Drying kinetics and rehydration characteristics of microwave-vacuum and convective hot-air dried mushrooms

Journal of Food Engineering 78 (2007) 512–521 www.elsevier.com/locate/jfoodeng

Drying kinetics and rehydration characteristics of microwave-vacuum and convective hot-air dried mushrooms S.K. Giri, Suresh Prasad

*

Department of Agricultural and Food Engineering, Post Harvest Technology Centre, Indian Institute of Technology, Kharagpur 721 302, India Received 5 March 2005; accepted 27 October 2005 Available online 20 December 2005

Abstract Microwave-vacuum dehydration characteristics of button mushroom (Agaricus bisporus) were evaluated in a commercially available microwave oven (0–600 W) modified to a drying system by incorporating a vacuum chamber in the cavity. The effect of drying parameters, namely microwave power, system pressure and product thickness on the drying kinetics and rehydration characteristics were investigated. The drying system was operated in the microwave power range of 115–285 W, pressure range of 6.5–23.5 kPa having mushroom slices of 6–14 mm thickness. Convective air drying at different air temperatures (50, 60 and 70 C) was performed to compare the drying rate and rehydration properties of microwave-vacuum drying with conventional method. Microwave-vacuum drying resulted in 70–90% decrease in the drying time and the dried products had better rehydration characteristics as compared to convective air drying. The rate constants of the exponential and Page’s model for thin layer drying were established by regression analysis of the experimental data which were found to be affected mainly by the microwave power level followed by sample thickness while system pressure had a little effect on the drying rate. Rehydration ratio was significantly affected by the system pressure. Empirical models are also developed for estimating the drying rate constant and rehydration ratio as a function of the microwave-vacuum drying process parameters.  2005 Elsevier Ltd. All rights reserved. Keywords: Microwave-vacuum drying; Button mushroom; Drying rate; Scanning electron microscopy; Rehydration ratio

1. Introduction Mushrooms are edible fungi of commercial importance and their cultivation and consumption has increased substantially due to their nutritional value, delicacy and flavor. The button mushroom (Agaricus bisporus) is the most widely cultivated and consumed mushroom throughout the world and it contributes about 40% of the total world production of mushroom. Mushrooms are extremely perishable and the shelf life of fresh mushroom is only about 24 h at ambient conditions. Various physiological and morphological changes occur after harvest, which make these mushrooms unacceptable for consumption. Hence, they should be consumed or processed promptly after harvest *

Corresponding author. Fax: +91 3222 282288. E-mail addresses: [email protected] (S.K. Giri), sp@agfe. iitkgp.ernet.in (S. Prasad). 0260-8774/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.10.021

and for this reason the mushrooms are traded mostly in processed form in the world market. Dehydration is one of the important preservation methods employed for storage of mushroom and dehydrated mushrooms are valuable ingredients in a variety of sauces and soups. As mushrooms are very sensitive to temperature, choosing the right drying method can be the key for a successful operation. Mushroom growers continue to dry mushroom under sun, which yields unhygienic and poor quality product. The conventional hot-air drying of mushrooms normally involves thermal and/or chemical pretreatment and drying at temperature maintained between 50 and 70 C. Due to long drying time and overheating of surface during hot-air drying, the problems of darkening in colour, loss in flavour and decrease in rehydration ability occur. Freeze drying produces a high quality product, but being an expensive process, its application for mushroom drying is limited. Vacuum drying is another

