Effects of electrical and sonication pretreatments on the drying rate and quality of mushrooms

LWT - Food Science and Technology 69 (2016) 197e202

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Effects of electrical and sonication pretreatments on the drying rate and quality of mushrooms
lu, Hamza Bozkır*, Ahsen Rayman Ergün, Taner Baysal Ruken S¸eyda Çakmak, Onur Tekeog Food Engineering Department, Faculty of Engineering, Ege University, 35100 Izmir, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2015 Received in revised form 13 January 2016 Accepted 14 January 2016 Available online 18 January 2016

Application of pre-treatments such as electrical methods, and ultrasound are being used for decreasing the processing time and/or preserving the quality of the foodstuffs in drying technology. In this study, electroplasmolysis (EP) and ultrasound pre-treatments (US) were implemented for mushrooms. Elecroplasmolysis was performed at 100 V for 40 s and ultrasound was performed at 35 kHz for 30 min. Then samples were dried in a tray dryer at 50
C with a 1.5 m/s air velocity. Drying rate and quality properties were investigated. As a result, the combination of pretreatments (EP þ US) increased the drying rate around 37.10%. Ultrasound treatment preserved the phenolic content and color values better compared to electroplasmolysis. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Mushroom Electroplasmolysis Ultrasound Quality Drying rate

1. Introduction Mushrooms are known as a valuable food material and have € been widely consumed for centuries (Ozyürek, Bener, Güçlü, & Apak, 2014). Most mushrooms sold in supermarkets today have been commercially grown on mushroom farms. Agaricus bisporus contains 92.81 g water, 1.63 g protein, 0.33 g fat, 5.24 g carbohydrates and gives 30 kcal of energy per 100 g of vegetable (Manzi, Aguzzi, & Pizzoferrato, 2001). With their phenolic content they have antioxidant properties (Barros, Ferreira, Queiros, Ferreira, & Baptista, 2007; Woldegiorgis, Abate, Haki, & Ziegler, 2014). Mushrooms can be consumed with soups, pizza, sauce, and as food € flavoring materials or as an alternative to meat (Ozyürek et al., 2014). Extracts of this vegetable can be used as a drug in medin et al., cine for diseases, such as cardiovascular disorders (Guillamo 2010). Fresh mushrooms are perishable with a high moisture content, around 90 percent (Kotwaliwale, Bakane, & Verma, 2007). Drying them is an effective way to both prolonge their shelf-life and preserve their flavor and nutrients (Marshall & Nair, 2009). There are so many studies about drying of mushrooms by using different methods such as hot air, microwave, vacuum, infrared, freeze

* Corresponding author. Tel.: þ90 2323113044; fax: þ90 2323427592. E-mail address: [email protected] (H. Bozkır). http://dx.doi.org/10.1016/j.lwt.2016.01.032 0023-6438/© 2016 Elsevier Ltd. All rights reserved.

drying, solar assisted heat pump, and electrohydrodynamic (Dinani, Hamdami, Shahedi, & Havet, 2014, 2015; Kotwaliwale ~ a, Rodríguez, & Ruiz, et al., 2007bib_Dinani_et_al_2015; Lombran 2010; Motevali, Minaei, Khoshtaghaza, & Amirnejat, 2011; €
an, & Ozyürek et al., 2014; Pei et al., 2014; S¸evik, Aktas, Dog Koçak, 2013; Walde, Velu, Jyothirmayi, & Math, 2006; Xiao-hui, Xia, Yu-rong, Long, & Jian, 2014). But there are limited studies about the pretreating of this vegetable by electric and ultrasound before drying (Jambrak, Mason, Paniwnyk, & Lelas, 2007). Electroplasmolysis increases the cell permeability and mass transfer coefficients of plant tissues. These changes have an important influence on the extraction and dehydration processes (Bazhal, Ngadi, & Raghavan, 2003). It provides the transfer of water to the surface so the drying becomes faster in vegetable slices (Baysal, Rayman, & Bozkır, 2012). Ultrasound technology which is among the emerging technologies is commonly used in combination with other technologies. The mechanical energy provided by the application of power ultrasound contributes to the reduction of both the internal and the external resistances to the mass transfer, being that the water transfer mainly improved by alternating the expansion and compression cycles (Rodríguez et al., 2014). In previous studies, this application was used as a pretreatment in many drying processes rez, Riera, & Mulet, 2007; (Bantle & Eikevik, 2014; C arcel, García-Pe Fernandes & Rodrigues, 2007; Garcia-Perez, Ozuna, Ortuno, Carcel, & Mulet, 2011; Kek, Chin, & Yusof, 2013; Noshad, Mohebbi, Shahidi,

