Drying of ultrasound pretreated apple and its selected physical properties

Journal of Food Engineering 113 (2012) 427–433

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Drying of ultrasound pretreated apple and its selected physical properties Małgorzata Nowacka ⇑, Artur Wiktor, Magdalena S´ledz´, Natalia Jurek, Dorota Witrowa-Rajchert Warsaw University of Life Sciences, Faculty of Food Science, Department of Food Engineering and Process Management, Nowoursynowska 159c, 02-776 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 22 March 2012 Received in revised form 7 June 2012 Accepted 22 June 2012 Available online 11 July 2012 Keywords: Ultrasound Drying Physical properties Rehydration Microstructure Apples

a b s t r a c t The aim of this work was to investigate the utilization of ultrasound as a mass transfer enhancing method prior to drying of apples tissue. Ultrasound power was provided at a frequency of 35 kHz for 10, 20 and 30 min in the ultrasound bath. Apple cubes were dried using convection method in 70 °C and at air velocity of 1.5 m/s. The effects of ultrasound pre-treatment upon drying were investigated. The ultrasound treatment caused reduction of the drying time by 31% in comparison to untreated tissue. The ultrasound treated apples exhibited between 9% and 11% higher shrinkage, 6–20% lower density, and porosity of 9–14% higher than untreated samples. Considerable differences in the density and porosity of the dried apple with and without ultrasonic application were confirmed by scanning electron microscopy image analysis of the investigated tissue. Moreover, ultrasound application caused alteration of rehydration properties in comparison to untreated sample. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Ultrasounds can be considered as the air vibrations of a frequency from 20 kHz to 100 MHz, and also caused by the mechanical waves propagated in solids, liquids and gases other than air. Ultrasounds have been used by nature for millions of years. Some cetaceans, use ultrasounds in a manner similar to sonar, to identify their prey. One of the advantages of ultrasounds, especially for analytical purposes, is their quick, precise, non-invasive action. Furthermore, they may be used in condensed and optically nontransparent systems (McClements, 1995; Kapturowska et al., 2011). Acoustic waves vibrating with a frequency inaudible for human (18–500 kHz) in materials containing water induce compression and expansion of the material (so called ‘‘sponge effect’’) which leads to formation of microchannels in a cell and causes leaking of intracellular liquid to the surroundings. Moreover, an application of ultrasound in liquids can lead to cavitation. Imploding bubbles induce very high and quick local changes in pressure and temperature, which lead to cells damage. Depending on applied ultrasound sequence and parameters, i.e., type of application (impulse or continuous sonication), impulse duration, time between impulses, number of impulses, their intensity and frequency, it is possible to obtain different effects: thermal effect, cavitation, both thermal and cavitation etc. The effect of ultrasound treatment on food is not always linearly dependent on ultrasound parameters e.g., intensity. Moreover, some phenomena can occur in material, even unexpected. For instance, cavitation may occur in a material ⇑ Corresponding author. Tel.: +48 22 593 75 77; fax: +48 22 593 75 76. E-mail address: [email protected] (M. Nowacka). 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.06.013

where gas was not detected. Acoustic waves inducing compression and expansion of material can cause protein denaturation what results in enzymes activity reduction. However, short ultrasound impulses may also increase enzymes activity by fragmentation of large molecular structures, which enables substrates to be more available by enzymes (Knorr et al., 2004; Fernandes and Rodrigues, 2007; Rastogi, 2010). Drying can be considered as the most common method of food preservation (Vega-Mercado et al., 2001; Lewicki, 2006; Deng and Zhao, 2008). Conventional air-drying is the most popular method of drying in food industry (Chou and Chua, 2001). Unfortunately, this method is energy-intensive and consequently an expensive method of food preservation (Sharma and Prasad, 2006; Fenandes and Rodrigues, 2008a). Some pre-treatment operations can be applied to reduce the initial water content or to modify the fruit tissue structure in the way that air-drying time becomes shorter (Kobus, 2005; Fernandes and Rodrigues, 2007). An application of ultrasound of a high intensity and lower frequency causes cells damage by their membranes continuity interruption. This behavior increases mass transfer rate between the cell and its extracellular surroundings. The connection of ultrasounds activity with drying, both during the process and as a preliminary treatment before removing water, profitably influences the properties of the obtained product. For instance, ultrasound applied during the course of process can improve the quality of dried food, due to its non-thermal character. Moreover, drying time can be reduced and the process can be performed at lower temperature, which is notably significant for products containing thermolabile substances e.g. persimmon, carrot or lemon peels (Carcel et al., 2007; Gallego-Juarez et al., 2007; Garcia-Perez et al., 2007).

