pretreatments and drying methods on

7/28/2019 pretreatments and drying methods on

http://slidepdf.com/reader/full/pretreatments-and-drying-methods-on 1/8

Effects of pretreatments and drying methods ondehydration of mushroom

S.G. Walde a,*, V. Velu b, T. Jyothirmayi b, R.G. Math b

a CFTRI Resource Centre, Chinhat Deva Road, UPSIDC Ind. Area, Lucknow 226 019, Indiab CFTRI Resource Centre, Habshiguda, Uppal Road, Hyderabad 500 007, India

Received 8 July 2004; accepted 12 February 2005Available online 27 September 2005

Abstract

Dehydration of button mushrooms (Agaricus bisporus) and oyster mushrooms (Pleurotus flavus) were carried out with variouspretreatments like blanching, blanching followed by soaking in potassium metabisulphite (KMS), fermented whey, curds, etc.and dried in different dryers viz, hot air cabinet dryer, fluidized bed dryer, vacuum dryer and microwave oven. The drying timeswere less in the case of oyster mushrooms (7200–8100 s) compared to button mushroom (8700–10800 s) with cabinet drying. Forboth oyster and button mushrooms using pretreatment by dipping in curds or fermented whey the time of drying was less comparedto other treatments in all types of dryers. The effect of drying methods was expressed by a polynomial equation and the regressioncoefficients were determined. The time taken for drying from 7.5% (db) moisture to 2.0% (db) was in the order of vacuumdryer > cabinet moisture dryer > fluidized bed dryer > microwave oven. However, fluidized bed drying seems to be a promisingmethod for drying mushrooms, when comparing the lower drying time and good quality products to the faster microwave drying.The diffusion coefficients evaluated were also found in the same order. In case of oyster mushroom, the diffusion coefficient wasfound maximum (469.7 · 10À6 m2/s) for the whey treated microwave dried mushroom and minimum (2.609 · 10À6 m2/s) for thecontrol cabinet tray dried sample. The diffusion coefficient was maximum (331.02 · 10À6 m2/s) for the blanched button mushroomdried by microwave drying and minimum (0.3225 · 10À6 m2/s) for the control sample dried by vacuum oven.Ó 2005 Published by Elsevier Ltd.

Keywords: Mushrooms; Dehydration; Pretreatment; Drying methods; Diffusion coefficient

1. Introduction

Mushrooms are a special group of macroscopicfungi, lacking chlorophyll and hence requiring a

substrate for their own absorptive nutrition. They pro-duce enzymes, which degrade complex organic matterand absorb the soluble substances (Chang & Miles,1989). One of the promising and important sourcesof unconventional protein is the single cell protein.Mushroom cultivation is a major fermentation industry,which involves the bioconversion of cellulose waste

into edible biomass. The cultivation of mushroom hasa great potential for the production of protein richquality food and for recycling of cellulose agro-residuesand other wastes. Out of 38,000 known mushroom

varieties, the most popular are Agaricus bisporus (whitebutton mushroom), Lentinus edodes (Shitake orJapanese mushroom), Pleurotus species (Oyster mush-room), Volvarella volvacea (paddy straw mushroom),Flammulna velutipis (winter mushroom) and theAuricularia polytricha (JewÕs ear mushroom) (Singh,Sharma, Kumar, & Singh, 1999).

India is known globally for its exotic mushrooms.Traditionally, the cultivation of mushrooms was mostlyin the hilly region. Currently the adoption of scientific

0260-8774/$ - see front matter Ó 2005 Published by Elsevier Ltd.doi:10.1016/j.jfoodeng.2005.02.008

* Corresponding author. Tel./fax: +91 522 2818126.E-mail address: [email protected] (S.G. Walde).

www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 74 (2006) 108–115

mailto:[email protected]

mailto:[email protected]



approaches in the cultivation methods has led to im-proved quality and yields in mushroom (Chang, 1991;Chang & Buswell, 1996; Indian Food Industry, 1994).In recent times, there has been a tremendous increasein the production of edible mushrooms increasing from4000 ton in 1985–1986 to 30,000 ton in 1996–1997(Rama & Jacob John, 2000). Button mushroom consti-tutes 37.7% of the worldÕs mushroom production interms of production and consumption followed by oys-ter mushroom with a share of 24.1% (Singh, Kumar, &Singh, 1995).

