Journal of Food Engineering 97 (2010) 267–274

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Journal of Food Engineering

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Rehydration and sorption properties of osmotically pretreatedfreeze-dried strawberries

Ciurzynska Agnieszka *, Lenart AndrzejFaculty of Food Sciences, Department of Food Engineering and Process Management, Warsaw University of Life Sciences, SGGW, Warsaw, Poland

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 May 2009Received in revised form 7 September 2009Accepted 14 October 2009Available online 20 October 2009

Keywords:StrawberriesRehydrationSorptionIsothermsKineticFreeze-dryingOsmotic dehydration

0260-8774/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2009.10.022

* Corresponding author. Tel.: +48 22 59 37 577; faxE-mail address: agnieszka_ciurzynska@sggw.pl (C.

The aim of this work was to investigate the influence of osmotic dehydration and type of osmotic solutionon selected physical properties of freeze-dried strawberries. Frozen Senga Sengana strawberries weredehydrated in osmotic solution with water activity of about 0.9 (sucrose and glucose solutions and starchsyrup). Osmotically dehydrated fruits were frozen and freeze-dried at heating shelf temperature of 30 �Cfor 24 h.

Rehydration, sorption isotherms and adsorption rate were determined for the freeze-dried strawber-ries. A decrease in rehydration capacity and adsorption rate was observed in the case of freeze-driedstrawberries that were osmotically dehydrated in sucrose and glucose solution. Osmotic dehydrationin glucose solution resulted in flatter sorption isotherms than osmotic dehydration in sucrose and starchsyrup solution.

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1. Introduction Investigations made in recent years have proved that applica-

Strawberries are very sensitive to chemical and microbial dete-rioration during post-harvest storage and handling, therefore, theyhave a rather limited shelf life in a fresh form (Duxbury, 1992;Parakash et al., 2004). Strawberries can be consumed fresh or inmany other forms (juice, jam, jelly, dried and rehydrated with yo-gurt and bakery products) (El-Beltagy et al., 2007). Freezing thefruit improves its availability, but despite increased cost, the prod-uct quality is poor (Agnelli and Mascheroni, 2002).

In recent years, a variety of drying methods have been tried andmuch attention has focused on the quality of the products obtainedby these methods (Jena and Das, 2005; Matuska et al., 2006). Somestudies have been carried out into the production of conventionallyair dried berry fruits such as strawberries (Alvarez et al., 1995),blueberries (Lim et al., 1995) or mulberries (Maskan and Gögüs,1998) which leads to elaborate freeze-drying technology (Tsamiand Katsioti, 2000).

Thus, there is a need to modify the freeze-drying method so asto limit its adverse influence, especially on fragile and delicatestructures. One possible solution is to apply osmotic dehydration,which involves the immersion of fruit in osmotic solution resultingin the removal of water from tissue, and replacing it with solublesolids (Montserrat and Wet, 2003).

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: +48 22 59 37 576.Agnieszka).

tion of osmotic dehydration to fruit and vegetable pre-treatmentyields very good results in decreasing water content in the prod-ucts, and significantly increases dry mater content (Kowalska andLenart, 2001). But it has to be noted that because there is a simul-taneous influx of osmotic solution into the plant tissue as water isremoved, the process may influence nutritional and organolepticqualities of the tissue (Bonazzi et al., 1996). Accordingly, the osmo-tic treatment has been used mainly as pre-treatment to some con-ventional processes such as freezing, vacuum drying, and airdrying, in order to improve final quality of products, reduce energycosts, or even to develop new products (Sereno and Hubiner, 2001).

Osmotic dehydration introduces changes in chemical composi-tion. Prothon (2003) observed that it caused a decrease in waterabsorption capacity during rehydration of vacuum-dried apples.This fact might be related to smaller porosity of the material result-ing from saturation of intercellular space and cell walls by sugar.However, Lewicki et al. (1998) found that immersing dehydratedonion in starch syrup resulted in better rehydration capacity. So,it appears that osmotic dehydration conditions before drying areof great consequence for rehydration and water vapour sorption.

The aim of this study was to investigate influence of osmoticdehydration and type of osmotic solution on the chosen physicalproperties of freeze-dried strawberries. Various conditions of os-motic dehydration were taken into account. An attempt was madeto define pre-treatment conditions before freeze-drying of straw-berries which could affect rehydration and water vapour sorptionof dried fruit.