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Ultrasound applied as preliminary treatment before drying may modify the course of the process and tissue properties. Fernandes and Rodrigues (2007) and Rodrigues and Fernades (2007) demonstrated, that ultrasound pretreatment of banana and melon before convective drying reduced drying time by 25%, and in the case of pineapple by over 30% (Fernandes et al., 2008a), when compared with the control sample. Additionally, the content of sugars in the banana and melon tissue decreased during ultrasound treatment. The changes in properties of dried material after ultrasound treatment were noted also by Jambrak et al. (2007), who claim that ability of rehydration of ultrasound processed, dried mushrooms, brussels sprouts and cauliflower germs was higher when compared to reference samples. The aim of this study was to investigate the effect of ultrasound on the mass transfer and water removal from apple cubes. The influence of ultrasound treatment on the physical properties including shrinkage, porosity, rehydration ability and microstructure were analyzed as well. 2. Material and methods 2.1. Preparation of samples Apples (var. Idared) were collected from the Experimental Fields (Orchards) of the Faculty of Horticulture and Landscape Architecture (Warsaw University of Life Sciences) and stored at 5–8 °C and at 90% air humidity for 2 weeks after harvest. The skin was removed from the apples and they were cut into cubes of 0.01 m side. Afterwards, the material was immersed in the 0.1% citric acid solution to prevent enzymatic browning reactions. Finally, the cubes were blotted with filter paper and pre-treated by ultrasound. 2.2. Ultrasound pretreatment The samples were immersed in distilled water and placed in an ultrasonic bath (InterSonic, Olsztyn, Poland, model IS-3, internal dimensions: 240  135  100 mm). The cubes were placed next to each other in the bath and covered with the metal net to avoid flow out of the samples. After that the distilled water was added into the ultrasonic bath. The pre-treatment was carried out at room temperature (25 °C). The ratio of raw material to water was set at 1:4, as recommended by Fernandes and Rodrigues, 2008a,b,c). The ultrasound frequency was 35 kHz. The ultrasound energy was applied for 10, 20 and 30 min. After the treatment the plant materials were blotted with filter paper and spread on the dryer’s screens. Before and after ultrasound treatment the mass of the samples, dry matter content and water temperature were measured. The temperature increase during the experiments was equal 3, 5 and 10 °C after 10, 20 and 30 min of ultrasound treatment, respectively. The experiments were conducted in duplicate for each drying process. 2.3. Convective drying Convective drying was carried out at 70 °C and air velocity 1.5 m/s in laboratory dryer (Warsaw, Poland). These parameters were set according to the most popular parameters used in industries. The dryer was pre-heated to temperature set point and then was loaded with 0.1 kg (1.13 kg/m2) of apple cubes spread on nets in a single layer. Each drying process was used around 100 cubes. The air flow was parallel to the screens and the drying process continued until constant mass was reached. During drying the mass of the material was recorded continuously, every 1 min, using program ‘‘Pomiar’’ (Radwag, Radom, Poland). The drying processes were conducted in duplicate.

The dry matter content in the raw, pre-treated and dried apples was measured according to Polish Standard PN-90/A-75101/03 by drying to the constant weight at 105 °C. Relying on the measurements of mass losses of the sample during process, drying curves were plotted as functions of dimensionless water content (MR) versus time. MR was computed according to the following equation:

MR ¼

u  ue u0  ue

ð1Þ

where: u – water content in the course of drying (kg H2O/kg d.b.). u0 – initial water content (kg H2O/kg d.b.). ue – equilibrium water content (kg H2O/kg d.b.) The drying rates were calculated relying on the first derivative of the Midilli et al. (2002) equation:

MR ¼ a  expðk  sn Þ þ b  s

ð2Þ

where: s – drying time (min). Regression analysis of the drying rate was conducted by means of Table Curve 2D software. Coefficient of determination R2, root mean squared error (RMSE), and the reduced v2 values were calculated from the following equations:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 i¼1 ðMRi;p  MRi;e Þ RMSE ¼ N

v2 ¼

PN

 MRi;p Þ2 Nn

i¼1 ðMRi;e

ð3Þ

ð4Þ

where: MRi,p– model-based value of relative water content. MRi,e– experimentally obtained value of relative water content. N number of observations. n – number of constant parameters in the model equation. The effective water diffusion coefficient Deff for each sample, were computed with the Table Curve 2D software according to Simplified Fick’s second law for cube equation:

MR ¼

8

 exp 2

p

  p2  Deff  s  3s2

ð5Þ

where s is size of the sample (m). 2.4. Analysis of physical properties Dried apple cubes were stored in a hermetic container for 3 days in order to equilibrate the water distribution of cubes. The volume of both raw and dried slices was measured by the displacement method using toluene (Mazza, 1983). Based on measurements of the volume of material, shrinkage was calculated using following equation:

S¼

  VK  100% 1 V0

ð6Þ

where S – shrinkage (%). Vk – the final volume of dried cube (cm3). V0 – volume of cube before drying (cm3). The shrinkage was estimated by measuring the volume of the apple cubes. The measurement of the mass of the samples made it possible to calculate the apparent density. Porosity was calculated according to Andrés et al. (2004) by applying the following equations:

e¼

qt  qa qt

qt ¼

1 xw 1000

wÞ þ ð1x 1590

ð7Þ

ð8Þ

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where:e - porosity ()qt - true density (kg/m3)qa - apparent density (kg/m3)xw – water content (). 2.5. Rehydration

3. Results and discussion

Rehydration was carried out at 20 °C. The kinetics of the process were analysed in a range of 0–1 h. Dried apple was immersed in distilled water for 5, 15, 30 and 60 min. After the certain period of time the samples were separated from water with the sieve, blotted with a blotting paper and weighed. Rehydration experiments were repeated twice for each rehydration time and for each dried material and the average moisture content, which was expressed as the dimensionless moisture (Eq. 9), was used every time for drawing the rehydration curves.

X¼

Xs X0

ð9Þ

where Xs is moisture of rehydrated apple at time s (kg H2O/kg db), and X0 is initial moisture of fresh sample (kg H2O/kg db). The soluble solid loss was measured by mass method. In rehydrated samples was measured dry matter content. The soluble solid loss was measured using following equation:

SSL ¼

ms  dms md  dmd

ð10Þ

where ms– material weight after rehydration time s (g). dms – dry matter content of sample after rehydration time s (%). md – dried material weight before rehydration (g). dmd – dry matter content of dried sample before rehydration (%). The course of rehydration and course of loss of water soluble components of dry matter was described by the Peleg’s model (Planinic´ et al., 2005):

X ¼ X0 

s K1 þ K2  s

ð11Þ

where X is the moisture content at time s, X0 is the initial moisture content, K1 is the Peleg rate constant, and K2 is the Peleg capacity constant. The ‘‘ ± ’’ becomes ‘‘ + ’’ if the process is absorption/ adsorption (water), or becomes ‘‘-’’ if the process is desorption (soluble solid). The coefficient of determination R2, the root mean squared error (RMSE) and the reduced v2 values were calculated for rehydration and loss of water soluble components of dry matter from the Eqs. (3) and (4). Moreover, the equilibrium value for rehydrated samples was calculated using relations between equilibrium moisture content and K2 constant, when s tends to infinity:

Xjs!1 ¼ X e ¼ X 0 

differences were identified using Duncan’s multiple range tests with the probability level set at 0.05.