Mushrooms are liked for their delicious flavour, lowcalorific value and high protein contents, vitamins of

B-group and minerals. Mushroom contains 20–40%proteins on a dry weight basis (Bano & Rajarathnam,1988) and no cholesterol, and is almost fat free. Mush-room proteins can serve the population in developingcountries (FAO, 1970).

Mushrooms are rapidly perishable commodities, andthey start deteriorating immediately within a day afterharvest. In view of their highly perishable nature, thefresh mushrooms have to be processed to extend theirshelf life for off-season use. Among the various methodsemployed for preservation, canning is the most fre-quently adopted method in commercial scale. Mush-room can be processed in many other ways to extendtheir shelf life such as drying, pickling, etc. It is reportedthat drying is a comparatively cheap method (Rama &Jacob John, 2000) and dried mushrooms, packed in air-tight containers can have a shelf life of above one year(Bano, Rajarathnam, & Shashi Rekha, 1992).The fac-tors that affect drying rate are temperature, thicknessof mushroom, method of drying and moisture diffusivity(Yapar, Helvaci, & Peker, 1990). Sulphitation followedby drying is one of the pretreatment methods for mush-rooms (Singh et al., 1995).

Mushrooms dehydrated by combining hot air andmicrowave treatment yielded a quality product of satis-

factory re-hydration and flavour retention (Riva,

Schiraldi, & Cesare, 1991). Yang and Maguer (1992)have studied osmotic dehydration of mushrooms.

It may be mentioned that pretreatments of mush-rooms before drying in one form or other viz, washingin water, potassium metabisulphite (KMS), sugar, salteither alone or in combination help in checking enzy-matic browning, stabilizing colour, enhancing flavourretention and maintaining textural properties (Singh,Narain, & Kumbhar, 2001). Curd and whey containinglactic acid bacteria were tried with bitter gourd, dried(Walde, Jyothirmayi, Velu, & Math, 2001) and gaveencouraging results with regards to maintain colour,

flavour and texture. Therefore, in addition to variouspretreatments like blanching, blanching followed bysoaking in potassium metabisulphite and citric acid,whey fermentation and curd fermentation using lacticacid bacteria also were tried and the treated mushroomswere dried in hot air cabinet tray dryer, fluidized beddryer, vacuum dryer and microwave oven for compara-tive study.

2. Methodology

2.1. Materials

Fresh button mushrooms (A. bisporus) with moisturecontent 15% (db) were procured from M/s TechtranAgro Industries, Medak District of Andhra pradesh.Oyster mushroom (Pleurotus flavus) having moistureof 11% (db) were procured from M/s Vaibhav AgroFarms Pvt. Ltd, Hyderabad. The button mushroomswere separated from the stems, and vertically cut intofour pieces for dehydration in order to increase the sur-face area. The oyster mushroom stems were removed,and only the leafy petal structure was taken fordehydration.

Nomenclature

c0 constant (%db)c1 constant (%db/s)c2 constant (%db/s2)

D diffusion coefficient (m

2

/s)r radius of button mushroom (m)h half thickness of oyster mushroom (m)M moisture content, kg of water per kg of dry

matter (%db)t drying time (s)t drying rate (kg of water/kg of dry matter/s)x position in the sample where the moisture

content (m)

dM change in moisture content (kg of water /kgof dry matter)

dt change in time (s)

dx change in position in the sample where themoisture content (m)

Subscripts

n experimentt at any time0 initialf equilibrium

S.G. Walde et al. / Journal of Food Engineering 74 (2006) 108–115 109



2.2. Equipments

2.3. Methods

2.3.1. Preparation of sample

The fresh oyster and button mushrooms were washedthoroughly in running water with 2% chlorine. Themushrooms were allowed to drain and divided into sixbatches for various treatments. The control sampleswere used without any pretreatment for drying, whereas5 samples were treated by soaking in fermented milk i.e.curd; blanching; blanching followed by soaking incurd; soaking in fermented whey and blanching fol-lowed by soaking in fermented whey. The blanchingtreatment was conducted with 2% salt solution at

90 °C followed by 1000 ppm potassium metabisulphitetreatment. The oyster mushrooms were blanched fora period of 3 min whereas, button mushrooms wereblanched for 4 min.

