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International Journal of Food Properties

ISSN: 1094-2912 (Print) 1532-2386 (Online) Journal homepage: http://www.tandfonline.com/loi/ljfp20

Shrinkage of Mirabelle Plum during Hot Air Dryingas Influenced by Ultrasound-Assisted OsmoticDehydration

Jalal Dehghannya, Rasoul Gorbani & Babak Ghanbarzadeh

To cite this article: Jalal Dehghannya, Rasoul Gorbani & Babak Ghanbarzadeh (2016)Shrinkage of Mirabelle Plum during Hot Air Drying as Influenced by Ultrasound-AssistedOsmotic Dehydration, International Journal of Food Properties, 19:5, 1093-1103, DOI:10.1080/10942912.2015.1055362

To link to this article: http://dx.doi.org/10.1080/10942912.2015.1055362

Accepted author version posted online: 09Jul 2015.Published online: 09 Jul 2016.

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Shrinkage of Mirabelle Plum during Hot Air Drying asInfluenced by Ultrasound-Assisted Osmotic Dehydration

Jalal Dehghannya, Rasoul Gorbani, and Babak Ghanbarzadeh

Department of Food Science and Technology, University of Tabriz, Tabriz, Iran

Convective drying in hot air is still the most popular method applied to reduce the moisture content offruits and vegetables. Conventional hot-air drying of Mirabelle plum is considered to be a slow andenergy intensive process. This is due to the fact that the waxy skin of Mirabelle plum has lowpermeability to moisture, a fact which results in high shrinkage. The aim of this study was to investigatethe effect of ultrasound-assisted osmotic dehydration pretreatment on shrinkage of Mirabelle plum as afunction of moisture content with the end goal of optimizing operating conditions that minimizeshrinkage of the produce during drying. Results showed that application of ultrasound-assisted osmoticdehydration led to a significant (p < 0.05) decrease in shrinkage (from 76.41 to 64.05%). A linearrelation between moisture loss and shrinkage was observed. Results indicated that shrinkage may beeasily estimated from changes in moisture content, and independent of the drying rate. Inversely,determination of shrinkage would provide an indirect indication of moisture content.

Keywords: Air drying, Mirabelle plum, Ultrasound-assisted osmotic dehydration, Moisture content,Shrinkage.

INTRODUCTION

Convective drying in hot air is still the most popular method applied to reduce the moisture contentof fruits and vegetables. However, this method has several disadvantages and limitations; forinstance, it requires relatively long times and high temperatures, which cause degradation ofimportant nutritional substances as well as color alteration. Another disadvantage is shrinkage,which results from tissue collapse caused by volume reduction, and is due to the loss of moisture aswell as the presence of internal forces.[1]

Shrinkage is important not only for quantification of the quality of dehydrated foodstuffs butalso in the characterization of textural properties of materials.[2,3] It is known that mass transfer rateis affected by shrinkage of the product and volume changes are dependent of several factors suchas geometry, drying method and experimental conditions. Physical properties such as bulk densityand porosity change and transport properties like thermal and mass coefficient of diffusion arerelated to changes in material shrinkage during dehydration.[4–7] Major shrinkage can indicate

Received 28 February 2015; accepted 22 May 2015.Address correspondence to Jalal Dehghannya, Department of Food Science and Technology, University of Tabriz, 29

Bahman Blvd., Tabriz 51666-16471, Iran. E-mail: J_dehghannya@tabrizu.ac.irColor versions of the figure in the article can be found online at www.tandfonline.com/ljfp.

International Journal of Food Properties, 19:1093–1103, 2016Copyright © Taylor & Francis Group, LLCISSN: 1094-2912 print/1532-2386 onlineDOI: 10.1080/10942912.2015.1055362

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structural damage because it implies the collapse of the tissue’s structural organization.[8] Duringdrying, shrinkage is rarely negligible. Furthermore, it is necessary to take it into account whenpredicting moisture content profiles in the material undergoing dehydration.[9,10] Values of theeffective diffusivities estimated while taking shrinkage into consideration were smaller than thoseobtained without considering this phenomenon.[7] Therefore, any attempt to characterize dryingbehavior must inevitably address physical parameters—such as shrinkage—of the material.[11]

