Drying kinetics and drying shrinkage of garlic subjected to vacuum-microwave dehydration (Figiel).pdf

Acta Agrophysica, 2006, 7(1), 49-58

DRYING KINETICS AND DRYING SHRINKAGE OF GARLIC SUBJECTED

TO VACUUM-MICROWAVE DEHYDRATION

Adam Figiel

Agricultural Engineering Institute, Agricultural University

ul. Chełmo ń skiego 37/41, 51-630 Wrocław

e-mail: figiel@imr.ar.wroc.pl

A b s t r a c t . Whole garlic cloves and cut into slices were subjected to pulsed vacuum-micro-

wave drying at three microwave power levels: 240, 480 and 720 W. The pressure in the dryer drum

was from 4 to 6 kPa. It was found that the process of drying can be divided into three periods

described by power, linear and exponential functions. Increase of microwave power caused increase

in the rate of drying, and had no effect on drying shrinkage. Cutting garlic cloves into slices resulted

in longer drying time. The rate of drying sliced garlic was lower compared with that of drying whole

cloves in all the three periods at 480 W power. At microwave power of 240 and 720 W the rate of

drying sliced garlic was greater at the end of second period and in the third one. A relative volume

increase, as a result of the puffing phenomenon, was observed only when whole cloves were drying.

K e y w o r d s : garlic, vacuum-microwave drying, drying kinetics, shrinkage

INTRODUCTION

Garlic has been and is cultivated all over the world because of its culinary and

medicinal advantages. That vegetable spice is produced and consumed in

abundant quantities. However, like other biological crops, garlic is exposed to

waste due to respiration and microbial spoilage during storage. Dehydration is an

alternative method of biological material preservation. Hot air drying method is

currently applied to reduce the moisture content of garlic [3]. The method has

several disadvantages and limitations. It takes long time during the falling rate

period even at high temperature. Air dryer temperatures usually need to be in the

range of 60 to 90

C. Such a drying regime generates degradation of important

flavour and nutritional substances as well as colour alteration. Hot air drying

causes structural changes in foods due to local hardening and tissue collapse and

A. FIGIEL

this way provides shrinkage. Vacuum microwave dehydration has been reported to

reduce these limitations [5,8]. In that process of dehydration microwaves penetrate

to the interior of the food causing water to boil within the food at relatively low

temperature. This creates a large vapour pressure in the centre of the product,

allowing rapid transport of moisture out of the product and preventing structural

collapse. This process, referred to as the puffing phenomenon, creates a porous

texture of the food [10].

Microwave energy has been already applied to convective drying of garlic

[1,11,12]. However, no work has been reported on the vacuum-microwave drying

of garlic. The evolution of this method to produce high quality dried garlic in

a short time could make a significant contribution to the vegetable processing

industry. Therefore the aim of the work was to determine the effect of microwave

power on the time and rate of drying whole and sliced garlic cloves with vacuum-

microwave method.

MATERIALS AND METHODS

Whole garlic cloves and cut into slices of 4.5 mm thickness were subjected to

vacuum–microwave drying in a VM 200 Plazmatronika dryer that had two

magnetrons of 1200 W combined power and a revolving drum of approximately

0.18 m radius and 0.27 m length. Three levels of microwave power were applied:

240, 480 and 720 W. Garlic was dried with 60-second microwave pulses followed

by 5-second breaks. Investigations conducted by Gunasekaran showed that pulse

drying of cranberry is more effective than continuous drying [6]. The pressure in

the drum revolving at 6 rev min -1 was from 4 to 6 kPa. Excessive lowering of

pressure, necessitating the use of more expensive vacuum pumps [7], did not

result in significant shortening of the drying time [2]. The kinetics of garlic drying

was determined on the basis of mass losses of 60 g portions of garlic with 66%

initial moisture content. With successive portions of fresh material longer and

longer drying times were applied. Therefore, the number of measurement points

necessary for drying curve fitting amounted to the number of fresh material

portions. Absolute values of derivatives of the functions describing garlic drying

allowed the drying rate determination and estimation of the critical points K1 and

K2. The critical points divided the drying process into three periods – I, II and III

(Fig. 1). Assuming that the drying rate in period II should assume a constant

value, it was necessary to reduce the other two ranges I and III to the same value

to ensure continuity of the drying curve. Sometimes in period II, characterised by

constant drying rate, only one point remained. Obviously, the only one point

could not be used to fit the linear function which was a part of the drying curve

between the critical points K1 and K2.

DRYING KINETICS AND DRYING SHRINKAGE OF GARLIC SUBJECED

Moisture

content M

(

)

(

)

K 1

K 2

III

(

)

Rate

of drying

Time t

III

Time t

Fig. 1. Drying rate determination and estimation of the critical points K1 and K2 (I, II, III – drying

periods)

Drying shrinkage ( S ) was determined as a function of dry mass ( D ) by

calculating the ratio of garlic volume after drying ( V ) to garlic volume before

drying ( V 0 ):

(1)

The volume of fresh garlic was measured with a graduated cylinder filled with

distilled water, while that of dried garlic with the use of toluene.

A. FIGIEL

RESULTS AND DISCUSSION

Based on the measurement points obtained from the drying experiments, it was

found that the process of garlic cloves drying with the vacuum-microwave method

can be divided into three periods separated by the critical points K1 and K2,

irrespective of the microwave power applied (Fig. 2). The decrease in moisture

content in the first drying period, between the starting point and K1, was described

by a power function, in the second period, between points K1 and K2 – by a linear

function, and in the third period, between K2 and the final point – by an exponential

function (Tab. 1). Increased microwave power from 240 to 720 W resulted in

shorter drying time – from 21 to 6.6 min at final moisture content of 7%.

240 W

480 W

720 W

Time t (min)

Fig. 2. Drying curves of garlic cloves obtained at different microwave power levels

Size reduction of the material under drying by the method of natural or forced

convection usually causes a shortening of drying time as a result of increased areas

for mass and energy exchange. However, the results of the investigations performed

indicate that size reduction of the raw material caused an increase in drying time with

the vacuum-microwave method (Fig. 3). The larger deference in drying time of whole

and sliced garlic cloves was observed for the lowest power level 240 W. For other

power levels, particularly 480 W, the difference was not significant. This is only

a tendency which was not statistically proved. Nevertheless, the tendency was con-

firmed by the results of microwave drying of banana, reported by Maskan [9].

DRYING KINETICS AND DRYING SHRINKAGE OF GARLIC SUBJECED

Table 1. Parameters of functions describing the decrease in moisture content of whole garlic cloves

and cut into slices in the three drying periods at different microwave power levels

Microwave Power (W)

Period

Function

Parameter

240

480

720

cloves slices cloves slices cloves slices

66.3 65.7 65.8 65.4 65.8 65.6

–1.09 –0.66 –2.19 –2.13 –4.05 –2.66

1.38 1.48

1.5 1.39 1.51 1.59

75.5

76 71.8 72.3 77.5 81.8

–3.62 –3.19 –6.08 –5.95 –11.1 –10.3

–11.6 –2.64 –12.4

0 –32.2 –42.4

III

146

238

309

517

144

157

10.1

7.4 3.89 2.75

5.1 6.44

Cloves Slices

Gloves Slices

240 W

480 W

720 W

Time t (min)

Fig. 3. Drying curves of whole garlic cloves and cut into slices obtained at different microwave

power levels

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