2.3.2. Sample size and drying methods

Cabinet tray drying : The known quantities of samples(Table 2) were loaded in perforated trays of cabinet traydryer by noting initial weights and subjected to drying at55 ± 2 °C. The initial temperature was maintained at55 °C for 1 h and then gradually raised to maintain at60 ± 2 °C till complete dehydration. The readings weretaken at regular intervals of 10, 15, 30 min for first, sec-ond and subsequent hour, respectively. Since the initialdrying rate is faster, the trays were circulated from topto bottom at each reading interval so as to maintain uni-form air flow on drying.

Fluidized bed drying : The known quantities of sam-ples (Table 2) were loaded in wire mesh perforated traysby noting initial weights and subjected to drying at inlettemperature of 60 ± 2 °C. The outlet temperature was43 ± 2 °C. The readings were noted at 10, 15, 30 minof interval for first, second and subsequent hour, respec-tively. The trays were also circulated through 360° so asto have uniformity in all the samples during drying.

Vacuum oven drying : The samples of 70 g each were

loaded on perforated trays by noting initial weightsand subjected to vacuum oven drying at vacuum of À0.5 · 104 kg/m2 and temperature of 50 ± 2 °C. Thereadings were noted at 10, 15, 30, and 60 min of intervalfor first, second and subsequent hour, respectively.

Microwave oven drying : The microwave oven wasmaintained at high-level (H) i.e. 700 W. The individualsamples were loaded to the microwave oven for all thetime interval readings at 10, 20, 30, 45, 60, 75, 90, 105,120, 150 and 180 s. The initial readings were noted forall individual experiment.

The button mushrooms were subjected to all treat-

ments and drying methods. However, the oyster mush-rooms were dried only in cabinet dryer and microwaveoven. The moisture content vs. time data were notedand drying curves were plotted to calculate the diffusivity.

2.3.3. Physical measurement

Diameter: The diameter measurements of the oysterand button mushrooms were done at two directionsusing vernier calipers (Mitutoyo, Japan) with an accu-racy of 0.02 mm.

Thickness: Oyster mushrooms were measured at fourplaces to an accuracy of 0.01 mm using a micrometer(Mitutoyo, Japan) to have a mean average thickness.

Weight: Mushroom samples were weighed accuratelyusing a monopan analytical balance (AEG 220, Shima-dzu, Japan) with an accuracy of ±0.1 mg.

Volume: The samples were immersed in water andsample volume was measured by the water displacementmethod with an accuracy of ±0.05 ml.

2.3.4. Analytical method

Moisture: The moisture content was determined byoven drying method (AOAC, 1990) having samples of 10 g in triplicate and average values reported as kg of water/kg of dry matter.

S. No. Equipment name and address Specification

1 Cabinet tray dryer, Chemida, Mumbai, India 137 cm (l) · 95cm (h) · 56 cm (depth)with 12 trays capacity fitted with 1/2 HPmotor for air circulating fan, 6 insulated

heating coils (1.5 kW) each2 Fluidized bed dryer M/s. Alliance Engg. Co.,

Mumbai, IndiaModel TW 30 with MS trolley. SS container(40 kg capacity) with perforated bottomand SSI Dutch weave mesh, 5 HP motorwithout air ducting

3 Vacuum oven M/s Pharmalab Industries,Mumbai, India

Chamber size 27.30 cm · 30.5 cm andtray size 23.82 cm · 27.30 cm

4 Microwave oven EssentiaTM Classic Model,Pune, India

Maximum power level of 700 W andchamber capacity of 31 l

110 S.G. Walde et al. / Journal of Food Engineering 74 (2006) 108–115



3. Results and discussion

Mushrooms were subjected to various pretreatmentsand then dried in different dryers. The data obtainedon drying were fitted by the regression analysis to poly-nomial equations (Table 1).

3.1. Effect of pretreatments on the drying characteristics

of mushrooms

The experimental data on moisture content present atany time versus the drying time were fitted to a firstorder polynomial model with a small modification asgiven by Korokida, Oreopoulou, and Maroulis (2000).