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& Mortazavi, 2012; Nowacka, Tylewicz, Laghi, Dalla Rosa, & Witrowa-Rajchert, 2014). In literature there is a lack of information about using the combination of electrical treatments with ultrasound before drying. The aim of this research was to investigate the effects of electroplasmolysis and ultrasound pretreatments and also their combinations on the drying rate and quality characteristics of mushrooms. 2. Material and methods

2.2.1.3.1. Rate of drying curves. Data acquired from the drying equipment is usually obtained as the total weight (W) of the wet solid at different times (t) over the drying period. Ws is the weight of the dry solid in kilograms then:

Xt ¼

W  Ws ðkg total water=kg dry solidÞ Ws

(1)

For the given constant drying conditions, the equilibrium moisture content Xe (kg equilibrium moisture/kg dry solid) is identified and the free moisture content X (kg free water/kg dry solid) is calculated for each value of Xt.

2.1. Material

X ¼ Xt  Xe White mushrooms (A. bisporus) were purchased from a local supermarket in Bornova, Izmir, Turkey. The raw material was stored in a refrigerator at þ4
C at 80e90% relative humidity for a maximum of 24 h before processing. The mushrooms were washed, and the stems were removed. 2.1.1. Chemicals Ethanol, NaOH, hydrogen peroxide, and HCI (Merck, Darmstadt, Germany) along with the FolineCiocalteu, gallic acid standards (SigmaeAldrich Corp., St. Louis, MO) were used. 2.2. Method 2.2.1. Processing method 2.2.1.1. Electroplasmolysis. The electroplasmolysis (EP) was applied by a drum type electroplasmolyzator which was designed in a € ven, 2007). The previous research (Baysal, Icier, Yıldız, & Demirdo electroplasmolyzator has two cylinders with stainless steel pins, a voltage control unit that gives alternative electric current to the system, and a feeding unit which provides the contact between pins and samples. The distance between the cylinders was 6.5 cm, the distance between the pins was 4.5 cm, and the rotating rate of cylinders was 25 s/cycle. The EP was treated with different parameters (20 V-90 s, 40 V-60 s, and 100 V-40 s). An optimum condition of 100 V-40 s was chosen which provides the lowest drying time. The EP treatment was applied to all the samples (30
C) and then the samples were sliced in a thickness of 4 mm. Before the drying application the sliced mushrooms were immersed in an SO2 solution (300 mg/kg) for 30 s to minimize the color degradation. The production scheme for dried mushroom is given in Fig. 1. 2.2.1.2. Ultrasound. An Ultrasonic bath (Sonorex Super RK-106, 100% power: 480 W, 35 kHz, volume: 5.6 L) was used for the ultrasound treatment. A 250 g sample of the sliced (4 mm thickness) mushrooms was then placed directly into the ultrasonic bath containing 1 L of water. The sonication pretreaments were carried out for 10, 20, and 30 min at 30
C and 30 min was chosen for the lowest drying time. Before the drying applications the mushrooms were immersed in an SO2 solution (300 mg/kg) for 30 s to minimize the color degradation. 2.2.1.3. Drying process. The drying experiments were performed in a laboratory scale hot air drier (Armfleld Ltd., Model UOP8 Hampshire, England) operating at an air velocity of 1.5 m/s. Generally, a temperature range of 50e60
C is preferred in the drying of foods for minimum damage to the product, in this study 50
C was selected. Weight loss was measured at every 5 min during drying. The initial moisture contents of the fresh mushroom slices were determined as 91 g/100 g using infrared moisture equipment (Shimadzu MOC-63U). The final moisture content of the samples was 5 g/100 g (Kumar, Singh, & Singh, 2013).

(2)

The drying rate (R) is calculated for each drying time increment using:

    Ws dx * R¼ A dt

(3)

Where A is the exposed surface area for drying in m2 (Geankoplis, 2003). The moisture ratio: In thin layer drying, the moisture ratio during drying was calculated using equation (4).

