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Convective Drying of Ganoderma tsugae Murrill and Effect of Temperature onBasidiosporesSiew Kian Chin a; Chung Lim Law a; Christina Vimala Supramaniam b; Poh Guat Cheng c; Arun S. Mujumdar d

a School of Chemical and Environmental Engineering, The University of Nottingham, Malaysia Campus,Semenyih, Selangor Darul Ehsan, Malaysia b School of Bioscience, Faculty of Science, The University ofNottingham, Malaysia Campus, Semenyih, Selangor Darul Ehsan, Malaysia c Vita Agrotech Sdn. Bhd., JalanLaut Membiru, Tanjong Sepat, Selangor Darul Ehsan, Malaysia d Department of Mechanical Engineering,National University of Singapore, Singapore

Online Publication Date: 01 December 2008

To cite this Article Chin, Siew Kian, Law, Chung Lim, Supramaniam, Christina Vimala, Cheng, Poh Guat and Mujumdar, ArunS.(2008)'Convective Drying of Ganoderma tsugae Murrill and Effect of Temperature on Basidiospores',Drying Technology,26:12,1524— 1533

To link to this Article: DOI: 10.1080/07373930802464018

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Convective Drying of Ganoderma tsugae Murrilland Effect of Temperature on Basidiospores

Siew Kian Chin,1 Chung Lim Law,1 Christina Vimala Supramaniam,2

Poh Guat Cheng,3 and Arun S. Mujumdar4

1School of Chemical and Environmental Engineering, The University of Nottingham, MalaysiaCampus, Semenyih, Selangor Darul Ehsan, Malaysia2School of Bioscience, Faculty of Science, The University of Nottingham, Malaysia Campus,Semenyih, Selangor Darul Ehsan, Malaysia3Vita Agrotech Sdn. Bhd., Jalan Laut Membiru, Tanjong Sepat, Selangor Darul Ehsan, Malaysia4Department of Mechanical Engineering, National University of Singapore, Singapore

In this study, drying of Ganoderma tsugae Murrill slices wascarried out at different temperatures in a hot air circulating oven.The objectives were to investigate the drying characteristic anddrying kinetics of the Ganoderma slices under constant air velocityand slice thickness. In addition, the effect of drying temperatures onthe size and surface structure of Ganoderma tsugae basidiosporeswere studied. Using statistical analysis, a two-term model wasfound to fit best with the experimental data. Convective drying ofGanoderma tsugae slices at 60�C shows the minimum heat defor-mation to the size and surface structure of basidiospores.

Keywords Activation energy; Diffusion coefficient; Ganodermatsugae; Thin-layer dehydration Modeling

INTRODUCTION

Ganoderma tsugae Murrill, commonly known as Ling-Chih or Reishi in Japan, is a famous mushroom used astraditional Chinese medicine in China. It has a varnisheddark red fruit body with a circular and kidney-shapedcap. Ganoderma has appeared in the form of complimen-tary and alternative medicine since 3000 years ago.[1] Ithad been crowned as the Miraculous King of Herbs dueto its valuable pharmaceutical constituents such as poly-saccharides, triterpenoids, nucleosides, proteins, organicgermanium, and other micro elements.[2,3] These werefound to be medically active in several therapeutic effects,including anticancer, antitumor, antiinflammatory, anti-viral, and other physiological benefits such as blood pres-sure regulation and kidney tonic.[3] In addition to itstherapeutic effects, the methanolic extracts from both

Ganoderma lucidum and Ganoderma tsugae were foundto be high in antioxidant abilities.[4]

Ganoderma is usually dried to prolong its shelf-lifewithout using chemical preservative. In most of theGanoderma processing industry, drying was carried outat low temperature until the product contains moistureless than 4% before the extraction of medicinal valuefrom the fruiting body and sprays dried into granuleform.[5] However, choosing the right drying conditioncan be the key for a successful operation since mush-rooms are very sensitive to temperature.[6] According toGiri and Prasad[6] solar drying of mushrooms yield anunhygienic and poor-quality product. This may be dueto the unpredictable weather and the disturbance ofinsects on the samples during the drying process. Thus,the most common drying method used in the Ganodermaindustry is a continuous hot air circulating oven as thismethod is economically feasible, effective, and simple.However, the publications on the drying characteristicsof Ganoderma are rather scarce.

