Effect of microwave power, air velocity and temperature on the final drying of osmotically dehydrated bananas

Journal of Food Engineering 81 (2007) 79–87 www.elsevier.com/locate/jfoodeng

Effect of microwave power, air velocity and temperature on the final drying of osmotically dehydrated bananas Na´dia R. Pereira a, Antonio Marsaioli Jr. a, Lı´lia M. Ahrne´ b,* a

Department of Food Engineering, College of Food Engineering, State University of Campinas (UNICAMP), P.O. Box 6121, 13083-970, Campinas, SP, Brazil b SIK, The Swedish Institute for Food and Biotechnology, Department of Environment and Process Engineering, Box 5401, 40229 Go¨teborg, Sweden Received 20 May 2006; received in revised form 7 September 2006; accepted 8 September 2006 Available online 22 December 2006

Abstract This paper reports on a study of the final stage of microwave-hot air drying of osmotically dehydrated bananas, focusing on the effects of microwave power, air temperature and air velocity on drying kinetics and product quality, evaluated in terms of colour, apparent volume and porosity. The drying process was divided into three periods: phase I (760 W; 2 kgmoisture/kgdry matter); phase II (380 W; 0.67 kgmoisture/ kgdry matter); and phase III (0 W, 76 W, 150 W or 230 W up to the final sample moisture of 0.17 kgwater/kgd m). Three conditions for the hot air were tested: 50 °C and 3.3 m/s; 70 °C and 3.3 m/s; 70 °C and 5.7 m/s. The results show that increasing the microwave power in phase III increased the drying rate, thus making the drying time shorter. However, higher microwave power also caused temperature runaway leading to charring on the dried product. Air flow cools the product surface and improves product quality by reducing charring. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Microwave final drying; Fruits; Porosity; Product quality

1. Introduction The quality of a dried product is strongly dependent on the drying process and the processing conditions. Hot air drying is commonly used to dehydrate food products. Although this is a simple process, the low thermal conductivity of food materials and internal resistance to moisture transfer yield that the efficiency of heat transfer is low and the quality of the dried fruits is generally reduced and often unsatisfactory. The main problem with hot air drying of fruit begins when the moisture content reaches 0.67 kgmoisture/ kgdry matter and the rate of moisture loss starts to decrease. Two-thirds of the drying time may be spent removing the last one-third of the moisture content in the final stage of drying. Because of their physical/chemical composition and structure (Sankat, Castaigne, & Maharaj, 1996), starch-rich fruits like bananas dry much more slowly than *

Corresponding author. Tel.: +46 31 335 5600; fax: +46 31 833782. E-mail address: [email protected] (L.M. Ahrne´).

0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.09.025

other fruits. But prolonged drying times increase shrinkage and toughness, reduce the bulk density and rehydration capacity of the dried product and cause serious damage to the flavour, colour and nutrients (Al-Duri & McIntyre, 1992; Maskan, 2000). The drying time can be reduced by using microwave energy, which is rapidly absorbed by the product water molecules and consequently results in rapid evaporation of water and thus higher drying rates. The interior temperature of dried microwave-heated food is higher than the surface temperature and moisture is transferred to the surface more dynamically than during convective drying (Ohlsson, 1990; Torringa, Esveld, Scheewe, van den Berg, & Bartels, 2001). A combination of microwave drying and hot air drying creates a synergistic effect in terms of heating uniformity and brings significant advantages in regards to processing time and quality (Berteli & Marsaioli, 2005; Drouzas & Schubert, 1996; Funebo & Ohlsson, 1998; Smith, 1979). Osmotic dehydration of fruit in a sugar solution prior to drying decreases the water content while simultaneously