S.K. Giri, S. Prasad / Journal of Food Engineering 78 (2007) 512–521

alternative method and is especially suitable for products that are prone to heat damage such as fruits and vegetables. However, in vacuum process requiring heat, transfer of heat energy to the workload becomes difficult, as convection is ineffective at low pressure. Most conventional vacuum dryers rely on conduction heat transfer from hot plates, which is slow, difficult to control and requires a large surface area and therefore, conventional vacuum drying has high operating and installation cost (Woodroof & Luh, 1986). The desire to prevent significant quality loss and to achieve fast and effective dehydration has resulted in increasing use of microwave heating for food drying. Microwave drying is rapid, more uniform and energy efficient compared to conventional hot-air drying. In recent years, Microwave-vacuum drying (MVD) has been investigated as a potential method for obtaining high quality dried food products, including fruits, vegetables and grains. Microwave-vacuum drying combines the advantages of both microwave heating and vacuum drying. The low temperature and fast mass transfer conferred by vacuum combined with rapid energy transfer by microwave heating generates very rapid, low temperature drying and thus it has the potential to improve energy efficiency and product quality. Some fruits and grains have been successfully dried by microwave-vacuum drying techniques (Cui, Xu, & Sun, 2003; Drouzas & Schubert, 1996; Durance & Wang, 2002; Lin, Durance, & Scaman, 1998; Wardsworth, Velupillai, & Verma, 1990; Yongsawatdigul & Gunasekaran, 1996a, 1996b). Despite those investigations, there is scanty information available either in terms of the drying kinetics or quality of button mushroom undergoing microwave-vacuum drying technique. The drying kinetics is often used to describe the combined macroscopic and microscopic mechanisms of heat and mass transfer during drying, and it is affected by drying conditions, types of dryer and characteristics of materials to be dried. Since on-line measurement of temperature and moisture is difficult and time-consuming for microwave assisted heating and drying, the drying kinetics models are essential for equipment design, process optimization and product quality improvement. The effect of vacuum in microwave drying operation is system specific, and for successful design and operation of an industrial microwave-vacuum drying system, knowledge of the drying characteristics of the material to be dried under a range of condition is vital (McLoughlin, McMinn, & Magee, 2003). The aim of the present work was to investigate microwave-vacuum drying characteristics of button mushroom slices and to compare with convective hot-air drying in respect to drying kinetics, rehydration qualities and micro structural changes of the dried products. 2. Material and methods 2.1. Materials Fresh button mushrooms (A. bisporus) were obtained from market and kept in cold storage at 4–5 C. Prior to

513

dehydration, mushrooms were thoroughly washed to remove the dirt and graded by size to eliminate the variations in respect to exposed surface area. Slices of desired thickness were obtained by carefully cutting mushrooms vertically with a vegetable slicer and the slices from middle portions with characteristics mushroom shape were used for drying experiments without any pretreatments. They were immediately weighed and placed into the dryer. Moisture content of the samples was determined in a vacuum oven at 70 C for 14–16 h (AOAC, 1984). The initial moisture content of the slices was ranged from 92% to 93% (w.b.). 2.2. Drying 2.2.1. Hot-air drying The mushroom slices were hot-air dried at air temperature of 50, 60 and 70 C in a cross-flow type dryer with air flow rates of 1.5 m/s. Air was heated electrically before entering the heater. Slices were spread in a single layer on the tray. During air drying, weight and temperature of the sample were recorded at regular interval of times. 2.2.2. Microwave-vacuum drying The experimental setup used for the microwave-vacuum drying of the samples is depicted in Fig. 1, which consists of a microwave oven (IFB, model electron) of rated capacity of 600 W at 2.45 GHz. The oven is modified to give variable power output (from 0 to 600 W) by incorporating a 230 V AC variac in the circuit (Sharma & Prasad, 2001). A glass container containing the material to be dried was placed inside the microwave cavity and a vacuum pump was connected to the container for maintaining the desired levels of vacuum inside it. Vacuum in the container was monitored by using a vacuum gauge and a pressure regulating valve to maintain the pressure at desirable levels. An air tight condenser was also used in the vacuum line for condensing the water vapour released from the samples. About 100 g of sliced mushrooms were taken for each drying experiments in the microwave-vacuum dryer at different microwave power and pressure levels. The sample remained in the container for a specified time interval while drying took place. The weight of the sample was recorded at every 5 min intervals by switching off the microwave oven and after releasing the vacuum, which took about 40 s for each observation. The samples were dried till the moisture content was reduced to 6–6.5% (w.b.). The variables chosen for microwave-vacuum drying experiments were microwave power (Q), system pressure (P) and thickness of the slices (T). Response surface methodology was used to determine the relative contributions of the above three variables to the drying characteristics and rehydration ratio. Twenty experiments were performed according to a central composite rotatable design with the three variables and with five levels of each variable. The maximum and minimum variable levels

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Pressure regulating valve

Microwave oven Vacuum gauge Product Vacuum pump

Condenser

Balance

Fig. 1. Experimental microwave-vacuum drying apparatus.

responses due to extraneous factors. The center point in the design was repeated six times to calculate the reproducibility of the method.

