indicator

of the degree of easiness/or difficulty for the drop to

reach the safe drying regime. The smaller the ratio, the

faster the drop enters the safe drying regime.

Fig. 11 presents the Tg histories of the surface layers

of these drops, drying at 95 �C, along with the drop

temperature history. The drops contain 40% w/w initialsolids concentration of sugars and maltodextrin at the

ratios of (4:1), (1:1) and (1:4). Since the difference in the

temperature histories of these mixtures is much less than

the difference in Tg histories, the temperature history of

the 50% maltodextrin solution is used to represent the

temperature histories of all the mixtures. Fig. 11 shows

that Tg of the drop containing 20% maltodextrin, i.e.

(F+G+S)/M (4:1), remains far below the drop tem-perature and it will remain sticky even when the drop is

completely dry. When the proportion of maltodextrin in

solids is increased to 50%, i.e. (F+G+S)/M (1:1), Tg ofthe surface layer still remains below the drop tempera-

ture. This indicates that the presence of 50% maltodex-

trin in solids fails to overcome the stickiness. Table 5

shows that w values of both drops indicate that the

addition of maltodextrin up to 50% fails to overcomethe sticky problem. When the proportion of maltodex-

trin is further increased to 80%, i.e. (F+G+S)/M (1:4),

the drop enters the safe drying regime when the w value

Table 4

Prediction of surface stickiness of drops of 120 lm initial diameter simulated at drying condition of 63 �C, 1 m/s air velocity, 2.5± 0.5% relative

humidity

Materials u range Time at Tg ¼ Td þ 10 (s) ðwÞ ¼ tNS=ttotal Remarks

F/M (4:1) 1.5–0.05 Inf Inf sticky

G/M (4:1) 1.5–0.05 Inf Inf sticky

S/M (4:1) 1.5–0.05 Close to reach >1 Unsuccessful spray drying

C/M (4:1) 1.5–0.05 Inf Inf Sticky

F/M (1:1) 1.5–0.05 17.6 17.6/67¼ 0.26 Successful spray drying

G/M (1:1) 1.5–0.05 7.2 7.2/84 ¼ 0.086 Successful spray drying

S/M (1:1) 1.5–0.05 6.6 6.6/100¼ 0.066 Successful spray drying

C/M (1:1) 1.5–0.05 Close to reach >1 Unsuccessful spray drying


G/M (1:4) 1.5–0.05 1.8 1.8/110¼ 0.016 Successful spray drying


C/M (1:4) 1.5–0.05 2.2 2.2/99¼ 0.022 Successful spray drying

Maltodextrin 1.5–0.05 1 1/150¼ 0.007 Most successful spray drying

Drops contain fructose (F), glucose (G), sucrose (S), and maltodextrin (M) and their mixtures. tNS ¼ time to enter the non-sticky regime (s),

ttotal ¼ time required to reach u ¼ 0:05 (s), inf¼ infinite time required.

Table 3

Prediction of surface stickiness of drops of 120 lm initial diameter simulated at drying condition of 95 �C, 1 m/s air velocity, 2.5± 0.5% relative

humidity. Drops contain fructose (F), glucose (G), sucrose (S), and maltodextrin (M) and their mixtures. tNS ¼ time to enter the non-sticky regime (s),

ttotal ¼ time required to reach u ¼ 0:05 (s), inf¼ infinite time required

Materials u range Time at Tg ¼ Td þ 10 (s) ðwÞ ¼ tNS=ttotal Remarks

F/M (4:1) 1.5–0.05 Inf Inf Sticky

G/M (4:1) 1.5–0.05 Inf Inf Sticky

S/M (4:1) 1.5–0.05 Inf Inf Sticky


F/M (1:1) 1.5–0.05 Inf Inf Sticky

G/M (1:1) 1.5–0.05 Close to reach >1 Unsuccessful spray drying

S/M (1:1) 1.5–0.05 Close to reach �1 Marginally successful (00)



G/M (1:4) 1.5–0.05 2.2 2.2/16¼ 0.14 Successful spray drying


C/M (1:4) 1.5–0.05 2.7 2.7/13.5¼ 0.2 Successful spray drying

Maltodextrin 1.5–0.05 1 1/20¼ 0.05 Most successful spray drying


is 0.153 which means that the drop enters the safe drying

regime after 15.3% of the total drying time.