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N.R. Pereira et al. / Journal of Food Engineering 81 (2007) 79–87

increasing the dry matter content. It also offers some advantages with regard to fruit quality, including retention of natural colour and high retention of volatile compounds during subsequent drying. The combination of osmotic dehydration with a mild drying process such as microwave drying is a new concept that has the potential to improve the overall quality of dried fruit products in a shorter time than other drying methods (Ahrne´, 2005; Piotrowski, Lenart, & Wardzyski, 2004; Venkatachalapathy & Raghavan, 1998). Venkatachalapathy and Raghavan (1998) concluded that osmotic dehydration prior to microwave-assisted drying led to a dried blueberry that was comparable to a freeze-dried berry in a much shorter time. According to Erle and Schubert (2001) osmotic pretreatment makes microwave drying more efficient both in terms of removed water and achieved water activity. The main reasons for this may be the higher dielectric losses caused by sucrose. Padua (1993) observed that gels prepared of highly concentrated sucrose solutions absorbed microwave energy more efficiently showing surface heating and a relatively cold center as opposed to gels with no sucrose, which absorbed less energy, showed pronounced central heating and a relatively cold surface. Maskan (2000) studied microwave finish drying of banana slices and observed good results in terms of fruit quality. Souza, Pitombo, Da Silva, and Marsaioli (2006) studied the effect of air parameters on microwave-assisted drying of bananas, focusing on sensory quality and found that high sensory quality could be achieved by using air temperature higher than 40 °C (Sousa, Marsaioli Jr, & Rodrigues, 2004). However, the interaction between air parameters and microwave power in the final microwaveassisted drying still requires further studies. Therefore, the aim of the work reported in this paper was to study how microwave power density, air temperature and velocity during the final drying of osmotically dehydrated bananas affect drying kinetics and product quality. 2. Materials and methods 2.1. Raw material Ripe but firm Cavendish bananas from Costa Rica were purchased from the local market. They were chosen on the basis of their peel colour (yellow but still green on the stem). These bananas were hand peeled and cut into 10 mm thick slices using a cutting device designed for this purpose. The composition of the raw material is showed in Table 1. The values represent the average of the samples used in all experiments.

Table 1 Composition of fresh and osmotically dehydrated banana

Moisture content (kgmoisture/kgprod) X (kgmoisture/kgdry matter) °Brix aw

Fresh

Osmo-dehydrated

74.54 ± 0.88 2.93 ± 0.01 18.8 ± 1.4 0.970 ± 0.005

70.00 ± 1.63 2.33 ± 0.01 23.2 ± 1.3 0.956 ± 0.011

bic acid at 40 °C. The banana slices and osmotic solution were placed in a shaking water bath at 100 rpm for 90 min. The mass ratio of the raw material to the osmotic solution was 1:3. Afterwards, the samples were lightly rinsed with water to remove excess sugar solution, drained, and wiped with tissue paper to remove excess water. The banana slices were stored overnight at 5 °C, wrapped in a film to allow for better distribution of water and sugar concentration within the banana slices after the osmotic dehydration. Before the microwave hot-air drying experiments, °Brix, water content and water activity were measured (see Table 1). 2.3. Microwave hot-air drying oven The drying experiments were performed in a special prototype dryer (Fig. 1) designed by Risman (Microtrans, Sweden) in collaboration with SIK (INCO-DEV project ICA4-CT-2002-10034). The microwave cavity has a diameter of 80 cm diameter and a height of 10 cm. This dryer uses both microwaves and hot air. It consists of a microwave generator with an output power of 760 W (according to IEC Standard 60705 (IEC, 2004), described below) at a frequency of 2450 MHz, connected to a forced hot-air system. The cavity of the drier is cylindrical and accommodates a Teflon tray and a built-in scale. The size of the cavity was optimized by modelling the microwave field distribution for one kilogram of fruit pieces. To simplify measurement of weight and temperature during drying, the load does not rotate. Inlet air velocity and the temperature inside the equipment are controlled by a separate adapted console. The air flow inlet and outlet are located in the middle of the chamber, and the air is distributed and directed through the lid into the samples placed inside the cavity. The air velocity was measured on the air pipe leaving the microwave cavity using a rotating vane anemometer. The average air velocity inside the cavity is estimated to be

2.2. Osmotic dehydration The osmotic dehydration conditions were selected according to our preliminary reporting (Ahrne´, 2005). Osmotic dehydration was carried out in a 55° Brix sucrose solution containing 1% citric acid and 0.6% ascor-

Fig. 1. Schematic illustration of the microwave drying cavity.