M t ¼ c0 þ c1t þ c2t 2 ð1Þ

Fig. 1 shows the drying characteristics of mushroomswith different pretreatment and dried with microwaveoven. As expected, water evaporation rate was fasterat the first 40 s. The moisture content was 9.2% (db) at20 s, which was very less in whey treated samples fol-lowed by curd, blanched + whey containing lactic acid

bacteria, control, blanched, and blanched + curd con-taining lactic acid bacteria treated samples up to 90 s.The experimental data was correlated using Eq. (1)and the regression coefficients were in the range of 0.97–0.99. From Fig. 1, it was clear that at a highermoisture content, rate of evaporation was faster as com-pared to evaporation at moisture content <2–4% (db) inmicrowave drying. After 90 s of microwave drying time,the moisture loss was constant. The values of constantsin Eq. (1) are shown in Table 1.

Table 1Drying data of oyster and button mushroom

Type of mushroom and drying Treatment Drying curve polynomial equation fit(g of H2O/g of dry matter)

Regressioncoefficients (R2)

c0 c1 c2

Button mushroom—cabinet drying Control 14.6130 À1.5167EÀ03 5.5556EÀ08 0.9995Blanched 10.8750 À1.1767EÀ03 5.5556EÀ08 0.9996Curd 9.9366 À1.1450EÀ03 5.5556EÀ08 0.9995Blanched + curd 9.9439 À1.2117EÀ03 5.5556EÀ08 0.9995Fermented whey 9.7476 À1.2550EÀ03 5.5556EÀ08 0.9980Blanched + whey 7.6493 À9.5000EÀ04 5.5556EÀ08 0.9877

Button mushroom—fluidized bed drying Control 12.1900 À2.3783EÀ03 1.9444EÀ07 0.9931Blanched 10.8750 À3.0533EÀ03 3.3333EÀ07 0.9885Curd 6.8296 À2.0017EÀ03 2.2222EÀ07 0.9863

Blanched + curd 6.8733 À1.6167EÀ03 1.6667EÀ07 0.9946Fermented whey 5.7826 À1.4933EÀ03 1.3889EÀ07 0.9944Blanched + whey 6.1711 À1.6450EÀ03 1.6667EÀ07 0.9939

Button mushroom—vacuum drying Control 12.4600 À4.4822EÀ04 6.2191EÀ09 0.9741Blanched 13.9370 À2.5464EÀ04 1.1111EÀ09 0.9967Curd 6.5832 À1.3864EÀ04 1.2500EÀ09 0.9839Blanched + curd 7.5886 À1.3011EÀ04 9.8765EÀ10 0.9902Fermented whey 6.8643 À1.0306EÀ04 7.3302EÀ10 0.9903Blanched + whey 7.9383 À1.8775EÀ04 1.6975EÀ09 0.9925

Button mushroom—microwave drying Control 14.8790 À2.1970EÀ01 9.0000EÀ04 0.9988Blanched 14.7570 À1.9820EÀ01 7.0000EÀ04 0.9822Curd 14.2730 À2.6210EÀ01 1.1000EÀ03 0.9855Blanched + curd 15.0070 À2.3030EÀ01 1.0000EÀ03 0.9868Fermented whey 15.3190 À3.5060EÀ01 2.0000EÀ03 0.9950

Blanched + whey 14.4690 À2.1150EÀ01 8.0000EÀ04 0.9741

Oyster mushroom—cabinet drying Control 11.6470 À1.9067EÀ03 1.1111EÀ07 0.9889Blanched 9.5413 À1.2450EÀ03 5.5556EÀ08 0.9989Curd 7.5810 À1.0383EÀ03 5.5556EÀ08 0.9978Blanched + curd 7.9502 À9.5167EÀ04 2.7778EÀ08 0.9979Fermented whey 7.5687 À9.4500EÀ04 2.7778EÀ08 0.9978Blanched + whey 7.5687 À9.4500EÀ04 2.7778EÀ08 0.9978

