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Effect of Air Temperature and Pre-treatment on ColorChanges and Texture of Dried Boletus edulis MushroomDimitrios Argyropoulos a , Muhammad Tahir Khan a & Joachim Müller aa Institute of Agricultural Engineering, Tropics and Subtropics Group, Universität Hohenheim,Stuttgart, Germany

Available online: 04 Oct 2011

To cite this article: Dimitrios Argyropoulos, Muhammad Tahir Khan & Joachim Müller (2011): Effect of Air Temperature andPre-treatment on Color Changes and Texture of Dried Boletus edulis Mushroom, Drying Technology, 29:16, 1890-1900

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Effect of Air Temperature and Pre-treatment on ColorChanges and Texture of Dried Boletus edulis Mushroom

Dimitrios Argyropoulos, Muhammad Tahir Khan, and Joachim MullerInstitute of Agricultural Engineering, Tropics and Subtropics Group, Universitat Hohenheim,Stuttgart, Germany

In the present work, the effects of drying air temperature andpre-treatment on color changes and texture of Boletus edulis (Bull.:Fr.) mushrooms were investigated during convective drying. Thedrying experiments were conducted in a laboratory hot-air dryerat temperatures of 50, 60, and 70 �C under thermal (water or steamblanching) and chemical (solution of potassium metabisulfite or cit-ric acid) treatments. Changes in color were evaluated by the CIE-LAB color system and the experimental color parameters (L� a�

b�) were fitted to a first order kinetic model. Values of total colordifference (DE�), hue angle (h�), and chroma (C�) were also calcu-lated. The texture of dried mushrooms was measured by an Instrontexture analyzer and expressed as hardness. The model fitted theexperimental a� and b� values sufficiently. The results showed thatduring convective drying, lightness decreased slightly while yellow-ness and redness increased. Chemical pre-treatments did not influ-ence the color of mushrooms positively while blanching causedintensive color deterioration. Moreover, the DE� value could con-tribute to the assessment of color change effectively. Hardnessincreased as drying proceeded and was found to be greater at highertemperatures. Mushrooms subjected to water or steam blanchingwere harder in texture. A temperature limit of 60�C is imposedfor the dehydration of B. edulis mushrooms. Thermal treatmentprior to drying is not recommended for B. edulis mushrooms whencolor and texture are desirable quality criteria.

Keywords Boletus edulis mushroom; Color; Convective drying;Pre-treatment; Texture

INTRODUCTION

Boletus edulis (Bull.: Fr.) mushrooms, commonly knownas porcini in Italy, cep in France, Steinpilz in Germany, andking bolete or penny bun in English-speaking countries, areone of the most frequently acquired wild edible mush-rooms. They occur naturally between summer and autumnin temperate forests of the Northern Hemisphere acrossEurope, Asia, North America, and have been introducedto South Africa[1] and New Zealand.[2] They are found inconiferous forests, usually forming mycorrhizal associationwith spruce (Picea spp.) or pine (Pinus spp.) trees. The fruit

body consists of a smooth brown cap, up to 30 cm in diam-eter, with very small white to olive brown colored poresbeneath. The stout and solid stipe, 3–23 cm long and3–11 cm thick, is white or greyish brown in color and finelycovered with a white or brownish reticulum over the upperhalf.[3]

Fruit bodies of B. edulis mushrooms are widely con-sumed due to their culinary and nutritive value. They havelow fat content, being rich in dietary fiber (mainly chitin[4])and characterized by a marvellous nutty and earthy flavor.The availability of fresh B. edulis mushrooms in inter-national markets is only seasonal; therefore they are tradedworldwide as frozen, brined, and dried products.[5]

Recently, the drying technologies for the post-harvestprocessing of bio-products have been extensively reviewedin the literature.[6] Although several drying systems andvarious pre-treatments have been evaluated for the dehy-dration of mushrooms,[7] air drying by convection in cabi-net or tunnel dryers is the most commonly employedmethod for several wild mushroom species such as Boletusedulis, Cantharellus cibarius (Chanterelle), Craterelluscornucopioides (Black trumpet), and Morchella spp.(Morel). The process usually involves thermal or chemicaltreatment prior to drying and an operational temperaturerange between 50 and 70�C. Correctly dried B. edulismush-rooms maintain or even intensify their distinctive flavor,including their odor[8,9] and taste,[10] without significantloss of their bright color, pleasant texture, and adequaterehydration in water.[11] However, improper applicationof the drying technique can cause undesirable changes incolor and texture of mushrooms. In particular, drying tem-peratures higher than the optimum will result in driedmushrooms of hard or rigid texture with visible effects ofbrowning. Furthermore, the flavor will also be affectedby inappropriate drying, which may result in a smoky ormusty product.