Attempts have been made to describe shrinkage of different products undergoing different dryingprocesses and conditions.[12]

Among various fruits and vegetables, conventional hot-air drying of the Mirabelle plum isconsidered to be a slow and energy intensive process. This is because its waxy skin has lowpermeability to moisture,[13] a fact which results in high shrinkage. Skin of this fruit consists of anunderlying amorphous wax layer adjacent to the cuticle proper, together with crystalline granulesof wax protruding from the surface.[14] Therefore, any pretreatment for plum drying processeswhich decreases shrinkage by reducing drying time through reducing the initial moisture contentand preserves the prune (dried plum) quality is of considerable interest.[15] Various pretreatmentssuch as blanching, freezing, piercing, abrasion and chemical additives have been used to increasemoisture transport from the plum surface. Methodologies such as ultrasound-assisted osmoticdehydration have also been implemented in a few studies as an alternative pretreatment to increasemoisture transport from the plum surface.[16–18] Reduction of drying time and, consequently,processing costs have been reported at the experimental scale after research was conducted onseveral fruits and vegetables. Osmotic dehydration pretreatment partially removes water from fruitsor vegetables immersed in a hypertonic solution.[19,20] Regarding low mass transfer rate duringosmotic treatment, ultrasound can be used to improve mass transfer rate and dehydration time.[18]

Ultrasonic waves can bring about a very rapid series of alternative compressions and expansions,similar to what a sponge does when it is squeezed and released repeatedly. Forces involved in thismechanical mechanism create microscopic channels that may ease moisture removal. In addition,ultrasound produces cavitation, which can be beneficial for removal of the moisture that is stronglyattached to the solid.[21,22]

Analysis of the relationship among process factors and shrinkage during drying could provide asolid base to optimize drying process.[16] Analyses of various experimental data have revealed thatshrinkage of food materials during drying could be represented only as a function of moisturecontent without any considerable dependency on inert material, air temperature, and velocity orsample length.[11] To our knowledge, there has been no study in the literature devoted toinvestigation of the effect of ultrasound-assisted osmotic dehydration as a pretreatment on shrink-age of the Mirabelle plum during hot-air drying. Therefore, the aim of this study was to investigatethe effect of ultrasound-assisted osmotic dehydration pretreatment on shrinkage of the Mirabelleplum as a function of moisture content searching for optimal operating conditions (sonication time,concentration of osmotic solution, and immersion time in the osmotic solution) that help minimizeshrinkage of the produce during drying.

MATERIALS AND METHODS

Preparation of the Samples

Mirabelle plums (Prunus domestica subsp. syriaca) were purchased from a local garden. They weresorted visually based on a relative standard of maturity, shape, size, and color. Such a sorting stagewas intended to select similar plums to be used in every experiment and to discard ripe and damagedsamples. Before experiments, plums were washed with tap water and were dried with a filter paper.

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Moisture content was gravimetrically measured by drying samples in an oven at 105ºC to reachconstant weight.[16] The average initial moisture content of the plums was 4.54 g water/g dry matter.

Ultrasound-Assisted Osmotic Dehydration Pretreatment

Pretreatments were structured in combinations of two ultrasonication times (both at 40 kHz): 10and 30 min; two osmotic solution concentrations: 50 and 70% sucrose in water (% w/w) and fourimmersion times in osmotic solution: 60, 120, 180, and 240 min. No pretreatment (neitherultrasonic nor osmotic treatment) was applied to control samples (Table 1). Results of kineticsstudies were obtained before these ultrasonication times were chosen. Results showed that effectsof ultrasound pretreatment started to influence the drying process after 10 min. After 30 min,changes inferred in the drying process became insignificant.[21]

Ultrasonic pretreatments were carried out using an ultrasonic bath (AS ONE Corporation, US-4R, Japan, capacity: 9.5 L, dimensions: 36.5 (height) × 30.5 (width) × 26.2 (depth) cm; oscillatingfrequency: 28 and 40 kHz, high frequency output: 160 W) without mechanical agitation. The bathwas operated at a frequency of 40 kHz. Water temperature inside the ultrasonic bath was main-tained constant at 25ºC. Temperature increase during the experiments was not significant (less than2°C) after 30 min of ultrasonic treatment.