MR ¼

Mt  Me Mi  Me

(4)

Where Mt is the moisture content at any time (kg water/kg dry solid), Mi is the initial moisture content (kg water/kg dry solid), and Me is the equilibrium moisture content of the mushroom samples (kg water/kg dry solid) (Doymaz, 2009). 2.3. Methods of analysis The total titratable acidity was determined by means of a potentiometric titration (pH meter, WTW InoLab). The results were expressed as g/100 g with reference to citric acid (AOAC, 1995). The moisture content was measured with 5 gr sliced (2 mm) sample by the Shimadzu MOC-63U infrared moisture measuring equipment at 105
C for 1 h. The sulfur content was measured by a
lu total sulfuric acid analysis according to the method of Cemerog (2007). The amount of alkali spent in the titration gives the amount of SO2 in the sample correlated with equation (5).

SO2 ¼

3200  ð0:1NaOHÞ ðmg=kgÞ sample

(5)

The total phenolic contents was measured using the FolineCiocalteu method (Franke, Chless, Sılverıa, & Robensam, 2004). The water activity was measured using TestoAG 400 (Germany) water activity equipment. The color (L*, a*, b*) values of the samples were measured with a HunterLab Colourflex colourimetre (Hunter Associates Laboratory Inc., Reston, VA, USA). The rehydration capacity was made with a 3 g dry sample with 50 ml distilled water in a beaker after 24 h in a water bath, the sample was then filtered for removing excess water and then weighed (Cui, Li, Song, & Song, 2008). The rehydration capacity was calculated using formula 6.

Rehydration capacity ¼

Wr Wd

Wr: weight after rehydration (g) Wd: weight of dry sample (g)

(6)

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199

Mushrooms

Elimination classification and washing

Drum type Electroplasmolysis

Slicing (4 mm)

Drum type Electroplasmolysis

Slicing(4 mm)

Slicing (4 mm)

Ultrasound (35 kHz, 30 min.)

Slicing (4 mm)

Drying (50°C)

Drying (50°C)

Drying (50°C)

Ultrasound (35 kHz, 30 min.)

Group control

Group EP

Group US

Drying (50°C)

Group EP+US

Fig. 1. Dried mushrooms production schema.

The bulk density was determined with 2 g of a dry sample which was sliced and the volume measured with a graduated cylinder. The bulk density was calculated using formula 7 (Goula & Adamopoulos, 2005).

rbulk

m ¼ ðg=mlÞ V

(7)

2.4. Statistical analysis The results were statistically analyzed by analysis of variance (ANOVA) using the software spss 18 (SPSS Inc., Chicago, IL, USA) with the Duncan test to evaluate the differences between the treatments at a level of significance P  0.05. Each experiment was

Fig. 2. Drying curves of the mushroom groups.

repeated at least three times, the means and standard deviations of the results were calculated. 3. Results and discussion 3.1. Evaluation of drying rate Drying curves and moisture ratio versus time were given in Figs. 2 and 3, respectively. The MR decreased exponentially with time in the samples. Our results were in agreement with the different studies which different materials were dried (Bas¸lar,
dıç, & Arici, 2014; Markowski, Bialobrzewski, & Kılıçlı, Toker, Sag Modrzewska, 2010; Perea-Flores et al., 2012). The drying time was shorter after pretreatments when compared to the control group. The drying time were found to be as 3.91 h for control group, whereas, after the EP application it took 2.83 h. The drying time was found as 2.66 h in the US application group. In the combined group it was found as 2.5 h. During the constant rate period, the rate of

Fig. 3. Moisture ratio versus time for drying of the mushroom groups.