Oyster mushroom—microwave drying Control 12.3550 À2.1910EÀ01 1.4000EÀ03 0.9918Blanched 9.7737 À1.4930EÀ01 9.0000EÀ04 0.9928Curd 7.9262 À1.5260EÀ01 1.0000EÀ03 0.9990Blanched + curd 8.5539 À1.4860EÀ01 8.0000EÀ04 0.9918Fermented whey 8.8231 À1.4880EÀ01 9.0000EÀ04 0.9985Blanched + whey 8.5512 À1.6380EÀ01 1.0000EÀ03 0.9950




3.2. Drying rates

The drying rate (t) of mushrooms was evaluated bydifferentiating the polynomial equations with respectto time (Table 1).

t ¼ Àd M =dt

where M is the moisture content at any time (t) per unitweight of dry matter.

The drying rate curves are shown in Fig. 2. Most of the treatments in different drying systems have shown

typical drying rate curves (Fig. 2) with characteristicconstant rate and falling periods except in the case of vacuum drying system in which the time of drying wasvery long (>20 h). A single falling rate period with char-acteristic linearity indicated that the moisture was diffus-ing out by the capillary forces at which stage the partialpressure of moisture was less than the vapor pressure(Keey, 1972).

3.3. Diffusivity

According to FickÕs second law of diffusion in onedimension, the change in moisture concentration canbe represented by Eq. (2).

d M

dt ¼

Dd2 M

d x2 ð2Þ

The solution of diffusion Eq. (2) was available(Newman, 1931) for three regular shapes (slab, cylinderand sphere) in the form of concentration–time–locationequation, for negligible surface resistance. This can beused to predict the concentration at any instance,throughout drying of mushroom for ease of application.The average moisture concentration can be normalizedby Eq. (3);

M 0 À M t

M 0 À M f

¼Final average free moisture content

Initial uniform free moisture contentð3Þ

where M f = final moisture content.

3.4. Diffusivity of oyster mushroom

Assuming the slice is a slab of infinite extent, i.e.the radius is much greater than the thickness, andNewmanÕs solution is

M 0 À M t

M 0 À M f

¼8

p2

expÀ1p2

4

Dt

h2

þ

1

9exp

À9p2

4

Dt

h2

þ1

25exp

À25p2

4

Dt

h2

þ Á Á Á

¼8

p2

X1n¼1

1

2n þ 1exp

Àð2nþ 1Þ2p

2

4

Dt

h2

!" #

ð4Þ

Neglecting the higher order terms in the above equation,

M 0 À M t

M 0 À M f

¼8

p2

expÀp2

4

Dt

h2

ð5Þ

Which on rearranging

À lnp

2

8

M 0 À M t

M 0 À M f

¼

p2

4

Dt

h2

ð6Þ

The first guess value of D was made by Eq. (6), by trun-cating the highest turn in Eq. (4). The exact value of D

was calculated subsequently by trial and error by Eq. (4)by considering first 6 terms (Fig. 3) for oyster mushroomtreating it as slab by taking the average half thickness(1.9 · 10À3 m). The diffusion coefficient (Table 3) wasfound maximum (469.7 · 10À6 m2/s) for the whey trea-ted microwave dried mushroom and minimum(2.609 · 10À6 m2/s) for control cabinet tray dried oystermushroom sample. The overall diffusion coefficients of microwave drying for oyster mushrooms were muchhigher than that of cabinet tray drying.

Blanched+ Whey

Whey

Blanched + Curd

Curd

Blanched

Control

16

14

12

10

8

6

4

2

0

M o i s t u r e c o n t e n t ( g o f H 2

O / g o f d r y m a t t e r )

20 40 60 80 100 120 140 160 2000

Time (s)

180

Fig. 1. Drying curves of button mushroom for microwave drying.

0.35

Blanched + Whey

Whey

Blanched + curd

Curd

Control

Blanched

0.3

0.25

0.2

- d M / d t

0.15

0.1

0.05

00 2 4 6 8 10 12 14

(Moisture content (g of H2O/g of dry matter)

Fig. 2. Drying rate curves of microwave dried button mushroom.