Dried B. edulismushrooms are graded according to theirquality level in international markets. The commercialvalue of B. edulis mushrooms is dependent on the colorand size of the dried slices. This implies that large and

Correspondence: Dimitrios Argyropoulos, Institute ofAgricultural Engineering, Tropics and Subtropics Group, Univer-sitat Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany;E-mail: dimitrios.argyropoulos@uni-hohenheim.de

Drying Technology, 29: 1890–1900, 2011

Copyright # 2011 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2011.594194

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uniform creamy white color pieces will tend to be moreexpensive than smaller or darker ones. In addition, thedried product should be free of microbial contaminationwithout mold and parasitic infestation. Although mush-rooms in general do not contain nicotine, a recent problemthat came to light was the unexpected presence of nicotinein dried B. edulis mushrooms imported to Germany fromChina.[12] Consequently, the European Food SafetyAuthority (EFSA) has imposed a temporary maximumresidue level (MRL) of 2.3mg of nicotine per kg of driedB. edulis mushrooms. A work published by Cavalieriet al.[13] has reported reasons for the significant amountof nicotine and found that the drying process is not a factorthat could influence the nicotine content in dried wildmushrooms.

To improve the quality or increase the microbiologicalstability of mushrooms several treatments are frequentlyapplied, involving aquatic solutions of enzyme inhibitorssuch as sodium metabisulfite[14] and citric acid[15] on anindustrial scale. The use of metabisulftites is a typical pre-liminary step for processing mushrooms since it retardsbrowning reactions and prevents color degradation duringdrying and storage.[16] Blanching in water or in aquaticsolutions containing antioxidative substances at tempera-tures of 90–98�C is another treatment used prior to dryingof mushrooms. The main reasons for water or steamblanching include the inactivation of enzymes and areduction in microbiological count; nevertheless, the qual-ity of mushrooms such as nutritive value,[17] color,[18,19]

rehydration capacity,[18] and texture[19] is affected bythermal treatments.

Color is considered to play a significant role in determin-ing the quality of food materials since it has a great influ-ence on visual appearance, processing, and acceptability.It is often an important criterion for consumers to decideon purchasing the foodstuff or not. Tristimulus color-imetry based on the CIELAB color system (CommissionInternationale de l’Eclairage) has been widely accepted asa rapid and simple method for quantifying visual percep-tion and describing the color change in food.[20] The prin-ciple of measurement relies on the optical assessment ofthe material’s surface, which can be indirectly related tothe quality deterioration of the product due to enzymaticor non-enzymatic browning reactions caused by processing.

Numerous research studies on the kinetics of colorchanges of food materials during drying have been pub-lished by several researchers. Devahastin and Niamnuy[21]

have put forward a review on modelling quality changesof fruits and vegetables including color degradationkinetics. More specifically, the influence of hot-air dryingon color changes has been studied by applying zero or firstorder kinetic models for basil,[22] apple,[23] hazelnut,[24]

potato,[25] avocado,[26] prune,[26] strawberry,[26] grape juiceand leather,[27] and onion[28] while the impact of microwave

drying for kiwi,[29] okra,[30] spinach,[31] and basil[32] hasbeen studied. The effects of the drying medium,[33]

pre-treatment,[34] and drying method[35] on the kinetics ofcolor changes have also been investigated for somefruits and vegetables. However, no studies on color kineticsof mushrooms during convective drying have beendocumented in the literature.

Moreover, texture is one of the main characteristics ofB. edulis mushrooms. Although the influence of prelimi-nary processing on the textural properties of rehydrated,[36]

frozen,[37] fresh and canned[38] B. edulis mushrooms hasbeen reported in the literature, no information is availablefor the effects of drying on the hardness of thosemushrooms.

