Quality Starchy Food_microwave

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Quality and structural changes in starchy foodsduring microwave and convective drying

M.A.M. Khraisheh a,*, W.A.M. McMinn b, T.R.A. Magee b

a Department of Civil and Environmental Engineering, University College London, Chadwick Building, Gower Street, London WC1E 6BT, UKb Food Process Engineering Research Group, School of Chemical Engineering, Queens University Belfast, Belfast BT9 5AG, UK

Received 7 July 2003; accepted 11 November 2003

Abstract

This study was conducted to evaluate the quality and structural changes in potatoes during microwave and convective drying.

A modified microwave oven, operated in either the microwave or convective drying mode, was used to dry the samples. The quality

attributes of the dehydrated potato samples were investigated on the basis of the ascorbic acid retention (vitamin C) and rehy-

dratibility, and the structure in terms of the shrinkage behaviour. Ascorbic acid is an important indicator of quality and its selection

was due to its heat labile nature. Ascorbic acid deterioration demonstrated first-order kinetic behaviour, and was further found to

depend on air temperature, microwave power and moisture content. Reduced vitamin C destruction was found in the microwave

dried samples. The volumetric shrinkage of the samples exhibited a linear relation with moisture content. With convective

processing, the samples exhibited uniform shrinkage throughout, however, with microwave drying two shrinkage periods were

observed. Microwave dried samples had higher rehydration potential.

2004 Elsevier Ltd. All rights reserved.

Keywords: Convective drying; Microwave drying; Potato cylinder; Rehydration; Shrinkage; Vitamin C

1. Introduction

During microwave processing, food quality is one of

the most important consumer concerns. The microwave

drying of foodstuffs gives rise to complicated chemical

conversions and reactions. Such reactions can cause

degradation of vitamins, lipid oxidation and browning

reactions, with the mechanisms being influenced by

factors such as concentration, temperature and water

activity (aw

) (Bruin & Luyben, 1980). Several research

reports have investigated vitamin losses during micro-

wave cooking. Rosen (1972) discussed the effect of mi-

crowaves on food and related materials. The quantum

energy of microwaves, in contrast to some other types of

electromagnetic radiation (X- and c-rays), was reported

to be too low, by several orders of magnitude, to cause

chemical changes by the direct interaction with mole-

cules and chemical bonds. Gerster (1989) used heat

sensitive and water-soluble vitamins C, B1 and B2 as

indicator nutrients for qualitative changes. The reten-

tion of vitamins during blanching, cooking and reheat-

ing of foods in a microwave oven was found to be

comparable to the retention using conventional methods

of heating.

The rate of ascorbic acid destruction was found to

increase with increasing aw and was more rapidly de-

stroyed in a desorption system due to the decrease in

viscosity (Labuza, McNally, Gallagher, & Hawkes,

1972). Kirk, Dennison, Kokoczka, and Heldman (1977)

studied the stability of ascorbic acid in a dehydrated

model food system as a function of water activity,

moisture content, oxygen and storage temperature.

Under the storage conditions used in the study, the

ascorbic acid losses conformed to a first-order kinetic

function. Lin, Durance, and Scaman (1998) reported a

higher vitamin C content in vacuum microwave dried

carrots than those prepared by air drying. El-Din and

Shouk (1999) also reported reduced ascorbic acid de-

struction in okra by using microwave drying.

* Corresponding author. Tel.: +44-20-7679-7224/7994; fax: +44-20-

7380-0986.

E-mail address: [email protected] (M.A.M. Khraisheh).

0963-9969/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodres.2003.11.010

Food Research International 37 (2004) 497503

www.elsevier.com/locate/foodres
http://mail%20to:%[email protected]/http://mail%20to:%[email protected]/


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The dissipation of electromagnetic energy inside a

material creates a thermal imbalance state producing

different reactions than those observed during classic

drying processes. The improved drying rates obtained by

microwave application can be explained by taking into

account the pressure gradients induced by microwave

application. These greatly accelerate the thermo-migra-tion mechanism and thereby modify the physical prop-

erties of the product. The shrinkage of a porous material

during drying is very sensitive to the internal vapour

pressure. The quality of such a product depends on the

shrinkage behaviour. Shrinkage during drying takes

place simultaneously with moisture diffusion and thus

affects the moisture removal rate. Shrinkage also affects

the physical properties of a material, e.g., apparent

density. Hence, a study of the shrinkage phenomena is

important for a better understanding of the drying

process and to control the product characteristics.

