www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 74 (2006) 484–489

Drying of vegetable starch solutions on inert particles:Quality and energy aspects

Marzouk Benali *, Mouloud Amazouz

Natural Resources Canada, CANMET Energy Technology Centre, 1615 Lionel-Boulet Blvd., P.O. Box 4800, Varennes, Que., Canada J3X1S6

Received 28 November 2003; received in revised form 9 December 2004; accepted 3 January 2005

Available online 10 May 2005

Abstract

The paper deals with the energy efficiency of drying vegetable starch solutions on inert particles and the product quality. The

experimental results performed in a continuous conical jet spouted bed with Teflon particles have shown that, for a given feed rate

of vegetable starch solution, the product lightness varies with the initial moisture content and the position of an atomizing device.

The starch damage index was below 2.5% and the lightness of the dried starch was in the range 94.1–96.1. A drying efficiency of

90 ± 3% is achieved with the proposed drying method.

Crown Copyright � 2005 Published by Elsevier Ltd. All rights reserved.

Keywords: Color; Starch damage; Powder; Energy efficiency; Drying efficiency

1. Introduction

A drying operation involves inter-phase mass (i.e.

moisture) transfer from the wet material to the gaseous

drying agent (heated air), which may be illustrated as atransport of moisture from the material core to its sur-

face, followed by evaporation from the surface of the

material, and dissipation of water vapor into a bulk of

the gaseous drying agent. There are almost as many ap-

proaches to drying as there are materials processed. One

key characteristic is the fact that biological materials (i.e.

foodstuffs) are thermally sensitive, which imposes operat-

ing constraints on drying. Three types of information aretherefore needed: how much water is in the material, how

fast this water will evaporate, and how fast the product

will change or degrade. In most of thermal drying pro-

cesses used for heat-sensitive materials, the loss of quality

is the key issue. Quality may be qualified by flavor, tex-

0260-8774/$ - see front matter Crown Copyright � 2005 Published by Elsev

doi:10.1016/j.jfoodeng.2005.01.045

* Corresponding author. Tel.: +1 450 652 5533; fax: +1 450 652 5177.

E-mail address: mbenali@nrcan.gc.ca (M. Benali).

ture and color, and determined using a wide range of

methods. If color is unacceptable to the consumer, the

product is rejected, regardless of its taste and texture.

Color gives a quick feedback on possible protein modifi-

cation and physical property changes. In addition, theunavoidable challenge is to optimize the interaction be-

tween product quality and energy efficiency. Besides,

the condition of minimum energy use is not often the

optimal solution because of the capital cost require-

ments. Drying on inert particles is considered here as a

potential alternative to conventional direct drying for

preventing food degradation since it modifies the drying

cycle (Benali, 2004). In addition, it offers the advantageof obtaining powdery products in a dryer of much smal-

ler volume and floor area than spray dryer. Drying of

solutions on inert solid particles, as heat carriers (i.e.

heating intermediate) is a relatively novel technique to

produce free-flowing powder, which was developed in

the former USSR towards the end of the 1950s. Even

though this technique was used in industrial processing

of fine chemicals and biological materials in the formerUSSR, it was not deployed in other countries. Because

ier Ltd. All rights reserved.

Nomenclature

cp specific heat, J/kg �C_m mass flow rate, kg/sM molar mass, kg/mol

q heat, J/s

h enthalpy of air, J/kg dry air

T temperature, �CX moisture content, w/w (wet basis)

Y 0 absolute air humidity, kg water/kg dry air

Greek symbols

�sp specific energy consumption, J/kg water

g drying efficiency, %

g actual energy efficiency, %

kv latent heat of vaporization (J/kg)

Subscripts

da dry air

ev evaporation

g gas

‘ liquidmi mixing

pp pumping

r required

str starch solution

ss solid content of starch solution

v vapor

w wall

wb wet-bulb1 ambient air

2 inlet of dryer

3 outlet of dryer

4 inlet of starch solution

5 outlet of dry starch

M. Benali, M. Amazouz / Journal of Food Engineering 74 (2006) 484–489 485

of its ability to produce powders from a large variety of

solutions at volumetric evaporative rates competitive

with spray drying, drying on inert particles became an

active R&D area since the 1980s in Australia (Barrett

& Fane, 1989), Brazil (Oliveira & Freire, 1996;

Spitzner-Neto, Cunha, & Freire, 1982), Canada (Benali

& Amazouz, 2002; Kudra & Mujumdar, 2002),

New Zealand (Pham, 1983), Poland (Markowski &Kaminiski, 1983; Markowski, 1992), Russia (Kutsakova

& Bogatyrev, 1987; Kutsakova & Usvyat, 1985;

Kutsakova, Utkin, & Kupanov, 1990), and United

Kingdom (Ochoa-Martinez, Brennan, & Niranjan,

1993a, 1993b). The aim of this paper is to provide some

experimental results on controllability of quality, and en-

ergy aspects when drying vegetable starch solution on in-

ert particles.