Table 5 also summarizes w values for the same drops

drying at 63 �C. Contrary to the 95 �C case, the dropcontaining 50% maltodextrin, i.e. (F+G+S)/M (1:1)

enters the safe drying regime after 9.5% of total drying

time. The drops containing 80% maltodextrin in solids

enters the safe drying regime at about 2% of the total

drying time. This is much earlier than the 95 �C case,

where the same drop enters the safe drying regime after

15% of total drying time. This suggests that lower dryer

outlet temperatures make it easier for sugar-rich foodsto overcome the stickiness.

5. Drying kinetics of model sugar and acid-rich foods/

maltodextrin solutions

A model mixture was prepared to investigate the

drying kinetics of fruit juices. It contained all of the

major constituents of fruit juices such as fructose, glu-

cose, sucrose and citric acid. Citric acid was chosen to

represent the organic acids content of the fruit juices. All

of these sugars and citric acid were mixed in equalproportions. This formulation does not conform to any

real fruit juice composition. However, it contains all of

the major components that are believed to contribute to

stickiness (Section 1). This is probably the simplest

composition that allows an investigation of the drying

kinetics of a multi-component sugar/acid mixture (for

example fruit juice). Maltodextrin, in varying propor-

tions, was then added to this mixture to investigate theeffect of drying aids.

5.1. Drying kinetics

5.1.1. Moisture history

Fig. 12(a) presents the experimental and predicted

moisture histories for drops of 50% w/w citric acid

(C) + fructose (F) + glucose (G)+ sucrose (S), to which is

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

)

(F+G+S)/M (4:1)

(F+G+S)/M (1:1)

(F+G+S)/M (1:4)

Model predictions

(a)

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Dro

p te

mpe

ratu

re (

o C)

(F+G+S)/M (4:1)

(F+G+S)/M (1:1)

(F+G+S)/M (1:4)

Model predictions

Water

Air

(b)

Fig. 9. Experimental and predicted (a) moisture and (b) temperature

histories for drops of 50% w/w solutions of fructose (F)+ glucose

(G)+ sucrose (S) and maltodextrin (M) at the ratios of (4:1), (1:1) and

(1:4) drying at 63±1 �C, 1 m/s air velocity and 2.5± 0.5% relative

humidity.

Fig. 10. Morphological features of drops of fructose (F)+ glucose

(G)+ sucrose (S) and maltodextrin (M) solutions at the ratios of (4:1),

(1:1) and (1:4), drying at 63± 1 �C, 1 m/s air velocity and 2.5± 0.5%

relative humidity. Time given to each image is the time at which it was

captured.

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Time (s)

Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, o C)

(F+G+S)/M (4:1)

(F+G+S)/M (1:1)

(F+G+S)/M (1:4)

— Drop temperature ( oC)

Fig. 11. Histories of glass transition temperature ðTgÞ of surface layersand drop temperature histories of a initially 120 lm diameter drops of

fructose (F)+ glucose (G)+ sucrose (S) and maltodextrin (M) at ratios

of (4:1), (1:1) and (1:4) containing 40% w/w solute initially simulated at

95 �C, 1 m/s air velocity and 2.5± 0.5% relative humidity. The tem-

perature histories are represented by using (F+G+S)/M (1:1) case.


added maltodextrin at (C+F+G+S)/M ratios of (4:1),

(1:1) and (1:4) and is subjected to drying at 63 �C. Thisfigure shows that the effect of the increasing proportion

of maltodextrin on the moisture history shows a similar

pattern to those obtained in the case of sugars/maltod-

extrin mixtures. The moisture history plots of(C+F+G+S)/M, in Fig. 12(a), are slightly steeper

compared to the corresponding (F+G+S)/M plots in

Fig. 9(a). It suggests that the addition of citric acid en-

hances the rate of moisture loss. The predicted moisture

histories agree with the experimental ones within aver-

age and maximum absolute relative errors of 6% and

10%, respectively. Fig. 13(a) presents the experimental

and predicted moisture histories of the drops containingof these drops during drying at 95 �C. These moisture

histories resemble the moisture histories at 63 �C, butare steeper and the moisture values are distributed

within a narrower range. The predicted moisture histo-

ries agree with the experimental ones within 5% and 11%

average and maximum relative absolute errors, respec-

tively. The experimental moisture histories are lower

than the predicted ones because of increased moistureevaporation due to morphological developments.

5.1.2. Temperature history

Fig. 12(b) presents the temperature histories (at 63

�C) corresponding to the moisture histories given in Fig.