In each ultrasound-assisted osmotic dehydration pretreatment trial, an experimental set of plumsamples were immersed in four separate beakers (one for each immersion time in osmotic solution:60, 120, 180, and 240 min) filled with osmotic pretreatment solution and were then placed in theultrasonic bath for 10 and 30 min. Experiments were carried out in separate beakers to avoidinterference between samples and runs. Osmotic solutions were prepared through mixing food-grade sucrose with distilled water until concentrations (% w/w sucrose in water) of 50 and 70%were obtained. The weight ratio between fruit and the osmotic solution was 1:4. This ratio wasused to avoid dilution effects.[17,21]

TABLE 1Abbreviations utilized for different treatments

Abbreviation U* B** T***

Control 0 0 0U10-B50-T60 10 50 60U10-B70-T60 10 70 60U30-B50-T60 30 50 60U30-B70-T60 30 70 60U10-B50-T120 10 50 120U10-B70-T120 10 70 120U30-B50-T120 30 50 120U30-B70-T120 30 70 120U10-B50-T180 10 50 180U10-B70-T180 10 70 180U30-B50-T180 30 50 180U30-B70-T180 30 70 180U10-B50-T240 10 50 240U10-B70-T240 10 70 240U30-B50-T240 30 50 240U30-B70-T240 30 70 240

*U: Ultrasonication time at 40 kHz (min);**B: Osmotic solution concentration (Brix) [Sucrose in water (% w/w)];***T: Immersion time (min).

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After completion of the ultrasound-assisted osmotic dehydration pretreatment for the intendedtime (10 and 30 min), all the beakers were removed from the ultrasonic bath and the remainingtime for osmotic dehydration pretreatment was passed under ambient temperature (25°C) andwithout mechanical agitation. Total immersion times of the samples in osmotic solutions were60, 120, 180, and 240 min, considering both the time with and without ultrasound. After reachingthe desired time, samples were removed from the beakers, washed with distilled water, and blottedwith absorbent paper to remove excess solution on the surface. All experiments were carried out induplicate.

Hot-Air Drying

After the completion of the osmotic dehydration pretreatment, samples were placed in Petri dishesin a single-layer arrangement and were dried in a pilot plant hot-air drier (UOP 8 Tray dryer,Armfield, UK). Air temperature in the drier was set at 80ºC.[23] Cross-flow air moved from side toside of the dryer at 1.4 m/s, flowing parallel to the drying surface of the samples. Moisture loss wasrecorded at 30 min interval by a digital balance of 0.01 g accuracy. Drying process continued untilan average moisture content of 0.57 g water/g dry matter was obtained.

Determination of Shrinkage

Shrinkage represents a relative or reduced dimensional change of volume and is represented by:[24]

S ¼ 1� Vt

V0

� �� 100

where S is the shrinkage (%), Vt is the apparent volume of the sample at a certain degree of drynessafter time t and V0 is the apparent volume of the raw sample. Toluene displacement method wasused to measure the volume of the samples gravimetrically.[25,26] Based on this method, sampleswere transferred into a flask half filled with toluene after being weighed precisely. The flask wasthen filled with toluene, the level of solvent being carefully adjusted to ensure consistency, and wasweighed. Sample volume (V) was calculated using:[25]

V ¼ Vf �Msf

ρs

Msf ¼ Mtþs �Mf �M

where Vf is the volume of the flask; Msf is weight of toluene added to fill the flask; Mt+s is theweight of the flask plus the sample and the solvent; Mf is the weight of the flask; M is the weight ofthe sample; and ρs is the density of toluene (0.87 g/cm3 at 20ºC).

