Drying of garlic (Allium sativum) cloves by microwave±hot air

combination

G.P. Sharma, Suresh Prasad

*

Agricultural & Food Engineering Department, Indian Institute of Technology, Kharagpur 721302, India

Received 4May 2000; accepted 20 November 2000

Abstract

Garlic cloves were dried with hot air and combined microwave±hot air drying methods in an experimental dryer. The combined

microwave±hot air drying experiments were carried out with 100 g sample sizes at temperatures of 40°C, 50°C, 60°C and 70°C at air

velocities of 1.0 and 2.0 m/s, using continuous microwave power of 40 W. For comparison of hot air drying, the same sample sizes

were taken for experiments and the drying air temperatures and air velocity were 60°C and 70°C, and 2.0 m/s respectively. The total

drying time, the color and ¯avor strength of dried garlic cloves were used to evaluate the performance of the combined microwave±

hot air drying and the conventional hot air drying processes. Combined microwave±hot air drying resulted in a reduction in the

drying time to an extent of 80±90% in comparison to conventional hot air drying and a superior quality ®nal product. Ó 2001

Elsevier Science Ltd. All rights reserved.

Keywords: Garlic cloves; Combined microwave±hot air drying; Drying time; Air temperature; Microwave power

1. Introduction

Garlic has been cultivated for centuries all over the

world on account of its culinary and medicinal proper-

ties. It is mainly used as a condiment in various food

preparations such as ¯avoring mayonnaise and tomato

ketchup-sauce, salad dressing, meat sausages, stews,

spaghetti, chutney pickles, etc. Though garlic is pro-

duced abundantly and consumed as such, little e€orts

have so far been made to produce dehydrated garlic and

garlic powder. Garlic is a semi-perishable vegetable spice

and nearly 30% of the crop is wasted due to respiration,

microbial spoilage during storage (Anon, 1993).

Drying is an alternative to minimize the losses to a

considerable extent. Garlic cloves with approximately

1.85 g water/g dry matter are dried to a safe moisture

content of 0.06 g water/g dry matter. Currently hot air

drying method is used for drying the garlic (Prakash,

Dekshinamurthy, & Shukla, 1994; Dash & Bhatnagar,

1991 and Dawn & Shreenarayanan, 1998).

A major disadvantage associated with hot air drying

is that it takes long time even at high temperature, which

results in degradation of the dried product quality.

Compared with hot air drying, combined microwave±

hot air could greatly reduce the drying time of biological

materials without damaging the quality attributes of the

®nished products (Ren & Chen, 1998). The food mate-

rials reportedly have been dried using mirowave±con-

vective technique include corn (Gunasekaran, 1990),

grapes (Tulasidas, Raghavan, & Norris, 1993), carrots

(Prabhanjan, Ramaswamy, & Raghavan, 1995) apples

and mushrooms (Funebo & Ohlsson, 1998), and various

medicinal plants (Ren & Chen, 1998). However, no

work has been reported on the drying of garlic. The

evolution of a microwave±hot air drying process to

produce high-quality dried garlic in a relatively short

time could make a signi®cant contribution to the garlic

processing industry. Therefore, the main objective of

this study was to explore the possibility of using com-

bined mirowave±convective drying technique for pro-

cessing of garlic and assessment of the quality of ®nished

product.

2. Materials and methods

Fresh garlic (Allium sativum) bulbs of Mainpuri va-

riety were used in the investigation which were procured

in bulk from the local market of Kharagpur in the state

of West Bengal (India). The garlic bulbs had initial

Journal of Food Engineering 50 (2001) 99±105

www.elsevier.com/locate/jfoodeng

*

Corresponding author.

E-mail address: [e-mail] (S. Prasad).

0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 0 - 8 7 7 4( 0 0 ) 0 0 2 0 0 - 4

moisture content ranging from 1.89 to 1.85 g water/g dry

matter and were stored in a cold storage chamber

maintained approximately at a temperature of 1°C and

70% relative humidity.