12(a). The pattern of temperature rise resembles the

patterns observed in (F+G+S)/M drops presented in

Fig. 9(b). The comparison of the experimental tempe-

rature histories with the predicted ones reveals that the

predicted values are accurate within a ±1 �C averagedifference. The three plots converge after 300 s. Simi-

larly, Fig. 13(b) presents the experimental and predicted

temperature histories of these drops at 95 �C. These

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

)

(C+F+G+S)/M (4:1)

(C+F+G+S)/M (1:1)

(C+F+G+S)/M (1:4)

Model predictions

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600

Time (s)

Tem

pera

ture

( o C

)

(C+F+G+S)/M (4:1)

(C+F+G+S)/M (1:1)

(C+F+G+S)/M (1:4)

Model predictions

Water

Air

(b)

(a)


histories for drops of 50% w/w citric acid (C)+ (F) + glucose

(G)+ sucrose (S) and maltodextrin (M) in solutions at ratios of (4:1),

(1:1) and (1:4), drying at 95± 2 �C, 1 m/s air velocity and 2.5± 0.5%

relative humidity.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

Time (s)

Moi

stur

e (k

g w

ater

/kg

solid

). (C+F+G+S)/M (4:1)

(C+F+G+S)/M (1:1)

(C+F+G+S)/M (1:4)

Model predictions

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600

Time (s)

Tem

pera

ture

( o C

)

(C+F+G+S)/M (4:1)

(C+F+G+S)/M (1:1)

(C+F+G+S)/M (1:4)

Model predictions

Water

Air

(a)

(b)


histories for 50% w/w solutions of citric acid (C) + fructose (F) + glu-

cose (G)+ sucrose (S) and maltodextrin (M) at the ratios of (4:1), (1:1)

and (1:4), drying at 63± 1 �C, 1 m/s air velocity and 2.5± 0.5% relative

humidity.

Table 5

Prediction of surface stickiness of 120 lm diameter drops during spray drying at 95 �C and 63 �C, 1 m/s air velocity, 2.5 ± 0.5% relative humidity

Materials u range Time (s) at Tg ¼ Td þ 10 w ¼ tNS=ttotal Remarks

Air temperature 95 �C(F+G+S)/M (4:1) 1.5–0.05 Inf Inf Sticky

(F+G+S)/M (1:1) 1.5–0.05 Inf Inf Sticky

(F+G+S)/M (1:4) 1.5–0.05 2.3 2.3/15¼ 0.153 Successful spray drying

Air temperature 63 �C(F+G+S)/M (4:1) 1.5–0.05 Inf Inf Sticky



Drops contain 40% w/w initial solids of fructose (F)+ glucose (G)+ sucrose (S) and maltodextrin (M) at ratios of (4:1), (1:1) and (1:4). tNS ¼ time to

enter the non-sticky regime (s), ttotal ¼ time required to reach u ¼ 0:05 (s), inf¼ infinite time required.


temperature histories also resemble their corresponding

temperature histories at 63 �C. The difference is that therise in temperature is much more rapid from the very

beginning. This is expected as larger temperature gra-

dients are established from the onset of drying and that

higher amount of heat energy, convected to the drop,

goes to increase the drop temperature. The predicted

moisture histories follow the experimental ones with an

average difference of ±1 �C.

5.1.3. Morphology changes

The features of morphological changes that takeplace during drying at 63 �C are presented in Fig. 14.

These features are, in many ways, similar to the ones

obtained in sugars/maltodextrin mixtures, shown in

-120-100-80-60-40-20

0

20406080

100120140

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

time (s)

Gla

ss tr

ansi

tion

tem

pera

ture

(T g

, o C)

CFGS/M (4:1)

CFGS/M (1:1)

CFGS/M (1:4)

— Drop temperature ( oC)

Fig. 15. (Surface) glass transition temperature ðTgÞ and drop tempe-

rature histories of a initially 120 lm diameter drops of citric acid (C)

fructose (F)+ glucose (G)+ sucrose (S) and maltodextrin (M) at ratios

of (4:1), (1:1) and (1:4) containing 40% w/w initial solids, simulated at

95 �C, 1 m/s air velocity and 2.5± 0.5% relative humidity. The tem-

perature histories are represented by drops of (C+F+G+S)/M (1:1).

Fig. 14. Morphological features of drops 40% w/w citric acid

(C) + fructose (F) + glucose (G)+ sucrose (S) and maltodextrin (M)

solutions, at ratios of (4:1), (1:1) and (1:4), drying at 63± 1 �C, 1 m/s

air velocity and 2.5± 0.5% relative humidity. Time given to each image

is the time at which it was captured.

