Drying Behavior of Cultured Mushrooms

DRYING BEHAVIOR OF CULTURED MUSHROOMS

SONER ÇELEN1, KAMIL KAHVECI2, UGUR AKYOL1,3 andAYSEN HAKSEVER1

1Mechanical Engineering DepartmentÇorlu Engineering FacultyNamık Kemal University

59860 Çorlu/Tekirdag, Turkey

2Mechanical Engineering DepartmentFaculty of Engineering and Architecture,Trakya University, 22030 Edirne, Turkey

Accepted for Publication June 6, 2008

ABSTRACT

In this study, the drying behavior of cultured mushrooms with an initialmoisture content of 93% (drying basis [d.b.]) was investigated experimentallyfor different slice thicknesses and drying air temperatures, and the suitabilityof various drying models in defining the drying behavior of mushrooms wasdetermined by statistical analysis. Drying operation was carried out at tem-peratures of 40, 45, 50 and 60C and at a fixed air velocity of 2 m/s. The slicethicknesses of mushrooms were taken as 2, 4 and 6 mm. The experimentalresults show that the drying temperature has a significant effect on the mois-ture removal from mushrooms. However, it is also observed that increasing thetemperature above a certain value for large values of slice thickness does nothave a considerable effect on the drying rate. It may also be concluded fromthe experimental results that the increase in the slice thickness slows down thedrying rate significantly. Furthermore, the results of statistical analysis showthat the most suitable model in defining the drying behavior of mushrooms isthe diffusion approach model.

PRACTICAL APPLICATIONS

Mushrooms are soft textured and extremely perishable. They begin todeteriorate shortly after harvest. Because of their short shelf life under normalambient conditions of temperature and humidity, their preservation has assumed

3 Corresponding author. TEL: 90-282-6529475; FAX: 90-282-6529372; E-MAIL: [email protected]

Journal of Food Processing and Preservation 34 (2010) 27–42.DOI: 10.1111/j.1745-4549.2008.00300.x 27© 2009 The Author(s)Journal compilation © 2009 Wiley Periodicals, Inc.

importance. The most common species that is also grown on a large scale inTurkey is the cultivated mushroom, Agaricus bisporus. They are low in calorieand have a delicate, appealing flavor. They contain about 91% water. Drying isthe most common preservation method, especially for mushrooms to be used asingredients for special sauces and soups. Preservation of mushrooms throughdrying makes it possible to limit microbial growth or other reactions by reducingmoisture content, and allows efficiency in transportation and storage.

INTRODUCTION

Mushrooms are edible fungi of commercial importance, and their culti-vation and consumption has increased substantially due to their nutritionalvalue, delicacy and flavor. The button mushroom (Agaricus bisporus) is themost widely cultivated and consumed mushroom throughout the world, and itcontributes about 40% of the total world production of mushroom. Mushroomsare extremely perishable, and the shelf life of fresh mushrooms is only about24 h at ambient conditions. Various physiological and morphological changesoccur after harvest that make these mushrooms unacceptable for consumption.Hence, they should be consumed or processed promptly after harvest. There-fore, mushrooms are traded mostly in processed form in the world market (Giriand Prasad 2007).

Having a high protein value, mushrooms are foodstuffs that are usedas garniture in vegetable and meat dishes, as well as in soup making. Also,ground mushrooms as flour are used in the medical industry. In order to keepmushrooms safe and unspoiled for a long time, it is vital to remove the bacteriacausing fermentation or decomposition from their environment and/or fromtheir structure. So, this bacterium should be killed and/or their contact withmushrooms cut. In order to provide this, the water should be removed from thebody of the mushroom. When the mushroom is dried, it keeps almost all of itstaste and other features. In addition, dehydrated mushrooms occupy very littlespace. The drying process of mushrooms is generally based on sending theheated dry air over the mushroom in order to remove the moisture inside it. Butthe temperature of the air should be below 50C, because mushrooms risklosing protein and taste at this temperature.