3.5. Diffusivity of button mushroom

Assuming the button mushroom as a hemisphere of infinite extent, NewmanÕs solution can be given as

M 0 À M t

M 0 À

M f

¼ 1 À6

p2 X

1

n¼1

6

n2exp À Dn

2p

2 t

r 2 " # ð7Þ

Neglecting higher order

M 0 À M t

M 0 À M f

¼ 1 À6

p2

expÀ Dp2

t

r 2

ð8Þ

Which on rearrangement gives

lnM 0 À M t

M 0 À M f

¼

1

2þ Dp

2t

r 2ð9Þ

The diffusivities (D) for button mushroom (Table 4)were evaluated considering the hemisphere with an

Time, min

20 18040 60 80 100 120 140 160 20000

Blanched + Whey

Whey

Blanched + Curd

Curd

Blanched

Control

-0.2

L n ( M 0 - M ) / ( M

0 - M i )

-0.4

-0.6

-0.8

-1

-1.2

-1.4

Fig. 3. Diffusivity of fluidized bed dried button mushroom.

Table 2Comparative data of time taken for various drying methods and treatments for oyster and button mushroom

Type of mushroom and drying Treatment Batch of sample (g) IMC (db) FMC (db) Time of drying (s)

Button mushroom—cabinet drying Control 1022 8.0 2.0 10,800Blanched 957 8.0 2.0 10,200Curd 400 8.0 2.0 10,200Blanched + curd 400 8.0 2.0 09600Fermented whey 400 8.0 2.0 08700Blanched + whey 400 8.0 2.0 10,200

Button mushroom—fluidized bed drying Control 200 6.0 2.0 05700Blanched 209 6.0 2.0 04680Curd 156 6.0 2.0 04200

Blanched + curd 200 6.0 2.0 04200Fermented whey 152 6.0 2.0 04200Blanched + whey 213 6.0 2.0 04200

Button mushroom—vacuum drying Control 130 7.5 2.0 39,600Blanched 130 7.5 2.0 34,200Curd 130 7.5 2.0 75,600Blanched + curd 130 7.5 2.0 90,000Fermented whey 130 7.5 2.0 90,000Blanched + whey 130 7.5 2.0 90,000

Button mushroom—microwave drying Control 5–5.5 7.5 2.0 00058Blanched 5–5.5 7.5 2.0 00084Curd 5–5.5 7.5 2.0 00041Blanched + curd 5–5.5 7.5 2.0 00102Fermented whey 5–5.5 7.5 2.0 00025Blanched + whey 5–5.5 7.5 2.0 00068

Oyster mushroom—cabinet drying Control 980 7.5 2.0 07200Blanched 1003 7.5 2.0 07200Curd 990 7.5 2.0 08100Blanched + curd 1000 7.5 2.0 07500Fermented whey 965 7.5 2.0 07500Blanched + whey 1000 7.5 2.0 07500

Oyster mushroom—microwave drying Control 11–12 7.5 2.0 00084Blanched 11–12 7.5 2.0 00075Curd 11–12 7.5 2.0 00049Blanched + curd 11–12 7.5 2.0 00058Fermented whey 11–12 7.5 2.0 00056Blanched + whey 11–12 7.5 2.0 00049




average radius (26.17 · 10À3 m) by Eq. (9). During ini-tial stages of drying, rapid moisture loss was observeddue to surface moisture evaporation. As drying time in-creased, drying rate decreased. The diffusion coefficientwas maximum (331.02 · 10À6 m2/s) for the blanchedbutton mushroom dried by microwave drying and min-

imum (0.3225 · 10À6

m2

/s) in case of control sampledried by vacuum oven.

3.6. Effect of pretreatments

The effect of various pretreatments was assessed onoyster mushroom and button mushroom in cabinet traydryer. From Table 2, it was noticed that various pre-treatments did not affect the drying time to decreasemoisture from 7.5% (db) to 2.0% (db) except with awhey and blanched + curd treatment for button mush-rooms. The diffusivity is at the close proximity to eachother. However, in cabinet dryer the drying time was less

in the case of oyster mushroom (7200–8100 s) comparedto button mushroom (8700–10800 s). This may be due tothe surface open cellular structure of oyster mushroomas compared to button mushroom which was evidentfrom the apparent density data (1660 and 731.5 kg/m3,respectively). The difference also could be noted in diffu-sivity in cabinet tray dried samples from Tables 3 and 4which is almost double in oyster mushroom to that of button mushroom.