Therefore, the aim of this work was to examine theeffects of drying temperature and pre-treatment on kineticsof color changes of B. edulis mushrooms. The research wasalso intended to assess the influence of the above para-meters on the texture of dried mushrooms and to giverecommendations for final product quality, taking intoaccount both properties.

EXPERIMENTAL SET-UP, MATERIALS, AND METHODS

Boletus edulis Mushroom

Fresh raw B. edulis mushrooms, free of visual defectsand characteristic pests, collected in the ‘‘Black Forest’’of Germany, were purchased from the local market inStuttgart, Germany, and stored at 2�C and 90% relativehumidity in a refrigerator for a maximum of three days.The mushrooms with initial moisture content of89.1� 1.1% in wet basis (8.21� 0.88 g=100 g d.b.) wereselected according to the diameter of the cap (ca. 50mm)and stipe (ca. 30mm) and cleaned thoroughly to removeadhering soil. Then, slices of 5� 0.18mm thickness fromthe middle section were obtained by cutting mushroomsvertically with an electrical vegetable slicer.

Laboratory Dryer

The thin layer drying experiments were conducted usinga high precision hot-air laboratory dryer designed atthe Institute of Agricultural Engineering, University ofHohenheim (Stuttgart, Germany), which allowed the con-trol of the desired drying conditions over a wide range ofoperating parameters. A detailed description of the experi-mental dryer (Figure 1) has been given by Argyropouloset al.[39] It essentially consists of four units: (i) an air flowcontrol unit; (ii) an air conditioning unit with athermostat-controlled water bath and sprayed Raschig-ring bed; (iii) a heating control unit with primary and sec-ondary heating elements; and (iv) two drying compart-ments to provide either through-flow or over-flow airstream for convective drying of products. Each unit is elec-tronically controlled by PID control. The weight measuring

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system comprised different load cells (type PC6, FlintecLtd, Vasteras, Sweden) for each drying chamber. The dryeris connected to an industrial computer using PLC softwareallowing pre-programming of the set drying conditions andmonitoring of temperature, humidity, velocity, and massdata during the drying process.

Experimental Set-Up

Prior to drying, mushroom slices were divided into dif-ferent groups and subjected to two thermal and two chemi-cal treatments. The thermal pre-treatments were performedby water or steam blanching of the mushrooms in distilledwater at 98�C for 3min. The chemical pre-treatments werecarried out by soaking the mushroom slices for 10min insolutions of 0.25% citric acid (C6H8O7) or 0.25% potassiummetabisulfite (K2O5S2) at room temperature. Additionally,an untreated batch of slices was used for control. Sampleswith a mass of approximately 200 g mushroom slices wereused for each drying trial. The dryer was warmed up for30min to reach the defined set points. Then, the materialwas evenly distributed on a round perforated tray (diam-eter: 340mm) and placed in the through-flow mode ofthe dryer. The experiments were conducted at air tempera-tures of 50, 60, and 70�C and the corresponding relativehumidity for each temperature, maintaining 10 g=kg ofabsolute humidity. The velocity of the air flowing throughthe mushrooms was kept constant at 0.9m=s. During thedrying process the weight change was recorded in ten-minute intervals for the determination of the drying curves.The drying experiments were replicated three times for eachdrying temperature and pre-treatment.

Determination of Moisture Content

The moisture content of mushrooms, expressed in drybasis (g=g d.b. or g=100 g d.b.), was determined by the ovenmethod (103� 2�C for 24 h) in triplicate and the averagevalue was recorded.

Determination of Water Activity

The water activity of dried mushrooms was determinedusing a digital hygrometer (Rotronic AG, Bassersdorf,Switzerland). The samples were finely ground in a labora-tory water-cooled mill and placed in a thermostatic cham-ber for 30min to allow for temperature equilibrium at25�C. Readings were obtained in the form of % equilibriumrelative humidity (ERH) and expressed as aw (ERH=100).The mean value of three measurements was recorded.