There are several studies in the literature related to the drying behavior ofmushrooms. For example, Pal and Chakraverty (1997) investigated dryingcharacteristics of untreated and treated mushrooms (oyster Pleurotus) at dif-ferent drying air temperatures of 45, 50 and 60C, and drying air velocities of0.9 and 1.6 m/s. Dogan (1991) examined the effect of drying temperature andpretreatment over the cultured mushrooms and found that the drying timesignificantly decreases when the temperature is increased from 40 to 70C. It

28 S. ÇELEN ET AL.

was also found in this study that the ascorbic acid has little effect on themushroom color. Suguna et al. (1995) examined the drying characteristics ofoyster mushrooms in three forms: sun drying, thin-layer drying and fluid beddrying. Color and rehumidification rate were used as criteria in this study toevaluate the characteristics of the product. The results show that the most idealbed drying is at a temperature of 50C and at a velocity of 35 m/min. Micro-wave vacuum dehydration characteristics of button mushrooms (A. bisporus)in a commercially available microwave oven (0–600 W) modified to a dryingsystem by incorporating a vacuum chamber in the cavity were investigated byGiri and Prasad (2007). Convective air drying at different air temperatures (50,60 and 70C) was also performed in this study to compare the drying rate ofmicrowave vacuum drying with conventional methods. Microwave vacuumdrying resulted in 70–90% decrease in the drying time as compared withconvective air drying. The dehydration characteristics of mushrooms were alsoexamined by Walde et al. (2006) by using different pretreatments and dryingsystems (hot air cabinet dryer, fluidized bed dryer, vacuum dryer and micro-wave oven). The results of this study show that microwave drying may not bea suitable method for the drying of mushrooms because higher drying timesresult in the charring of edges. In another study, Kotwaliwale et al. (2007)investigated the textural and optical properties of paddy straw mushrooms(Pleuratus spp.) in a cabinet tray. It was found that the whiteness index ofmushrooms decreases while yellowness index increases during drying, and thedrying temperature has an inverse effect on the whiteness of mushrooms.

In order to simulate the drying behavior of food products, usually, empiri-cal and semiempirical models are used. The main advantage of these kind ofmodels in drying simulations is that they are easy to apply and therefore areuseful for automatic control processes. These drying models (Table 1) areusually simplified versions of a general series solution of the Fick’s second law

TABLE 1.VARIOUS MATHEMATICAL MODELS USED IN MODELING OF DRYING

Name Model equation Reference

Newton mr = exp(-kt) O’Callaghan et al. (1971)Page mr = exp(-ktn) Page (1949)Henderson and Pabis mr = aexp(-kt) Henderson and Pabis (1961)Geometric mr = at-n Cihan et al. (2007)Wang and Singh mr = 1 + at + bt2 Wang and Singh (1978)Two-term exponential mr = aexp(-kt) + (1 - a) exp(-kat) Togrul (2005)Logarithmic mr = a0 + aexp(-kt) Cihan et al. (2007)Diffusion approach mr = aexp(-kt) + (1 - a) exp(-kbt) Cihan et al. (2007)Midilli mr = aexp(-ktn) + bt Midilli et al. (2002)Two-term mr = a1exp(-k1t) + a2exp(k2t) Sharaf-Eldeen et al. (1980)

29DRYING BEHAVIOR OF CULTURED MUSHROOMS

https://www.researchgate.net/publication/222428188_Modeling_of_intermittent_drying_of_thin-layer_rough_rice_J_Food_Eng?el=1_x_8&enrichId=rgreq-f93aa09c-ad38-44c8-86fd-f8503167d374&enrichSource=Y292ZXJQYWdlOzIzMDI4OTUxNztBUzoxODYyNjc3Njk3NzgxNzdAMTQyMTQyMTEwNTQxNA==


or modified versions of the simplified models (Kahveci and Cihan 2007).Therefore, the majority of these models are not arbitrarily selected models, andthey are the models based on the physiological bases. Several models given inTable 1 were used by Panchariya et al. (2002) to model the drying process ofblack tea. After a similar analysis, Doymaz (2005) concluded that the Vermamodel is the most appropriate model in defining the drying behavior of figsunder the sun. Akpinar and Biçer (2002) looked at the drying behavior ofpotato slices in a convective cyclone dryer and found that drying can besimulated by the diffusion approach model. Ertekin and Yaldiz (2004) simu-lated the drying of eggplants by the Midilli model. Togrul and Pehlivan (2002)concluded from their analyses that the solar drying of apricots can be definedby the logarithmic model well. Cihan et al. (2007) found that the Midilli modelis the most appropriate model in defining the drying behavior of intermittentdrying of rough rice.