Experimental Design and Statistical Analysis

A 2×2×4 factorial experiment in a randomized complete block design with two replicates was usedto study the effects of ultrasonication time, osmotic solution concentration, and immersion time inosmotic solution on shrinkage as a response variable until an average moisture content of 0.57 gwater/g dry matter was obtained. Independent variables were ultrasonication time at two levels: 10and 30 min; osmotic solution concentration at two levels: 50 and 70% (w/w); and immersion timein osmotic solution at four levels: 60, 120, 180, and 240 min. For control samples, no pretreatment[ultrasonication time (0 min), osmotic solution concentration (0%) or immersion time in osmoticsolution (0 min)] was utilized. Values in the analysis of variance (ANOVA) table were calculatedusing the Proc GLM Model procedure of SAS (SAS Software v. 9.1, SAS Institute Inc., Cary, NC,

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USA). Significant differences within pretreatments were determined at p < 0.05 (95% confidencelevel). Duncan’s multiple range test was employed to compare means where significant differencesoccurred within the pretreatment combinations in terms of shrinkage response.

RESULTS AND DISCUSSIONS

Table 2 presents drying time, moisture content and shrinkage during hot-air drying of control andpretreated Mirabelle plum samples as influenced by ultrasonication time, osmotic solution con-centration, and immersion time in osmotic solution. Drying continued until reaching an averagemoisture content of 0.57 g water/g dry matter. Shrinkage rate decreased along with decrease inmoisture contents of all the samples (Table 2). This can be deduced from the small gradients ofshrinkage between two different moisture contents when reaching the end of the process. Thisobservation can be related to a higher effective moisture diffusivity,[27] case-hardening of thesurface and the fixation of the volume of the sample[25,28] at the earlier stage of the dryingprocess.[9] This observation is in agreement with the results of Niamnuy et al.[29] who noticedthat faster drying rate induced extensive cellular shrinkage.

The highest shrinkage was observed in the control sample (Table 2a). Generally, the shrinkageof the pretreated samples was significantly (p < 0.05) decreased by increasing ultrasonication timefrom 10 to 30 min at different immersion times (60, 120, 180, and 240 min) in osmotic solutions(Table 2). Shrinkage of the pretreated samples was also decreased by increasing osmotic solutionconcentration from 50 to 70% at different immersion times in osmotic solutions; however, thisdecrease was not statistically significant (p > 0.05). This could be due to the stronger influence ofultrasonication time compared to the concentration of the osmotic solution. In accordance with theresults obtained in this study, Koc et al.[9] also reported that the extent of shrinkage is generallyhigher for air drying than for osmotic dehydration. With respect to the solution concentration, asmaller moisture content, and a consequently higher shrinkage were observed for samples withlower osmotic solution concentrations. This is due to the formation of a dense layer of solutes inthe surface of the fruit when concentrated solutions are used. This layer makes transfers betweenthe fruit and the solution more difficult.[30] Fante et al.[30] also observed lower shrinkage values byincreasing sucrose solution concentration during plum drying. However, Nowacka et al.[24] andSchössler et al.[26] observed that ultrasound treatment had no significant effect (p > 0.05) onproduct shrinkage.

At constant ultrasonication time and osmotic solution concentration, increasing immersion timefrom 60 to 240 min decreased the shrinkage. Many aspects of cell structure are affected duringosmotic dehydration of fruits, such as alteration (deformation) of cell walls, splitting of the middlelamella, lysis of membranes (plasmalemma and tonoplast), and tissue shrinkage.[31] During osmoticdehydration, plasmolysis is also accompanied by a loss in the turgor pressure, pectin solublization,and solute uptake in the cells.[32] These tissue changes, which strongly alter the cellular compart-mentalization, wall matrix, and membrane permeability, could greatly influence the transportproperties of the product during processing.[31] Because of the complex situation in the micro-structure of plant tissue, the phenomena observed during osmotic dehydration cannot always beexplained just in terms of osmotic processes in which cell membranes act as a semipermeablebarrier and allow the passage of water. Disruption of cell membranes during osmotic dehydrationputs an end to the osmotic mechanism and from then on, diffusion, capillarity or free convectionbecome the mechanisms that control the mass transfer as the process advances.[4] This, in turn,could lead to a higher moisture diffusivity, lower drying time and lower shrinkage.