Shrinkage during drying has usually been assumed

negligible to facilitate solving heat and mass transfer

equations, however, such an assumption is not valid for

all substances in all moisture ranges (Madamba, Dris-

coll, & Buckle, 1994). It has been shown that both vol-

umetric shrinkage (Lozano, Rotstein, & Urbicain, 1983)

and dimensional shrinkage (Rahman & Potluri, 1990)

are dependent on moisture content. Preliminary exper-

iments showed that shrinkage of potatoes is not negli-

gible under the experimental conditions used. Therefore,

mathematical models relating shrinkage to moisture

content are required. The theoretical basis for shrinkage

should involve mechanical laws which take into account

material stresses and deformations during dehydration(Ratti, 1994). However, analysis of foods is extremely

complicated because of the multiphase and cellular na-

ture of the system. In order to model shrinkage of foods

from this point of view, a knowledge of the structural,

mechanical and elastic properties of each phase of the

system, and the variation with water content and tem-

perature, is required. Therefore, a practical approach to

the study of food shrinkage is experimentally based.

Current research has indicated that degree of rehy-

dration is dependent on processing conditions, sample

preparation, sample composition and the extent of

structural and chemical disruption induced during dry-

ing (Okos, Narsimhan, Singh, & Weitnauer, 1992).

Studies to assess the relationship between the duration

and severity of the drying process and the rate and de-

gree of rehydration, indicate more rapid and complete

rehydration with decreased drying time. This reflects less

shrinkage, and therefore the presence of well-defined

intercellular voids which promote increased rehydration

rates (Haas, Prescott, & Cante, 1974). Maskan (2001)

reported that microwave dried kiwifruit slices exhibited

lower rehydration capacity and faster water absorption

rate than hot air and microwave-assisted hot air drying.

Durance and Wang (2002) examined the rehydration

capacity of tomatoes dehydrated in a batch convection

air dryer, a vacuum microwave system and by combi-

nation processes. Samples finish-dried using microwave

vacuum drying exhibited a puffed structure and thus,

faster rehydration. El-Din and Shouk (1999) also re-

ported an increased rehydration ratio in okra samples

dehydrated using microwave drying.The aim of this work is to examine the vitamin C

degradation, shrinkage and rehydration characteristics

of potato cylinders during microwave and convective

drying.

2. Materials and methods

The microwaveconvective drying system used in this

work is a modified microwave oven (Brother, Hi-speed

cooker, Model No. MF 3200 d13) of variable power

output settings and rated capacity of 650 W at 2.45

GHz. The equipment consists of two parts; a hot-air

drying unit and a laboratory microwave oven (func-

tioning as the drying chamber). Ambient air is drawn

through the duct assembly by a centrifugal fan, passed

through an electric heating element, and then mixed in

the reduction section, before being introduced into the

drying chamber.

Cylindrical (radius 13.5 mm, length-to-radial ratio

4:1) potato samples, of approximate initial moisture

content 4.5 kg kg1 (dry basis), were dried in the ex-

perimental dryer. The system was operated in convective

mode at an air velocity of 1.5 m s1 and air temperatures

(30, 40 and 60 C), and in the microwave mode at var-ious output power levels between 90 and 650 W (cor-

responding to absorbed power levels of 10.538 W).

The measurement of power output of the microwave

oven was determined calorimetrically (Khraisheh,

Cooper, & Magee, 1997) that is the change of temper-

ature of a known mass of water for a known period of

time. The basic equation is

MWabs 4:187mCpDT

Dt; 1

where MWabs is the power absorbed by the sample (W);

m is the mass of sample (g); Cp is the specific heat of the

material (kJ kg1 C1 or kJkg1 K1); DT is the tem-

perature rise in the water load (C); Dt is the time mi-

crowave power was on (s).

Eq. (1) assumes that the power absorbed was solely

due to the microwave energy, there was no heat gain or

loss to the surroundings, and Cp of water did not change

with temperature.

Deionised water weighing 1000 g and equilibrated at

a temperature of 5 C below room temperature, was

heated in the microwave oven at full power. Heating was

continued for a period of time until the final tempera-

ture of a water load reached 5 above room temperature.

498 M.A.M. Khraisheh et al. / Food Research International 37 (2004) 497503


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The water temperature before and after heating was

measured using a type K thermocouple probe after

thoroughly mixing with a spatula.