2.1. Moisture content

The vacuum oven method was used to determine the

moisture content of the garlic cloves. Garlic samples of

approximately 15 g were placed in pre-dried aluminum

dishes in a vacuum oven, with sulfuric acid as a desic-

cant. The operating temperature was 70°C with a gauge

pressure of 85 kPa and the sample was kept for 24h

(Madamba, Driscoll, & Buckle, 1995). The samples were

then taken out of oven, cooled in a dessicator and

weighed using an ANAMED top pan electronic balance

with a sensitivity of 0.01 g. The fresh and bone-dried

weights were used to calculate the moisture content

which was expressed as g water/g dry matter.

2.2. Microwave±hot air dryer

A 600 W, 2450 MHz microwave oven (IFB make,

model Electron) having inside chamber dimensions of

300 (W) 240 (D) 210 (H) mm

3

was modi®ed and

developed into a microwave±hot air drier. The schematic

diagram of the microwave±hot air dryer is shown in

Fig. 1. An opening of 10 cm diameter was made on top of

the microwave chamber in the center to supply hot air to

the chamber and a 12 cm diameter stainless steel wire

mesh was placed over the opening to prevent microwave

leakage. An inverted cone made of mild steel sheet was

put on the wire mesh and fastened to the top of the

chamber. The other end of the cone was connected to a

galvanized iron (GI) pipe of about 42 mm inside diam-

eter for supplying hot air into the chamber. A duct made

of Perspex tube having 10 cm inner diameter and 10 cm

length was ®tted to the inside top of the chamber for

carrying hot air to the sample holder. The sample holder,

for accommodating the material to be dried, was made of

Perspex tube having 10 cm diameter and 4cm length,

with nylon wire mesh at the bottom. The sample holder

was supported by three legs made of Perspex rod of 0.6

cm diameter and 5 cm length and placed beneath the hot

air supply duct. The sample holder rested on a rotating

platform made of Perspex sheet of size 12 cm diameter

and 0.5 cm thickness. A gap of about 1 mm was kept

between the hot air supply duct of perspex and the

sample holder to facilitate its rotation for uniform

heating of garlic cloves to be dried. The platform was

connected to another circular plate of similar dimensions

placed outside the drying chamber by a Te¯on rod of 1

cm diameter and 10 cm length passing through a 1.2 cm

hole in the bottom of the chamber. An electric motor of

3.5 W with 2.5 r.p.m. was mounted on a separate plat-

form of Perspex sheet of dimension 140 140 mm

2

and

its shaft ®tted snugly to the bottom of the circular plate

to enable the rotation of the sample holder assembly. The

whole assembly was placed over a top loading electronic

balance (Make: ANAMED; 2000 0:01 g) beneath the

drying chamber for intermittent recording of sample

weight. The arrangements enabled on-line measurement

of the sample weight on the balance by diverting the air

supply into the atmosphere temporarily for few seconds

while measuring the weight of the sample. The balance

took about 5 s to get stabilized.

A small air blower of 2.5 m

3

/min capacity and an

electrical heater of 1.5 kW were used to supply hot air

Fig. 1. Schematic diagram of microwave±hot air dryer.

100

G.P. Sharma, S. Prasad / Journal of Food Engineering 50 (2001) 99±105

into the experimental dryer. A gate valve was also

connected after the heater, to regulate the supply of air.

Two stainless steel ball valves were ®tted to facilitate air

supply and cut o€ the supply to the chamber as and

when needed.

The output power of the magnetron of the microwave

oven was regulated by varying the anode current as

described by Tong, Lenz, and Lund (1993) A high

voltage transformer was incorporated in the circuit to

supply current to the anode of magnetron separately. A

variac of 15 A/230 V rating was also placed on the

primary side of the high voltage transformer to regulate

the anode current, thus varying the output power of the

magnetron between 0 and 600 W. The output power of

the magnetron was measured by the calorimetric meth-

od (Mataxas & Meredith, 1983; Tulasidas et al., 1993).

1000 g of distilled water was taken in a glass beaker and

its temperature was measured by a mercury thermome-

ter. The beaker was kept in the center of the microwave

oven and was exposed to microwaves for a predeter-

mined period of time (62 s). The water was taken out

immediately and stirred quickly by a glass rod and its

temperature was again measured by the thermometer.

The output power of magnetron was estimated by esti-

mating the power absorbed by water.