1 K2

ð12Þ

2.6. Microstructure The microstructure was examined using a scanning electron microscope (Phenom G2 Pro, Eindhoven, Netherlands). In order to produce SEM images, small pieces were taken from the samples and put into the vacuum chamber. The image were made using 265 magnification. 2.7. Statistical analysis Replicates of treatment and drying was conduct duplicate. All measurements was conduct in three repetitions. Therefore, the number of samples was six for each measurements and four for rehydration. All results were subjected to the analysis of variance (ANOVA) using Statgraphics Plus 4.1 software. Individual group

3.1. Ultrasound treatment Ultrasound causes a series of rapid contractions and expansions of tissue, which can remove water from raw material to the surrounding environment. The use of ultrasound as a pre-treatment resulted in weight loss of apple cubes. After 30 min of ultrasound treatment the weight loss were the smallest (0.8 ± 0.37%), and larger weight loss for samples exposure to ultrasound for a shorter period of time were observed. Samples subjected to ultrasound treatment for 10 and 20 min lost 2.3 ± 0.08 and 3.0 ± 0.19% of weight, respectively. Probably, during the long residence time in the water, apple tissue during pre-treatment, apart from the effect of ultrasound causing removing water from the tissue, took place ingress of water to the inside of material, due to osmotic concentration differences. Such a process was made possible by the high porosity of apple tissue and could be the reason for the minimum mass loss during the ultrasonic treatment, which lasted the longest for 30 min. For papaya after 90 min the fruit subjected to ultrasound was the largest loss of water (11.92%), while at shorter ultrasound treatment time the water loss was lower (Fernandes et al., 2008b). Moreover, weight changes in raw materials during the pretreatment were associated with changes in dry matter content in apple tissue. With the increase of the applied treatment time followed loss of dry matter. After 10, 20 and 30 min of ultrasonic treatment apple tissue contain 11.7 ± 0.06, 10.8 ± 0.06 and 10.1 ± 0.06% of dry matter content, respectively. This changes were statistically significant in comparison to untreated sample. 3.2. Drying characteristic In the course of the experiment, apple cubes were dried from the initial water content of about 7.5 kg H2O/kg d.b. The process resulted in obtaining dried apples characterized by dry matter content of 91.4 ± 0.45% for untreated sample, and 90.1 ± 0.58%, 91.1 ± 0.51%, and 91.6 ± 0.71% for samples treated for 10, 20 and 30 min, respectively (Table 1). Moreover, the water activity of the dried samples were measured (Table 1) and results obtained in experiment indicates that equilibrium state has been reached (Rza˛ca, 2009) (see Table 1). Time required to dry the apple slices to water content of 0.11 kg H2O/kg d.b. (u/u0 = 0.015) for untreated apple cubes, 10, 20 and 30 min ultrasound treated samples was 165, 108, 114 and 99 min, respectively (Fig. 1). The ultrasound treatment reduced the drying time by 31–40%. Similarly, Fernandes et al. (2008a), who dried ultrasound treated pineapple reported the reduction of the drying time over 30% in comparison with the reference sample. Similar effects were described by Rodrigues and Fernades (2007) for melon, Jambrak et al. (2007) for Brussels sprouts and

Table 1 Drying time of fresh apple tissue and treated with US to obtain 0.11 kg H2O/kg d.b. and its dry matter content. Treatment

Drying time [min]

Dry matter [%]

Water activity []

Untreated 10 min US 20 min US 30 min US

165 ± 2a 108 ± 1b 114 ± 1c 99 ± 1d

91.4 ± 0.45b 90.1 ± 0.58a 91.1 ± 0.51b 91.6 ± 0.71b

0.248 ± 0.01ab 0.285 ± 0.01bc 0.335 ± 0.01d 0.393 ± 0.01e

a, b, c, d, e: the same letters show homogeneous groups.

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Fig. 1. Dimensionless moisture content in untreated and ultrasound treated apples during drying, MR – relative water content, a, b, c, d: the same letters show homogeneous groups at the end of process.