The main objective of this work was to investigate the drying behavior ofsliced mushrooms under forced convection of hot air. Also, the suitability ofvarious models in the simulation of the drying behavior of mushrooms wasdetermined by statistical analysis.

MATERIALS AND METHODS

The experimental setup used in the drying experiments consists of acentrifugal fan, a drying chamber, an electrical heater, an air supply channel,four drying sieves and other devices in order to measure the temperature,velocity and volumetric flow rate of drying air. The experimental setup isshown in Fig. 1.

Drying air is directed to the drying chamber through horizontal ducts bya 2-kW fan supplying 500 m3/h. A sliding gate is used at the inlet of the fan tocontrol the flow rate of air. The drying air is heated by three 1-kW and three1.5-kW Cr-Ni electrical heaters fixed at the outlet of the fan. A thermostatattached to one of the 1-kW heaters is used to control the temperature of the air.The other heaters are controlled manually by switches at the control panel.Steel pipes 20 cm long and 6 mm in diameter are installed after the heaters tomake the airflow uniform along the ducts. The flow meter is at a point that is2 m from the flow regulator and 1.5 m from the drying chamber. The air ductis connected to a 2 m ¥ 1.5 m ¥ 1.5 m drying chamber. The humidity and thetemperature at the drying chamber are digitally monitored at the control boardthrough the signals of a humidity sensor and a thermocouple, which are placedin the chamber. Four measuring pipes are connected to the drying chamber.The measuring pipes are 10 cm in diameter and 1 m in length to make the airflow coming from the chamber uniform. They are insulated to prevent heat

30 S. ÇELEN ET AL.



loss. Drying sieves made of PVC pipes are used for drying the mushroomsamples. Meshes are used under the sieves to allow passage of the drying air.The meshes are tacked to the PVC pipe by a ring. The rim of the ring extendsabout 10 mm. The ring is also fitted to the measuring pipe, and the extensionis used to attach the sieves to the measuring pipe. The attachment is alsowrapped around by an elastic material to prevent any leakage. At the controlboard, there is one temperature converter and indicator, one relative humidityconverter and indicator, and four switches that control the heaters. The tem-perature converter and indicator help to observe the temperature of the dryingair during the experiment by a Ni-Cr thermocouple placed in the dyingchamber. It is possible to adjust the temperature to a certain value at the controlboard using the thermostatic switch. The thermostat keeps the temperaturewithin a narrow range of the adjusted value during the experiment. The relativehumidity is continuously observed at Elimko 4000 relative humidity indicatorgetting input from an E–RH–101 humidity sensor placed in the dryingchamber. Air flow rate is monitored by a digital anemometer with a sensitivityof 0.1 m/s at the outlet of the sieves. The mushroom samples used in this studywere bought from the local supermarket. The mushroom samples were cutat 2, 4 and 6 mm thicknesses (Fig. 2). No pretreatment was applied to thesamples before the experimental study. Drying experiments were carried outfor drying air temperatures of 40, 45, 50 and 60C, drying air velocity of 2 m/s

differentialmanometer

insulation

flow regulator

supply

sliding gatefan

rubber pipeheater

power supply

switches

switchmain

heater power

thermostatic switch

humidity indicator

temperature indicator

control panel

thermocouple

orifice

sensorhumidity

chamberdrying

drying sieves

sieve stands

insulation

FIG. 1. A SCHEMATIC VIEW OF THE EXPERIMENTAL SETUP


and for each thickness taken into consideration. Drying tests were repeatedthree times for each experimental condition in order to minimize the uncer-tainties in the results. The progress of the drying process was followed byweighting the sieves containing mushrooms at regular intervals of time on adigital scale with an accuracy of �0.001 g. The dry weights of mushroomswere obtained by placing the samples in an oven at 110C and keeping themthere for 48 h. Using these weight values, the experimental moisture ratiosduring the drying period were determined for each experimental conditiontaken into consideration (Çelen 2004; Çelen et al. 2007).

