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Quality and structural changes in starchy foods during microwave and convective drying
Food Research International 37 (2004) 497–503 www.elsevier.com/locate/foodres
Quality and structural changes in starchy foods during microwave and convective drying M.A.M. Khraisheh a
a,*
, W.A.M. McMinn b, T.R.A. Magee
b
Department of Civil and Environmental Engineering, University College London, Chadwick Building, Gower Street, London WC1E 6BT, UK b Food Process Engineering Research Group, School of Chemical Engineering, Queen’s 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 rehydratibility, 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 microwave cooking. Rosen (1972) discussed the effect of microwaves 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-
*
Corresponding author. Tel.: +44-20-7679-7224/7994; fax: +44-207380-0986. 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
cules and chemical bonds. Gerster (1989) used heat sensitive and water-soluble vitamins C, B1 and B2 as indicator nutrients for qualitative changes. The retention of vitamins during blanching, cooking and reheating 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 destroyed 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 destruction in okra by using microwave drying.
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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-migration mechanism and thereby modify the physical properties 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, Driscoll, & Buckle, 1994). It has been shown that both volumetric shrinkage (Lozano, Rotstein, & Urbicain, 1983) and dimensional shrinkage (Rahman & Potluri, 1990) are dependent on moisture content. Preliminary experiments showed that shrinkage of potatoes is not negligible 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 nature 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 temperature, is required. Therefore, a practical approach to the study of food shrinkage is experimentally based. Current research has indicated that degree of rehydration is dependent on processing conditions, sample preparation, sample composition and the extent of structural and chemical disruption induced during drying (Okos, Narsimhan, Singh, & Weitnauer, 1992). Studies to assess the relationship between the duration and severity of the drying process and the rate and degree 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 combination processes. Samples finish-dried using microwave vacuum drying exhibited a puffed structure and thus, faster rehydration. El-Din and Shouk (1999) also reported 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 microwave–convective 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 (functioning 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 experimental 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 various output power levels between 90 and 650 W (corresponding to absorbed power levels of 10.5–38 W). The measurement of power output of the microwave oven was determined calorimetrically (Khraisheh, Cooper, & Magee, 1997) that is the change of temperature of a known mass of water for a known period of time. The basic equation is 4:187mCp DT ; ð1Þ Dt 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 kJ kg1 K1 ); DT is the temperature rise in the water load (C); Dt is the time microwave 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 temperature of a water load reached 5 above room temperature. MWabs ¼
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A graphical representation of the predicted and experimental vitamin C degradation behaviour in the potato samples during convective and microwave drying is shown in Figs. 1 and 2, respectively. As shown, the total ascorbic acid content decreases progressively with increasing processing time, at a constant temperature or absorbed microwave power level. At a specific drying time, 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 product 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: d½CTAA ¼ kTAA ½CTAA ; dt
30C 40C 60C Predicted
70 60 50 40 30 20 10 0 0
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 temperatureand moisture-sensitivity. The effect of moisture content and drying conditions, namely air temperature and microwave 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 dehydroascorbic acid (DHAA) contents.
1
2
3 Time (hr)
4
5
6
Fig. 1. Vitamin C retention characteristics of convective dried potato samples.
10.5 W 38 W
90 80
CTAA (mgl-1)
3. Results and discussion
ð2Þ
where CTAA is the concentration of ascorbic acid (mg l1 ); 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 ln½dCTAA =dt against
CTAA (mgl-1)
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 intervals throughout the experimental run for shrinkage and vitamin C content measurements. Rehydration tests were performed on samples dried to a final moisture content 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 additional 25 ml of water. The rehydration tests were conducted as recommended by Prabhanjan, Ramaswamay, and Raghavan (1995). The moisture content of each sample was determined by drying in a convective oven at 105–110 C for 8–10 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 liquid chromatography (HPLC) technique, as detailed in McMinn and Magee (1997). Further information on the equipment and experimental procedures adopted are detailed in Khraisheh (1996).