3. Experimental procedure

Garlic bulbs were taken out of cold storage and al-

lowed to equilibrate with ambient condition for 12 h and

the cloves were hand peeled thereafter. A sample of

about 100 g of peeled garlic cloves of uniform size

having 210 mm length and 72 mm diameter (each clove

weighing between 0.9 and 1.1 g) was used for drying

experiments. The moisture content of the sample was

determined by the vacuum oven method as described

earlier. The dryer was run without the sample for about

30 min to set the desired drying conditions before each

drying experiment. The inlet and outlet air temperatures

as well as the air velocity were measured by iron±con-

stantan thermocouple (26 gage) coupled to a digital

temperature indicator with the sensitivity of 1°C and

an anemometer (Make: Kanomax, Japan) with a least

count of 0.1 m/s, respectively. Hot air drying experi-

ments on the garlic cloves were carried out at 60°C and

70°C maintaining the air velocities of 2 m/s. Preliminary

experiments of microwave±hot air drying of garlic cloves

resulted in charring of the product towards the end of

drying at power level higher than 40 W. The combined

microwave±hot air drying experiments were thus con-

ducted with a continuous microwave power of 40 W, in

conjunction with hot air at 40°C, 50°C, 60°C and 70°C

temperatures at air velocities of 1.0 and 2.0 m/s. The

microwave power was applied until the weight of the

sample reduced to a level corresponding to a moisture

content of about 0.06 g water/g dry matter. The drying

was terminated thereafter. The weight of sample was

measured every 5 min during ®rst hour, 15 min during

second hour and at 30 min intervals during rest of the

drying period for hot air drying whereas the observa-

tions were taken every 5 min intervals for combined

microwave±hot air drying experiments with three repli-

cations until the weight of the sample was reduced to a

level corresponding to a moisture content of about 0.06

g water/g dry matter moisture content. The sample

weight loss during drying was co-related with moisture

content on dry basis and expressed as g water/g dry

matter. The drying data were analyzed to study the

drying behavior of garlic cloves.

3.1. Color measurement

The chromaticity of the dried garlic cloves was mea-

sured in terms of L (the degree of the lightness), a (degree

of redness) and b (degree of yellowness) values, using a

Hunter lab colorimeter. The colorimeter was calibrated

against a standard calibration plate of a white surface

with L, a, b values of 91.10, )0.64and )0.43, respectively.

The measurements of color were replicated ®ve times after

shaking the dried samples and the average values of L, a,

and b were reported. The shaking was primarily done to

take into account the variation in the color with the ori-

entation of garlic cloves, if any in the sample.

3.2. Flavour strength

The volatile oil comprising of sulfur compounds

which are responsible for the pungency of garlic was

determined by Chloramine-T method (Shankaranara-

yana, Abraham, Raghavan, & Natrajan, 1981; Dawn &

Shreenarayanan, 1998). The volatile oils in the samples

were expressed as mg oil/g dry matter.

4. Results and discussion

A typical drying curve, exempli®ed by Fig. 2, exhibits

the change in the moisture content of sample with time

Fig. 2. Drying curves for garlic cloves under convective drying con-

dition at air velocity 2.0 m/s.

G.P. Sharma, S. Prasad / Journal of Food Engineering 50 (2001) 99±105

101

under di€erent drying conditions. In hot air drying, the

drying rates were more in the beginning of the process

when the air stream easily evaporates moisture from the

garlic cloves. The moisture content decreased exponen-

tially with the drying time and the drying took place in

falling rate period. The drying time required to reduce

the moisture from initial moisture content of about 1.85

g water/g dry matter to a desired moisture in the ®nal

product, approximately 0.06 g water/g dry matter was

6.5 and 11.5 h at drying air temperatures of 70°C and

60°C, respectively. The drying rate curve for the con-

vective drying process of the garlic cloves is shown in

Fig. 3. The drying rates were higher in the beginning of

the drying process at both the temperatures studied. The

drying rate decreased with the decrease in moisture

content, signifying that drying of garlic cloves occurred

in falling rate period. As expected, drying rates increased

when the drying air temperature was increased from

60°C to 70°C.