Color Measurement

The color of mushrooms was measured periodically by aMinolta Colorimeter (model CR-400 Minolta Co, Ltd.,Japan). The instrument was calibrated with a standardwhite plate at D65 illumination before taking measurements(Y¼ 93.7, x¼ 0.3158, y¼ 0.3324). Two readings were per-formed per mushroom surface by placing the colorimeterhead directly above the slice. The mean value of ten mea-surements was taken for each experiment. Color para-meters were expressed as L� describing lightness (L� ¼ 0for black, L� ¼ 100 for white), a� describing intensity ingreen-red (a� < 0 for green, a� > 0 for red), b� describingintensity in blue-yellow (b� < 0 for blue, b� > 0 for yellow).Furthermore, other color components such as C�, h� andDE� were also calculated. Chroma (C�) indicates color

FIG. 1. Schematic diagram of the laboratory hot-air dryer and its components.

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saturation that is proportional to its intensity and is calcu-lated as follows:

C� ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia�2 þ b�2

pð1Þ

For the h� value, an angle of 0� or 360� indicates red hue,while angles of 270�, 180�, and 90� indicate blue, green, andyellow hue, respectively. The hue angle (h�) is calculated as:

h� ¼ tan�1 b�

a�

� �ð2Þ

The total color difference is expressed as:

DE� ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDL�2 þ Da�2 þ Db�2

pð3Þ

A larger DE� denotes greater color change from the refer-ence material. Color differences were defined for lightness,redness, and yellowness as follows:

DL� ¼ L�0 � L�

t ð4Þ

Da� ¼ a�0 � a�t ð5Þ

Db� ¼ b�0 � b�t ð6Þ

Subscript ‘‘0’’ refers to initial color values and ‘‘t’’ values atselected time intervals during drying.

Kinetic Considerations

The evaluation of color change during drying was basedon the assumption that each parameter (L� a� b�) followsfirst order kinetics. The conventional equation for firstorder kinetics is given as:

Z ¼ Z0 expðkZ � tÞ ð7Þ

Considering that each color parameter reaches a finalequilibrium value, the rate of color change can be describedusing the model proposed by Krokida et al.[33] Therefore,the average values of L�, a� and b� were fitted to thefollowing equation:

Z� Ze

Z0 � Ze¼ expðkZ � tÞ ð8Þ

Z ¼ ðZ0 � ZeÞ � expðkZ � tÞ þ Ze ð9Þ

where Z is the value of the color parameter at regular timeintervals during drying, Z0 and Ze are the correspondinginitial and equilibrium values, kZ is the rate constant(min�1), t is the drying time (min). The values of the para-meters were determined by fitting the model to the experi-mental data using the non-linear least squares procedure(Matlab v. R2010b, MathWorks, Inc.). The fitting

accuracy was evaluated by R-squared (R2) and sum ofsquares due to error (SSE). A good fit is considered whenhigh values of R2 and low values of SEE were obtained.

Texture Measurement

Texture was determined as puncture force, which is anindicator of the hardness of the material’s surface. In parti-cular, the puncture force quantifies the extent of case hard-ening occurred during drying.[40] The test was carried out atroom temperature (25�C) using a texture analyzer (InstronUniversal Testing Machine, Model 4301, Norwood, USA).A 5mm diameter needle probe was used to measure hard-ness as the force required to puncture an individual mush-room slice placed over a plate with an 8mm diameter hole.The crosshead speed was maintained at 100mm=min. Themean value of ten measurements was recorded for eachtrial.

Statistical Analysis

Two-way analysis of variance (ANOVA) was performedto assess any interaction between data sets of drying tem-perature and pre-treatment. Differences among the meanvalues of the experimental data for each treatment wereevaluated by one-way analysis of variance using Tukey’stest at P< 0.05 significance level.

RESULTS AND DISCUSSION

Moisture Content Changes during Drying

Fresh untreated B. edulis mushrooms with a moisturecontent of 8.214� 0.88 g water per g dry matter were driedto a moisture content of 0.069� 0.006 g=g dry basis.Figure 2 shows changes in moisture content of the slicesdried at temperatures of 50, 60, and 70�C. As expected,the moisture content decreased gradually until the desiredfinal moisture content was reached. Furthermore, the total

FIG. 2. Change of moisture content during drying of untreated B. edulis

mushrooms at & 50, 4 60 and � 70�C.

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time of the process increased substantially with a decreaseof drying air temperature. The influence of variouspre-treatments on the drying characteristics of B. edulismushroom as a function of time is shown in Figure 3.The initial moisture content of the samples subjected toeither thermal or chemical treatments was higher than theuntreated batch of slices used as a control. The mushroomsindicated a tendency to absorb water after the pretreat-ments resulting in slightly different drying behavior. Never-theless, the total drying time required to reach the targetvalue of moisture content was not affected by thepre-drying treatments employed.