N.R. Pereira et al. / Journal of Food Engineering 81 (2007) 79–87

2.4. Microwave-hot-air drying In order to better understand the effect of microwaves and hot air on the final drying period, the drying process was divided into three periods as shown in Fig. 2. The first stage (phase I) was identical for all experiments. It involved 100% output power (760 W) and input air flow at 80 °C and 5.7 m/s for 5 min. In the second stage (phase II) 380 W of output power was supplied and air flow involved three conditions: 50 °C and 3.3 m/s; 70 °C and 3.3 m/s; 70 °C and 5.7 m/s, up to the point when the sample reached about 0.67 kgmoisture/kgdry matter. The third stage (phase III) was run under the same conditions as the second stage, except that the microwave power levels were 0 W, 76 W, 150 W and 230 W, until the final sample moisture was 0.17 kgmoisture/kgdry matter. 2.5. Quality evaluation The following quality parameters were evaluated to assess the effect of drying conditions on the dried samples: moisture content, water activity, °Brix, colour, apparent volume, and porosity. The percentage of charred pieces, which represent loss of yield due to non-uniform heating, was also quantified. Water content was measured using AOAC Method 934.06 (1997), which involves drying the samples in a 70 °C vacuum oven at 100 mmHg until a constant weight is reached. Water activity was measured with a water activity meter at 24 °C (AquaLab Series 3 – Decagon). To determine the soluble solids (Brix), 30 g of banana pulp was blended with 90 ml of distilled water. For dried samples, this blend was performed in a dilution of 1 g/6 ml. The mixture was filtered through filter paper (Whatman no. 44) and a drop of the filtrate was placed on the prism of a refractometer (Digit0–32, Ceti Optical Instruments). Colour measurements were carried out directly on the outer surface of the banana slices using a colour reader (Minolta Colour Reader CR-10). One reading was taken

PHASE II

PHASE I

PHASE III

0

5

tII

2.3

2.0

XII = 0.67

80ºC - 5.7 m/s 760 W

t f (min) 0.17 ⎛⎜ kg moisture ⎜ kg dry matter

⎝

50ºC - 3.3 m/s 380 W

70ºC - 3.3 m/s 380 W

70ºC - 5.7 m/s 380 W

⎛ ⎜ ⎜ ⎝

about 1.95 times lower than the outlet air velocity. In the present study, one kilogram of osmotically dehydrated banana slices were distributed evenly in a circular path with five rows. In order to avoid overheating, no samples were placed near or opposite the antenna. The equipment allowed us to control process variables such as microwave power, air temperature and velocity. In addition, product temperature and weight were monitored during drying. The internal product temperature was measured by inserting fibre optic temperature probes (ReFLexTM, Neoptix, Canada) into four samples placed at different spots so as to cover distinct conditions of electric field strength. The acquisition system recorded the assigned variables at every second during drying. The efficiency of the microwave drier was assessed by two tests: (1) measurement of microwave power available to the load, and (2) measurement of microwave absorption by the load of bananas during drying as determined by external resonance phenomena. For the first test, it was used the IEC Standard Method 60705 (IEC, 2004), with some modifications. Cold ethanol was used instead of water to verify the microwave heating efficiency. This liquid was chosen due to its low dielectric constant (e0 ) and high loss factor (e00 ), which represents good absorption and low reflection of microwave energy. This test showed that the efficiency of the cavity was around 76%. For the second test, the magnetron was replaced by a SWR (Standing wave ratio) autotester (Wiltron – model 5400-6N50), which was connected to a Scalar Measurement System (Wiltron – model 5411A). The resonance was measured in decibels in the range of 2400–2500 MHz. The matching was checked with a load of banana slices at six different moisture contents during the drying process: fresh bananas, 67, 50, 40, 25 and 15 kgmoisture/kgproduct. According to this test, the load showed good microwave absorption throughout the drying process (around 98% absorption), which means that the load impedance was well matched to the microwave system impedance (resonant).