It has been found that Ganoderma basidiospore containsmost of the pharmaceutical components,[7] and the efficacyof the basidiospores was found to be 75 times greater thanthat of the Ganoderma fruiting body. Thus, it is importantto optimize the hot air temperature of the drying process inthe current work since heat treatment is found to affect thenormal surface structure of the basidiospores (the core wallis built from b-1,3-glycochitin).[8,9] In addition, the dryingprocess can affect the viability and the effective pharma-ceutical constituents in the basidiospores and fruiting bodyof Ganoderma, as mentioned by Cotter and George,[9] whoworked on the germination and mitochondrial damage inspores of Dictyostelium discoideum, and by Cui et al.[10]

on dehydration of concentrated Ganoderma luciduum

Correspondence: Siew Kian Chin, School of Chemical andEnvironmental Engineering, The University of Nottingham,Malaysia Campus, 43500, Semenyih, Selangor Darul Ehsan,Malaysia; E-mail: Siew-Kian.Chin@nottingham.edu.my

Drying Technology, 26: 1524–1533, 2008

Copyright # 2008 Taylor & Francis Group, LLC

ISSN: 0737-3937 print/1532-2300 online

DOI: 10.1080/07373930802464018

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extraction using different drying methods and conditions.Therefore, the objectives of this study were to study thedrying characteristics, which include the drying kineticsof Ganoderma tsugae slices at three different temperatures(50, 60, and 70�C) with constant air velocity and slice thick-ness in a hot air circulating oven. In addition, this studywas to investigate the effect of drying temperatures onthe morphology of basidiospores.

METHODOLOGIES

Materials and Methods

The matured fruiting body of Ganoderma tsugae Murrillwas supplied by Ganofarm Sdn. Bhd., Tanjung Sepat,Selangor Darul Ehsan, Malaysia. The production ofGanoderma tsugae Murrill involves a minimum of 60 days’cultivation in plastic bags for fruiting bodies. As the fruit-ing bodies are easily dehydrated by ambient air, they areutilized immediately for experimental purposes onceexcised from the plastic bags.

Convective Hot Air Drying

The fresh fruiting body of Ganoderma tsugae was thor-oughly washed to remove dirt. Slices of desired thickness(0.5� 0.1 cm) were obtained by cutting the Ganodermavertically with a vegetable slicer to obtain half of toris-pherical shape (height¼ 2.3� 0.1 cm, length¼ 5.3� 0.1 cm).The initial weight of each slice was measured with anAdventure OHAUS electronic balance (model AR3130,Pine Brook, NJ, USA, range 0–310 g with accuracy of�0.001 g). A total of six Ganoderma slices with averageweight of 3.00�0.01 g were immediately dried in a warm-wall mechanical convection oven (Memmert, Model no.KH-52200–06, Germany, range 30–250�C with accuracyof �0.5�C) at air temperatures of 50, 60, and 70�C, con-tinuously. The relative humidity in the oven was measuredusing a humidity probe (Hygromax, Switzerland, rangefrom 0 to 100%), while the air velocity was measuredusing an anemometer (Hygrolog, Switzerland, with accu-racy �0.01 m s�1). The incoming air was warmed in a pre-heated chamber and entered the chamber throughventilation slots in the chamber side wall; the fan at theback of the chamber wall produced greater air throughputand a more intensive horizontal forced circulation of air todry the slices with natural convection. The average weightof dried samples was recorded at 15-min intervals untilequilibrium moisture content (EMC) was achieved. Thedry weight of the samples was determined in the oven at105�C overnight.[11] The drying process was carried outin six replicates for each drying temperature. Averageinitial moisture content of the slices was 203.66% (d.b.).Hot air velocity was maintained at 1.401 m s�1 and surfacearea of each slice was 2.261� 10�3 m2. Average relative

humidities were 12.50, 6.73, and 4.83% for drying tem-peratures of 50, 60, and 70�C, respectively.