Samples were removed at predetermined time inter-

vals throughout the experimental run for shrinkage and

vitamin C content measurements. Rehydration tests

were performed on samples dried to a final moisturecontent of 0.5 kg kg1 (dry basis). A 5-g sample of the

dried potato was added to 150 ml of distilled water. The

beaker was then placed on a hot plate and covered with

a watch-glass. The water was brought to boiling point,

taking approximately 3 min, and then boiled for the

specified time period. At the end of the rehydration

period, the sample was transferred to a Buchner funnel,

covered with No. 4 Whatman filter paper, and the excess

water removed by applying a slight vacuum. The sample

was then removed and weighed. The aforementioned

procedure was repeated for boiling times of 10, 20, 30

and 45 min, with the latter two tests requiring an addi-

tional 25 ml of water. The rehydration tests were con-

ducted as recommended by Prabhanjan, Ramaswamay,

and Raghavan (1995).

The moisture content of each sample was determined

by drying in a convective oven at 105110 C for 810 h.

The shrinkage of the sample was evaluated on the basis

of volume change. The volume changes were determined

using the method proposed by Lozano, Urbicain, and

Rotstein (1980). This is based on the buoyancy forces

which act on a body submerged in a liquid. The vitamin

C content of the fresh, dried and partially dried potato

samples was determined using a high-performance li-

quid chromatography (HPLC) technique, as detailed inMcMinn and Magee (1997). Further information on the

equipment and experimental procedures adopted are

detailed in Khraisheh (1996).

3. Results and discussion

3.1. Vitamin C

Nutritional quality deterioration during drying was

assessed in terms of vitamin C (ascorbic acid, AA)

content, which was selected due to its high temperature-

and moisture-sensitivity. The effect of moisture content

and drying conditions, namely air temperature and mi-

crowave power, on the stability of the vitamin C was

determined using an HPLC technique. Destruction of

ascorbic acid may occur by a number of pathways,

however, irrespective of the actual mechanism, the loss

can be described as (Kirk et al., 1977):

AA $ DHAA ! Products

The total ascorbic acid content was determined from a

summation of the ascorbic acid (AA) and dehydroa-

scorbic acid (DHAA) contents.

A graphical representation of the predicted and ex-

perimental vitamin C degradation behaviour in the po-

tato samples during convective and microwave drying is

shown in Figs. 1 and 2, respectively. As shown, the total

ascorbic acid content decreases progressively with in-

creasing processing time, at a constant temperature or

absorbed microwave power level. At a specific dryingtime, the loss of vitamin C increases with increasing air

temperature, as expected, due to the heat liable nature of

ascorbic acid. Under microwave drying conditions, an

increase in absorbed power causes an increase in prod-

uct temperature and, as a consequence, a greater rate of

vitamin C loss.

It has been suggested that the kinetics of vitamin C

degradation may be expressed by the first-order equation:

d CTAA

dt kTAA CTAA ; 2

where CTAA

is the concentration of ascorbic acid

(mgl1); t is the time (min) and kTAA is the rate constant

(min1).

The experimental data conformed to a first-order rate

function, as verified by a plot of lndCTAA=dt against

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6

Time (hr)

CTAA(mgl-1)

30C

40C

60C

Predicted

Fig. 1. Vitamin C retention characteristics of convective dried potato

samples.

0

10

20

30

40

50

60

7080

90

0 20 40 60 80 100

Time (min)

CTAA

(mgl-1)

10.5 W 15 W

38 W Predicted

Fig. 2. Vitamin C retention characteristics of microwave dried potato

samples.

M.A.M. Khraisheh et al. / Food Research International 37 (2004) 497503 499


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CTAA giving a straight line, with the slope representing

the rate constant, kTAA.

Alternatively, the degradation behaviour can be

written as

CTAA C0 exp kTAAt; 3

where C0

is initial ascorbic acid concentration (mg l1).

Eq. (3) was fitted to the experimental data. The pre-

dicted characteristics are shown in Figs. 1 and 2 and the

corresponding constants detailed in Table 1.