81

0W 76 W 150 W 230 W 0W 76 W 150 W 230 W 0W 76 W 150 W 230 W

Fig. 2. Schematic illustration of the experimental conditions tested. tII = time to dry banana slices up to 0.67 kg w/ kg dm (min); tf = time to dry banana slices up to 0.17 kg w/kg dm (min).

N.R. Pereira et al. / Journal of Food Engineering 81 (2007) 79–87

for every five slices. The CIE chromaticity coordinates (L*, a* and b*) were measured to give an indication of the degree of discolouration occurring. The total colour (DE*) was calculated using Eq. (1) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ð1Þ DE ¼ ðDL Þ þ ðDa Þ þ ðDb Þ The apparent volume was measured by under-solvent weighing (oil; qoil = 0.916 g/ml). A hook was attached to the lower side of an analytical balance and weighed in air (Wh) and immersed in oil (Wh,o). One slice was placed on the hook and weighed in air (Ws+h) and submersed in oil (Ws+h,oil). The measurement was repeated for three different samples. Care had to be taken that the hook was always immersed to the same depth. The volume was calculated using Eq. (2) V apparent

ðW sþh  W oil Þ  ðW sþh;oil  W h;oil Þ ¼ qoil

ð2Þ

The porosity was calculated by Eq. (3), which takes into account the apparent volume and the volume of solids e¼1

V solids V apparent

ð3Þ

The volume of solids takes into account the volume of samples without any pores. For this purpose, the volume of solids of ground samples was measured by a pycnometer at 20 °C. The charred pieces were collected after every drying test and weighed. The amount was calculated in gram based on the final weight of all samples. In order to minimize subjective error, a standard of charred pieces was previously defined. 3. Results and discussion In order to assess the effect of microwave power, air temperature and air velocity on the drying of osmotically dehydrated bananas, the results were analysed in terms of the kinetics of drying and the quality of the dried bananas. The drying process was divided into three phases (I, II and III), as shown in Fig. 2, according to the microwave power supplied. The microwave power was decreased following water loss during drying. Typical drying curves for the whole process (all phases) with and without the use of microwaves in the last stage of drying are illustrated in Fig. 3. As this work focuses on the final stage of drying, the results for phase III were analysed separately from those for phases I and II. 3.1. Kinetics of drying 3.1.1. Phases I and II The evolution of the product temperature during phases I and II and the respective drying rate curves are illustrated in Fig. 4a. Phase I represents a simultaneous rapid warming-up and drying period. As expected, the temperature

I

II

III

0W 230 W

2.5

X (kgmoisture/kgdry matter)

82

2.0 1.5 1.0 0.5 0.0 0

50

100

150

200

250

300

350

400

Overall drying time (min) Fig. 3. Typical drying curve of banana slices at 70 °C and 3.3 m/s with and without microwave power applied in the last stage of drying.