Table 1 Levels of microwave-vacuum drying process variables Variable

Name (units)

Level 1.68

1

0

1

1.68

Q P T

Microwave power (W) System pressure (kPa) Thickness (mm)

115 6.5 5.8

150 10 7.5

200 15 10

250 20 12.5

285 23.5 14.2

2.3. Modeling drying data

Table 2 Central composite design showing variable level combinations

Various transport properties like moisture and thermal diffusivity and mass and heat transfer coefficients, describe completely the drying kinetics, although in the literature sometimes the drying rate constant is used as their combination and as a trial for standardizing a process description independently from the controlling mechanism (Mujumdar, 1995). The exponential model, Eq. (1) and empirical Page’s model, Eq. (2), have been used to describe the drying kinetics of various agricultural materials in convective and microwave-convective drying (Jasna, Sander, & Skansi, 2001; Pravanjan, Ramaswamy, & Raghavan, 1995; Sharma & Prasad, 2001). Both the models were tested for their validity to mushroom under microwave-vacuum drying:

Experiment no.

MR ¼ expðKtÞ;

were selected on the basis of preliminary drying experiments. Table 1 gives the levels of variables in coded and actual levels and Table 2 shows the combination of variable levels used in the central composite design. Experiments were randomized in order to minimize the effects of unexplained variability in the observed

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Variable levels

n

ð1Þ

Q

P

T

MR ¼ expðkt Þ;

1 1 1 1 1 1 1 1 1.68 1.68 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 0 0 1.68 1.68 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 0 0 0 0 1.68 1.68 0 0 0 0 0 0

where MR = moisture ratio = (M  Me)/(M0  Me); M = moisture content (kg water/kg dry matter) at time t; Me = equilibrium moisture content (kg water/kg dry matter); M0 = initial moisture content (kg water/kg dry matter) at time = 0; K = drying rate constant (min1); k and n are the parameters of Page’s model; and t = drying time in min. The equilibrium moisture content during microwavevacuum drying is considered zero, due to vacuum conditions of the process (Kiranoudis, Tsami, & Maroulis, 1997). The parameters K, k and n of the above equations were evaluated through non-linear regression analysis using SYSTAT computer program. The goodness of fit of the tested mathematical models to the experimental data was evaluated from the coefficient of determination (R2). The higher the R2-value, the better is the goodness of fit.

ð2Þ

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Rehydration ratio (RR) which is a measure of rehydration characteristics of dried mushroom slices was determined by immersing 5 g of dried samples in distilled water at 30 and 100 C temperatures. The water was drained and the samples weighed at every 30 min intervals for those immersed at 30 C and at every 2 min intervals for those at 100 C. Triplicate samples were used. Rehydration ratio was defined as the ratio of weight of rehydrated samples to the dry weight of the sample. 2.5. Scanning electron microscopy (SEM) The structure of the dehydrated mushroom slices was examined using a scanning electron microscope (JEOL JSM-5800) coupled with an energy dispersive X-ray micro analytical system (OXFORD ISIS-300). Thin slice of about 1 mm thick was cut from the dried samples and fixed on the SEM stub, which were subsequently coated with gold in order to provide a reflective surface for the electron beam. Gold coating was carried out in a sputter coater (BIO-RAD E-5200) under a low vacuum with the presence of the inert gas, argon. The gold-coated samples were subsequently viewed under the microscope. 3. Results and discussion 3.1. Drying kinetics The drying curves for thin layer drying of mushroom slices under microwave-vacuum drying and hot-air drying conditions are shown in Fig. 2, where the moisture content of the samples at various time intervals and for varying microwave power level and at various drying air temperatures is recorded. The time required to reduce the moisture content to any given level in microwave-vacuum drying