499
15 W Predicted
70 60 50 40 30 20 10 0 0
20
40 60 Time (min)
80
100
Fig. 2. Vitamin C retention characteristics of microwave dried potato samples.
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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 ½CTAA ¼ ½C0 exp ð kTAA tÞ;
ð3Þ 1
where C0 is initial ascorbic acid concentration (mg l ). Eq. (3) was fitted to the experimental data. The predicted 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 concentration as a function of moisture content for samples dried under selected convective and microwave processing conditions. Such representation with respect to moisture content facilitates comparison between different modes of drying. As shown, samples dried under microwave conditions retain a much greater concentration of ascorbic acid as compared with air-dried samples, 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 Table 1 Ascorbic acid degradation characteristics Drying conditions
C0
kTAA (h1 )
r2
Air
30 C 40 C 60 C
64.4 56.3 52.0
0.107 0.100 0.099
0.914 0.993 0.907
Microwave
10.5 W 15.0 W 38.0 W
83.8 73.4 60.3
0.240 0.252 0.414
0.826 0.851 0.869
100
CTAA (mgl-1)
80 60
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 30–100% during the processing of raw potatoes to dehydrated flakes on a pilot-scale, while using commercial equipment the loss was approximately 50%. As shown 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 content decreases. The low rate of loss at the start of the drying process may be attributed to the physical structure of the material; the membrane integrity of the potato tissue is substantially intact, and thus provides protection from deleterious cell components. In addition, endogenous antioxidative constituents may be responsible 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 moisture availability appears to be related to the dilution of reactants in the aqueous phase. As the moisture content decreases, 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 temperature is also elevated, in comparison to that at the early stages, and swelling of the solid matrices may expose 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 preprocessing histories and the system dependent nature of the reaction slight variations in the kinetic data are observed.
40
3.2. Shrinkage 20
60 C
10.5 W
38 W
0 0
0.2
0.4 0.6 0.8 Moisture Content (kg.kg-1)
Fig. 3. Vitamin C retention as a function of moisture content.
1
Food samples undergo volume changes, i.e., shrinkage, 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
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the drying process reveals that the shrinkage is not perfectly homogeneous. In the initial stage of drying, the samples keep the original geometry, i.e., cell structure appears to be intact. As drying proceeds, however, the shrinkage is accompanied by particle deformation. Quantitative evaluation of the shrinkage was performed on the basis of a bulk shrinkage coefficient (Sb ), i.e., ratio of sample volume at time, t, to initial volume (V/ V0 ). Fig. 4 shows Sb as a function of dimensionless moisture content (dry basis, X/X0 ) for potato cylinders dried at varying air temperatures. The experimental data show a linear behaviour between bulk shrinkage coefficient and moisture content, which suggests that the shrinkage is predominantly due to the volume of water removed. A linear relation between bulk shrinkage coefficient and water content was fitted to the experimental data: Sb ¼ a
X þ b; X0
ð4Þ
where X0 is the initial moisture content (kg kg1 , dry basis). The constants, a and b, for various drying conditions are shown in Table 2. The linear relationship proposed by Kilpatrick, Lowe, and Van Arsdel (1955): Sb ¼
X þ 0:80 X0 þ 0:80
ð5Þ
was also fitted to the experimental data. This provided a good description of the data (Fig. 4). The linear shrinkage behaviour of food materials was reported by a number of researchers including Wang and Brennan (1995), Lozano et al. (1983) and Sjoholm and Gekas (1995). 1.0 0.8
Sb
0.6 30˚C 0.4
40˚C 60˚C Kilpatrick (1995)
0.2 0.0 0
0.2
0.4
0.6
0.8
1
X/X0
Fig. 4. Variation in bulk shrinkage coefficient during convective drying of potato samples. Table 2 Characteristic parameters for Eq. (3)
0.2 6 X =X0 6 1
Air temperature (C)
a
b
r2
30 40 60
0.861 0.880 0.819
0.127 0.100 0.152
0.998 0.997 0.991
501
Although the overall magnitude of the volume change during air drying is relatively small, the value of the constant ‘a’ (Eq. (4), Table 2) indicates a slight increase in volume change during low temperature processing. The effect of temperature on shrinkage may be attributed to the temperature dependence of the elastic and mechanical properties. During low temperature processing, a more uniform moisture distribution exists within the sample, with little difference being expected between the centre and surface. This may reduce the internal stresses and so allow the sample to continue to shrink until the latter stages of drying, when the shape of the material becomes fixed. In contrast, at higher air temperatures, case hardening of the surface may occur and the volume of the sample becomes fixed at an earlier stage and, as a result, a reduced degree of shrinkage is observed. The change in volume of samples exposed to a microwave environment was also examined. Application of microwave energy, during the drying