ANALYSIS AND MODELING PROCEDURE

Ten different drying models given in Table 1 have been taken into accountfor determining the most appropriate model for the drying simulation ofmushrooms. The moisture ratio (mr) in the model equations is defined asfollows:

mrm m

m m=

−−

e

o e

(1)

where m, mo and me are the instantaneous, initial and equilibrium moisturecontents, respectively. The equilibrium moisture content of mushrooms isdefined by Pal and Chakraverty (1997) as follows:

mTe = −( )

− ×( )⎡⎣⎢

⎤⎦⎥−

ln

.

.1

2 072 10 5

11 654ϕ (2)

where j is the relative humidity of air and T (C) is the ambient temperature.The least squares method is used to simulate the drying behavior of

mushrooms by the models taken into consideration. In this method, the

FIG. 2. MUSHROOM SAMPLES USED IN THE EXPERIMENTS (2 MM, 4 MM AND 6 MM)

32 S. ÇELEN ET AL.

coefficients in the models are determined by minimizing the sum of the squareddifferences between the experimental moisture ratios and the theoretical ones.The coefficient of correlation (r) is one of the primary criteria in determining thebest equation. In addition to the correlation coefficient, the standard deviation(es) and mean squared deviation (c2) are used to determine the suitability of thefit. These parameters are defined as follows (Cihan et al. 2007):

r

n mr mr mr mr

n mr

i ii

n

ii

n

ii

n

=−

= = =∑ ∑ ∑o pre pre

o pre

o o o

, exp, , exp,1 1 1

,, , exp, exp,ii

n

ii

n

ii

n

mr n mr mr( ) − ⎛⎝⎜

⎞⎠⎟

( ) −= = =∑ ∑ ∑2

1 1

22

1

o o o

pre o iii

n

=∑⎛

⎝⎜⎞⎠⎟1

2o

(3)

e

mr mr

n

i ii

n

s

pre

o

o

=−( )

=∑ , exp,

2

1 (4)

χ2

2

1=−( )

−=∑ mr mr

n n

i ii

n

pre

o c

o

, exp,

(5)

where mrpre,i is the ith predicted moisture ratio, mrexp,i is the ith experimentalmoisture ratio, no is the number of observations and nc is the number ofcoefficients in the drying model.

RESULTS AND DISCUSSION

Curve fitting computations was carried on the 10 drying models relatingthe drying time and moisture ratio. The results are given in Table 2. Theacceptability of the drying model is based on a value for the correlationcoefficient r, which should be close to 1, and low values for the standard errores and the mean squared deviation c2. The results show that the most appro-priate model in describing drying curves of cultured mushrooms is the diffu-sion approach model, with r in the range of 0.9981 to 1.0000, and with es inthe range of -0.7018 ¥ 10-2 to 0.2072 ¥ 10-1, and with c2 in the range of0.1188 ¥ 10-4 to 0.5902 ¥ 10-3. The diffusion approach model yields a betterfit compared with the other models because of the form of the model equationand because of the higher number of parameter it has. Although the diffusion


TAB

LE

2.ST

AT

IST

ICA

LA

NA

LYSI

SO

FT

HE

MO

DE

LS

FOR

DR

YIN

GT

EM

PER

AT

UR

ES

Mod

elw

Coe

ffici

ents

T=

40C

T=

45C

T=

50C

T=

60C

New

ton

(r=

0.98

90–0

.999

9)2

k=

1.34

9k

=1.

624

k=

1.87

3k

=3.

353

4k

=0.

606

k=

0.99

9k

=1.

120

k=

1.39

96

k=

0.48

5k

=0.

844

k=

0.91

4k

=1.

055

Page

(r=

0.99

63–0

.999

9)2

k=

1.30

4;n

=0.