In case of fluidized bed drying of button mushrooms,the type of pretreatments did not show any effect. All thetreated samples of button mushroom took approxi-

mately 4200 s to decrease the moisture from 6.0% to2.0% (db), whereas the control required 5700 s withthe higher diffusion coefficient of 1.3317 · 10À6 m2/s

compared to other treatments. Fig. 3 shows the effectof pretreatment on diffusivity of fluidised bed dried but-ton mushroom. The microwave drying showed signifi-cant effect on blanching in all treatments with higherdrying time compared to unblanched samples (Table2). However, the trend was not consistent in case of

oyster mushroom. Pretreatment by soaking in simplefermented curds or fermented whey took less time of drying compared to other treatments in all types of dry-ers both for oyster and button mushrooms. This couldbe due to lactic acid bacteria alone in treatment by wheyand curd makes the body of mushroom opening to thepores which could induce faster dehydration in thosesamples. However, in other blanched samples, due tothe excess absorption of moisture during blanching bymushroom subsequently by treatment, dehydration isdelayed unlike other vegetables which loose moistureon blanching inducing faster dehydration.

3.7. Effect of drying methods

The typical experimental drying data on moisturecontent vs. time is presented in Fig. 1. The data werecompiled by a polynomial equation and the regressioncoefficients were tabulated (Table 1). The time requiredfor drying was in the order of vacuum dryer > cabinetdryer > fluidized dryer > microwave dryer to decreasethe moisture content from 7.5% to 2.0% (db). The dataare given in Table 2. Drying in microwave oven was notsatisfactory as there was no temperature control. Highertime of exposure was resulting in charring of mushroom

particularly at the edges. Even though it was taking veryless time for drying, microwave drying was not a suit-able method for drying of materials like mushroomswhere case hardening was a problem to be reckonedwith. Microwave drying is good for materials like gums,etc. in which case hardening takes place during conven-tional drying and thus does not allow moisture to dif-fuse/ ooze out (Walde, Balaswamy, Shivaswamy,Chakkaravarthi, & Rao, 1997).

The vacuum drying was taking very long timefor drying which is clear evidence of lower diffusioncoefficient among all drying methods studied in thepresent studies even though the product quality wasgood. In case of fluidized bed drying, the mushroomquality was good and the time of drying was also shorter.

Table 3Diffusivity of oyster mushroom with respect to different treatments anddrying systems

Treatments Cabinet traydrying (m2/s)

Microwave ovendrying (m2/s)

Control 2.609 · 10À6 272.1 · 10À6

Blanched 3.828 · 10À6 210.7 · 10À6

Curd 2.629 · 10À6 412.6 · 10À6

Blanched + curd 2.755 · 10À6 400.9 · 10À6

Fermented whey 3.097 · 10À6 469.7 · 10À6

Blanched + whey 2.707 · 10À6 428.7 · 10À6

Table 4Diffusivity of button mushroom with respect to different treatments and drying systems

Treatments Cabinet tray drying (m2/s) Fluidized bed drying (m2/s) Vacuum oven drying (m2/s) Microwave drying (m2/s)

Control 1.2175 · 10À6 1.3317 · 10À6 0.3225 · 10À6 221.44 · 10À6

Blanched 1.2175 · 10À6 1.0273 · 10À6 0.3970 · 10À6 331.02 · 10À6

Curd 1.1795 · 10À6 0.8180 · 10À6 0.3266 · 10À6 53.65 · 10À6

Blanched + curd 1.5599 · 10À6 1.1414 · 10À6 0.4204 · 10À6 47.94 · 10À6

Fermented whey 1.3888 · 10À6 0.9132 · 10À6 0.4943 · 10À6 150.67 · 10À6

Blanched + whey 1.2746 · 10À6 1.1034 · 10À6 0.3291 · 10À6 25.11 · 10À6




4. Conclusion

Dehydration of mushroom was carried out with dif-ferent treatments and various drying systems. The dry-ing rate curves showed a characteristic constant rateperiod and falling rate period except in the vacuum dry-

ing where the drying rates were very slow. Microwavedrying may not be a suitable method for drying of mushrooms where higher drying times resulted in thecharring of edges. In view of the drying time and qualityof products, the fluidized bed drying system was better.