Mathematical Models for Thin-Layer Dehydrationof Ganoderma tsugae Murrill

Mathematical models are useful tools for the predictionof drying kinetics of heat-sensitive bio-origin materials.Drying kinetics are often used to describe the combinedmacroscopic and microscopic mechanism of heat and masstransfer during drying and are affected by drying con-ditions such as type of dryer and characteristics of dryingmaterials. It is sufficient to use single-layer semi-empiricalexpressions, which can adequately describe the dryingkinetics when the external resistance to heat and masstransfer is eliminated or minimized.[12] Thin-layer dryingmodels such as Newton model,[13] Page model,[14] Hendersonand Pabis model,[15] logarithmic model,[16] two-termmodel,[17] Wang and Singh model,[18] and Midilli model[12]

were used to fit the experimental data obtained from dryingof Ganoderma. The models coefficients (e.g., k, a, and n) weredetermined using nonlinear regression analysis while thevalidity of the thin-layer drying models was evaluated basedon statistical analysis.

The parameters used to evaluate the goodness of fit ofthe models to the experimental data are the root meansquare error (RMSE), coefficient of correlation (r), chi-square (v2),[19] and mean bias error (MBE).[20]

Effective Diffusivity and Activation Energy

From Fick’s second law of diffusion in one dimension,the change in moisture concentration can be represented by:

dM

dt¼ D

d2M

dx2ð1Þ

The analytical solution of the diffusion model for infiniteextent slab geometry was derived by Crank.[21] By assuminguniform initial moisture distribution, constant moisture dif-fusivity, negligible shrinkage effect (radius of the slices aremuch greater than the thickness), and negligible heat trans-fer effect on moisture loss (isothermal condition in the inter-nal and surface of samples), Eq. (1) is solved and yields:

MR ¼ M �Meq

Mo �Meq

¼ 8

p2

X1n¼0

1

ð2nþ 1Þ2exp �ð2nþ 1Þ2p2Deff t

L2

!ð2Þ

where Deff is the effective moisture diffusivity (m2 min�1); Lis the thickness of the slices (m); n is a positive interger; MRis the moisture ratio; and M, Meq, and Mo are the average,equilibrium, and initial moisture contents, respectively.Equation (2) can be used to predict the moisture ratio ofthe slices throughout the drying process.[22] It is simplified

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by taking the first term of the series solution for long dryingperiods or Defft=L2 greater than 0.2:

MR ¼ 8

p2exp � p2Deff t

L2

� �ð3Þ

By applying a natural logarithm at both sides of Eq. (3), theeffective diffusivity can be calculated from the slopeobtained from ln MR versus time (t) plot:

lnMR ¼ ln8

p2� p2Deff t

L2

� �ð4Þ

slope ¼ p2Deff

L2ð5Þ

The effective diffusivity obtained from Eq. (5) is theinitial guess value of Deff in Fourier series as the exact valueof Deff was calculated by trial and error, taking into accountthe first six terms in Eq. (2).[22] The RMSE and R2 betweenthe diffusion model and experimental data were determinedfor every trial of Deff until RMSE converged to thelowest value and highest for R2. The effective diffusivitydetermination process is simplified in Fig. 1.

The effective diffusivity is affected by the drying con-dition such as temperature, which in turn can be describedby an Arrhenius-type equation given in Eq. (6).[23]

Deff ¼ Do exp � Ea

RðT þ 273:15Þ

� �ð6Þ

where Do is the preexponential factor of the Arrheniusequation or diffusivity constant (m2 min�1), Ea is theactivation energy for the moisture diffusion (kJ mol�1), Ris the ideal gas constant (kJ mol�1 K�1), and T is the

absolute drying temperature (�C). The activation energywas determined by plotting ln Deff versus the reciprocal ofthe temperature [1=(Tþ 273.15)] according to Eq. (6).

Spores Morphology

Basidiospores of Ganoderma tsugae Murrill were col-lected directly from the filter paper (Whatman 40, diameter¼ 125 mm, pore size¼ 8mm), which contained spore prints ofthe fruit bodies of the plants cultured in plastic bags. The fil-ter paper was placed underneath the fruiting body for 1–2weeks. The fresh spores were dried on a microscope slideglass with cover slip in a convective oven at drying conditionsstated above. The drying process was continued until EMCwas achieved. Two replicates were used for each drying tem-perature. The detailed surface structure and size of the freshand dried spores due to hot air drying were observed usingenvironmental scanning electron microscope (ESEM; PhilipsXL30, Eindhoven, Holland). Images were taken for freshspores and spores dried at different temperatures.