Influence of Drying Temperature on Color

Table 1 presents the estimated initial and equilibriumvalues of the individual color components predicted by afirst order kinetic model for all drying temperatures andtreatments tested along with the corresponding statisticalvalues of R2 and SSE. The model described well the experi-mental a� and b� values as it can be observed from the highR2 and the relatively low SSE; on the other hand, theresults of the L� values indicated an uncertainty due to highexperimental error. The results are in agreement with pre-vious works reported in the literature where an irreversiblefirst order kinetic model fitted the experimental values ofredness and yellowness adequately for some fruits andvegetables dried at different temperatures[33] and variouspre-treatments.[34] Convective air drying influenced signifi-cantly the color of B. edulis mushrooms. The freshuntreated mushroom slices possessed a creamy white coloras indicated by the high initial L� ¼ 91.17� 2.64 values andlow a� ¼�035� 0.64, b� ¼ 9.63� 1.35 values. As the dry-ing process progressed, the lightness of mushroomsdecreased slightly while yellowness and redness increased

for all examined temperatures. The experimental results(points) and predicted curves (solid lines) for the L� a� b�

values for the untreated mushrooms dried at 50, 60, and70�C are shown in Figure 4.The decreasing value of L�

indicates that the samples turned darker. On the otherhand, a� and b� values tended to increase gradually duringdrying. For instance at a temperature of 50�C, the initialexperimental L� value of 91.17 decreased to 79.97(Fig. 4a) while the b� value increased from 9.63 to 12.33(Fig. 4c). Furthermore, a negative a� value of -0.35 at thebeginning of the process changed to positive and reacheda value of þ0.65 at the end of drying (Fig. 4b). Decreaseof lightness and increase of yellowness were also observedin a previous study for hot-air dried A. bisporus mushroomslices.[41] In addition, work by Krokida et al.[33] noted anincrease of both a� and b� values for potato, apple, andbanana when subjected to conventional hot-air drying.This change in color could be ascribed to non-enzymaticMaillard browning reactions and the formation of brownpigments that took place during drying.[42,43] Color satu-ration indicated by the C� value increased during dryingand closely followed the b� value. On the contrary, the h�

value gradually decreased although only a small changewas documented. The results for C� and h� values are notshown. Moreover, the total color difference (DE�) is aparameter extensively used to evaluate the overall colorvariation between the fresh and dried samples. The individ-ual color components obtained from the fresh untreatedsamples were taken as a reference in order to assess theextent of color change. Discoloration was more severe atlow moisture contents, in particular at the end of the dryingperiod when the product temperature was raised due to lessevaporative cooling. Samples dried at 50�C showed theleast total color change (DE� ¼ 12.08). An increase of dry-ing temperature caused intensive browning of the samplesindicating a greater sensitivity of mushrooms to highertemperatures. A similar effect was also noticed byKotwaliwale et al.[19] for hot-air dried oyster mushrooms(Pleurotus spp.). More specifically, considerable colordeterioration was observed at temperatures above 60�Cand especially at 70�C where the final product with an aver-age moisture content of 6.595� 0.242 g=100 g d.b. possessedthe lowest L� value and the highest a�, b� and DE� values.This can be attributed to the fact that the material mighthave been exposed to a drying temperature higher thanits glass transition temperature (Tg). A work by Karmaset al.[44] pointed out that the increase of molecular mobilityabove Tg may increase the reaction rate of non-enzymaticbrowning. According to the statistical analysis there is asignificant effect of the progressive increase in temperatureup to 70�C on the changes of color parameters. In parti-cular, the a� and b� values appear to be the most sensitiveparameters for the perception of the color variation of B.edulis mushrooms during convective drying. Moreover,

FIG. 3. Change of moisture content during drying of B. edulis mush-

rooms at 60�C, subjected to �control, ~ potassium metabisuflite, & citric

acid, 4 water blanching and & steam blanching.