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0.696 kg H2O/h.m2 for the US þ EP group. In addition the drying rate increased by 22.96%, 34.78%, and 37.10% with the EP, US, and EP þ US combination respectively. Drying occurs in the falling rate period not in the constant rate period as shown in Fig. 4. It was observed that the drying rate increases in apple slices treated by an ultrasound assisted air drying system (Rodríguez et al., 2014). Wang and Sastry (2002) reported that electroplasmolysis can enhance the mass transport because of the electroporation of the cell membrane and the thermal denaturation of the membranes. Therefore, the drying rate increased when compared to the control. 3.2. Evaluation of quality characteristics The final moisture contents of the mushroom samples were below 5 g/100 g as seen in Table 1. There is no significant difference between the moisture contents of the control and US group (P > 0.05). In addition, there is no significant difference between the EP and EP þ US groups (P > 0.05). After the drying process the SO2 content was not found in all groups. This shows us that the SO2 solution (300 mg/kg) was removed during drying. Drying of mushroom by combining freezedrying and mid-infrared radiation had a significant effect on aroma retention and caused an increase in sulfur compounds such as dimethyl, trisulfide, and 1,2,4-trithiolane (Wang, Zhang, & Adhikari, 2015). In another study, the time required to obtain the desired final dry matter for nonsulphited apricots was greater than sulphited apricots (Karabulut, Topcu, Duran, Turan, & Ozturk, 2007). They also pointed out that hot air drying of apricots causes a significant decrease in the SO2 concentration in the samples. In another study, similar results were found that hot air drying decreased the SO2 concentration of dried apricots (Ozkan & Cemeroglu, 2002). But it was mentioned that the drying of apricots with both sun drying and hot air drying without sulphur caused a more undesirable color (browning) (Karabulut et al., 2007). The phenolic contents decreased after the applications compared to the control group (Table 1). The difference between the phenolic contents of groups was found statistically significant (P < 0.05). The US application preserved the phenolic content better when compared to the other pretreated groups. The total phenolic contents was changed in the range of 3.39e14.6 mg GAE/g for the different species of mushrooms (Woldegiorgis et al., 2014). Raw mushrooms have 8.16 ± 0.93 g GAE total polyphenols and after blanching the total polyphenols decrease to 3.22 ± 0.14 g GAE (Jaworska, Pogo, Berna, Skrzypczak, & Kapusta, 2014). Mattila, €m, and To €rro €nen (2006) stated that A. bisporus type Hellstro mushrooms have a lower polyphenol content than other types. There is no study about the phenolic content of mushrooms pretreated with EP þ US before drying but significant differences were found in the total phenolic contents of apple slices dried with ultrasound assisted air drying at 50
C (Rodríguez et al., 2014). The acidity and pH values are also shown in Table 1. The control group has a pH value of 6.540 ± 0.01. The pH value of fresh mushrooms was denoted as 6.44 in previous studies (Borchert et al., 2014; Jaworska, Bernas, Biernacka, & Maciejaszek, 2010). The EP application increased the pH value the most. There is no significant difference between the pH values of the EP þ US and US

Fig. 4. Variation of the drying rate with the moisture ratio of mushroom slices in different groups.

moisture removal was rapid due to the amount of moisture on the surface of the mushroom slices. This situation was explained by Dinani et al., (2014) after finding a high drying rate with pretreatments. The surface of the mushroom slices is saturated with water for only a very short time and consequently no constant rate period is observed. In another study, during the drying of mushrooms with different hot air systems, it was decided that the shorter the time to reach the material balance, the higher the efficiency of dehumidifications. In addition, it was stated that the curve is steeper at the beginning of drying, and the curve becomes gradually gental as the rate of dehydration is slower (Xiao-hui et al., 2014). In addition, other researchers, Chen and Barthakur (1991) also reported that the constant rate period of potato drying was not observed due to the high rate of evaporation. The drying rate increases when the process temperature increases in both convective and vacuum drying. For the same temperature, vacuum drying of mushrooms was faster rather than in the convective drying. For high moistures, convective drying occurred at higher rates than the vacuum drying at all temperatures assayed (Zecchi, Clavijo, Garreiro, & Gerla, 2011). The application of ultrasound preosmotic treatments prior to hot-air drying reduced the drying time significantly by 11e33% when compared to the drying time needed for the hot-air drying only. The drying rate constant of the pre-osmotic ultrasound treated dried guava in osmotic solutions increased by 37e42% when compared with the drying rate constant of the hot-air dried guava (Kek et al., 2013). Overall, drying took place mainly in the falling rate period followed by a constant rate period after a short heating period. Jambrak et al., (2007), found that the drying time after ultrasound treatment was shortened for all samples, as compared to the untreated samples. Decreasing the drying time after ultrasound application was demonstrated by the asymmetric implosions of the cavitation bubbles close to a solid surface that generate microjets in the direction of the surface which can affect mass transfer (Knorr, Zenker, Heinz, & Lee, 2004). The average drying rate of the control group was found to be as 0.507 kg H2O/h.m2 where the average drying rate was calculated as 0.624 kg H2O/h.m2 for the EP group, 0.684 kg H2O/h.m2 for the US group, and Table 1 Physical and chemical analyses of the dried mushroom samples. Samples