Environmental Scanning Electron Microscope (ESEM)

ESEM experiments were carried out on basidiosporescollected from fresh spore prints on a filter paper and driedat 50, 60, and 70�C. Aluminum foil sheets (2 cm� 2 cm)was coated with fresh albumin and dried in a 60�C ovenfor 2 h. Another drop of albumin was pipetted onto thedried aluminum foil sheets, which were then placed facingdown on the filter paper that contained the fresh basidio-spores print. They were then placed in a Petri dish andexposed to 1% osmium tetroxide vapor for 24 h. Thealuminum foil sheets were then dehydrated through agraded ethanol series (30, 40, 50, and 70% (v=v) for 5 minmin each, and then 80, 90, and 95% (v=v) for 10 min eachand finally 100% with four changes of ethanol at 15-minintervals). The specimens were then transferred into speci-men baskets and put into critical point dryer (Baltec CPD)for 30 min. Consequently, the specimens were attachedonto the stub using double-sided tape and gold coated insputter coater (Baltec SCD 30, Balzers, Switzerland). Thespecimens were viewed using ESEM 10,000� magnifi-cation and an accelerating voltage of 20 kV. The proce-dures were repeated for dried basidiospores.

RESULTS AND DISCUSSION

Drying Characteristics and Modeling for Drying Kineticsof Ganoderma Slices

The drying curves for thin-layer drying of Ganodermatsugae slices under different hot air drying temperaturesare shown in Fig. 2. At 70�C drying temperature, the timerequired to achieve equilibrium moisture content (EMC) at4.23% (d.b.) was about 140 min, which is the fastest com-pared to 50 and 60�C. Ganoderma slices dried at 50�Crequired 340 min to achieve EMC at 7.25% (d.b.) while aFIG. 1. Flow diagram of effective diffusivity determination.

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total of 235 min was needed to dry the slices to EMC at4.84% (d.b.) for drying temperature of 60�C. Thus, for con-stant air velocity of 1.401 m s�1, increasing the temperaturefrom 50 to 60�C decreases the total drying time to about32%. Further increase of drying temperature to 70�Cdecreased the total drying time to about 40 and 60% com-pared to those at drying temperatures of 60 and 50�C,respectively. Higher drying temperature increases the heattransfer, which in turn intensifies the drying rate and leadsto a shorter drying time. Results from Table 1 show thatthe equilibrium moisture content decreases with increasingdrying temperature at constant air velocity. The lowestEMC is normally obtained when using the highest tem-perature among the set of conditions, which is in agreementwith the results of drying characteristic of Ganoderma slicesin this study and apple slices reported by Kaya et al.[24]

Higher drying temperature enhances the state of excitationof molecules, increasing the molecules distance apart, anddecreases the attraction force between the water molecules,which consequently decreases the degree of water sorptionat a given relative humidity.[24] Besides, lower relative

humidity at higher temperature such as 70�C (4.833%)compared to 50�C (12.5%) and 60�C (6.733%) stimulatesthe moisture transfer from the inner part of material andcontributes to lower equilibrium moisture content. Theequilibrium moisture content of Ganoderma slices rangesfrom 4.230 to 7.251% (d.b) with total drying timevarying between 140 to 340 min for 50, 60, and 70�C hotair temperatures.

The drying rates of Ganoderma slices at different dryingtemperatures are shown in Fig. 3. It is apparent that thedrying rate increased at higher drying temperature in theinitial stage. The average drying rate for drying tempera-ture of 50�C is 3.141 g H2O m�2 min�1, while the averagedrying rate for 60 and 70�C are 3.786 and 4.650 gH2O m�2 min�1, respectively. The drying curves reveal thata constant drying rate period did not exist in all three dry-ing temperatures tested in this study. Thus, the drying ofGanoderma slices occurred in the falling rate drying period.During the falling rate period, the moisture transfer wasgoverned by diffusion. Similar characteristics of mushroomdehydration using hot air drying method were reported byGiri et al.[6]

Statistical analysis was conducted based on the rootmean square error (RMSE), coefficient of correlation (r),chi-square (v2), and mean bias error (MBE), to evaluatethe goodness of fit between the predicted and experimentaldata. It can be concluded that a two-term model couldadequately describe the thin-layer hot air drying kineticsof Ganoderma tsugae slices. The two-term model gives thehighest correlation coefficient (r) and lowest root meansquare error (RMSE), chi-square (v2), and mean biaserror (MBE) among the models tested in this study. Ther and RMSE values of the resulting model varybetween 0.9995–0.9997 and 0.0067–0.0093, while v2 andMBE values ranged from 5.858� 10�5–1.119� 10�4 and4.862� 10�3–6.846� 10�3, respectively. The drying

FIG. 2. Variation of moisture ratio with drying time at different drying

temperatures.