was dependent on the power level, being the highest at 115 W and lowest at 285 W. Mushroom having 92% initial moisture content dried to a final moisture content of 6% within 45 and 20 min at microwave power level of 115 and 285 W, respectively, at pressure level of 15 kPa. By comparison, similar mushroom samples under convective hot-air drying took 270 and 180 min at drying air temperature 50 and 70 C, respectively. Thus, the microwavevacuum drying times were around 90% shorter at 285 W and 75% shorter at 115 W than the corresponding convective air drying times. Figs. 3–5 further show the effect of changing the process variables, namely microwave power, system pressure and sample thickness on drying time during microwave-vacuum drying. It is evident that the drying time decreases with the increase in power output. When microwave power level and product thickness remain constant, the drying rate at higher vacuum level was slightly greater. However, the effect of system pressure on drying time was not as significant as that of microwave power. Concerning the effect of the sample size on drying time, mushroom slices of smaller

Moisture content, kg/kg dry matter

2.4. Rehydration

515

14 12

285 W 200 W

10

115 W

8 6 4 2 0 0

10

20 30 Drying time, min

40

50

Fig. 3. Effect of microwave power levels on drying time for 10 mm thick mushroom slices during MVD at 15 kPa system pressure.

12 Moisture content, kg /kg dry matter

Moisture content, kg/kg dry matter

14

AD: 50 °C

10

AD: 70 °C MVD: 115 W, 15 kPa

8

MVD: 285 W, 15 kPa

6 4 2 0 0

50

100

150 200 Drying time, min

250

300

350

Fig. 2. Drying curves of air-dried (AD) and microwave-vacuum dried (MVD) mushroom slice (10 mm thick).

14 12

23.5 kPa

10

15 kPa 6.5 kPa

8 6 4 2 0 0

10

20 30 40 Drying time, min

50

60

Fig. 4. Effect of pressure levels on drying time for 10 mm thick mushroom slices during MVD at 200 W microwave power.

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1990). As the values of dielectric constant and loss factors are higher at higher moisture content of the material, obviously the material absorbs more microwave power and heating is faster at high moisture content. As drying progressed, the loss of moisture in the product decreases the absorption of microwave power and resulted in a fall in the drying rate during the later part of drying (Kharaisheh, Cooper, & Magee, 1995; Sharma & Prasad, 2001). Microwave heating under vacuum resulted in large increase in drying rates (almost three to five-folds) as compared to hot-air drying throughout the drying process.

Moisture content, kg /kg dry matter

14 6 mm

12

10 mm

10

14 mm

8 6 4 2 0 0

10

20 Drying time, min

30

40

Fig. 5. Effect of slice thickness on drying time during MVD of mushroom slices at 200 W microwave power and at 15 kPa system pressure.

thickness exhibited more rapid dehydration. This can be attributed to enhanced water transport phenomena within the sample, which favour the thin samples. The influence of microwave power on drying rate is illustrated in Fig. 6. There were no constant rates drying under any of the test conditions. Although high moisture foods like mushrooms can be expected to have a constant rate of drying, this was not observed in the present study, may be because of the thin (single) layer arrangement and too rapid heating by microwaves, providing instant drying. As expected, higher drying rates were obtained with higher microwave power. It can be seen from the figure that the influence of power on drying rate is markedly higher when the moisture is higher. At moisture content of less than 4.0 kg/kg d.b. there is no difference in the drying rates among different power levels, indicating the significance of internal resistance to mass transfer at low water content in the material. The amount of microwave energy absorbed by the material depends upon its dielectric properties and the electric field strength (Mudgett,

Drying rate, kg water/kg dry matter-hr

80 70

MVD: 285 W, 15 kPa MVD: 200 W, 15 kPa

60

MVD: 115 W, 15 kPa AD: 70 °C

50

AD: 50 °C

40 30 20 10 0 0

2

4 6 8 10 Moisture content, kg/kg dry matter

12

14

Fig. 6. Comparison of drying rate for microwave-vacuum drying and air drying of 10 mm thick mushroom slices.