Moisture content (g/100 g) Titrable acidity (%) pH

CONTROL EP US EP þ US

4.715 3.470 4.690 3.570

± ± ± ±

0.145a 0.02b 0.24a 0.08b

0.728 0.494 0.557 0.727

± ± ± ±

0.026a 0.026c 0.039b 0.026a

6.540 6.665 6.630 6.620

Water activity ± ± ± ±

0.01c 0.005b 0.0001a 0.01a

Different letters (a to d) within columns are significantly different (P < 0.05).

0.394 0.293 0.319 0.344

± ± ± ±

0.001a 0.0005d 0.0001c 0.0005b

Total phenolic contents (mg/kg) Rehdyration rate Bulk density (g/ml) 5.382 3.096 4.989 4.081

± ± ± ±

0.19a 0.037d 0.049b 0.138c

3.403 3.534 3.838 3.905

± ± ± ±

0.014d 0.02c 0.011b 0.015a

0.200 0.187 0.169 0.159

± ± ± ±

0.0005a 0.006ab 0.014bc 0.016d

R.S¸. Çakmak et al. / LWT - Food Science and Technology 69 (2016) 197e202 Table 2 Color values of the dried mushroom groups. Samples

L*

CONTROL EP US EP þ US

58.925 67.565 77.840 47.810

a* ± ± ± ±

0.045c 0.12b 0.11a 0.13d

4.935 5.135 3.495 6.180

b* ± ± ± ±

0.055c 0.025b 0.015d 0.05a

15.820 22.105 18.985 16.730

± ± ± ±

0.02d 0.045a 0.045b 0.02c

groups (P > 0.05). The acidity changed according to the pH values. The EP application decreased the acidity the most. But previous studies determined that the pH values were low after the EP applications. The transfer of ions by cell degradation increased the _ acidity (Baysal, Içier, Rayman, Cos¸gun, & Petek, 2013; Rayman & Baysal, 2011). But in this case there is a loss in the acidity of mushrooms. After the drying process losses can be seen in the acidity so the pH value increases. This result is in agreement with the previous studies for example after the drying of red bell pepper the acidity decreased and the pH value increased compared to the , Lopes, Barroca, & Ferreira, 2009). fresh sample (Guine The water activity values were found in the range of 0.293e0.394. The applications have a significant effect on the water activity values. The differences could be a result of differences in final water content. The suggested water activity value, which is a very important criteria for the storage of products, should be less than 0.6 (S¸evik et al., 2013). The water activity value was constant after drying at 50
C, but, increased after drying at different temperatures 70, 80, and 90
C during the drying of red pepper (Vegalvez et al., 2009). Indirect sonication with a ultrasonic bath at Ga 1.75 kW and 60 min of immersion time in a 70
Brix osmotic solution when applied to guava (0.53e0.61) had higher water activity values than the hot-air dried guava (0.48) (Kek et al., 2013). The EP þ US has the most rehydration capacity value. There is a significant difference between the values of the groups (P < 0.05). It can be said that EP þ US caused the minimum physical damage. Previous studies also showed that rehydration behavior of plant food could also be enhanced by the use of ultrasound (Jambrak et al., 2007). Similar to our study Kulshreshtha, Singh, and Vipul (2009) found the rehydration rate of dried mushroom slices between 3.184 and 4.015%. The rehydration properties were found to be the best for freeze-dried (FD) samples (Jambrak et al., 2007). In another study, FD samples have 5.45 ± 0.107, whereas, the combination of the mid infrared drying (MIRD-10 min) and the FD application has a 4.95 ± 0.102 rehydration capacity. They also stated that the rehydration rate of FD (6 h)eMIRD mushrooms was 5.297 which was not significantly different (P > 0.05) compared to the RR value of the FD sample (Wang et al., 2015). During the air-drying the rehydration ratio (RR) decreased with temperature, showing a lower RR of 4.24 ± 0.12 (g absorbed water/g d.m.) at 90
C (Vegalvez et al., 2009). Ga All groups have lower bulk densities compared to the control group also the pretreatments decreases the shrinkage. The effect of pretreatments on the bulk density was found statistically significant (P < 0.05). Giri and Prasad (2013) found that the bulk density of mushroom samples which were dried with hot air to be 0.150 g/ cm3 which is comparable with the EP þ US group in our study (0.159 g/cm3). Dinani et al., (2015) measured the solid density of mushrooms after drying with hot air combined with an electrohydrodynamic (EHD) drying system. They indicated that the solid density values were significantly influenced by voltage (P  0.05), whereas, the electrode gap factor, and the interaction of the electrode gap and voltage were not significant (P  0.05). They explained this situation with the fact that moisture removal tends to decrease in the constituents of the remaining solids, with the exception of fat, have a solid density higher than 1000 kg/m3 (fat