On the other hand, Rodriguez et al.[33] studied the effect of ultrasound on the microstructure ofapple tissue during drying by means of scanning electron microscopy (SEM). Microphotographs offresh and dried apples showed that during drying, one of the most important phenomena is cell

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TABLE 2Drying time, moisture content and shrinkage of different samples (Table 1) until reaching an average moisturecontent of 0.57 g water/g dry matter pretreated at four immersion times in osmotic solution: (a) 60 min; (b) 120

min; (c) 180 min; and (d) 240 min

(A)

Treatment Time (min) MC (g water/g dry matter) Shrinkage (%)

Control 0 4.54 ± 0.06 0.00ν ± 0.00120 2.96 ± 0.04 22.18ζηθι ± 2.22240 2.03 ± 0.12 45.71uvwxy ± 5.96360 1.41 ± 0.07 65.52fghijkl ± 2.44480 0.96 ± 0.09 75.49a ± 5.43625 0.57 ± 0.01 76.41a ± 1.36

U10-B50-T60 0 4.50 ± 0.04 0.00ν ± 0.00130 2.88 ± 0.10 20.89ηθικ ± 1.71230 2.11 ± 0.09 44.02vwxy ± 2.10350 1.42 ± 0.07 63.12hijklmn ± 2.75480 0.90 ± 0.04 73.95abcd ± 4.18565 0.60 ± 0.03 76.05a ± 1.42

U30-B50-T60 0 3.98 ± 0.03 0.00ν ± 0.0070 2.85 ± 0.06 17.33θικλμ ± 1.88150 2.08 ± 0.06 35.76zαβγδ ± 4.28250 1.40 ± 0.03 52.16qrstu ± 0.55350 0.91 ± 0.03 63.44hijklmn ± 3.26500 0.55 ± 0.00 68.99abcdefghij ± 3.74

U10-B70-T60 0 4.41 ± 0.06 0.00ν ± 0.00120 2.88 ± 0.02 18.78θικλ ± 1.57210 2.11 ± 0.00 40.55xyzα ± 0.01330 1.42 ± 0.01 58.89lmnopq ± 0.98460 0.89 ± 0.00 70.24abcdefghi ± 2.47590 0.57 ± 0.03 75.21a ± 3.24

U30-B70-T60 0 3.80 ± 0.14 0.00ν ± 0.0080 2.86 ± 0.04 14.33κλμ ± 4.25170 2.06 ± 0.05 32.50βγδε ± 3.76280 1.39 ± 0.08 48.49stuvw ± 0.85390 0.90 ± 0.05 60.76klmnop ± 4.75525 0.56 ± 0.01 67.55bcdefghijk ± 0.93

(B)

Treatment Time (min) MC (g water/g dry matter) Shrinkage (%)

U10-B50-T120 0 4.45 ± 0.04 0.00ν ± 0.00110 2.91 ± 0.01 22.12ζηθι ± 4.37210 2.09 ± 0.02 44.30vwxy ± 2.40320 1.43 ± 0.05 62.42jklmno ± 1.33440 0.90 ± 0.08 73.29abcde ± 4.84565 0.57 ± 0.02 74.99ab ± 4.35

U30-B50-T120 0 3.84 ± 0.06 0.00ν ± 0.0070 2.87 ± 0.05 13.28λμ ± 1.74140 2.11 ± 0.07 33.53αβγδε ± 3.47250 1.39 ± 0.01 50.94rstuv ± 6.58350 0.92 ± 0.06 62.52jklmno ± 5.18490 0.55 ± 0.01 66.69defghijk ± 2.58

(continued )

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TABLE 2(Continued)

(B)

Treatment Time (min) MC (g water/g dry matter) Shrinkage (%)

U10-B70-T120 0 4.34 ± 0.04 0.00ν ± 0.00120 2.94 ± 0.02 18.62θικλ ± 1.76220 2.12 ± 0.08 40.57xyzα ± 2.26340 1.42 ± 0.01 58.79lmnopq ± 6.10460 0.91 ± 0.01 69.88abcdefghij ± 1.51580 0.59 ± 0.01 73.27abcde ± 2.07

U30-B70-T120 0 3.63 ± 0.04 0.00ν ± 0.0050 2.90 ± 0.16 13.14λμ ± 0.28140 2.06 ± 0.12 27.68εζη ± 0.13240 1.42 ± 0.11 42.81wxyz ± 1.97360 0.89 ± 0.04 56.71nopqr ± 2.41495 0.55 ± 0.01 65.54fghijkl ± 0.20