2. Experimental apparatus and methods

The experiments were performed in a 0.035 m3

(laboratory-scale unit) and 0.660 m3 (pilot-unit) insu-

lated continuous conical jet spouted bed dryer (Fig. 1)

with 5-mm Teflon cubes as inert particles. The experi-ments carried out to date with glass beads, PVC pellets,

spherical resin particles, ceramic balls, silica gel spheri-

cal particles and Teflon cubes and spheres proved that

Teflon is the most suitable because of no attrition and

high capacity to accumulate heat. Virgin Teflon was

used for all tests carried out on both laboratory and

pilot units. An equivalent particle diameter of 3–6 mm

is required to maintain hydrodynamic stability of thebed. The thermal conductivity of Teflon is 0.9 kJ/

m h �C, and the coefficient of linear thermal expansion

is 13.2 · 10�5 cm/cm/�C within the temperature range

25–250 �C. The static bed height was varied from 0.05

to 0.10 m for the laboratory-unit, and from 0.10 to

0.35 m for the pilot-unit. As shown in Fig. 1, the dryer

consists of a drying chamber, a mixer to homogenize

the starch solution, a 1200 L-feeding reservoir, a vari-

able 50–400 kW electric heater, and a powder dischargesystem composed of a cyclone and a cartridge filter. A

conical grid located at the top of a disengaging chamber

prevents Teflon particles from being entrained from the

drying chamber. The initial solid content of vegetable

starch solution varies from 25% to 40%, w/w and its

density is 1180 ± 35 kg/m3. This starch solution is atom-

ized into droplets by means of a pneumatic spray nozzle

located at the axis of the dryer either at the bottom or onthe top. A 1/3-HP positive displacement pump with

speed control drive regulates the feed rate to the nozzle.

The inlet heated air temperature varied from 140 to

240 �C while the starch solution was fed at 10–15 �C.In continuous operation, the spray of fine droplets of

dispersed starch covers the surface of inert particles that

are heated by convection by the spouting air stream

after a period of ‘‘peeling off’’ of the dry material. Thiswet coat is dried by convective heat transfer from the

upward hot air stream and by conduction of heat stored

in the Teflon particles. The drying process continues un-

til the moisture content drops to a critical value, after

which the layer is dry enough to become brittle. The

dry layer is then broken and peeled off from the surface

of Teflon due to particle-to-particle collisions, and par-

ticle collisions with the dryer walls and the conical grid(abrasion and/or attrition). The steady state condition

1

14

10

57

6

42

10

9

8Exhaust

15

Drain

Air5

1311

3

1. blower; 2. flowmeter; 3. electric heater; 4. wind box; 5. atomizing device; 6. conicaldrying chamber; 7. grid; 8. cyclone; 9. cartr idge f i l ter; 10. powder col lector;11. raw material storage; 12. rotary pump; 13. paddle-type mixer; 14. feeding reservoir;15. positive displacement pump

12

Fig. 1. General view of experimental drying apparatus.

486 M. Benali, M. Amazouz / Journal of Food Engineering 74 (2006) 484–489

is reached when the temperature of outlet gas is

constant.

The color is determined using Hunter-Lab portable

system. The lightness varies from 0 to 100: 0 being black

and 100 being white or colorless. The level of starch

damage directly affects the water absorption and doughmixing properties of the starch. Method 76-31 of Amer-

ican Association for Cereal Chemists is used to measure

the starch damage. This method provides the extend to

which starch has been damaged. A lower percentage

indicates smaller damage and for the material tested

here, the damage index should be less than 2.5%.

3. Results and discussion

3.1. Quality aspects

Vegetable starch is a difficult to handle product be-

cause of its tendency to settle very quickly if being not

constantly in motion; this can cause pipe clogging.