The stability and retention of vitamin C is not only

dependent on drying conditions but also on sample

moisture content. Fig. 3 shows ascorbic acid concen-

tration as a function of moisture content for samples

dried under selected convective and microwave pro-

cessing conditions. Such representation with respect to

moisture content facilitates comparison between differ-

ent modes of drying. As shown, samples dried under

microwave conditions retain a much greater concentra-

tion of ascorbic acid as compared with air-dried sam-ples, at a specific moisture content. For example, to

attain a moisture level of approximately 0.3 (dry basis)

(chosen as commercial potato flakes have a moisture

content of 5.5% (wet basis)) (Wang, Kozempel, Hicks, &

Sieb, 1992) using microwave drying at 10.5 W, the

samples had approximately 75% of the initial vitamin C

content. In contrast, potatoes dried under air conditions

(30 C) have retained less than 30%. Even under more

severe microwave processing conditions (absorbed

power of 38 W) vitamin C retention exceeds 45%. This

demonstrates one of the advantages of using microwave

power for drying processes.

Significant vitamin C degradation during classical air

drying is not unusual. Wang et al. (1992) reported losses

of 30100% during the processing of raw potatoes to

dehydrated flakes on a pilot-scale, while using com-

mercial equipment the loss was approximately 50%. Asshown in Fig. 3, there is an initial low rate of vitamin C

loss at relatively higher moisture contents, followed by a

period of more rapid degradation as the moisture con-

tent decreases. The low rate of loss at the start of the

drying process may be attributed to the physical struc-

ture of the material; the membrane integrity of the po-

tato tissue is substantially intact, and thus provides

protection from deleterious cell components. In addi-

tion, endogenous antioxidative constituents may be re-

sponsible for this slow reaction rate (Mishkin, Saguy, &

Karel, 1983).

Clearly water content is of great importance in the

reduction of vitamin C, however, the mechanisms by

which water controls the reaction is complex (Lee &

Labuza, 1975). Water content can affect the dilution of

ascorbic acid; as the moisture content increases ascorbic

acid concentration is lowered which in turn induces a

relatively reduced degradation rate. An increase in water

content may, however, make the reaction easier if the

aqueous phase becomes less viscous, with the presence

of water also affecting the level of oxygen absorbed by

the material and hence, the destruction. Based on these

considerations, the decrease in reaction rate with mois-

ture availability appears to be related to the dilution of

reactants in the aqueous phase. As the moisture contentdecreases, the degree of dilution decreases and thus, the

reaction rate increases to give lower vitamin C retention.

In the latter stages of drying, the internal sample tem-

perature is also elevated, in comparison to that at the

early stages, and swelling of the solid matrices may ex-

pose new catalytic sites, both of which may attribute to

the decreased ascorbic acid content. Such phenomena

are more apparent during microwave processing as a

wider range of moisture levels can be achieved.

The degradation characteristics are, for the most

part, comparable with the work of Mishkin, Saguy, and

Karel (1984) and Villota and Karel (1980). However,

due to the complexity of sample structures, varying pre-

processing histories and the system dependent nature of

the reaction slight variations in the kinetic data are

observed.

3.2. Shrinkage

Food samples undergo volume changes, i.e., shrink-

age, on water loss. Such shrinkage affects the physical

attributes and the transport properties of the solids. The

volume change during drying is not an easily predictable

function. Visual examination of the samples throughout

Table 1Ascorbic acid degradation characteristics

Drying

conditions

C0 kTAA (h1) r2

Air 30 C 64.4 0.107 0.914

40 C 56.3 0.100 0.993

60 C 52.0 0.099 0.907

Microwave 10.5 W 83.8 0.240 0.826

15.0 W 73.4 0.252 0.851

38.0 W 60.3 0.414 0.869

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

Moisture Content (kg.kg-1)

CTAA

(mgl-1)

60 C 10.5 W 38 W

Fig. 3. Vitamin C retention as a function of moisture content.



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analysis are shown in Table 3. The magnitude of pa-

rameter a indicates that the degree of shrinkage is

greater in the initial stages. In the lower moisture con-

tent range (X=X0 < 0:45) the reduced degree of volumechange is due to the fact that, in addition to shrinkage

due to the loss of water, air-filled pores are being

formed, i.e., puffing occurs to counter the shrinkageaffect. Smith (1976) reported puffing of pasta when dried

in a microwave field, with this being at a maximum at a

moisture content of approximately 20%.