process, accelerates mass transfer through the product towards the surface. The air then facilitates mass transfer to the surrounding media. However, the drying process does not only consist of moisture transfer from the body to its surroundings, but also complex reactions within the product due to the microwave energy to which it is subjected. Hence, microwave energy application may lead to differing physical changes in the product in comparison with those observed during classical air drying. For the purpose of this work, the samples were considered to be homogeneous, however, some heterogeneity is in fact present. Such heterogeneity may lead to non-uniform energy absorption due to differing sensitivity levels of the electromagnetic receptors in the product. As a result of this selective heating, local temperature gradients may be set-up, leading to a thermal imbalance; unlike traditional drying which is based on a global thermal equilibrium. Thus, it may be possible to induce qualities that are specific to the microwave treatment. The presence of temperature gradients, due to the application of microwave energy, in turn generates pressure gradients which create different degrees of stress which the material may relieve through shrinking. The shrinkage data obtained during microwave drying were also analysed in terms of the bulk shrinkage coefficient. Fig. 5 shows the volume change as a function of moisture content. As shown, sample shrinkage again exhibits a linear relationship with moisture content. However, uniform shrinkage does not exist throughout the entire moisture content range, rather two stages can be identified with a transitional normalized moisture content of 0.45. Ratti (1994) also reported two periods of differing linear shrinkage. Eq. (3) was fitted to the experimental data in each section and the results of the
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stresses of the sample reduced by shrinking by a relatively 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.
1.0 0.8
Sb
0.6 10.5 W 15 W 38 W Predicted
0.4 0.2
3.3. Rehydration
0.0 0
0.2
0.4 X/Xo
0.6
0.8
1
Fig. 5. Variation in bulk shrinkage coefficient during microwave drying of potato samples.
Table 3 Characteristic parameters for Eq. (3) Absorbed power (W)
a
b
r2
X =X0 < 0:45
10.5 15 38
0.579 0.610 0.574
0.418 0.373 0.413
0.981 0.936 0.983
X =X0 P 0:45
10.5 15 38
0.200 0.400 0.282
0.589 0.484 0.544
0.893 0.893 0.849
analysis are shown in Table 3. The magnitude of parameter ‘a’ indicates that the degree of shrinkage is greater in the initial stages. In the lower moisture content range (X =X0 < 0:45) the reduced degree of volume change 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 shrinkage affect. 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 limited. For relatively high drying rates (38 W), the frictional 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
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 rehydration ratio is defined as the ratio of the mass of the rehydrated sample to the mass of the dried sample. The coefficient of rehydration is calculated using Prabhanjan et al. (1995): COR ¼
mrh ð100 X0 Þ ; mdh ð100 Xdh Þ
ð6Þ
where mrh is the mass of the rehydrated sample (kg); mdh the 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 properties 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 observed and this reduces the degree of rehydration. The rehydration characteristics of samples dried in a microwave environment may be explained on consideration of the shrinkage behaviour. With samples dried at high microwave power levels, the outer layers of the Table 4 Rehydration characteristic of convective and microwave dried potato samples Drying conditions
COR
RR
Air
40 60
0.354 0.361
2.57 2.60
Microwave
10.5 15 38
0.479 0.515 0.464
2.75 2.89 2.64
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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 diffusion and hence, rehydration. The observations are in agreement 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 least twice the vitamin C content of convective-dried samples (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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Quality and structural changes in starchy foods during microwave and convective drying M.A.M. Khraisheh a
a,*
, W.A.M. McMinn b, T.R.A. Magee
b
Department of Civil and Environmental Engineering, University College London, Chadwick Building, Gower Street, London WC1E 6BT, UK b Food Process Engineering Research Group, School of Chemical Engineering, Queen’s 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 rehydratibility, 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 microwave cooking. Rosen (1972) discussed the effect of microwaves 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-
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Corresponding author. Tel.: +44-20-7679-7224/7994; fax: +44-207380-0986. 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
cules and chemical bonds. Gerster (1989) used heat sensitive and water-soluble vitamins C, B1 and B2 as indicator nutrients for qualitative changes. The retention of vitamins during blanching, cooking and reheating 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 destroyed 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 destruction in okra by using microwave drying.