increased rapidly because of the internal heat generation caused by the high initial microwave power (760 W) and the strong flow of heated air (80 °C and 5.7 m/s). The product temperature rose from around 17–80 °C in 5 min, which represents a heating rate of 7.6 °C/min. This rapid warming increased the drying rate in the range of the dimensionless product moisture (X/X0) from 1 to 0.94 (Fig. 4b). In phase II, the drying process was carried out with the microwave power set to 380 W. The heating effect of the microwaves was supplemented by the effect of the air temperature. However, the influence of air velocity was found to be negligible. The maximum difference in product temperature when the air flow was set to 50 or 70 °C was found to be around 25 °C, and occurred at the end of the drying period, revealing that at the lower air temperature (50 °C) the cooling effect on the slices was more prominent than when the air temperature was 70 °C. In this phase, the drying rate of the banana slices decreased compared with phase I. At the beginning (between X/X0 = 0.9 and 0.8), there was a rapid decrease in the drying rate curves from approximately 0.06– 0.04 kgmoisture/kgdry matter  min1. This sudden drop-off was due to the 50% reduction in the microwave power applied to the product. In addition, the rate of water loss decreased from 0.06 to 0.02 kg w/kg dm/min for air at 70 °C and from 0.06 to 0.01 kg w/kg dm/min for 50 °C. The increasing of air temperature from 50 and 70 °C (3.3 m/s) shortened the time taken to dry one kilogram of bananas to 0.67 kgmoisture/ kgdry matter from 77 min to 63 min. Air velocity of 3.3 or 5.7 m/s had a negligible impact on the drying kinetics in this range of water content. 3.1.2. Phase III Fig. 5 illustrates the effect of phase III drying conditions on the internal temperature of banana slices. There was clearly an increase in the product temperature up to the end of drying for all conditions. The level of microwave power applied to the load exerted a major influence on product temperature. Power levels of 150 and 230 W induced a sharp rise in temperature, which suggests that there may be a run-away effect, which could have a lower-

N.R. Pereira et al. / Journal of Food Engineering 81 (2007) 79–87

I

II

T (˚C)

100 80 60 50˚C and 3.3 m/s 70˚C and 3.3 m/s 70˚C and 5.7 m/s

40 20 0

10

20

30 40 t II (min)

50

60

70

(b)

Drying rate (kg moisture /kgdry matter *min)

(a) 120

80

83

0.07

II

50˚C and 3.3 m/s 70˚C and 3.3 m/s 70˚C and 5.7 m/s

0.06

I

0.05 0.04 0.03 0.02 0.01 0.00 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

X /X 0

Fig. 4. Effect of air conditions on (a) product temperature evolution and (b) drying rate curves of banana slices during Phases I and II.

100

100

50˚C and 3.3 m/s

90

230 W

150 W

60 76 W

70

76 W 60

0W

50

40

40

30

30

0

100

200

300

400

0

500

100

tIII (min)

200

300

400

0W

60

40

0W

76 W

70

50

50

230 W 150 W

80 150 W

T (˚C)

T (˚C)

70

70˚C and 5.7 m/s

90

80

80

T (˚C)

100

70˚C and 3.3 m/s 230 W

90

30

500

0

100

tIII (min)

200

300

400

500

tIII (min)

Fig. 5. Product temperature evolution of Phase III during microwave drying with different microwave power, air temperature and velocity.

ing effect on the quality of the product quality by causing charring. At an air temperature of 70 °C, the product temperature was slightly higher with higher air velocity at the 76 W and 150 W microwave power levels. But when the microwave power level was 230 W, air velocity had a lowering effect on product temperature. To better analyse the effect of microwave power and air conditions on the last stage of drying (phase III), the drying kinetics were modelled mathematically using a two-term exponential model (Eq. (4)). The standard error (SE) was estimated by Eq. (5). X ¼ X 1 ek1 tIII þ X 2 ek2 tIII X 2 SE ¼ ðX icalc  X i exp Þ

ð4Þ ð5Þ

X = moisture content (dry bases) (kgmoisture/kgdry matter) at time tIII X1 = moisture content associated with the drying rate k1 X2 = moisture content associated with the drying rate k2 tIII = drying time from 0.67 to 0.17 kg moisture/kgdry matter = tf  tII (min). The calculated moisture content and time were used to test the model. Table 2 shows the calculated kinetic parameters and Fig. 6 illustrates the fitted drying curves for all drying conditions. The model gave a good fit for the experimental data, particularly at higher microwave power levels for all air conditions. Both kinetic constants (k1 and k2) increased with power level. Fig. 7 shows a linear increase of k1 with microwave power, while k2 is almost constant

Table 2 Two-terms exponential fitting model parameters of Phase III Tair (°C)

50

70

70

vair (m/s)