Therefore, the position of a nozzle plays a key role for

such materials. Since the nozzle is in contact with the

hot air stream from the dryer inlet the solid deposit of

vegetable starch can be formed in the feeding pipe. A

water jacket was designed to cool down the nozzle and

prevent deposit and clogging. Liquid injection from

the bottom reveals better product quality since the light-ness (L) and damage index (DI) are in the ranges 95.0–

96.1, and 1.86–188, respectively when atomizing from

the bottom of the dryer. These values are in the ranges

94.1–94.4, and 1.98–2.22 when atomizing from the top

of the dryer. The industrial quality target for vegetable

starch is L P 93. The comparison of the proposed dry-

ing method with a commercial flash pneumatic dryer

where lightness and damage index are respectively inthe ranges L = 90–92, and DI = 3.5–5.1 demonstrates

the competitiveness of the proposed drying method in

terms of product quality.

The feed rate-temperature effect on the final moisture

content (FMC) of dry starch is illustrated in Fig. 2. As

expected, the FMC increases with feed rate for a given

inlet air temperature, and it decreases with increasing

inlet air temperature, for a given feed rate of vegetable

38.3 (%, w/w)

38.3 (%, w/w)

37.2 (%, w/w)

39 (%, w/w)

39 (%, w/w)Upper acceptable limit

Feed rate (kg/h)

40 60 80 100 120 140 160 180

Fina

l moi

stur

e co

nten

t of d

ry s

tarc

h (%

, w/w

)

4

6

8

10

12

14

16

18

Teflon cubes: 12 kg

upper acceptable limit

(Tair)2 = 160°C(Tair)2 = 180°C(Tair)2 = 210°C (Tair)2 = 220°C(Tair)2 = 240°C

Fig. 2. Effect of inlet air temperature on FMC.

M. Benali, M. Amazouz / Journal of Food Engineering 74 (2006) 484–489 487

starch solution. Increasing the inlet air temperature

from 180 to 240 �C increased the damage index by

approximately 25–30%. Consequently, the optimal inlet

air temperature should be in the range from 160 to

180 �C. The measurements of FMC were repeated three

times with the same measurement procedure, the sameoperator, and the same measurement instruments, used

under the same conditions. A 92–97% of the closeness

of agreement was found.

The effect of feed rate and Teflon load on FMC is

shown in Fig. 3. At an inlet air temperature of 180 �C,the effect of Teflon load on the FMC is insignificant

above 8 kg. Indeed, even tough the heat transfer should

be more efficient when the amount of inert particles in-creases (i.e. heat transfer area increases), there is, how-

ever, an upper limit, above which the internal

circulation of inert particles is unstable and the spouting

of the bed ceases.

The dried starch was subject to laser diffraction anal-

ysis and the results showed that the size of dried starch

Feed rate (kg/h)

0 20 40 60 80 100 120 140 160

Fina

l moi

stur

e co

nten

t of d

ry s

tarc

h (%

, w/w

)

2

4

6

8

10

12

14

16

18

20

(Tair)2 = 180oC

upper acceptable limit

mTeflon = 3.6 kgmTeflon = 8 kgmTeflon = 12 kgmTeflon = 16 kg

Fig. 3. Effect of load of Teflon particles on FMC.

ranged from 5 to 80 lm, with median diameter of

45.6 lm when the initial solid content was 36.2%, w/w.

The size of dried starch ranged from 10 to 120 lm, with

a median diameter of 59.7 lm when the initial solid con-

tent was 38.3%, w/w.

3.2. Energy aspects

Energy consumption in the continuous conical jet

spouted bed with inert particles arises from the follow-

ing processes:

• heating of the inert particles and vegetable starch

solution to the required temperature and to compen-sate for heat losses,

• mixing and pumping of vegetable starch solution, and

• blowing air.