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TABLE 1Estimated parameters, R2 and SSE values of the model for L�, a�, b� under various drying temperatures

and pre-treatments

Control K2O5S2 C6H8O7 Water blanching Steam blanching

50�CL0 91.48 78.16 79.77 73.70 70.82Le 74.82 77.02 75.22 58.16 59.04kL �0.0057 �0.0690 �0.0137 �0.0178 �0.0179R2 0.9423 0.6902 0.7323 0.9274 0.9763SSE 8.79 1.742 6.473 16.72 2.99a0 �0.3528 �1.111 �0.5114 �0.5997 �0.2744ae 0.6459 1.048 0.5245 4.20 4.177ka 0.0620 0.0105 0.0552 0.0185 0.0160R2 0.9671 0.9616 0.9491 0.9772 0.9492SSE 0.02944 0.1461 0.04998 0.4776 0.9285b0 9.631 9.393 10.66 9.514 9.991be 12.28 15.35 13.17 20.66 21.93kb 0.0339 0.0343 0.0608 0.0297 0.0480R2 0.9529 0.9803 0.9395 0.9855 0.9855SSE 0.3036 0.6335 0.3915 1.646 1.553

60�CL0 91.45 77.93 80.48 73.26 70.73Le 53.44 69.58 69.96 52.10 51.93kL �0.0034 �0.0111 �0.0107 �0.0044 �0.0042R2 0.9638 0.9621 0.8479 0.9819 0.9879SSE 8.991 1.613 11.33 1.929 0.9613a0 �0.3698 �1.072 �0.4948 �0.1959 0.1173ae 1.324 1.22 1.43 5.528 6.132ka 0.0364 0.0210 0.0157 0.0072 0.0065R2 0.9567 0.9933 0.9966 0.9637 0.9882SSE 0.1044 0.002675 0.00889 0.5093 0.1602b0 9.602 9.433 10.69 9.581 9.806be 12.99 16.02 13.42 20.80 20.92kb 0.0299 0.0249 0.0296 0.0254 0.0273R2 0.989 0.9878 0.9622 0.9825 0.9855SSE 0.101 0.428 0.2872 1.798 1.553

70�CL0 91.74 78.27 79.76 72.95 71.02Le 50.32 66.77 64.83 53.19 49.2kL �0.0054 �0.0092 �0.0112 �0.0078 �0.0069R2 0.9837 0.9922 0.9799 0.8975 0.8684SSE 4.071 0.2934 1.591 10.31 14.66a0 �0.3148 �0.9834 �0.6411 �0.6121 �0.1217ae 1.905 2.343 4.377 8.675 10.26ka 0.0225 0.0088 0.0095 0.0100 0.0086R2 0.9782 0.9487 0.9601 0.9755 0.9879SSE 0.06321 0.161 0.307 0.6685 0.3447b0 9.634 9.394 10.76 9.945 10.14be 14.12 20.92 23.96 29.17 29.95kb 0.0405 0.0446 0.0130 0.0120 0.0132R2 0.9822 0.9881 0.9952 0.9956 0.9954SSE 0.2529 0.1687 0.3335 0.6012 0.6737

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the DE� value could contribute significantly to the assess-ment of color change of the final product as a morepractical oriented indication.

Influence of Pretreatment on Color

The results of color kinetics for the samples subjected tofour treatments prior to drying are depicted in Figure 5.Theinitial L� value of the untreated samples was higher thanthat of all treatments. Nevertheless a gradual decreasewas observed over the entire duration of drying. Fresh B.edulis mushrooms treated either with chemicals or in aphysical way indicated a noticeable loss of brightness,which was greater for the blanched samples. Both waterand steam blanching resulted in darker dried mushrooms,which it was visually apparent at all examined tempera-tures. Improved L� values were obtained for the samplesimmersed either in a solution of potassium metabisulfiteor citric acid. The mean L� values between the chemicaltreated mushrooms were statistically identical (p> 0.05).It is worth mentioning that these samples indicated a

remarkable stability of lightness during drying, particularlyat lower temperatures (Fig. 5a). A study published byMartinez-Soto et al.[18] reported a similar observation forthe L� value of hot-air dried oyster mushrooms (Pleurotusostreatus) immersed either in a solution of 1 g=l sodiummetabisulfite or citric acid.