798

k=

1.48

7;n

=0.

716

k=

1.70

4;n

=0.

806

k=

2.96

4;n

=0.

859

4k

=0.

698;

n=

0.73

6k

=1.

019;

n=

0.83

1k

=1.

118;

n=

0.88

3k

=1.

392;

n=

0.96

86

k=

0.60

8;n

=0.

709

k=

0.85

5;n

=0.

952

k=

0.92

6;n

=0.

925

k=

1.06

5;n

=0.

861

Hen

ders

onan

dPa

bis

(r=

0.98

87–0

.999

5)2

k=

1.26

6;a

=0.

949

k=

1.52

3;a

=0.

946

k=

1.82

1;a

=0.

975

k=

3.33

2;a

=0.

992

4k

=0.

517;

a=

0.90

1k

=0.

927;

a=

0.94

3k

=1.

070;

a=

0.96

3k

=1.

385;

a=

0.99

06

k=

0.39

9;a

=0.

881

k=

0.82

4;a

=0.

979

k=

0.88

1;a

=0.

970

k=

1.00

8;a

=0.

962

Geo

met

ric

(r=

0.70

45–0

.931

5)2

n=

0.12

7;a

=0.

177

n=

0.12

0;a

=0.

192

n=

0.12

2;a

=0.

188

n=

0.17

2;a

=0.

093

4n

=0.

086;

a=

0.32

1n

=0.

103;

a=

0.24

9n

=0.

094;

a=

0281

n=

0.10

8;a

=0.

229

6n

=0.

082;

a=

0.34

3n

=0.

094;

a=

0.28

2n

=0.

089

a=

0.30

3n

=0.

100;

a=

0.25

7

Wan

gan

dSi

ngh

(r=

0.87

49–0

.998

9)2

a=

-0.5

73;

b=

0.07

2a

=-0

.784

;b

=0.

138

a=

-0.9

39;

b=

0.19

9a

=-1

.090

;b

=0.

247

4a

=-0

.355

;b

=0.

030

a=

-0.5

88;

b=

0.08

2a

=-0

.754

;b

=0.

140

a=

-0.8

62;

b=

0.17

36

a=

-0.2

99;

b=

0.02

2a

=-0

.551

;b

=0.

074

a=

-0.6

29;

b=

0.09

9a

=-0

.652

;b

=0.

101

Two-

term

expo

nent

ial

(r=

0.99

47–0

.999

9)2

k=

5.69

91;

a=

0.19

0k

=5.

4946

;a

=0.

230

k=

5.06

8;a

=0.

277

k=

14.0

89;

a=

0.19

84

k=

2.90

2;a

=0.

168

k=

5.56

3;a

=0.

150

k=

8.42

9;a

=0.

115

k=

1.48

4;a

=0.

775

6k

=2.

379;

a=

0.16

4k

=16

.232

;a

=0.

049

k=

11.9

86;

a=

0.07

0k

=4.

789;

a=

0.17

9

34 S. ÇELEN ET AL.

Log

arith

mic

(r=

0.98

63–0

.999

5)2

k=

1.34

2;a 0

=0.

016;

a=

0.94

1k

=1.

745;

a 0=

0.03

9;a

=0.

924

k=

1.99

8;a 0

=0.

029;

a=

0.95

5k

=3.

470;

a 0=

0.01

2;a

=0.

982

4k

=0.

567;

a 0=

0.02

6;a

=0.

885

k=

0.95

5;a 0

=0.

008;

a=

0.93

8k

=1.

051;

a 0=

0.00

6;a

=0.

967

k=

1.37

1;a 0

=-0

.003

;a

=0.

992

6k

=0.

446;

a 0=

0.03

1;a

=0.

862

k=

0.79

8;a 0

=0.

010;

a=

0.98

6k

=0.

829;

a 0=

0.02

1;a

=0.

984

k=

1.04

2;a 0

=0.

010;

a=

0.95

5

Dif

fusi

onap

proa

ch(r

=0.

9981

–1.0

000)

2k

=1.