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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-migration mechanism and thereby modify the physical properties 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, Driscoll, & Buckle, 1994). It has been shown that both volumetric shrinkage (Lozano, Rotstein, & Urbicain, 1983) and dimensional shrinkage (Rahman & Potluri, 1990) are dependent on moisture content. Preliminary experiments showed that shrinkage of potatoes is not negligible 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 nature 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 temperature, is required. Therefore, a practical approach to the study of food shrinkage is experimentally based. Current research has indicated that degree of rehydration is dependent on processing conditions, sample preparation, sample composition and the extent of structural and chemical disruption induced during drying (Okos, Narsimhan, Singh, & Weitnauer, 1992). Studies to assess the relationship between the duration and severity of the drying process and the rate and degree 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 combination processes. Samples finish-dried using microwave vacuum drying exhibited a puffed structure and thus, faster rehydration. El-Din and Shouk (1999) also reported 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 microwave–convective 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 (functioning 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 experimental 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 various output power levels between 90 and 650 W (corresponding to absorbed power levels of 10.5–38 W). The measurement of power output of the microwave oven was determined calorimetrically (Khraisheh, Cooper, & Magee, 1997) that is the change of temperature of a known mass of water for a known period of time. The basic equation is 4:187mCp DT ; ð1Þ Dt 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 kJ kg1 K1 ); DT is the temperature rise in the water load (C); Dt is the time microwave 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 temperature of a water load reached 5 above room temperature. MWabs ¼
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A graphical representation of the predicted and experimental vitamin C degradation behaviour in the potato samples during convective and microwave drying is shown in Figs. 1 and 2, respectively. As shown, the total ascorbic acid content decreases progressively with increasing processing time, at a constant temperature or absorbed microwave power level. At a specific drying time, 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 product 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: d½CTAA ¼ kTAA ½CTAA ; dt
30C 40C 60C Predicted
70 60 50 40 30 20 10 0 0
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 temperatureand moisture-sensitivity. The effect of moisture content and drying conditions, namely air temperature and microwave 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 dehydroascorbic acid (DHAA) contents.
1
2
3 Time (hr)
4
5
6
Fig. 1. Vitamin C retention characteristics of convective dried potato samples.
10.5 W 38 W
90 80
CTAA (mgl-1)
3. Results and discussion
ð2Þ
where CTAA is the concentration of ascorbic acid (mg l1 ); 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 ln½dCTAA =dt against
CTAA (mgl-1)
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 intervals throughout the experimental run for shrinkage and vitamin C content measurements. Rehydration tests were performed on samples dried to a final moisture content 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 additional 25 ml of water. The rehydration tests were conducted as recommended by Prabhanjan, Ramaswamay, and Raghavan (1995). The moisture content of each sample was determined by drying in a convective oven at 105–110 C for 8–10 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 liquid chromatography (HPLC) technique, as detailed in McMinn and Magee (1997). Further information on the equipment and experimental procedures adopted are detailed in Khraisheh (1996).
499
15 W Predicted
70 60 50 40 30 20 10 0 0
20
40 60 Time (min)
80
100
Fig. 2. Vitamin C retention characteristics of microwave dried potato samples.