after 30 min ultrasound application. Lower drying rates equal to 0.048 and 0.047 kg/(kg d.bmin) were observed for samples ultrasound treated for 10 and 20 min, respectively. Untreated samples were characterized by the longest drying time and their drying rate was the smallest 0.042 kg/(kg d.bmin). However, the effect of ultrasound treatment duration on plant tissue is not explicit. In our investigations, the samples treated with ultrasound for 30 min were dried faster, than apples treated for 10 min. Moreover, the material after 20 min of ultrasonic treatment exhibited the longest drying time of all sonicated samples. The influence of sonication time is different for various type of raw material i.e., drying of pineapples (Fernandes et al., 2008a) and bananas (Rodrigues and Fernades, 2008) was faster after 20 min of ultrasound application than after 10 and 30 min. However, the Sapota fruits submitted to 30 min of ultrasound pretreatment dried faster as compared with the sample treated for 10 min (Fenandes and Rodrigues, 2008c). The calculated values of the effective water diffusion coefficients for each individual samples are presented in Table 3. This parameter can be used as a specific indicator of drying rate. Hence, the highest value of the effective moisture diffusion coefficient was noticed in sample treated with ultrasound for 10 and 30 min. Therefore, it was more than 18% higher compare with the reference sample. It is obvious, that the effective moisture diffusion coefficients corresponded to the time of drying. For instance, the intact samples exhibited both the lowest value of this parameter and longest time of drying. Although there were no differences in the effective water diffusion coefficient for samples treated by ultrasound for 10 and 30 min, the drying times were different (95 and 88 min, respectively). It is worth noticing, that also other non-thermal pretreatment methods lead to increase of this parameter, e.g., pulsed electric field (PEF) treatment (Ade-Omowaye et al., 2003). 3.3. Physical properties

Fig. 2. Drying rate curves of convective drying of untreated and ultrasound treated apples obtained within first derivative calculated from the Midilli et al. model.

cauliflower, García-Pérez et al. (2009) for lemon peel, Ortuño et al. (2010) for orange peel. The drying rate of ultrasound treated samples was greater in comparison to untreated one (Fig. 2). Table 2 shows the regression coefficients (R2) and the statistical parameters of drying rate curves for investigated samples. Midilli models was found to fit the experimental data very well with R2 value equal 0.999, RMSE and v2 values equal or lower than 5.1  103 and 2.6  105, respectively. Drying rate curves confirmed that ultrasonic treatment enhanced water loss during drying. The highest drying rate, 0.052 kg/ (kg d.bmin), was noticed at the beginning of the drying for apples

In the performed investigations the apples without pretreatment exhibited the smallest shrinkage 60% (Fig. 3). No significant differences between ultrasound treated samples were observed in regard to this parameter. Nevertheless, it was 9–11% higher when compared with the intact dried sample. Shrinkage caused by water removal has an impact on the density of the material (Witrowa-Rajchert and Rza˛ca, 2009). Therefore, a similar trend in the apparent density was noticed. However, after 20 and 30 min of ultrasonic application the changes were statistically significant. Untreated dried apple exhibited the highest value of the apparent density equal to 427 ± 42 kg/m3 (Fig. 4). The apparent density of ultrasound treated samples was lower by 6–20% in comparison to the references sample, and the changes were significant for samples with 20 and 30 min ultrasound treatment. This was confirmed by the microscopic image analysis, where structural damage was observed, especially for apples pretreated by 30 min of ultrasound. Apples, obtained by convective drying without any preliminary treatment were characterized by the lowest porosity of 58% (Fig. 5), and the longest drying time. The porosity of ultrasound

Table 2 Parameters of Midilli et al. model describing the kinetics of drying rate of apples. Treatment

Equation parameters y = a exp(ktn) + b s a

Untreated 10 min US 20 min US 30 min US

0.997 1.001 1.001 1.012

k 0.049 0.055 0.055 0.064

n 0.905 0.902 0.890 0.857

Statistical parameters R2

b 4

0.4  10 1.4  104 1.3  104 2.8  104

0.999 0.999 0.999 0.999

v2

RMSE 3

5.1  10 3.2  103 3.4  103 3.5  103

2.6  5 1.1  105 1.2  105 1.3  105

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M. Nowacka et al. / Journal of Food Engineering 113 (2012) 427–433 Table 3 The effective moisture diffusion coefficient during drying of apple tissue depending on sonication parameters. Treatment

Untreated 10 min US 20 min US 30 min US

D  109 [m2/s]

1.829 2.168 2.058 2.168

Statistical parameters R2

RMSE

v2

0.997 0.999 0.999 0.998

0.015 0.075 0.016 0.017

2.4  104 5.8  103 2.6  104 3.1  104

Fig. 5. Porosity of dried apples untreated and submitted to ultrasonic treatment for 10, 20 and 30 min, a, b: the same letters show homogeneous groups.

Fig. 3. Shrinkage of dried apples untreated and 10, 20 and 30 min submitted to ultrasonic treatment, a, b: the same letters show homogeneous groups.