027;

a=

0.79

2;b

=10

.091

k=

1.00

2;a

=0.

631;

b=

6.02

6k

=1.

134;

a=

0.52

9;b

=3.

339

k=

2.81

8;a

=0.

818;

b=

49.3

994

k=

0.43

0;a

=0.

783;

b=

14.8

82k

=0.

816;

a=

0.84

8;b

=16

0.22

7k

=0.

969;

a=

0.88

5;b

=14

5.43

6k

=1.

339;

a=

0.95

8;b

=10

2.24

86

k=

0.33

5;a

=0.

772;

b=

17.9

09k

=0.

795;

a=

0.95

1;b

=17

3.10

9k

=0.

831;

a=

0.92

4;b

=16

7.59

0k

=0.

860;

a=

0.82

1;b

=5.

498

Mid

illi

(r=

1.00

00)

2k

=0.

371;

n=

0.89

0k

=0.

371;

n=

0.89

0k

=0.

371;

n=

0.89

0k

=0.

371;

n=

0.89

0a

=0.

998;

b=

0.02

6a

=0.

998;

b=

0.02

6a

=0.

998;

b=

0.02

6a

=0.

998;

b=

0.02

64

k=

0.33

7;n

=0.

899

k=

0.33

7;n

=0.

899

k=

0.33

7;n

=0.

899

k=

0.33

7;n

=0.

899

a=

0.99

9;b

=0.

028

a=

0.99

9;b

=0.

028

a=

0.99

9;b

=0.

028

a=

0.99

9;b

=0.

028

6k

=0.

299;

n=

0.90

4k

=0.

299;

n=

0.90

4k

=0.

299;

n=

0.90

4k

=0.

299;

n=

0.90

4a

=0.

999;

b=

0.02

7a

=0.

999;

b=

0.02

7a

=0.

999;

b=

0.02

7a

=0.

999;

b=

0.02

7

Two-

term

(r=

0.99

98–0

.999

9)2

k 1=

0.06

8;k 2

=0.

599

k 1=

0.06

8;k 2

=0.

599

k 1=

0.06

8;k 2

=0.

599

k 1=

0.06

8;k 2

=0.

599

a 1=

0.44

4;a 2

=0.

548

a 1=

0.44

4;a 2

=0.

548

a 1=

0.44

4;a 2

=0.

548

a 1=

0.44

4;a 2

=0.

548

4k 1

=0.

048;

k 2=

0.53

2k 1

=0.

048;

k 2=

0.53

2k 1

=0.

048;

k 2=

0.53

2k 1

=0.

048;

k 2=

0.53

2a 1

=0.

435;

a 2=

0.55

8a 1

=0.

435;

a 2=

0.55

8a 1

=0.

435;

a 2=

0.55

8a 1

=0.

435;

a 2=

0.55

86

k 1=

0.04

8;k 2

=0.

489

k 1=

0.04

8;k 2

=0.

489

k 1=

0.04

8;k 2

=0.

489

k 1=

0.04

8;k 2

=0.

489

a 1=

0.47

0;a 2

=0.

523

a 1=

0.47

0;a 2

=0.

523

a 1=

0.47

0;a 2

=0.

523

a 1=

0.47

0;a 2

=0.

523


approach with three parameters and the two-term exponential model with fourparameters yielded the same suitability, the diffusion approach model wasaccepted as the best fit. The reason behind this selection can be given asfollows. The two-term model is a model proposed for the materials having twocompartments. The moisture transfer in each compartment is expressed by oneof the two exponential terms. On the other hand, there is no extra compartmentin the mushroom, and therefore, there is no need for using an extra exponentialterm in the simulation. This case can be seen by examining the results oftwo-term models. As it can be observed, the coefficients a1 and a2 are notindependent from each other. Therefore, the model with a lower number ofterms was selected as the best fit. Furthermore, the results show that the Midillimodel gives better results than the diffusion approach for some drying condi-tions. However, the coefficient number in this equation is again much morethan the diffusion approach model. Therefore, this model was not consideredas the best fit, but the diffusion approach model with a lower number ofparameters. Among the models considered here, the geometric model yieldedthe worst fit.