The drying curves of the garlic cloves by combined

microwave±hot air drying process at air velocities of 1.0

and 2.0 m/s at temperatures of 40°C, 50°C, 60°C and

70°C are shown in Figs. 4and 5 respectively. The

combined microwave±hot air drying behavior of garlic

followed the similar trend as that of the conventional

hot air drying, in that the drying occurred mainly in the

falling rate period. This indicated a di€usion-controlled

type mechanism of drying (Feng & Tang, 1998). The

drying time in microwave±hot air drying was found to

be higher at air velocity of 2.0 m/s as compared to the

velocity of 1.0 m/s at all temperatures. The food absorbs

more microwave heat at the beginning of drying because

of having high moisture. This results in a steep rise in the

temperature of product, sometimes reaching near

the boiling point of water. The hot air ¯owing across the

product is at temperature lower than the temperature of

the product during microwave drying which in turn

picks up the heat from the product and thus imparts

cooling e€ect. At higher velocity, heat removal from the

product increased because of higher ®lm heat transfer

coecient (h / v

0:4

for cross air¯ow). This may be the

reason for enhanced drying time when the air velocity

was increased from 1.0 to 2.0 m/s. Tulasidas et al. (1993)

also reported increase in the drying time with increase in

air velocity at a constant microwave power.

Drying rate curves for garlic by microwave±hot air

drying process at di€erent air velocities are shown in

Figs. 6 and 7. The drying rates were higher for all

temperatures for the combined microwave±hot air dry-

ing than convention hot air drying. The accelerated

drying rates may be attributed to internal heat genera-

tion and the so-called `liquid movement' within the

material when it is exposed to microwaves (Lyons,

Hatcher, & Suderland, 1972). Microwave power ab-

sorption by the product depends on its moisture

Fig. 3. Drying rate curves for garlic by convective drying at drying air

velocity of 2.0 m/s.

Fig. 4. Drying curve for garlic by microwave±hot air drying at mi-

crowave power of 40 W at air velocity of 1.0 m/s.

Fig. 5. Drying curve for garlic by microwave convective drying at 40 W

at air velocity of 2.0 m/s.

Fig. 6. Drying rate curve for garlic by microwave±hot air drying at

microwave power of 40 W and air drying velocity of 1.0 m/s.

102

G.P. Sharma, S. Prasad / Journal of Food Engineering 50 (2001) 99±105

content. The moisture content of the garlic was rela-

tively high during the initial phase of the drying which

resulted in higher absorption of the microwave power

and led to an increased product temperature. This re-

sulted in higher drying rate due to higher moisture dif-

fusion. As the garlic drying progressed, the loss of

moisture in the product decreased the absorption of

microwave power and resulted in a fall in the drying rate

during the latter part of the drying (Khraisheh, Cooper,

& Magee, 1995). A similar phenomenon was observed

for both the air velocities studied.

4.1. Modeling drying data

Methods of describing the drying process with thin-

layer drying models are widely reported in the literature

for the purpose of simulation and scale up of the pro-

cess. The empirical Page's equation, Eq. (1) has been

used to describe the drying kinetics of grains and other

agricultural materials (Shivhare, Raghavan, & Bosisio,

1990; Daimante & Munro, 1991; Prabhanjan et al.,

1995).
MR ˆ exp… kt

n

†;

…1†

where, MR is the moisture ratio, …M

t

M

e

†=…M

o

M

e

†

is dimensionless, M

t

is the moisture content at time

t ˆ t, g water/g dry solid, M

o

is the moisture content at

time t ˆ 0, g water/g dry solid, M

e

is the equilibrium

moisture content, g water/g dry solid, k is the rate

constant, h

1

, n is the parameter of Page's equation, and

t is the time, h.

The drying data of garlic cloves by the combined

microwave±hot air process were used to test the appli-

cability of Eq. (1). The equilibrium moisture content was

assumed to be the ®nal moisture content (0.06 g water/g

dry matter) of each run (Prabhanjan et al., 1995; Pez-

zutti & Crapiste, 1997; Ren & Chen, 1998). The pa-

rameters `n' and `k' of the equation were evaluated

through non-linear regression analysis (using SYSTAT

8.0) and the values are tabulated in Table 1. The analysis

yielded high values of R

2

, implying a good ®tness of

model to the experimental data of mirowave±convective

drying of garlic. The ®tness of the model for di€erent air

velocities for microwave±hot air drying is illustrated in

Figs. 8 and 9.