3.3

3.3

5.7

MP (W)a

0

76

150

230

0

76

150

230

0

76

150

230

X1 k1 X2 k2 SE

0.403 0.005 0.158 0.000 0.18

0.351 0.007 0.113 0.004 0.56

0.359 0.011 0.136 0.002 0.14

0.262 0.013 0.279 0.013 0.02

0.539 0.007 0.181 0.000 0.06

0.338 0.011 0.317 0.005 0.06

0.552 0.013 0.179 0.003 0.02

0.498 0.016 0.309 0.016 0.01

0.472 0.010 0.189 0.000 0.46

0.362 0.012 0.155 0.001 0.47

0.484 0.015 0.156 0.005 0.13

0.503 0.017 0.153 0.017 0.09

a

MP = Microwave power applied to the drying of banana slices.

N.R. Pereira et al. / Journal of Food Engineering 81 (2007) 79–87 50˚C and 3.3 m/s

0W 76 W 150 W 230 W

0

100

200

300

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

400

70˚C and 3.3 m/s

0W 76 W 150 W 230 W

X/X II

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

X/XII

X/XII

84

0

100

200

t III (min)

300

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

400

70˚C and 5.7 m/s

0

100

200

t III (min)

0W 76 W 150 W 230 W

300

400

t III (min)

Fig. 6. Drying curves and fitting model of Phase III during microwave drying with different microwave power, air temperature and velocity.

0.018

0.018

50˚C and 3.3 m/s 70˚C and 3.3 m/s 70˚C and 5.7 m/s

0.016

0.014

0.014

0.012

0.012

k2 (min-1)

-1

k1 (min )

50˚C and 3.3 m/s 70˚C and 3.3 m/s 70˚C and 5.7 m/s

0.016

0.010 0.008

0.010 0.008 0.006 0.004

0.006

0.002

0.004

0.000 0

75

150

0

225

75

150

225

Microwave power (W)

Microwave power (W)

Fig. 7. Effect of microwave power, air temperature and velocity on kinetic rate constants (k1 and k2).

0W 76 W 150 W 230 W

0.012 0.010 0.008 0.006 0.004 0.002 0.000 0.2

0.3

0.4

0.5

0.6

X /XII

0.7

0.8

0.9

1.0

reflected in the drying time, which decreases as the microwave power increases (Fig. 9). Furthermore, when the microwave power is the same, air temperature plays a more important role in water loss than air velocity. This difference is even more pronounced when no microwaves are used in the process. When microwaves were not used, the bananas reached the desired moisture level (0.17 kg moisture/ kgdry matter) after about 520 min when the air temperature was 50° and its velocity 3.3 m/s, and after 320 min when the air temperature was 70 °C and its velocity 3.3 m/s. However, when microwave energy is brought into the process, the difference in drying time is reduced, and continues to decrease as the microwave power increases. Shorter drying times were found for all processes carried out with 70˚C and 3.3 m/s

0.014 0W 76 W 150 W 230 W

0.012 0.010 0.008 0.006 0.004 0.002 0.000 0.2

0.3

0.4

0.5

0.6

X /XII

0.7

0.8

0.9

1.0

Drying rate (kgmoisture/kgdry matter*min)

50˚C and 3.3 m/s

0.014

Drying rate (kgmoisture/kgdry matter*min)

Drying rate (kgmoisture/kgdry matter*min)

for microwave power of 76 and 150 W but increases significantly for 230 W. This effect may be the result of the temperature runaway observed at 230 W, but not at 76 or 150 W. The effect of air conditions on the kinetic constants is less noticeable than that of the microwave power, but clearly air temperature and air velocity have an effect on kinetic constant k1. The drying rates were calculated as the derivative of Eq. (4). Fig. 8 illustrates the corresponding drying rate curves as functions of normalized moisture contents X/XII (dimensionless), for all experiments. The effect of microwave power on the drying rate of banana slices was much stronger than that of the air temperature and velocity (Figs. 7 and 8). This difference is

70˚C and 5.7 m/s

0.014 0W 76 W 150 W 230 W

0.012 0.010 0.008 0.006 0.004 0.002 0.000 0.2

0.3

0.4

0.5

0.6

0.7

0.8

X /XII

Fig. 8. Drying rate curves of Phase III during microwave drying with different microwave power, air temperature and velocity.