The a� value for mushrooms was strongly affected bythe thermal treatments during drying (Fig. 5b). Theincrease of a� value signifies a deeper red chroma indicatingbrowning reactions, which were more intensive at elevatedtemperatures. An exception was observed for the samplesdipped in a solution of potassium metabisulfite or citricacid where a smaller increase of a� value was recorded.Nevertheless, the final mean a� values of the untreatedand chemical pre-treated mushrooms were not significantlydifferent. This implies that the chemical pre-treatments didnot influence the color of mushrooms during drying posi-tively, which contradicts with the result obtained byKrokida et al.[34] for sulfite pre-treated materialssuch as apple, banana, and potato. On the other hand,

FIG. 4. Course of lightness L� (a), redness a� (b) and yellowness b� (c) during drying of untreated B. edulis mushrooms at & 50, 4 60 and � 70�C, —model.

FIG. 5. Course of lightness L� (a), redness a� (b), and yellowness b� (c) during drying of B. edulis mushrooms at 60�C, subjected to � control, ~

potassium metabisuflite, & citric acid, 4 water blanching and & steam blanching, — model.

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water- and steam-blanched samples exhibited the greatestincrease of redness compared to all the other samples. How-ever, no significant differences were observed between thefinal a� values of water- and steam-blanched mushroomsdried at three temperatures (p> 0.05). Fresh mushroomsindicated a negative a� value (�0.35) before steam blanch-ing, which changed to positive values when dried at 60�Cand reached a final value of þ4.84 after drying. A similartrend to the behavior of the a� value was noticed for theb� value (Fig. 5c). This value increased significantly as dry-ing at 60�C proceeded and steam-blanched mushrooms pos-sessed the highest final b� value (20.2) compared to all othertreatments. The application of blanching resulted in anincrease of the initial moisture content of mushrooms, whichcould affect both water activity of the material and its mol-ecular mobility causing deteriorative reactions. In addition,the temperature as well as the duration of the treatmentmight have contributed to the color deterioration of themushroom slices. The concepts of water activity and glasstransition temperature as functions of moisture content areimportant in predicting product stability during proces-sing.[45] Regarding the samples soaked in a solution of pot-assium metabisulfite, the unexpected relatively high b� valuecan be ascribed to the presence of sulfur, which hasdeveloped a yellow hue on the mushroom surface. The equi-librium a� and b� values predicted by the model showed a

tendency to increase at higher drying temperatures for alltreatments employed. As far as the total color differenceDE� value is concerned, chemical treatments with citric acidor potassium metabisulfite at a temperature of 50�C causedthe least color change whereas steam blanching at 70�Ccaused the greatest color change among the pre-treatmentsand temperatures. However, the final DE� values of thechemical-treated samples were statistically identical(p> 0.05) with the DE� value obtained from the untreatedslices. According to the above changes in color, the chemicaltreatments did not show any significant improvement whencompared to untreated samples while thermal treatmentsdeteriorated the overall color of B. edulis mushrooms. Theexperimental mean DE� values of B. edulis mushrooms forall tests at the end of the drying process are presented inTable 2.

Texture

Texture of B. edulis mushrooms was affected by the dry-ing process. Convective air drying might have causedphysical and structural modifications of the mushroomtissue. The average moisture content of the dried slicesobtained for texture measurement was 6.969� 0.404 g=100 g d.b. Figure 6 shows the mean values of hardnessfor mushrooms dried at different temperatures. The punc-ture force required to penetrate a mushroom slice with low

TABLE 2Moisture content, water activity, hardness, and total color change of B. edulis mushrooms at the end of drying

Processing conditionMoisture content(g=100 g) dry basis Water activity, – Hardness, N Color change, DE

Control50�C 7.140� 0.246 0.362� 0.006 4.917� 0.533 12.08� 4.7660�C 6.973� 0.305 0.374� 0.005 5.874� 0.656 18.51� 3.4270�C 6.595� 0.242 0.350� 0.006 6.014� 0.804 19.33� 6.86

Potassium metabisulfite50�C 7.214� 0.281 0.374� 0.007 6.817� 2.145 15.07� 5.5460�C 6.874� 0.254 0.368� 0.004 8.994� 2.190 22.10� 4.3970�C 6.467� 0.415 0.360� 0.006 12.075� 3.704 23.47� 5.36