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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 ½CTAA ¼ ½C0 exp ð kTAA tÞ;
ð3Þ 1
where C0 is initial ascorbic acid concentration (mg l ). Eq. (3) was fitted to the experimental data. The predicted 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 concentration as a function of moisture content for samples dried under selected convective and microwave processing conditions. Such representation with respect to moisture content facilitates comparison between different modes of drying. As shown, samples dried under microwave conditions retain a much greater concentration of ascorbic acid as compared with air-dried samples, 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 Table 1 Ascorbic acid degradation characteristics Drying conditions
C0
kTAA (h1 )
r2
Air
30 C 40 C 60 C
64.4 56.3 52.0
0.107 0.100 0.099
0.914 0.993 0.907
Microwave
10.5 W 15.0 W 38.0 W
83.8 73.4 60.3
0.240 0.252 0.414
0.826 0.851 0.869
100
CTAA (mgl-1)
80 60
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 30–100% during the processing of raw potatoes to dehydrated flakes on a pilot-scale, while using commercial equipment the loss was approximately 50%. As shown 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 content decreases. The low rate of loss at the start of the drying process may be attributed to the physical structure of the material; the membrane integrity of the potato tissue is substantially intact, and thus provides protection from deleterious cell components. In addition, endogenous antioxidative constituents may be responsible 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 moisture availability appears to be related to the dilution of reactants in the aqueous phase. As the moisture content decreases, 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 temperature is also elevated, in comparison to that at the early stages, and swelling of the solid matrices may expose 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 preprocessing histories and the system dependent nature of the reaction slight variations in the kinetic data are observed.
40
3.2. Shrinkage 20
60 C
10.5 W
38 W
0 0
0.2
0.4 0.6 0.8 Moisture Content (kg.kg-1)
Fig. 3. Vitamin C retention as a function of moisture content.
1
Food samples undergo volume changes, i.e., shrinkage, 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
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the drying process reveals that the shrinkage is not perfectly homogeneous. In the initial stage of drying, the samples keep the original geometry, i.e., cell structure appears to be intact. As drying proceeds, however, the shrinkage is accompanied by particle deformation. Quantitative evaluation of the shrinkage was performed on the basis of a bulk shrinkage coefficient (Sb ), i.e., ratio of sample volume at time, t, to initial volume (V/ V0 ). Fig. 4 shows Sb as a function of dimensionless moisture content (dry basis, X/X0 ) for potato cylinders dried at varying air temperatures. The experimental data show a linear behaviour between bulk shrinkage coefficient and moisture content, which suggests that the shrinkage is predominantly due to the volume of water removed. A linear relation between bulk shrinkage coefficient and water content was fitted to the experimental data: Sb ¼ a
X þ b; X0
ð4Þ
where X0 is the initial moisture content (kg kg1 , dry basis). The constants, a and b, for various drying conditions are shown in Table 2. The linear relationship proposed by Kilpatrick, Lowe, and Van Arsdel (1955): Sb ¼
X þ 0:80 X0 þ 0:80
ð5Þ
was also fitted to the experimental data. This provided a good description of the data (Fig. 4). The linear shrinkage behaviour of food materials was reported by a number of researchers including Wang and Brennan (1995), Lozano et al. (1983) and Sjoholm and Gekas (1995). 1.0 0.8
Sb
0.6 30˚C 0.4
40˚C 60˚C Kilpatrick (1995)
0.2 0.0 0
0.2
0.4
0.6
0.8
1
X/X0
Fig. 4. Variation in bulk shrinkage coefficient during convective drying of potato samples. Table 2 Characteristic parameters for Eq. (3)
0.2 6 X =X0 6 1
Air temperature (C)
a
b
r2
30 40 60
0.861 0.880 0.819
0.127 0.100 0.152
0.998 0.997 0.991
501