Examination of the effect of microwave power on

shrinkage reveals that the shrinkage of samples dried at

10.5 W is comparable to that observed at a power level

of 38 W, however, at 15 W a relatively higher degree of

shrinkage occurs (Fig. 5 and Table 3). This behaviour

may be explained on consideration of the internal forces

and stresses produced by the pressure gradient. The

combined action of such stresses leads to a volume

change dependent on the extent of resorption of the

internal stresses. It can be assumed that the structure

tends towards stable states during moisture extraction,

with such states being attained either at low or high

drying rates. At a low microwave power, and hence low

drying rate, the induced forces are not strong enough to

break the structure and therefore, the shrinkage is lim-

ited. For relatively high drying rates (38 W), the fric-

tional forces increase rapidly, hardening the structure

before it retracts sufficiently and thereby yielding a small

degree of shrinkage, as in the former case. In contrast,

an intermediate drying rate (at 15 W) can exist during

which the structure can be broken down and the internal

stresses of the sample reduced by shrinking by a rela-

tively greater extent.

Further comparison of Figs. 4 and 5 indicates that

potato samples dried in a microwave field exhibit less

shrinkage that those undergoing classical air drying. A

further advantage of the application of microwave

technology for drying operations.

3.3. Rehydration

Rehydration involves a reversal of some of the

physiochemical changes that occur during drying. In

general, the rate of water absorption and the extent of

restoration of the dried product is influenced by the

degree of drying, i.e., disruption of cellular integrity. It

may be assumed that moisture movement during the

rehydration process is occurring by liquid diffusion

(Neubert, Wilson, & Miller, 1968), with water transfer

occurring from the rehydration liquid to the dry solid

until equilibrium is reached.

The rehydratibility of potato samples subjected to

both convective and microwave modes of drying was

quantified on the basis of the rehydration ratio (RR)

and the coefficient of rehydration (COR). The rehydra-

tion ratio is defined as the ratio of the mass of the re-

hydrated sample to the mass of the dried sample. The

coefficient of rehydration is calculated using Prabhanjan

et al. (1995):

COR mrh 100 X0

mdh 100 Xdh ; 6

where mrh is the mass of the rehydrated sample (kg); mdhthe mass of the dehydrated sample (kg) and Xdh the

moisture content of the dried sample (% wet basis).

The values of the COR and RR for the samples are

detailed in Table 4. As shown, the rehydration proper-

ties of the microwave dried samples are better than those

of convective dried samples. The extent of rehydration

also increases with increasing power level. However, at

high power levels (38 W) starch gelatinisation is ob-

served and this reduces the degree of rehydration. The

rehydration characteristics of samples dried in a mi-

crowave environment may be explained on consider-

ation of the shrinkage behaviour. With samples dried athigh microwave power levels, the outer layers of the

Table 3

Characteristic parameters for Eq. (3)

Absorbed power (W) a b r2

X=X0 < 0:45 10.5 0.579 0.418 0.981

15 0.610 0.373 0.93638 0.574 0.413 0.983

X=X0P 0:45 10.5 0.200 0.589 0.89315 0.400 0.484 0.893

38 0.282 0.544 0.849

Table 4

Rehydration characteristic of convective and microwave dried potato

samples

Drying conditions COR RR

Air 40 0.354 2.57

60 0.361 2.60

Microwave 10.5 0.479 2.75

15 0.515 2.89

38 0.464 2.64

0.0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1X/Xo

Sb

10.5 W15 W

38 WPredicted

Fig. 5. Variation in bulk shrinkage coefficient during microwave drying

of potato samples.



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sample become fixed in the early stages of the drying

operation. This more consolidated, rigid structure leads

to the absence of pathways for water entrance, i.e., low

rehydratibility. At lower drying rates, the sample shrinks

with little change in shape to produce a dense, closely

packed cellular structure with limited intercellular

spaces. This gives rise to restricted intercellular diffusionand hence, rehydration. The observations are in agree-

ment with other researchers who reported rehydration

attributes dependent on the physical properties of the

dried product (Jayaraman, Das Gupta, & Babu Rao,

1990).

4. Conclusions

Vitamin C degradation in potatoes during microwave

and convective drying was found to exhibit first-order

kinetics. Microwave-dried samples retained at leasttwice the vitamin C content of convective-dried sam-

ples (for comparable moisture contents).

Volumetric shrinkage exhibited a linear relation with

moisture content. Samples dried under convective

conditions exhibited uniform shrinkage throughout,

however, two shrinkage periods were observed during

microwave drying.

Microwave dried samples had improved rehydratibility.

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