Citric acid50�C 7.324� 0.373 0.369� 0.007 7.045� 1.736 14.70� 6.1360�C 6.979� 0.394 0.359� 0.004 9.423� 2.671 19.55� 4.4370�C 6.602� 0.257 0.348� 0.002 14.085� 3.161 26.71� 6.26

Water blanching50�C 7.920� 0.260 0.417� 0.011 23.094� 3.095 32.36� 7.2260�C 7.145� 0.364 0.392� 0.011 24.101� 3.071 30.27� 5.1170�C 6.872� 0.479 0.397� 0.006 27.278� 4.087 33.62� 5.31

Steam blanching50�C 7.216� 0.317 0.402� 0.007 23.684� 3.337 33.89� 7.2260�C 6.970� 0.435 0.384� 0.009 25.143� 3.357 31.60� 4.2070�C 6.239� 0.336 0.388� 0.005 28.987� 2.925 39.03� 6.03

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moisture content was higher than that of fresh samples. Inthe early phase of drying only the surface layer of themushroom slice was altered while the inner structureremained intact. As drying continued, cracks were formedand the material began to warp, resulting in a rigid finalproduct. More specifically, during the drying process,moisture migrates by diffusion from the internal part ofthe slice to the surface and carries water soluble compo-nents with it. Then, the surface moisture is continuouslyevaporated and the solutes are concentrated on the surface,forming a hard and dry layer.[46] In addition, loss of moist-ure and subsequent separation of components could causedamage and disruption of cell walls.[47] When air dryingwas performed at higher temperatures, a substantialincrease in force for puncturing mushroom slices wasobserved, possibly due to the formation of an intensivecrust layer on the surface. The mean value of the samplesdried at 70�C was significantly different from the value ofthe material dried at 50�C (p< 0.05) but not from the valueof the mushrooms dried at 60�C (p> 0.05).

Concerning the pre-drying treatments, both blanchingapplications exhibited the highest mean values among allthe other samples. The statistical analysis revealed no sig-nificant difference between blanching treatments on hard-ness (p> 0.05). Kotwaliwale et al.[19] also observed thatat the end of the drying process the hardness of blanchedoyster mushrooms was higher when compared to the otherdried samples. Figure 7 shows the mean values of hardnessfor the pre-treated B. edulis mushroom slices at the end ofdrying. Steam and water blanching resulted in loss of inter-nal pressure in fresh B. edulis mushrooms. The loss of tur-gor was accompanied with an apparent reduction of weightand volume of the blanched samples. However, the force

required to penetrate each blanched slice was higher whencompared with the other materials probably due to expul-sion of intercellular air within the mushroom itself andconsequently tighter organization of the hyphae withinthe tissue.[48] Immersion in a solution of either citric acidor potassium metabisulfite (p> 0.05) caused a smallincrease in hardness of dried B. edulis mushrooms, presum-ably because of the excess moisture uptake, which mighthave caused changes in the cell wall strength during chemi-cal treatments. Results of hardness for all the samplesexamined along with corresponding values of moisturecontent, water activity, and overall color difference aresummarized in Table 2.

CONCLUSION

Color changes of B. edulis mushrooms were evaluatedduring convective air drying by fitting the experimentaldata to a first order kinetic model. Lightness (L�) of mush-rooms decreased, while redness (a�) and yellowness (b�)increased during the drying process. Higher drying tem-peratures caused intensive color deterioration of the sam-ples. Immersion in an aquatic solution of citric acid orpotassium metabisulfite did not influence the color ofmushrooms positively, whereas blanching had a detrimen-tal effect on color quality. The DE� value as an indicatorfor practical applications could contribute to the assess-ment of the overall color variation of B. edulis mushroomseffectively. Texture of mushrooms expressed as a degree ofhardness, increased as drying progressed, and was found tobe greater at higher temperatures. Blanched mushroomswere harder in texture when compared to all the othersamples. A temperature limit of 60�C is proposed for the

FIG. 6. Hardness of B. edulis mushroom slices, fresh and dried at differ-

ent temperatures. FIG. 7. Effect of pre-treatment on hardness of B. edulis mushrooms

dried at 60�C.

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dehydration of B. edulis mushrooms in order to maintainquality. Physical treatment, in the form of either steam orwater blanching, is not recommended for thepre-processing of B. edulis mushrooms when color and tex-ture are desirable quality criteria of the dried product.

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