Table 6

Prediction of surface stickiness of 120 lm diameter drops during spray dryin

Materials u range Time (s) at Tg ¼ Td þ 10

Air temperature 95 �C(C+F+G+S)/M (4:1) 1.5–0.05 Inf

(C+F+G+S)/M (1:1) 1.5–0.05 Inf

(C+F+G+S)/M (2:3) 1.5–0.05 14.5

(C+F+G+S)/M (1:4) 1.5–0.05 2.3

Air temperature 63 �C(C+F+G+S)/M (4:1) 1.5–0.05 Inf

(C+F+G+S)/M (3:2) 1.5–0.05 70

(C+F+G+S)/M (1:1) 1.5–0.05 10.4

(C+F+G+S)/M (1:4) 1.5–0.05 1.8

Drops contain 40% w/w initial solids of citric acid (C) + fructose (F) + gluco

(1:4). tNS ¼ time to enter the non-sticky regime (s), ttotal ¼ time required to re


Fig. 10. However, the drops with 50% of maltodextrin,that is, (C+F+G+S)/M¼ 1:1, look more elongated.

This explains why there is a wider difference between the

experimental and predicted moisture history points in

Figs. 12(a) and 13(a). Furthermore, the surface of these

drops was smoother and more thermoplastic compared

to the (F+G+S)/M drops at the same maltodextrin

ratio. This supports the conclusion that the presence of

acid prolongs the surface stickiness of sugar-rich foods.

5.2. Prediction of surface stickiness of drops during spray

drying

Fig. 15 presents the simulated Tg histories of surface

layers of 120 lm diameter drops comprised of sugars,

citric acid and maltodextrin. It is simulated at 95 �C, 1m/s air velocity and 2.5 ± 0.5% relative humidity. Table

6 summarizes the outcome of simulations, at 95 �C and

63 �C. Since these drops contain all the components,

likely to contribute to the stickiness, the simulated re-

sults shown in Fig. 15 and summarized in Table 6 areassumed to be applicable to natural fruit juices.

Fig. 15 and Table 6 show that after addition of 20%

maltodextrin, the Tg of the surface layer of the fruit juiceremains well below the drop temperature. When the

proportion of maltodextrin in the solid fraction is in-

creased to 50%, the Tg of the surface layer still remains

below the drop temperature, hence the drop surface

remains sticky even if it is completely dry. Table 6 showsthat when the fraction of maltodextrin in solids is in-

creased to 60%, that is, (C+F+G+S)/M (2:3),

tNS=ttotalðwÞ is 1, which means that Tg of the surface layerjust attains Td þ 10 �C, towards the completion of dry-

ing. This is a limit or a cut off point. To summarize: if wis >1, it is impossible to successfully spray dry. If the

w < 1, it should be possible to spray dry. The smaller the

w ratio, the earlier the drop enters the safe drying re-gime. For example, at 95 �C, a fruit juice/maltodextrin

ratio of 2:3 represents the minimum amount required

for successful spray drying. From Table 6, such drops

g at 95 �C and 63 �C, 1 m/s air velocity, 2.5 ± 0.5% relative humidity

w ¼ tNS=ttotal Remarks

Inf Sticky

Inf Sticky

14.5/14.5¼ 1.00 Marginally successful spray drying

2.3/14.3¼ 0.16 Successful spray drying

Inf Sticky

70/70¼ 1.00 Marginally successful spray drying

10.4/74¼ 0.14 Successful spray drying

1.8/105¼ 0.017 Successful spray drying

se (G)+ sucrose (S) and maltodextrin (M) at ratios of (4:1), (1:1) and

ach u ¼ 0:05 (s), inf¼ infinite time required.


require 16% of their total drying time to enter the safedrying regime even if the solids contains 80% of mal-

todextrin.

When the dryer outlet temperature is maintained at

63 �C, however, the drops enter the safe drying regime

much earlier, which is conducive to spray drying. As

shown in Table 6, the fruit juice/maltodextrin ratio of

3:2, that is, 40% maltodextrin in solids, is the cut-off

point as w ¼ 1. This is the limit for marginally successfuldrying. All the formulations having a higher proportion

of maltodextrin than this ratio will allow successful

spray drying. Furthermore, a drop containing fruit juice/

maltodextrin ratio of 1:1 enters the safe drying regime

by about 14% of the total time at 63 �C. The corres-

ponding droplet would remain completely sticky at 95

�C. Similarly, a drop containing a fruit juice/maltodex-

trin ratio of 1:4 enters the safe drying regime within 2%of total drying time at 63 �C, which is much earlier than

14% of the total time required at 95 �C. These results

show that lower outlet temperatures are conducive for

spray drying of fruit juices using a minimal amount of

drying aids. Furthermore, it is impossible to spray dry

fruit juices in their pure form even at temperatures as

low as 63 �C.