TABLE 1Final moisture content and total drying time in accordance

with the drying conditions

Drying temperatures

Parameters 50�C 60�C 70�C

Relative humidity(% RH)

12.500 6.733 4.833

Initial moisture content(% d.b.)

208.807 207.473 194.685

Final moisture content(% d.b.)

7.251 4.840 4.230

Drying time (min) 340 235 140

FIG. 3. Variation of drying rate with average free moisture content at

different drying temperatures.

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constant (k) increases with increasing of drying tempera-ture, ranges from 0.0389 to 0.0677 min�1. This is attributedto higher drying temperature, which increases the drivingforce of heat and mass transfer. Figure 4 shows the mois-ture ratio versus drying time for both predicted and experi-mental data at different drying temperatures. The two-termmodel equations for drying of Ganoderma slices at differentdrying temperatures are expressed as below:

50�C: MR ¼ �2:849 exp ð�0:0389tÞþ 3:8438 exp ð�0:0320tÞ ð7Þ

60�C: MR ¼ �4:895 exp ð�0:0494tÞþ 5:8916 exp ð�0:0442tÞ ð8Þ

70�C: MR ¼ �4:893 exp ð�0:0677tÞþ 5:8931 exp ð�0:0609tÞ ð9Þ

Effective Diffusivity and Activation Energy

The effective diffusivity of the moisture content from theinterior to the surface of the slices at falling rate period wascalculated from Eq. (5). Graph ln (MR) versus dryingtime (t) is plotted and illustrated in Fig. 5, and the effectivediffusivities for different drying temperature are presentedin Table 2. It is obvious that the values of effective

diffusivity increased with increasing drying temperature.This is due to the fact that higher drying temperatureand lower relative humidity promote a greater moistureconcentration gradient between the surface and interiorstructure of the slices. This in turn increases the drivingforce of mass transfer and encourages the liquid to transferfrom the interior to the surface of the sample. Drying at70�C gave the highest effective diffusivity values, which is1.5097� 10�7 m2 �min�1.

The diffusivity constant, Do, and the activation energyvalues, Ea, for the dehydration of Ganoderma slices weredetermined based on Eq. (6). Figure 6 plots ln (Deff) versus1=(Tþ 273.15). The plot is linear where diffusivity constantand activation energy can be obtained. It was found thatthe activation energy is 42.2966 kJ mol�1 while the diffusiv-ity constant is 0.4115 m2 min�1. The effective diffusivitiesobtained from ln (MR) versus drying time (t) as statedin Table 2 do not give the best goodness of fit with theexperimental data. These values, however, can be used asan initial guess for the trial-and-error procedure depictedin Fig. 1 for obtaining effective diffusivities that are bestfit with experimental data. The effective diffusivities areshown in Table 3.

The effective diffusivities of the Ganoderma sliceswere ranged from 4.322� 10�8 m2 min�1 to 8.5270�10�8 m2 min�1 over the temperature range of 50–70�C.

FIG. 4. Variation of moisture ratio with drying time from two-term

model and experimental data at different drying temperatures.

FIG. 5. ln (MR) versus drying time (t) for the determination of effective

diffusivity at drying temperature of 50�C.

TABLE 2Effective diffusivity of Ganoderma slices at different drying temperature

Effective diffusivity, Deff (m2=min)Drying temperatures (�C) Experimental data Coefficient of correlation, R2 RMSE

50 6.0286� 10�8 0.9529 0.076960 9.4735� 10�8 0.9429 0.082470 1.5097� 10�7 0.9326 0.1002

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The values are within the general range for food materials,which is in the order of 10�8 to 10�10 m2 min�1.[25] Thesevalues are slightly lower than the diffusion coefficientreported by Akanbi et al.[26] for oven drying of tomatoslices in the range 2.232� 10�7–7.362� 10�7 m2 min�1,1.38� 10�7–5.46� 10�7 m2 min�1 for hot air–dried tomatoproducts,[27] 4.656� 10�8–5.601� 10�7 m2 min�1 for con-vective air drying of thinUlayer carrots,[28] and slightlyhigher than drying of okra,[29] which is 2.562� 10�8–7.8� 10�8 m2 min�1. The above experiments were conduc-ted in the same range of hot air drying temperatures.Figure 7 showed a good agreement between the diffusionmodel and experimental data, based on the effectivediffusivities found from trial-andUerror procedures. Thecoefficient of correlation (R2) was around 0.99 for allexperiments with RMSE ranging from 0.0399 to 0.0492.