The drying curves based on the diffusion approach model are presentedalong with the experimental moisture ratios in Figs. 3–5. As it can be seenfrom the figures, moisture removal is very fast at the beginning of the dryingprocess, and the drying rate slows down as the drying proceeds. Approxi-mately 50% of the moisture is dehydrated in 1.5 h even in the drying tempera-

0 1 2 3 4

0.0

0.2

T=40CT=45CT=50CT=60C

0.4

0.6

0.8

1.0

t (h)

mr

FIG. 3. DRYING CURVES FOR w = 2 MM AND VARIOUS VALUESOF DRYING TEMPERATURE

36 S. ÇELEN ET AL.

ture of 40C and the mushroom thickness of 6 mm, which is the slowest dryingprocess taken into consideration. In the initial stages of drying, there aresignificant differences in the moisture concentration between mushroomsurface and the drying air, and this difference begins to decrease as the drying

0 1 2 3 4 5 6 7

0.0

0.2

0.4

0.6

0.8

1.0

t (h)

mr



0 1 2 3 4 5 6 7 8

0.0

0.2

0.4

0.6

0.8

1.0

t (h)

mr




goes on. Moisture should be transferred from the inner surface to the outersurface to continue the drying. In other words, moisture transfer becomesprogressively more dependent to the internal moisture transfer. Thus, thedrying rate slows down. As it can also be observed from the figures, thetemperature of drying air has a major effect on the drying rate. As the dryingtemperature increases, a considerable increase occurs in the drying rate as aresult of a decrease in the relative humidity of drying air. Increasing thetemperature from 40 to 60C decreases the time required to reach the equilib-rium moisture content almost 62% for slice thickness of 2 mm, almost 45% for4 mm and almost 40% for 6 mm. As it can also be observed from the results,the effect of the drying temperature on drying rate decreases as the slicethickness increases. Increasing the drying temperature above 45C does notproduce noticeable effect on the drying rate for higher values of the slicethickness. As the mushroom thickness increases, the moisture transferbecomes much more internally controlled and decreases the effects of the airconditions in drying. The experimental study also shows that for the highvalues of drying temperature, a little blackness occurs on the surface of themushroom. This is an important factor affecting the quality of the product, andin order to avoid the blackness, it was concluded from the results that thedrying temperature should be kept under 60C.

To confirm the suitability of the diffusion approach, the predicted mois-ture contents were also compared with the observed values from all the tests.The performance of the diffusion approach model for different drying condi-tions is shown in Figs. 6–8. The predicted data are banded around the straightline, which shows the suitability of the model in describing the drying behaviorof cultured mushrooms.

CONCLUSIONS

In this paper, the drying behavior of cultured mushrooms was experimen-tally studied through the forced convection of hot air, and the suitability ofseveral drying models in defining the drying behavior was determined bystatistical analysis. The results of the experimental study show that with theincrease in the drying temperature, the drying rate increases significantly.However, with the increase in slice thickness, the effect of the temperature onthe drying rate begins to weaken. The results also show that a certain amountof blackness occurs on the surface of mushrooms for high drying temperatures.From the results of the statistical analysis, it may be concluded that the mostsuitable model in defining the drying behavior of cultured mushrooms is thediffusion approach model.

38 S. ÇELEN ET AL.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0mrexp

mr p

re


FIG. 6. EXPERIMENTAL AND PREDICTED MOISTURE RATIOS FOR w = 2 MM

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0

mrexp

mr p

re




NOMENCLATURE

a drying constantb drying constantes standard errork drying coefficient (h-1)m moisture content (g water/g dry matter)mr dimensionless moisture rationo number of observationsnc number of constants in the drying modeln exponentr correlation coefficientT temperature (C)t time (h)w slice thickness (mm)

Greek Letters

c2 mean squared deviationj relative humidity of air

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0m

r pre

mrexp



40 S. ÇELEN ET AL.

Subscripts

d drye equilibrium conditiono initial conditionexp experimentalpre predicted

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Drying Behavior of Cultured Mushrooms