6. Conclusions

The convective drying kinetics (moisture and tem-

perature histories) of multicomponent mixtures were

determined experimentally through single drop drying

experiments. Predictions were made by solving the dif-

fusion equation in a solute-fixed coordinate system. Theaverage and maximum absolute errors in the prediction

of moisture histories ranged from 4.5–6% and 9–11%,

respectively. The experimental moisture history points

were lower than the predicted ones due to morphologi-

cal changes. The average and maximum differences in

prediction of temperature histories were 0.7–0.8 �C and

2–3 �C, respectively. The error in prediction was higher

at higher temperatures and also in drops with higherproportion of maltodextrin.

The surface stickiness of the drop was determined

assuming that a skin/shell was formed at the drop

surface and that surface properties were different than

the bulk properties. The glass transition temperature

of the surface layer gave quite a reasonable prediction of

the surface stickiness of the mixture drops, which was

verified using results obtained from surface stickiness

experiments. The model results were extrapolated todetermine the safe drying regime (non-sticky regime)

and how early or late a drop entered it. It was found that

maltodextrin altered the surface stickiness of low mo-

lecular weight sugars and organic acids and made them

possible to enter the safe drying regime sooner and

hence acted as an effective drying aid.

References

Adhikari, B., Howes, T., Bhandari, B. R., & Truong, V. (2003a). In

situ characterization of stickiness of sugar-rich foods using a linear

actuator driven stickiness testing device. Journal of Food Engineer-

ing, 58(1), 11–22.

Adhikari, B., Howes, T., Bhandari, B. R., & Truong, V. (2003b).

Characterization of surface stickiness of fructose-maltodextrin

mixtures during drying. Drying Technology, 21, 17–34.

Adhikari, B., Howes, T., Bhandari, B. R., & Truong, V. (2003c).

Surface stickiness of drops of carbohydrate and organic acid

solutions during convective drying: experiments and modelling.

Drying Technology, 21, 839–873.

Adhikari, B., Howes, T., Bhandari, B. R., Yamamoto, S., & Truong,

V. (2002). Application of simplified method based on regular

regime approach to determine the effective moisture diffusivity of

mixture of low molecular weight sugars and maltodextrin during

desorption. Journal of Food Engineering, 54(2), 157–165.

Bhandari, B. R., Datta, N., Crooks, R., Howes, T., & Rigby, S.

(1997a). A semi-empirical approach to optimize the quantity of

drying aids required to spray dry sugar rich foods. Drying

Technology, 15(10), 2509–2525.

Bhandari, B. R., Datta, N., & Howes, T. (1997b). Problems associated

with spray drying of sugar-rich foods. Drying Technology, 15(2),

671–684.

Bhandari, B. R., Senoussi, A., Dumoulin, E. D., & Lebert, A. (1993).

Spray drying of concentrated fruit juices. Drying Technology, 11(5),

1081–1092.

Brennan, J. G., Herrera, J., & Jowitt, R. (1971). A study of some of the

factors affecting the spray drying of concentrated orange juice, on a

laboratory scale. Food Technology, 6, 295–307.

Dolinsky, A., Maletskaya, K., & Snezhkin, Y. (2000). Fruit and

vegetable powders production technology on the bases of spray

and convective drying methods. Drying Technology, 18(3), 747–

758.

Gordon, M., & Taylor, J. S. (1952). Ideal copolymers and the second-

order transitions of synthetic rubbers. I. Non-crystalline copoly-

mers. Journal of Applied Chemistry, 2, 493–500.

Gupta, A. S. (1978). Spray drying of orange juice. US patent no.

4112130.

Incropera, F. P., & DeWitt, D. P. (2002). Fundamentals of heat and

mass transfer (5th ed.). New York: John Wiley and Sons.

Van der Lijn, J., Kerkhof, P. J. A. M., Rulkens, W. H. (1972). Droplet

heat and mass transfer under spray-drying conditions. In Interna-

tional symposium on heat and mass transfer problems in food

engineering. Wageningen, The Netherlands, Paper F3.

Effect of addition of maltodextrin on drying kinetics and stickiness of sugar and acid-rich foods...

Documents

Transcript of Effect of addition of maltodextrin on drying kinetics and stickiness of sugar and acid-rich foods...