Fig. 6. Rehydration rate curves of dried apple untreated and submitted to ultrasonic treatment for 10, 20 and 30 min, X – dimensionless moisture, a, b: the same letters show homogeneous groups after 60 min of rehydration.

Table 4 Parameters of Peleg’s model describing the kinetics of rehydration of dried apples. Treatment

Untreated 10 min US 20 min US 30 min US

Equation parameters

Statistical parameters

K1

K2

R2

RMSE

v2

26.253 30.255 34.014 38.243

0.646 0.859 0.598 0.436

0.994 0.991 0.990 0.980

0.026 0.024 0.030 0.045

0.009 0.008 0.010 0.015

Fig. 4. Apparent density of dried apples untreated and submitted to ultrasonic treatment for 10, 20 and 30 min, a, b: the same letters show homogeneous groups.

treated samples was 9–14% higher in comparison to the intact sample. These apples, as it was mentioned earlier, exhibited shorter drying times than the untreated material. It is worth to notice that significant changes in porosity were observed for tissue treated with ultrasound for 20 min. 3.4. Rehydration properties After drying, products can be directly consumed or further processed (Vadivambal and Jayas, 2007). Drying causes the irreversible changes in the structure of the material. Thus, the capacity of water absorption and its maintenance is reduced (Nijhuis et al., 1998; Witrowa-Rajchert and Lewicki, 2006; WitrowaRajchert and Rza˛ca, 2009). Rehydration is one of the most important quality parameters of dried foods, often used to evaluate instant products (Sumnu et al., 2005). During the rehydration three processes are proceeding simultaneously: mass gain, volume increase of dried material and loss of soluble components of dry matter (Witrowa-Rajchert and Lewicki, 2006). It is generally accepted that the rehydration rate is dependent on the degree of disruption

Table 5 Parameters of Peleg’s model describing the kinetics of loss of water soluble components of dry matter of dried apples. Treatment

Untreated 10 min US 20 min US 30 min US

Equation parameters

Statistical parameters

K1

K2

R2

RMSE

v2

58 550 66.653 67.814 51.582

1.399 1.575 1.208 1.376

0.975 0.985 0.985 0.978

0.025 0.016 0.019 0.023

0.008 0.005 0.007 0.008

and destruction of cells and its structures (Prothon et al., 2003). Hence, it allows us to determine the degree of physical and structural changes that have occurred during drying (Witrowa-Rajchert and Lewicki, 2006). In this investigation the moisture uptake increased with increasing rehydration time, but at a decreasing rate up to saturation level (Fig. 6). The same behavior was observed by Deng and Zhao (2008) during rehydration of apple cylinders var. Fuji and by Jambrak et al. (2007) during rehydration of mushrooms, brussels sprouts and cauliflower.

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Table 6 Moisture content after 60 min of rehydration and calculated equilibrium value (over time tends to infinity). Treatment

Moisture content after 60 min (kg moisture/kg d.b.)

Equilibrium moisture content (kg moisture/kg d.b.)

Untreated 10 min US 20 min US 30 min US

5.32 ± 0.68b 4.23 ± 0.41a 5.00 ± 0.19b 5.48 ± 0.22b

6.87 5.39 6.67 7.77

a, b: the same letters show homogeneous groups.

Fig. 7. Loss of water soluble components of dry matter (SSL) curves of dried apple untreated and 10, 20 and 30 min submitted to ultrasonic treatment, a, b: the same letters show homogeneous groups after 60 min of rehydration.

Tables 4 and 5 shows the parameters of Peleg’s model describing the kinetics of rehydration and loss of water soluble components of dry matter of dried apples, regression coefficients (R2) and other statistical parameters for investigated samples. Peleg model shows a good applicability for rehydration process. The regression coefficients (R2) were in the range of 0.975–0.994. Moreover, the K2 parameters may be treated as a parameter characterizing the sorption properties of the material. The lower value of parameter K2 of rehydration shows better water absorption properties, which was confirmed by the moisture content in apples after 60 min of rehydration and the calculated equilibrium value (see Table 6). The amount of moisture in untreated apple, absorbed after 60 min of rehydration were equal 5.32 ± 0.68 kg moisture/kg d.b. (Table 6). The moisture content of rehydrated samples subjected to ultrasonic treatment for shorter time (10 and 20 min) were lower as compared to untreated one. However, only a samples subjected to ultrasound for 10 min was significantly different. The highest moisture content was obtained for 30 min ultrasound treated samples, however the rehydration was not completed, and the equilibrium was not obtained. Additionally, equilibrium values confirm the observed differences in samples with and without treatment, which were analyzed after 60 min of rehydration. During the rehydration, loss of water soluble components of dry matter is observed. The loss of water soluble components of dry matter increased with increasing rehydration time (Fig. 7). The lowest loss of water soluble components of dry matter was reported for samples treated by ultrasound for 10 min. The highest value was observed for samples submitted for ultrasound for