Figure 8 plots ln (Deff) versus 1=(Tþ 273.15), whereactivation energy, Ea, and diffusivity constant, Do, aredetermined from the plot. The activation energy anddiffusivity constant were 31.321 kJ mol�1 and 4.994�10�3 m2 min�1. The activation energy was lower than hotair drying of tomatoes (32.94 kJ mol�1)[23] and okra(51.26 kJ mol�1)[29] but slightly higher than convective airdrying of carrots (28.36 kJ mol�1)[28] with similar drying

temperature. The effect of temperature on effective diffu-sivity for dehydration process of Ganoderma slices can bewritten as:

Deff ¼ 0:0134 exp � 3962:0

ðT þ 273:15Þ

� �ð10Þ

Effect of Drying Temperatureson Basidiospores Morphology

The use of ESEM enables the measurement of basidio-spore size and other physical observations of basidiosporeafter the drying process at 50, 60, and 70�C.

Table 4 gives the dimension of basidiospores when freshand after drying at different temperatures. An average of20 basidiospores was measured in each drying conditionand the mean standard deviation was determined inTable 4. Figure 9 shows that there is no significant differ-ence of shape and size between spores dried at 60 and

FIG. 6. ln (Deff) versus 1=(Tþ 273.15) for the determination of diffusiv-

ity constant and activation energy.

TABLE 3Effective diffusivity of Ganoderma slices at different drying

temperatures after trial and error procedures

Effective diffusivity,Deff (m2=min)Drying

temperatures(�C)

Experimentaldata

Coefficientof correlation,

R2 RMSE

50 4.3220� 10�8 0.9899 0.049260 6.1210� 10�8 0.9911 0.042870 8.5270� 10�8 0.9908 0.0399

FIG. 7. Prediction of diffusion model for different drying temperatures

base on Deff from Table 4.

FIG. 8. ln (Deff) versus 1=(Tþ 273.15) for the determination of diffusiv-

ity constant and activation energy based on effective diffusivities from

Table 3.

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70�C as compared to fresh spores. However, the defor-mation of basidiospores dehydrated at 50�C is significant,which elongated at 9.34% (L) while the change of widthdue to heat treatment is not significant for all drying tem-peratures as compared to fresh sample. This indicates theeffect of total drying time at 50�C on basidiospores(Table 1), which is identical to the total time to achieveEMC condition for the Ganoderma slices, is significant toinduce differences on the physical appearance of basidios-pores, whereas short drying time for 60 and 70�C doesnot show any significant effect on the basidiospores sizes.Figure 10 relates the effect of drying temperatures andthe proportional drying time on the length of basidios-pores. A long drying time with heat supplied induces thelost of structural stability of basidiospore (e.g., expansiondue to swelling) and indicates the beginning of denaturingof DNA, which is clarified by common observation inbiological system.[30,31] Drying of basidiospores at 50�Cfor 340 min, which is the longest drying time, causes cellwall defects and induces excessive spore swelling, which is

FIG. 10. Effect of the drying process on the basidiospores dimension

(length and width).

FIG. 11. Effect of drying temperatures and the proportional drying time

on the length of basidiospores.FIG. 9. Fresh basidiospores of Ganoderma tsugae at (a) bar¼ 5mm and

(b) bar¼ 2mm.

TABLE 4Dimensions for fresh and dried basidiospores

Basidiospore dimensions

Drying temperatureLength

(mean, mm)Width

(mean, mm)

Fresh spore(30.6�C)

7.470� 0.355 4.749� 0.318

50�C 8.168� 0.560 4.772� 0.42160�C 7.561� 0.741 4.472� 0.57170�C 7.247� 0.550 4.375� 0.438

The values indicate mean standard deviation from 20 replica-tions of basidiospores.