Although the overall magnitude of the volume change during air drying is relatively small, the value of the constant ‘a’ (Eq. (4), Table 2) indicates a slight increase in volume change during low temperature processing. The effect of temperature on shrinkage may be attributed to the temperature dependence of the elastic and mechanical properties. During low temperature processing, a more uniform moisture distribution exists within the sample, with little difference being expected between the centre and surface. This may reduce the internal stresses and so allow the sample to continue to shrink until the latter stages of drying, when the shape of the material becomes fixed. In contrast, at higher air temperatures, case hardening of the surface may occur and the volume of the sample becomes fixed at an earlier stage and, as a result, a reduced degree of shrinkage is observed. The change in volume of samples exposed to a microwave environment was also examined. Application of microwave energy, during the drying process, accelerates mass transfer through the product towards the surface. The air then facilitates mass transfer to the surrounding media. However, the drying process does not only consist of moisture transfer from the body to its surroundings, but also complex reactions within the product due to the microwave energy to which it is subjected. Hence, microwave energy application may lead to differing physical changes in the product in comparison with those observed during classical air drying. For the purpose of this work, the samples were considered to be homogeneous, however, some heterogeneity is in fact present. Such heterogeneity may lead to non-uniform energy absorption due to differing sensitivity levels of the electromagnetic receptors in the product. As a result of this selective heating, local temperature gradients may be set-up, leading to a thermal imbalance; unlike traditional drying which is based on a global thermal equilibrium. Thus, it may be possible to induce qualities that are specific to the microwave treatment. The presence of temperature gradients, due to the application of microwave energy, in turn generates pressure gradients which create different degrees of stress which the material may relieve through shrinking. The shrinkage data obtained during microwave drying were also analysed in terms of the bulk shrinkage coefficient. Fig. 5 shows the volume change as a function of moisture content. As shown, sample shrinkage again exhibits a linear relationship with moisture content. However, uniform shrinkage does not exist throughout the entire moisture content range, rather two stages can be identified with a transitional normalized moisture content of 0.45. Ratti (1994) also reported two periods of differing linear shrinkage. Eq. (3) was fitted to the experimental data in each section and the results of the
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stresses of the sample reduced by shrinking by a relatively 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.
1.0 0.8
Sb
0.6 10.5 W 15 W 38 W Predicted
0.4 0.2
3.3. Rehydration
0.0 0
0.2
0.4 X/Xo
0.6
0.8
1
Fig. 5. Variation in bulk shrinkage coefficient during microwave drying of potato samples.
Table 3 Characteristic parameters for Eq. (3) Absorbed power (W)
a
b
r2
X =X0 < 0:45
10.5 15 38
0.579 0.610 0.574
0.418 0.373 0.413
0.981 0.936 0.983
X =X0 P 0:45
10.5 15 38
0.200 0.400 0.282
0.589 0.484 0.544
0.893 0.893 0.849
analysis are shown in Table 3. The magnitude of parameter ‘a’ indicates that the degree of shrinkage is greater in the initial stages. In the lower moisture content range (X =X0 < 0:45) the reduced degree of volume change 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 shrinkage affect. 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 limited. For relatively high drying rates (38 W), the frictional 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
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 rehydration ratio is defined as the ratio of the mass of the rehydrated sample to the mass of the dried sample. The coefficient of rehydration is calculated using Prabhanjan et al. (1995): COR ¼
mrh ð100 X0 Þ ; mdh ð100 Xdh Þ
ð6Þ
where mrh is the mass of the rehydrated sample (kg); mdh the 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 properties 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 observed and this reduces the degree of rehydration. The rehydration characteristics of samples dried in a microwave environment may be explained on consideration of the shrinkage behaviour. With samples dried at high microwave power levels, the outer layers of the Table 4 Rehydration characteristic of convective and microwave dried potato samples Drying conditions
COR
RR
Air
40 60
0.354 0.361
2.57 2.60
Microwave
10.5 15 38
0.479 0.515 0.464
2.75 2.89 2.64
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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 diffusion and hence, rehydration. The observations are in agreement 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 least twice the vitamin C content of convective-dried samples (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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