(C)

Treatment Time (min) MC (g water/g dry matter) Shrinkage (%)

U10-B50-T180 0 4.37 ± 0.07 0.00ν ± 0.0090 2.86 ± 0.01 23.04ζηθ ± 2.22180 2.11 ± 0.01 46.74tuvwx ± 4.34290 1.43 ± 0.01 64.49hijklm ± 5.16410 0.90 ± 0.02 73.18abcde ± 1.63545 0.57 ± 0.01 74.38abc ± 2.81

U30-B50-T180 0 3.72 ± 0.03 0.00ν ± 0.0050 2.91 ± 0.01 15.16ικλμ ± 1.62130 2.10 ± 0.04 34.25αβγδε ± 2.45230 1.40 ± 0.01 51.23rstuv ± 2.80340 0.90 ± 0.01 62.20jklmno ± 0.46490 0.56 ± 0.00 66.15efghijkl ± 2.61

U10-B70-T180 0 4.20 ± 0.28 0.00ν ± 0.00120 2.86 ± 0.13 17.92θικλμ ± 3.00210 2.12 ± 0.14 36.91zαβγ ± 2.79330 1.42 ± 0.06 55.26opqrs ± 2.55450 0.91 ± 0.02 67.07cdefghijk ± 2.91565 0.61 ± 0.00 72.18abcdefg ± 2.46

0 3.41 ± 0.04 0.00ν ± 0.0040 2.94 ± 0.11 13.90κλμ ± 1.43

U30-B70-T180 130 2.12 ± 0.07 31.44γδε ± 6.45240 1.38 ± 0.01 47.37tuvwx ± 3.24350 0.91 ± 0.05 59.00lmnopq ± 2.50505 0.56 ± 0.01 64.72ghijkl ± 3.32

(D)

Treatment Time (min) MC (g water/g dry matter) Shrinkage (%)

U10-B50-T240 0 4.27 ± 0.02 0.00ν ± 0.0090 2.91 ± 0.08 19.40θικλ ± 5.08190 2.14 ± 0.08 39.33yzαβ ± 8.71310 1.41 ± 0.06 57.08mnopqr ± 2.81430 0.90 ± 0.03 69.00abcdefghij ± 3.69560 0.59 ± 0.02 72.42abcdef ± 0.18

(continued )

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shrinkage, which leads to a major modification in the structure of the product and allows therelease of water. Through microstructural analysis, it was observed that ultrasound applicationdisrupted the cellular structure and resulted in pores which were larger than those in fresh samples.This fact could improve the drying rate by making an easier water pathway,[33] which, in turn,could lead to higher moisture diffusivity, lower drying time, and lower shrinkage.

Fernandes et al.[21] demonstrated that osmotic and ultrasound pretreatments increased moisturediffusion of melons through different effects. Ultrasonic waves created microscopic channels in thefruit; water could use these microscopic channels as an easier pathway to diffuse toward the surfaceof the fruit.[34] Fernandes et al.[21] verified in microscopic images that micro-channels were formedby the elongation and flattering of cells in some regions of the melons submitted to ultrasound.Besides, authors argued that no cell breakdown was observed in the samples. On the other hand,osmotic dehydration increased moisture diffusion by breaking down parts of the cell walls and,therefore, reducing the resistance for water to diffuse through the cells. In a similar study, Garcia-Noguera et al.,[17] in experiments with strawberries, showed that increasing the time of ultrasoundpretreatment reduced moisture content of the samples and consequently resulted in reduction of air-drying time. This result may be due to higher creation of microscopic channels in higherultrasonication time (30 min).

Table 2 shows that different samples needed various drying times to reach an average moisturecontent of 0.57 g water/g dry matter. Mirabelle plums treated with ultrasound for 30 min and dehydratedat osmotic solution concentration of 70% for 240 min (U30-B70-T240), prior to drying, were found tohave the lowest shrinkage (64.1%) compared to control (76.4%). Thus, processing conditions in termsof ultrasonication time, osmotic solution concentration and immersion time in osmotic solution can beoptimized to reduce shrinkage to a minimum, if it is desired from an industrial point of view.