Acknowledgments

The authors thank the Head, CFTRI Resource Cen-tre, Hyderabad and the Director, CFTRI, Mysore forpermission to publish the paper and wish to acknowl-edge the financial grant of AP-NL Project, Biotechnol-

ogy Unit, Institute of Public Enterprise, Hyderabad.

References

Anon (1994). Mushroom: A food for all. Indian Food Industry, 13(1),41–44.

AOAC (1990). Association of Official Analytical Chemists. Official

methods of analysis (15th ed.). Washington, DC.Bano, Z., & Rajarathnam, S. (1988). Pleurotus mushrooms Part II.

Chemical composition, nutritional value, post-harvest physiology,preservation and role as human foods. CRC Critical Review in Food

Science and Nutrition, 27 (2), 87–158.Bano, Z., Rajarathnam, S., & Shashi Rekha, M. N. (1992). Mushroom

as the unconventional single cell protein for a conventionalconsumption. Indian Food Packer, 46 (5), 20–31.Chang, S. T. (1991). Cultivated mushrooms. In D. K. Arora, K. G.

Mukerji, & E. H. Marth (Eds.). Handbook of applied mycology,

Vol. 3. Food and feeds. New York, USA: Marcel Dekkar.

Chang, S. T., & Buswell, J. A. (1996). Mushroom nutraceuticals.World Journal of Microbiology and Biotechnology, 12(5), 473–476.

Chang, S. T., & Miles, P. G. (1989). Part I—Mushroom science. Edible

mushroom and their cultivation. Boca-Raton, USA: CRC Press.FAO (1970). Amino acid contents of foods and biological data on

proteins. Nutritional Studies No. 24.Keey, R. B. (1972). Drying principles and practice (pp. 196–197).

Oxford: Pergamon Press.Korokida, M. K., Oreopoulou, V., & Maroulis, Z. B. (2000). Effect of

frying conditions on shrinkage and porosity of fried potatoes.Journal of Food Engineering, 43, 147–154.

Newman, P. (1931). The drying of pours solids diffusion calculations.American Institute of Chemical Engineers’ Transactions, 27 ,310–333.

Rama, V., & Jacob John, P. (2000). Effects of methods of drying andpretreatments on quality of dehydrated mushroom. Indian Food

Packer, 54(5), 59–64.Riva, M., Schiraldi, A., & Cesare, L. F. (1991). Drying of Agaricus

bisporus mushrooms by microwave hot air combination. Leben-

smittel-Wissenschaft und- Technologie, 24, 479–483.Singh, A. K., Sharma, H. K., Kumar, P., & Singh, B. (1999). Physico-

chemical changes in white button mushroom (Agaricus bisporus) atdifferent drying temperatures. Mushroom Research, 8(2), 27–30.

Singh, S. C., Kumar, G., & Singh, S. (1995). Production, processingand consumption patterns of mushroom. Indian Food Industry,

14(6), 38–47.Singh, S. K., Narain, M., & Kumbhar, B. K. (2001). Effect of drying

air temperatures and standard pretreatments on the quality of fluidized bed dried button mushroom (Agaricus bisporus). Indian

Food Packer, 55(5), 82–86.Walde, S. G., Balaswamy, K., Shivaswamy, R., Chakkaravarthi, A., &

Rao, D. G. (1997). Microwave drying and grinding characteristicsof Gum Karaya (Sterculia urens). Journal of Food Engineering,

31(3), 305–313.Walde, S. G., Jyothirmayi, T., Velu, V., & Math, R. G. (2001). Effect

of lactic acid bacteria on preservation quality of bitterguard ,unpublished data.

Yang, D. C., & Maguer, M. L. (1992). Mass transfer kinetics of osmotic dehydration of mushrooms. Journal of Food Processing

and Preservation, 16 (3), 215–231.Yapar, S., Helvaci, S. S., & Peker, S. (1990). Drying behavior of

mushroom slices. Drying Technology, 8(8), 77–79.


Documents

pretreatments and drying methods on