Fig. 8. Photos of dried apples made using scanning electronic microscopy at a magnification of 265; untreated samples (a), 10 min (b), 20 min (c), 30 min (d) ultrasound treated samples.

M. Nowacka et al. / Journal of Food Engineering 113 (2012) 427–433

30 min, what means that the loss of water soluble components of dry matter in this sample was the lowest. Also the K2 parameter of loss of water soluble components of dry matter of dried apples confirm this dependence (Table 5). 3.5. Microstructure The SEM photos in the section of dried apples showed a distinct difference in the microstructure of apples subjected to a different duration of ultrasound treatment. The microstructure of untreated dried apple slices was characterized by small cavities and high density. The microscopic image analysis showed that ultrasound treatment used before drying induced breakdown of cells. Of course, high temperature during the drying process may also cause structural damages and destroy the material in some places (Witrowa-Rajchert and Rza˛ca, 2009). Our study showed that the ultrasound application before drying may enhance structural damages caused by elevated temperature during drying. Moreover, increased temperature during the ultrasound treatment might have influence on the greater cell damages, especially when samples were subjected to 30 min of ultrasound treatment. A longer ultrasound treatment time resulted in greater destruction of structure of dried apple (Fig. 8a–d). Similarly, Fenandes and Rodrigues (2009) found that ultrasound treatment before drying causes disruption of cells and formation of microscopic channel in pineapple structure. After 30 min of the ultrasound application, researchers observed, that the cell becomes more distorted. However, when applied ultrasound during osmotic dehydration of melon for more than one hour caused noticeable breakdown of cells (Fenandes and Rodrigues, 2008b). Moreover, Deng and Zhao (2008) also obtained differences in the microstructure of apple subjected to the different treatment such as osmotic dehydration, osmotic dehydration with pulsed vacuum, ultrasound, hot-air drying and freeze drying. 4. Conclusions The use of ultrasound as pretreatment has been demonstrated to facilitate water loss during drying. The ultrasound treatment of apple cubes caused reduction of the drying time by 31–40% in comparison with untreated sample. Ultrasonic treated material showed between 9% and 11% higher shrinkage, 6–20% lower density and porosity of 9–14% higher than untreated samples. Considerable differences in the density and porosity of the dried apple with and without ultrasonic application were confirmed by analyzing the tissue images made with the scanning electron microscopy. Ultrasound treatment and drying induced changes of apples microstructure. Application of ultrasound before drying caused breakdown of cells and damage of the tissue, what can be enhanced by the drying process. Moreover, the results showed the time of ultrasound treatment influence on apples microstructure. A longer ultrasound treatment time resulted in greater destruction of structure of dried apple. Furthermore, ultrasound application caused alteration of rehydration properties in comparison to untreated sample. References Ade-Omowaye, B.I.O., Rastogi, N.K., Angersbach, A., Knorr, D., 2003. Combined effect of pulsed electric field pre-treatment and partial osmotic dehydration on air drying behaviour of red bell pepper. Journal of Food Engineering 60, 89–98. Andrés, A., Bilbao, C., Fito, P., 2004. Drying kinetics of apple cylinders under combined hot air–microwave dehydration. Journal of Food Engineering 63 (1), 71–78. Carcel, J.A., Garcia Perez, J.V., Riera, E., Mulet, A., 2007. Influence of high-intensity ultrasound on drying kinetics of persimmon. Drying Technology 25, 185–193. Chou, S.K., Chua, K.J., 2001. New hybrid drying technologies for heat sensitive foodstuffs. Trends Food Science and Technology 12 (10), 359–369.

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