The value of n increased with increase in temperature

at a constant air velocity. This signi®es that with in-

crease in temperature drying curve becomes steeper in-

dicating faster drying of the product. At constant

temperature, with increase in velocity value of n de-

creased signifying that the drying process became slower

with increase in air velocity during microwave±hot air

drying. The value of rate constant, k is expected to de-

pend on the mirowave±convective drying conditions.

Table 1

Page's equation parameters for drying of garlic by mirowave±con-

vective technique at microwave power of 40 W

Air velocity

(m/s)

Air temperature

(°C)

k

n

R

2

1.0

40

0.051

0.976

0.992

50

0.036

1.139

0.997

60

0.042

1.140

0.983

70

0.032

1.382

0.975

2.0

40

0.043

0.915

0.961

50

0.044

1.002

0.982

60

0.038

1.119

0.994

70

0.059

1.0940.984

Fig. 8. Moisture ratio vs time comparing experimental curve with the

predicted one (-) through Page's equation for microwave±hot air

drying at air velocity of 1.0 m/s.

Fig. 9. Moisture ratio vs time comparing experimental curve with the

predicted one (-) through Page's equation for microwave±hot air

drying at air velocity of 2.0 m/s.

Fig. 7. Drying rate curve for garlic by microwave±hot air drying at

microwave power 40 W and air drying velocity of 2.0 m/s.

G.P. Sharma, S. Prasad / Journal of Food Engineering 50 (2001) 99±105

103

There is no possible explanation for such a variation of

the parameter k in Eq. (1) because of the equation being

impirical in nature.

5. Quality assessment

The color and ¯avor are important attributes of the

dehydrated product, from the consumer acceptance

viewpoint. The e€ect of the drying methods on the

quality attributes of the dried garlic cloves is presented

in Table 2. It is evident from the table that the dried

garlic cloves were lighter in color when dried by com-

bined microwave±hot air drying process as compared to

hot air drying. It was found that with an increase of the

air temperature, the color became darker implying that

more browning of the cloves occurred. It is also reported

in the literature that browning increases with an increase

in the drying temperature and/or time (Ren & Chen,

1998). The retention of the volatile components re-

sponsible for ¯avor strength was also more in micro-

wave±hot air drying than hot air drying (keeping

microwave power zero). Thus, the microwave±hot air

drying gave superior quality dehydrated garlic cloves.

6. Conclusion

It is possible to dry garlic cloves by combined mi-

crowave±hot air drying technique. The drying technique

was more ecient than conventional hot air drying for

garlic cloves and resulted in saving to an extent of about

91% of drying time. The empirical Page's model ade-

quately described the thin layer combined microwave±

hot air drying process for garlic cloves. Good quality

dried garlic cloves were also obtained by the microwave±

hot air technique.

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Table 2

E€ect of drying methods on quality attributes of dried garlic cloves

Air velocity

(m/s)

Air temperature

(°C)

Microwave

power (W)

Color parameters

Flavor strength

(mg/g dry matter)

L

a

b

1.0

60

0

a

51.84(0.34)

b

8.97 (0.73)

21.99 (0.96)

3.31

70

0

a

52.55 (0.67)

79.32 (0.76)

23.43 (0.86)

3.27

40

40

45.53 (0.35)

6.77 (0.36)

18.75 (0.35)

5.43

50

40

47.25 (0.14)

7.21 (0.47)

19.97 (0.12)

4.78

60

40

49.02 (1.27)

5.61 (0.32)

19.03 (0.44)

4.86

70

40

48.07 (0.38)

6.01 (0.72)

20.03 (0.21)

4.06

2.0

60

0

a

52.34 (0.54)

8.97 (0.87)

21.45 (0.98)

3.22

70

0

a

53.2 (0.89)

9.87 (0.98)

24.31 (0.92)

3.24

40

40

41.65 (1.21)

4.87 (0.43)

18.24 (0.61)

4.06

50

40

43.62 (0.45)

5.98 (0.78)

19.02 (0.56)

5.14

60

40

48.58 (0.81)

6.09 (0.36)

19.53 (0.37)

5.57

70

40

48.07 (0.380)

6.01 (0.72)

20.03 (0.21)

4.91

a

Indicates the hot air drying.

b

Values in parenthesis indicate the S.D.

104

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G.P. Sharma, S. Prasad / Journal of Food Engineering 50 (2001) 99±105

105