201

900e970 kg/m3), carbohydrates from 1500 to 1600 kg/m3, and proteins 1250 kg/m3. So the overall solid density increases as the moisture (density 1000 kg/m3) decreased (Marques, Silveira, & Freire, 2006). The maximum L* value was found in the US group, and the minimum value was found in the EP þ US application as shown in Table 2. The lightness was detected in the US group was significant but, there is a more dark color in EP þ US group. There is no significant difference between the lightness of the control and the other groups (P > 0.05). The lightness of the mushrooms are important for the consumer and the L* value significantly decreases during the storage of the raw material. They measured the L* value of the white mushroom (A. bisporus) as 88.67 ± 0.06 (Khan et al., 2014). Shukla and Singh (2007) indicated the color values (L, a, and b) of fresh mushrooms as 80.90, 4.03, and 17.70 respectively. They found significant changes in the illuminance of the mushrooms after the osmo-convective air drying. Giri and Prasad (2013) found the L* value as 47.8 in mushrooms dried with air. Dinani et al. (2015) indicated that color of mushrooms changes during drying. They found that the whiteness in mushrooms decreases while yellowness increases during drying due to the increase in drying temperature. They decided that the sensitivity of mushrooms to higher temperatures may cause pigmentation in the mushrooms. They also denoted that pre-drying treatments had a significant effect on the whiteness of rehydrated mushrooms. There were no significant differences of the DE response of oven-dried mushroom slices in comparison to the EHD dried samples. Sulphitation is also effective on the color of mushrooms (Kotwaliwale et al., 2007). The dried guavas were darker with decreased L* values ranging from 76 to 80, redder with an increase ranging from 1.9 to 6.0, and more yellow with increased b ranging from 21 to 32. Karabulut et al., (2007) who dried apricots with hot air drying indicated that the color values of hot air dried samples were better in comparison to air drying. Drying at 60 and 70
C generated high L* levels as compared to other temperatures applications for sulphurated apricots. The a* value decreased after US pretreaments. The b* value was higher in the EP group and the b* value was less effected after the combined application. The difference between the b* values of groups was found statistically significant (P  0.05). The L* values of the hot air dried mushrooms ranged from 22.248 to 26.381, while the a* values ranged from 3.467 to 5.789, and the b* were values ranged from 11.891 to 14.789 (Xiao hui et al., 2014). Wang et al. (2015) found significant differences between the fresh shiitake mushroom caps freeze dried samples but the mid infrared drying of samples decreased the lightness of the samples. The b* values decreased after the application of freeze drying for 2 or 4 h, but there is no significant difference between the MIRD (10 and 25 min) treatments and fresh samples. They also determined the b* values after the combination of MIRD with FD applications. They found 9.85 ± 2.57 after MIRD (10 min) FD. Whereas, the FD (4 h)eMIRD has a 15.21 ± 19.8 b value. The value also changes from 6.39 to 7.41 whereas a 7.10 ± 0.41 value was denoted for the FD group. 4. Conclusion EP and US was found to be effective on the quality and drying rate of the mushrooms. Due to cell degredation and increases in the mass transfer and diffusivitiy, the drying time was decreased in pretreated groups when compared to the control group. The US application was effective on drying rate and on the quality parameters such as the phenolic content, and color values. In this group the rehydration capacity increased and acidity values preserved better. After drying, the pH values increased in all groups but there is no significant difference between the groups. Shrinkage

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