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thought to result from the inability of a weakened wall toresist the uniform outwardly directed force of turgor.[30]

This causes the expansion of the basidiospores dimensionbut shrinkage may occur at the spore core (containingDNA) due to mineralization and high relative dehydrationof the spore core, resulting in DNA denaturation.[31]

Spores dried at 60�C show insignificant expansion effectdue to shorter drying time compare to 50�C, while sporesdried at 70�C indicate minimal spore shrinkage due to cellwall lesions, which are caused by a heat shock effect (hightemperature with short drying time).[32]

Figure 11 shows the image of fresh basidiospores, andFigs. 12, 13, and 14 represent basidiospores dried at differ-ent drying conditions. Figure 12 shows that the surfacestructure of basidiospores dried at 50�C is uneven as com-pared to the surface structure of fresh basidiospore(Fig. 11). Fresh basidiospore is smooth and undeformed.According to Lundblad et al.,[33] the deformation of basi-diospores surface structure (b-1,3-glyco chitin) indicatesthe formation of chitinase, which is a cell wall degradingenzyme produced due to stresses caused by environmentalfactors such as temperature and drying duration. Thisstudy also showed that maximal activity of chitinase

occurred at 51�C, which was the optimal temperature forthe degradation of glycol chitin.[33] The least deformationwas observed on the basidiospores that were dried at60�C (Fig. 13). This minimal surface deformation wasprobably due to the enzyme chitinase produced when thebasidiospores reached the optimal temperature of enzymeactivity (51�C), but it was deactivated when the tempera-ture of the basidiospores reached 60�C.[33] At drying tem-perature of 70�C, the components of cell wall tear apart,leaving a ‘‘peeled’’ effect to indicate that a lesion occurred(Fig. 14). At this temperature, the cell wall experienced chi-tinase deformation (at 51�C) and demonstrated heat shockeffect, which normally occurs at high-temperature heatingand short drying time.[30] This cell wall morphology at70�C has been assumed to result from temperature-inducedinstability of proteins that fold normally at lower tempera-tures.[34] Furthermore, the basidiopsores dried at this tem-perature may not be viable since heat agents probably killspores through destruction of molecules other than DNA,such as proteins and=or membranes.[35]

CONCLUSION

Thin-layer convective drying of Ganoderma tsugaeMurrill was carried out with different drying temperatures

FIG. 12. Spores dried at 50�C: (a) bar¼ 5 mm and (b) bar¼ 2 mm.

FIG. 13. Spores dried at 60�C: (a) bar¼ 5mm and (b) bar¼ 2mm.

CONVECTIVE DRYING OF GANODERMA TSUGAE MURRILL 1531

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at constant slice thickness and air velocity. Higher dryingtemperature allows faster drying rate, lower equilibriummoisture content, and shorter drying time. The dryingcurve from the experiment showed that the falling rate dry-ing period prevailed and the drying process was controlledby diffusion. A two-term model was found to have goodagreement with the experimental data. The values of effec-tive diffusivities for drying of Ganoderma tsugae slices werefound in the range of 4.322� 10�8 m2 min�1 to 8.527�10�8 m2 min�1, while the activation energy and diffusivityconstant were 31.3213 kJ mol�1 and 4.9943�10�3 m2 min�1, respectively. Convective drying of Gano-derma tsugae Murrill at 60�C is considered the mostsuitable drying temperature since it produced the leastdeformation to the basidiospores based on the visualobservation from the images obtained from ESEM.

NOMENCLATURE

Deff Effective diffusivity (m2 s�1)Do Diffusivity constant (m2 s�1)Ea Activation energy (kJ mol�1)k Drying constant (min�1)L Slice thickness (m)

M Average moisture content (g H2O g�1 drymaterial)

MBE Mean bias errorMeq Equilibrium moisture content (g H2O g�1 dry

material)Mo Initial moisture content (g H2O g�1 dry solid)MR Moisture ration Positive integerR Ideal gas constant (kJ mol�1 K�1)RMSE Root mean square errorR2 Coefficient of determinationr Coefficient of correlationT Absolute air temperature (�C)t Drying time (min)

Greek Symbols

v2 Chi-square

Superscripts and Subscripts

a Activationd.b. Dry basiseff Effectiveeq Equilibriumo Initial

ACKNOWLEDGEMENTS

The authors acknowledge Vita Agrotech Sdn. Bhd.,Malaysia, for the supply of fresh Ganoderma tsugaeMurrill.

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