3.2. Modeling of the thin layer drying data of mushroom slices Table 3 lists the model constants obtained by application of exponential and Page’s equation to the experimental drying data. The empirical Page’s model gave better fit for the drying data under all conditions tested with higher R2 (coefficient of determination) values. A good agreement was found between the experimental and fitted values with the R2-values of greater than 0.99 (Fig. 7). The drying constant (K) increased from 0.017 to 0.03 min1 with the increase in drying air temperature from 50 to 70 C for 7.5 mm thick mushroom slices. It can be seen that K- and k-values for microwave-vacuum drying were higher than hot-air drying. In microwave-vacuum drying of mushroom slices of a particular thickness, these values were found to increase as the microwave power was increased and pressure was reduced. This can be attributed to the fact that higher microwave power, lower pressure and smaller sample size help in increasing the driving force of heat and mass transfer. For parameter n, it did not possess a clear trend. For air drying, the n-values were close to one where as in case of microwave-vacuum method the values were more than one. The values for parameters ‘K’, ‘k’ and ‘n’ are in close proximity to those reported in the literature for button mushrooms and other products (Arora, Shivhare, Ahmed, & Raghavan, 2003; Drouzas, Tsami, & Saravacos, 1999; Kar & Gupta, 2003; Pravanjan et al., 1995). The effect of process variables (microwave power, system pressure and thickness) on rate constants were studied by response surface method and fitting the data to a second order polynomial model. The analysis of variance (ANOVA) in Table 4 shows the significance of the models and individual model terms. The models are found highly significant (p < 0.05) with R2-values more than 0.86 (Eqs. (3) and (4)). The rate constant, K was highly affected by microwave power and slice thickness, as the linear as well as quadratic terms of these variables are significant. In case of Page’s model constant, k, the linear effect of all the process variables were found significant, with microwave power having strongest effect as can be seen from corresponding F-values. Significant lack of fit of the models implied that a high proportion of the variability was

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517

Table 3 Effect of various drying conditions on parameters of exponential and empirical Page’s model Drying method

Thickness (mm)

Air temperature (C)

Microwave power (W)

Pressure (kPa)

Model parameters Exponential

AD

7.5

12.5

MVD

7.5

12.5

Page

K (min1)

R2

k (min1)

n

R2

50 60 70 50 60 70

– – – – – –

– – – – – –

0.017 0.026 0.030 0.011 0.014 0.020

0.987 0.990 0.995 0.995 0.994 0.996

0.025 0.028 0.033 0.011 0.012 0.019

0.997 1.02 0.98 0.985 1.06 1.034

0.996 0.994 0.995 0.996 0.994 0.995

– – – – – – – –

150

10 20 10 20 10 20 10 20

0.110 0.092 0.175 0.157 0.082 0.080 0.137 0.129

0.997 0.995 0.997 0.997 0.985 0.986 0.994 0.990

0.074 0.055 0.099 0.085 0.024 0.023 0.064 0.048

1.168 1.197 1.289 1.281 1.482 1.471 1.345 1.443

0.999 0.996 0.999 0.999 0.998 0.997 0.999 0.999

250 150 250

AD—air drying; MVD—microwave-vacuum drying.

1.0

1.0 Q = 150 W P = 20 kPa

0.8

Q = 250 W P = 20 kPa

0.8 0.6

MR

MR

0.6 0.4

0.4

0.2

0.2

0.0

0

7

14

21

28

0.0

35

0

5

Time, min

10

15

20

25

Time, min

1.0

1.0 Q = 150 W P = 10 kPa

0.8

Q =250 W P =10 kPa

0.8 0.6

MR

MR

0.6 0.4

0.4

0.2

0.2

0.0

0.0 0

7

14

21

28

35

Time, min

0

5

10

15

20

25

Time, min

Fig. 7. Moisture ratio (MR) versus time comparing experimental curves with the predicted one (–) by Page’s equation for microwave-vacuum drying of mushroom.

not explained by the data. The variation of ‘K’ and ‘k’ with microwave power and slice thickness are depicted in Fig. 8.