Figure 1 shows the relationship between shrinkage and moisture content of all treated samplesshown in Table 2 (a, b, c, and d). As can be seen from Fig. 1, a uniform behavior was observed

TABLE 2(Continued)

(D)

Treatment Time (min) MC (g water/g dry matter) Shrinkage (%)

U30-B50-T240 0 3.54 ± 0.07 0.00ν ± 0.0040 2.96 ± 0.07 17.95θικλμ ± 3.56130 2.07 ± 0.13 36.77zαβγ ± 0.06240 1.39 ± 0.11 53.05qrst ± 0.18350 0.92 ± 0.10 62.90ijklmn ± 1.06490 0.60 ± 0.04 65.77efghijkl ± 7.50

U10-B70-T240 0 4.14 ± 0.03 0.00ν ± 0.00120 2.90 ± 0.06 15.95θικλμ ± 0.73220 2.09 ± 0.00 35.77zαβγδ ± 1.35340 1.41 ± 0.01 53.55pqrst ± 2.30470 0.91 ± 0.06 65.90efghijkl ± 4.60590 0.59 ± 0.03 70.77abcdefgh ± 5.230 3.37 ± 0.05 0.00ν ± 0.0040 2.88 ± 0.03 11.13μ ± 2.20130 2.06 ± 0.00 28.77δεζ ± 7.85

U30-B70-T240 240 1.39 ± 0.01 44.52vwxy ± 2.26350 0.92 ± 0.00 57.00mnopqr ± 5.55500 0.54 ± 0.01 64.05hijklmn ± 1.19

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between shrinkage and moisture content; such behavior is essentially independent of each set ofexperimental conditions and suggests a linear relation between moisture loss by the samples andshrinkage. The fundamental equation of shrinkage during drying is normally developed on the basis ofthe hypothesis that variation of the volume of the product corresponds to the volume of the evaporatedwater.[35] A linear relationship between shrinkage and moisture content during drying of various fruitsand vegetables, during the whole process or at least in part of it is reported in several works usingdifferent drying procedures. Results indicate that shrinkage may be easily estimated from changes inmoisture content of the sample, and independent of the drying rate. Inversely, determination ofshrinkage would give an indirect indication of moisture content of the product.[4] It has also beennoted that if development of pores during drying is not negligible, a linear model may not be adequateto model the shrinkage behavior. This is the case for drying at higher temperatures or lower moisturecontents.[36] Dissa et al.[5] stated that although experimental shrinkage curves were not strictly linear,they could be fitted by the fundamental linear model. In addition, analysis of the experimental data bySouraki et al.[7] revealed that the shrinkage of apple could be represented only as a linear function ofwater loss without any considerable dependency on the osmotic solution temperature and concentra-tion. Similar results were also obtained by Koc et al.[9] and Schössler et al.[26] for different fruits andvegetables. However, Panyawong and Devahastin[12] and Yan et al.[25] found the relationship betweenthe degree of shrinkage and the moisture content to be more or less of a second-order in nature at everytested condition. Shrinkage modelling by Aversa et al.[37] also revealed a non-linear dependence ofeggplant sample volume on the food’s moisture content.

CONCLUSION

Shrinkage of pretreated plum samples was decreased by increasing ultrasonication time from 10 to30 min and osmotic solution concentration from 50 to 70% at different immersion times (60, 120,180, and 240 min) in osmotic solutions. At constant ultrasonication time and osmotic solutionconcentration, increasing immersion time from 60 to 240 min decreased the shrinkage.Ultrasonication time, osmotic solution concentration and immersion time in osmotic solution allhad a significant effect (p < 0.05) on shrinkage of the samples. Mirabelle plums treated with

FIGURE 1 Relationship between shrinkage and moisture content of all treated samples shown in Table 2 (a, b, cand d).

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ultrasound for 30 min and dehydrated at osmotic solution concentration of 70% for 240 min (U30-B70-T240), prior to drying, were found to have the lowest shrinkage (64.1%) compared to control(76.4%). Thus, processing conditions in terms of ultrasonication time, osmotic solution concentra-tion and immersion time in osmotic solution can be optimized to reduce shrinkage to a minimum,in case it is desired from an industrial point-of-view.

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