0.9

1.0

Overall drying time (min)

N.R. Pereira et al. / Journal of Food Engineering 81 (2007) 79–87 550

50˚ C and 3. 3 m/s 70˚ C and 3. 3 m/s 70˚ C and 5. 7 m/s

500 450 400 350 300 250 200 150 100 0

75

150

225

Microwave power (W)

Water activity

Fig. 9. Influence of microwave power in the drying time at different air temperature and velocity.

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

50˚C and 3.3 m/s 70˚C and 3.3 m/s 70˚C and 5.7 m/s

85

230 W of microwave power: 154 min for air at 50 °C and 3.3 m/s, 123 min for air at 70 °C and 3.3 m/s, and 134 min for air at 70° and 5.7 m/s. The experiments indicate that microwave heating results in rapid mass transfer within the sample because the generation of heat creates a large vapour pressure differential between the centre and the surface of the product (Lin, Durance, & Scaman, 1998). However, as the water content decreases markedly in the final drying (mostly in the vicinity of 0.67 to 0.17 kgmoisture/kgdry matter), two parallel behaviours can be identified: a considerable decrease in the microwave power absorbed by the foodstuff (associated with a smaller e0 and e00 ), and a higher resistance to water transport as a result of dried cell layers and a reduction in the driving force caused by concentration and pressure gradients. According to the published literature (Drouzas & Schubert, 1996; Maskan, 2000), resistance to water transport during the drying of bananas increases when the product reaches a moisture content of around 0.67 kg moisture/kgdry matter, after which drying proceeds more slowly. 3.2. Quality analysis

0

75

150

225

Microwave power (W) Fig. 10. Effect of microwave hot-air drying conditions on product water activity.

Banana slices lost 4.5%wb of water content and gained 4.4% of sugar content during osmotic dehydration. After microwave hot-air drying, the sugar content of the samples had concentrated up to 50%. All dried samples reached a

Fresh 70˚C and 3.3 m/s

50˚C and 3.3 m/s 70˚C and 5.7 m/s

22

100

20

90

18

80 70

14

L* value

a* value

16 12 10 8

60 50 40

6

30

4

20

2

0

75

150

10

225

0

Microwave power (W)

75

150

225

Microwave power (W)

45 40

40

30

30 ΔE*

b* value

35

25

20

20 10 15 0

10 0

75

150

Microwave power (W)

225

0

75

150

Microwave power (W)

Fig. 11. Effect of microwave hot-air drying conditions on product colour.

225

N.R. Pereira et al. / Journal of Food Engineering 81 (2007) 79–87

water activity below 0.65 (Fig. 10). It was observed that the higher the microwave power, the higher the standard errors of the measurements of samples placed in different spots, mainly due to the lack of field uniformity inside the cavity. Osmotically dehydrated bananas after microwave drying showed darker colour (L* values) and more red colour (a* values) than fresh fruit. However, no differences were observed between the b* values of the microwave dried fruits and the fresh ones. The processing conditions tested had no influence on fruit colour values (L*, a* and b*) and total colour change (DE*) values (Fig. 11), probably because the dried bananas had good, stable colour following the osmotic treatment with added citric and ascorbic acids before microwave drying. Consequently, the dried fruits presented an appealing yellow/orange colour. These results are in agreement with those observed by Maskan (2000) and Sankat et al. (1996). The apparent volume and porosity of dried bananas tend to decrease with an increase in microwave power (Fig. 12). Increasing the air temperature from 50 to 70 °C decreased the apparent volume and porosity when microwave energy was not supplied during phase III, but an increase in apparent volume and porosity was observed at 76 W and 150 W, and no significant difference was observed at 230 W. Increasing the air velocity at 70 °C from 3.3 to 5.7 m/s seems to have an adverse effect on apparent volume and porosity (again, with 230 W representing an exception). The variations in apparent volume and porosity may be due to the temperature runaway, which at 50 °C was only observed at 230 W (Fig. 5). In terms of quality, the cooling of the sample surface by air flow can be advantageous during microwave drying. Although the application of microwave power for drying banana slices decreased the drying time, higher microwave power also increased the number of charred pieces (Fig. 13). Samples dried without microwave energy in phase III presented 9% of charred pieces even with air flow at 50 °C. This result indicates that samples probably started to char before reaching 0.67 kgmoisture/kgdry matter, due to the 380 W microwave power that was applied to