Regression analysis of the drying rate constant, K and k versus MW power (Q), system pressure (P) and thickness (T) yielded the following second order equations:

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Table 4 ANOVA showing the linear, quadratic and cross-product effect of the variables on rate constants and rehydration ratio Source of variation

df

K

k

RR

Mean square

F-value

Mean square

F-value

Mean square

F-value

9 1 1 1 1 1 1 1 1 1

1.85 · 103 1.10 · 102 2.01 · 104 9.98 · 104 1.57 · 103 6.50 · 104 2.60 · 103 9.11 · 105 9.11 · 105 6.12 · 106

14.46** 87.27** 1.57 7.78* 12.21** 5.06* 20.26** 0.71 0.71 0.048

9.21 · 104 3.58 · 103 7.92 · 104 2.30 · 103 4.54 · 104 5.13 · 104 8.63 · 104 1.25 · 105 1.25 · 105 3.21 · 105

6.93** 26.96** 5.96* 17.42** 3.43 3.87 6.50* 0.094 0.094 0.24

0.16 0.11 0.61 0.34 0.012 0.16 0.087 0.017 0.053 0.021

3.62* 2.44 14.13** 7.95* 0.27 3.75 2.02 0.40 1.22 0.49

Residual Lack of fit Pure error

10 5 5

1.28 · 104 2.54 · 104 2.70 · 106

94.07**

1.32 · 104 2.55 · 104 1.51 · 105

16.59**

0.043 0.079 0.0075

10.52**

Total

19

Model Q P T Q2 P2 T2 QP QT PT

df—Degree of freedom; F-ratio of variance estimate. * Significant at 1%. ** Significant at 5%.

0.090

0.143

0.068

k (1/min)

0.113

0.182

K (1/min)

0.221

0.104 0.065

14.0

0.045 0.022

14.0 285

12.0 10.0

T (mm)

(a)

158 6.0 115

243 10.0

200 8.0

285

12.0

243

T (mm) Q (W)

(b)

200 8.0

158 6.0

Q (W)

115

Fig. 8. Effect of microwave power and slice thickness on (a) drying rate constant (K) and (b) Page equations parameter (k), during microwave-vacuum drying at 15 kPa pressure.

K ¼ 0:434  1:03  103 Q  0:011  P  0:04  T þ 4:17  106 Q2 þ 2:7  104 P 2 þ 2:15  104 T 2 þ 1:35  105 QP  2:7  105 QT  7:0  105 PT ðR2 ¼ 0:93Þ;

ð3Þ

k ¼ 0:342  6:0  104 Q  9:28  103 P  0:034T þ 2:24  106 Q2 þ 2:38  104 P 2 þ 1:24  103 T 2  5:0  106 QP þ 1:0  105 QT þ 1:6  104 PT ðR2 ¼ 0:86Þ.

ð4Þ

3.3. Rehydration characteristics The rehydration curves of microwave-vacuum dried mushroom samples, both at ambient (30 C) and at

100 C water temperatures, as affected by microwave power and pressure levels are shown in Figs. 9 and 10. In all the cases the amount of moisture absorbed increases with rehydration time, but at a decreasing rate up to saturation level. The rehydration stabilized in about 10 min at 100 C and in 3 h at 30 C and the rehydration ratio was in the range of 2.3–3.4 under various drying conditions. Rehydration properties were improved by drying at lower system pressure and higher microwave power as indicated by higher values of rehydration ratio. Similar results were reported by Drouzas and Schubert (1996), Durance and Wang (2002), Pappas, Tsami, and Marinos-Kouris (1999). The regression model for RR relating the microwave-vacuum drying process variables was found as

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4.0

Rehydration ratio

4.0

3.0 Rehydration ratio

519

2.0

3.0

2.0 P = 23.5 kPa

1.0

P = 15 kPa P = 6.5 kPa

Q = 285 W

1.0

Q = 200 W

0.0

Q = 115 W

0

4 8 6 Rehydration time (min)

2

(a)

0.0 0

2

4

(a)

8

6

10

10

12

12

Rehydration time (min) 4.0

Rehydration ratio

Rehydration ratio

4.0

3.0

2.0

3.0

2.0 P = 23.5 kPa

1.0

P = 15 kPa P = 6.5 kPa

Q = 285 W

1.0

Q = 200 W

0.0

Q = 115 W

1

0

(b)

0.0 0

0.5

(b)

1

1.5

2

2.5

3

3.5

Rehydration time (h)

3 2 Rehydration time (h)

4

Fig. 10. Effect of system pressure on rehydration ratio of 10 mm thick mushroom slices dried at 200 W microwave power: (a) at 100 C and (b) at 30 C water temperature.