The key energy parameter from industrial point of

view is the specific energy consumption �sp of the dryingsystem for a given application, which is related to the

variation of the enthalpy of air defined as follows:

hair ¼ ðCpg þ Y 0CpvÞT air þ Y 0kv ð1ÞThe absolute air humidity is calculated from the follow-

ing equation:

Y 0 ¼ MH2O

Mda

� PH2O

P tot � PH2O

ð2Þ

Fig. 4 illustrates the characteristic changes of air temper-

ature and humidity during drying. Y 02 is equal to the

ambient absolute air humidity Y 01 since there is neither

moisture removal nor moisture addition during air heat-

ing. The energy required to reach the targeted final

moisture content is therefore:

qr ¼ _mda½ðhairÞ2 � ðhairÞ1� ð3ÞThe specific energy consumption �sp is given in kJ per kg

of evaporated water:

�sp ¼qr

_mfeedX 4�X 5

1�X 5

� � ð4Þ

Fig. 5 shows the specific energy consumption as a func-tion of the drying temperature for a given feed rate of

the vegetable starch solution. As expected, the specific

energy consumption increases with increasing inlet and

outlet air temperature difference, representing the extent

of heat usage during the drying operation. The maxi-

mum specific energy consumption is in the order of

3.8 MJ/kg H2O with initial solid content of 36.2%, w/w

while it is in the order of 5.3 MJ/kg H2O with initialsolid content of 38.3%. Such an expected increase of

the specific energy consumption with initial solid con-

tent can be explained by the plastic-like characteristics

of the starch solution appearing above 38–40%, w/w.

This finding is confirmed by microscope examination,

(Tair)2 - (Tair)3 (oC)

20 40 60 80 100 120 140 160 180

Spec

ific

Ener

gy C

onsu

mpt

ion

(MJ/

kg H

2O)

2

3

4

5

6

7

8

(Tair)2 = 180oC

(Tair)2 = 210oC

(Tair)2 = 220oC

(Tair)2 = 240oC

Teflon cubes: 12 kgFeed rate: 107.53 kg/h

Xini = 36.2%, w/w

Xini = 38.3%, w/w

Fig. 5. Specific energy consumption.

(Tair)2 - (Tair)3 (oC)20 40 60 80 100 120 140 160 180

Dry

ing

Effic

ienc

y (%

)

0

20

40

60

80

100

(Tair)2 = 180oC(Tair)2 = 210oC(Tair)2 = 220oC(Tair)2 = 240oC

(Xini) = 38.3%, w/w

(Xini) = 36.2%, w/w

45.6%

53.7%

64.8%

79.6%

Fig. 6. Drying efficiency.

Dryer

vegetable starch solution

dried starch

1 3 2ambient air drying hot air exhaust air

Absolute Humidity(kg water/kg dry air)

Temperature(Tair)2

(Tair)3

1Y 2 1Y Y

3Y

(Tair)1

(Tstr)4

5

4

(Tstr)5

electric heater

1

2

3

4

5

′ ′ ′

′

Fig. 4. Characteristic changes of drying parameters.

488 M. Benali, M. Amazouz / Journal of Food Engineering 74 (2006) 484–489

which shows liquid bridges similar to the capillary stateoccurring in wet agglomerates. As a consequence, it

could be expected that the effective interfacial area and

hence the volumetric heat transfer would be smaller.

Thus, an additional heat is needed for the evaporation

of water. However, since the concentrated starch solu-

tion is considered as a particular food polymer, rheolog-

ical and interfacial properties should be examined in

details to well describe the thermal resistances and the

drying mechanism.

The performance of the drying process can be defined

either as a drying efficiency (g) or as actual energy effi-

ciency ðgÞ:

g ¼ ðT airÞ2 � ðT airÞ3ðT airÞ2 � T wb

ð5Þ

and

g ¼ qevqsp þ qpp þ qmi þ qw

ð6Þ

where

qev ¼ X 4 _mfeedCp‘½ðT strÞ5 � ðT strÞ4� þ X 4 _mfeedkvþ ð1� X 4Þ _mfeedCp;ss½ðT strÞ5 � ðT strÞ4� ð7Þ

The power consumed for mixing and pumping the veg-

etable starch solution is 0.50 and 0.25 kW, respectively.

Since the drying system is well insulated and the hydro-dynamics are well optimized, the average energy lost

through dryer walls is only of the order of 2% of the

total energy supplied.