50˚C and 3.3 m/ s 70˚C and 3.3 m/ s 70˚C and 5.7 m/ s

35 30 25 20 15 10 5 0 0

75 150 Microwave power (W)

the process during phase II. Air flow slightly decreased the number of charred pieces for processes carried out with 150–230 W of microwave power in Phase III. This reduction in charring is due to the cooling effect of the air flow over the sample surface while the microwave energy heated the interior. This cooling effect slightly increased the drying time (Fig. 9). 4. Conclusions The results of this study indicate that increasing microwave power during the final drying of banana slices increases the drying rate and consequently decreases drying time. However, higher microwave power also causes a rapid rise in product temperature (temperature runaway) and consequently charring of the dried product. Thus, it is necessary to control the microwave power during the final drying phase in order to avoid temperature runaway and quality deterioration of the product. Air temperature and air velocity also have a positive effect on drying time, although the reductions with higher temperatures and velocities are not as great as those associated with microwave power. Air flow conditions have an interesting effect when combined with microwave drying,

50˚C and 3.3 m/s 70˚C and 3.3 m/s 70˚C and 5.7 m/s

0.6 0.5

3 Porosity

0.4 2

0.3 0.2

1 0.1 0.0

0 0

75

150

Microwave power (W)

225

225

Fig. 13. Influence of microwave power in the number of charred pieces at different air temperature and velocity.

50˚C and 3.3 m/s 70˚C and 3.3 m/s 70˚C and 5.7 m/s

4 Apparent volume (ml)

% Charred Pieces (g/g dry prod)

86

0

75

150

225

Microwave power (W)

Fig. 12. Effect of microwave hot-air drying conditions on product apparent volume and porosity.

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especially at high levels of microwave power. It was found that a lower air temperature or a high air velocity can cause cooling on the product surface. This interaction can be explored to improve product appearance and quality and to decrease the quantity of charred pieces. Product quality evaluated in terms of colour, porosity and apparent volume was not strongly affected by processing parameters such as microwave power, air temperature and velocity. No effects were observed on the colour of the dried banana, but some effects were observed with respect to porosity and apparent volume. Acknowledgements The authors would like to acknowledge receipt of a scholarship from CAPES (PDEE program – BEX 3423/ 04–2) and the financial support of the European Commission within CombiDry project (ICA4-CT-2001–10034, www.sik.se/combidry). The authors are grateful to PO Risman (www.por.se) for very useful discussions, and help on selection of tests to assess the efficiency of the microwave drier. References Ahrne´, L. (2005). CombiDry first annual report. European INCO-DEV project reference ICA4-CT-2002-10034. Al-Duri, B., & McIntyre, S. (1992). Comparison of drying kinetics of foods using a fan-assisted convection oven, a microwave oven and a combined microwave/convection oven. Journal of Food Engineering, 15(2), 139–155. AOAC (1997). Official methods of analysis. Washington: Association of Official Analytical Chemists. Berteli, M. N., & Marsaioli, A. Jr., (2005). Evaluation of short cut pasta air dehydration assisted by microwaves as compared to the conventional drying process. Journal of Food Engineering, 68(2), 175–183. Drouzas, A. E., & Schubert, H. (1996). Microwave application in vacuum drying of fruits. Journal of Food Engineering, 28(2), 203–209.

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