Fig. 9. Effect of microwave power on rehydration ratio of 10 mm thick mushroom slices dried at 15 kPa pressure: (a) at 100 C and (b) at 30 C water temperature.

RR ¼ 3:94 þ 0:0156Q  0:173P  0:244T  1:13  105 Q2 3.78

 6:5  104 QT þ 4:1  103 PT

3.43

ðR2 ¼ 0:77Þ.

ð5Þ

The response surface plot for rehydration ratio is shown in Fig. 11. From ANOVA (Table 4) it can be concluded that the rehydration ratio depends mainly on pressure level while sample thickness and microwave power had less effect. While the rehydration ratio is positively correlated with microwave power that of system pressure is negatively correlated. As the pressure level decreases, the rehydration ratio increases, owing to the increased drying rate and creation of pores that are induced by vacuum conditions (Kiranoudis et al., 1997). The higher RR at higher microwave power can be attributed to the development of greater internal stresses during drying at higher power levels. The quick microwave energy absorption causes rapid evaporation of water, creating a flux of rapidly escaping vapour which helps in preventing the shrinkage and case hardening, thus improving the rehydration characteristics

Rehydration Ratio

þ 4:24  103 P 2 þ 0:012T 2  1:85  104 QP

3.08 2.73

2.37

6.0

23.5

8.0

19.3 10.0

15.0

P (kPa)

12.0

10.8

T (mm)

6.5 14.0

Fig. 11. Effect of system pressure and thickness on rehydration ratio of mushroom slices dried at 200 W microwave power.

(Kharaisheh et al., 1995; Lyons, Hatcher, & Sunderland, 1972; Sharma & Prasad, 2001). It is also clear from

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Fig. 12 that the rehydration capacity, both in terms of final moisture content and speed is higher for the microwavevacuum dried samples in comparison to air-dried samples. Microwave-vacuum drying creates a porous structure with very little shrinkage than those obtained by air drying, thereby providing better rehydration char-acteristics.

Rehydration ratio

4.0

3.0

2.0

3.4. SEM MVD: 200W, 15 kPa

1.0

AD: 60°C

0.0 0

2

4 6 8 Rehydration time (min)

10

12

Fig. 12. Comparison of rehydration ratio of microwave-vacuum and airdried mushroom slices.

Effect of different drying methods and conditions on the structure of dried mushrooms was observed under scanning electron microscope. In air-dried samples (Fig. 13a) there is very less open structure and pores as compared to microwave-vacuum dried samples (Fig. 13b and c), indicating severe tissue shrinkage and collapse during air drying. The less shrinkage in the microwave-vacuum dried samples may be due to the shorter drying time, lower drying temperature and some tissue expansion from internal water vapour. Again the structure is more porous in samples dried under more vacuum conditions (Fig. 13c) during microwave-vacuum drying. 4. Conclusions Microwave-vacuum drying of mushroom was much faster than conventional hot-air drying, particularly towards the end of the drying process. The exponential as well as empirical Page’s model adequately described the microwave-vacuum drying data. Statistical analysis of the drying data showed that drying rate constant was highly influenced by microwave power while system pressure had highest influence on the rehydration characteristics of the dehydrated mushroom in a p 6 0.05. Sample thickness also appeared to be factor influencing the drying rate with thinner sample resulting in quick drying. Microwave-vacuum dried mushroom also created a more porous dehydrated product, which rehydrated more quickly and more completely than the air dried product. The button mushroom slices could be microwave-vacuum dried to 6% moisture content (w.b.) in 25–30 min, avoiding any burning of the product and with good rehydration characteristics. References

Fig. 13. Scanning electron micrographs of dried mushroom: (a) air-dried at 60 C, (b) microwave-vacuum dried at 20 kPa pressure and (c) microwave-vacuum dried at 10 kPa.

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