Fig. 6 shows the drying efficiency as a function of the

inlet and outlet air temperature difference. For a given

feed rate and initial solid content, it increases with an in-

crease in the temperature difference, i.e. for a fixed inlet

air temperature, (Tair)3 should be as low as possible withrespect to the product quality characterized mainly by

the residual moisture content and the damage index. In-

deed, lowering the outlet air temperature leads to a high-

er residual moisture content in the dried product

(powder). Handling this higher moisture content prod-

uct becomes a challenge since such powder cannot be

discharged continuously from the drying chamber be-

cause of cohesion. The high drying efficiency resultsfrom the rapid and intense movement of Teflon parti-

M. Benali, M. Amazouz / Journal of Food Engineering 74 (2006) 484–489 489

cles, the large contact area, and the high temperature

difference between the inlet and outlet air. Fig. 6 shows

also that the initial solid content affects slightly the dry-

ing efficiency over the parameter ranges studied. At a

temperature difference below 70 �C, the drying efficiency

decreases by approximately 3–9% when the initial solidcontent approaches the plastic-like characteristics of

the starch solution. Such decrease of drying efficiency

becomes higher above 70 �C.In addition, the experimental findings demonstrated

that the outlet air temperature influences the temperature

of the inert particles, which can be correlated as follows:

T Teflon ð�CÞ ¼ ½ðT airÞ3 � 17� � 2 �C ð8ÞSuch findings are similar to those obtained in dryer with

swirling counter-current streams where the temperature

difference between the inert particles and the outlet air isin the order of 15 ± 3 �C during drying of the protein

hydrolyzate (Kutsakova, 2004).

4. Concluding remarks

Drying of vegetable starch solution in a continuous

conical jet spouted bed of inert particles is a good tech-nique to produce a free-flowing powder of controlled

final moisture content. In addition, the results point

out the importance of outlet air temperature as a key

control variable to obtain the optimal product quality

and energy efficiency. For the material studied the max-

imum inlet air temperature should not exceed 180 �C,and the initial solid content should be in the order of

36 ± 1%, w/w.

Acknowledgements

The authors would like to thank Manitoba

Hydro and Program on Energy Research and Develop-

ment of Natural Resources Canada for their financial

support.

References

Barrett, N., & Fane, A. (1989). Drying liquid materials in a spouted

bed. In A. S. Mujumdar & M. Roques (Eds.), Drying�89(pp. 415–420). New York: Hemisphere Publishing Corporation.

Benali, M. (2004). Thermal drying of foods: Loss of nutritive content

and spoilage issues. In A. S. Mujumdar (Ed.), Drying of products of

biological origin. India: Oxford IBH and Science Publishers.

Benali, M., & Amazouz, M. (2002). Effect of drying aid agents on

processing of sticky materials. Developments in Chemical Engineer-

ing and Mineral Processing, 10(3/4), 1–14.

Kudra, T., & Mujumdar, A. S. (2002). Advanced drying technologies.

NY: Marcel Dekker Inc.

Kutsakova, V. E. (2004). Drying of liquid and pasty products in a

modified spouted bed of inert particles. Drying Technology, 22(10),

2343–2350.

Kutsakova, V. E., & Bogatyrev, A. N. (1987). Intensification of heat

and mass transfer in drying of food products. Moscow: Agroprom-

izdat (in Russian).

Kutsakova, V. E., Utkin, Y. V., & Kupanov, B. Y. (1990). Method for

drying of liquid materials, Russian Patent No. 1560948.

Markowski, A. S. (1992). Drying characteristics in a jet-spouted bed

dryer. Canadian Journal of Chemical Engineering, 70, 938–944.

Markowski, A., & Kaminiski, W. (1983). Hydrodynamic characteristic

of jet-spouted beds. Canadian Journal of Chemical Engineering, 61,

377–383.

Ochoa-Martinez, L. A., Brennan, J. G., & Niranjan, K. (1993a).

Spouted bed dryer for liquid foods. Food Control, 4, 41–45.

Ochoa-Martinez, L. A., Brennan, J. G., & Niranjan, K. (1993b).

Drying of liquids in a spouted bed dryer of inert particles: Heat

transfer studies. Journal of Food Engineering, 20, 135–148.

Oliveira, W. P., & Freire, J. T. (1996). Analysis of evaporation rate in

the spouted bed zones during drying of liquid materials using a

three region model. In Proceedings of the 10th international drying

symposium (IDS�96), Krakow-Poland (Vol. A, pp. 504–212).

Pham, Q. T. (1983). Behavior of a conical spouted-bed dryer for

animal blood. Canadian Journal of Chemical Engineering, 61,

426–434.

Spitzner-Neto, P. I., Cunha, F. O., & Freire, J. T. (1982). Effect of the

presence of paste in a conical spouted bed dryer with continuous

feeding. Drying Technology, 20, 789–811.