268 C. Agnieszka, L. Andrzej / Journal of Food Engineering 97 (2010) 267–274

2. Materials and methods

The objectives of analysis was strawberries of Senga Senganavariety, frozen, about 25–30 mm in diameter, with previously re-moved leaf stalks. They had been stored in plastic pouches of500 g each at the temperature of �18 �C for 3 months. The frozenstrawberries were then osmotically dehydrated in various sugarsolutions of water activity equal to 0.9 and different mass weight(MW) [sucrose: 61.5 g/100 g solution, glucose: 49.2 g/100 g solu-tion, starch syrup (glucose equivalent DE 30–35): 67.2 g/100 gsolution] in a water bath (ELPAN-357) at the temperature of30 �C for 3 h under atmospheric pressure. The ratio of material tosolution was 1:4 w/w. In the beginning of the osmotic dehydrationprocess water activity of the osmotic solution equal to 0.9 ensuresthe same driving force in mass exchange. Additionally, the wholesystem was being shaken with the frequency of 100 Hz and theamplitude of 10 Hz. After the specified period of time, the straw-berries were separated from the osmotic solution in a sieve, andrinsed twice in water. During the osmotic dehydration of strawber-ries, the temperature in the centre of the fruit changed from �10 to26.5 �C. The measurement was conducted using a thermocouplewhich was stuck in the centre of the examined fruit (Kowalskaand Lenart, 2001; Piotrowski et al., 2004; Matuska et al., 2006;Kowalska et al., 2008).

Next, the osmotically dehydrated strawberries were frozen in aNational Lab GmbH (ProfiMaster Personal Freezers PMU series)freezer at the temperature of �70 �C for 2 h. Both osmoticallydehydrated and unprocessed frozen strawberries were then driedfor 24 h in an ALPHA1-4 LDC-1m freeze-dryer (Christ, Germany)using contact heating under the pressure of 63 Pa, safety pressure103 Pa, the dryer shelves temperature being 30 �C. During this pro-cess of drying, the fruit temperature was being monitored by athermocouple which indicated that the temperature inside osmot-ically dehydrated strawberries had risen from �30 to 25 �C. Subse-quently, the fruit were put into jars and stored in a dark place atthe temperature of 25 ± 3 �C until the time of the planned exami-nation (1–2 months).

The degree of rehydration was estimated on the basis of freeze-dried fruit mass increase during a specified time of immersion inwater. The measurement was carried out at room temperature,and the whole procedure was repeated five times (Witrowa-Rajc-hert and Lewicki, 2006). For this purpose, a whole strawberry(about 1 g in weight) previously weighed on an analytical scaleswith the accuracy of ±0.001 g was submerged in 100 ml of distilledwater contained in each of the four beakers. After the periods of 5,30, 60, and 120 min, the fruit were consecutively drained, weighed,and their dry matter content was determined (Lenart, 1996).

To establish the isotherms of water vapour sorption (Kowalskaand Lenart, 2000), weighed whole strawberries (about 1 g each)had been put in seven chambers filled with salt solution withwater activity from 0.113 to 0.903 for 1 month. After that time,samples were weighed again and water activity of the strawberrieswas determined.

The measurement of water vapour sorption kinetics (Kowalskaand Lenart, 2000) was conducted in four repetitions for each typeof strawberry using a stand which ensured continuous measure-ment of mass increase in conditions of constant temperature andrelative humidity. Saturated NaNO2 solution was used to obtainconstant water activity of environment (0.648). The measurementwas carried out at the temperature of 25 ± 1 �C for 20 h. The inves-tigated samples consisted of whole dried strawberries, and theirmass increase was registered by means of the ‘‘measurement forDOS” computer software.

An exponential equation (Kowalska et al., 2006) was used formathematical interpretation of the obtained results:

u ¼ aþ b � 1� expð�c�sÞ� �ð1Þ

where u – water content [g H2O/g d.m.], a, b, c – constant parametersof equation, s – time [h].

Changes in water content (u) in time (s) during osmotic dehy-dration and rehydration of freeze-dried strawberries and water va-pour sorption rate were determined. Correlation coefficient R2;mean relative error MRE (Jamali et al., 2006); error of water con-tent estimation SEE (Jamali et al., 2006); relative squares sumRSS (Pagano and Mascheroni, 2005); and root mean square RMS(Lewicki, 2000) were also computed using the following equations:

u ¼ ð1� sÞs

ð2Þ

MRE ¼ 100n�X ue � up

ue

�������� ð3Þ

SEE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXðue � upÞ2

qð4Þ

RSS ¼Xðue � upÞ2 ð5Þ

RMS ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP ue�up

up

� �2

n

vuut� 100% ð6Þ

where s – dry matter content (g d.m./g), n – number of observations,e – experimental water content, p – predicted water content.

For osmotically dehydrated strawberries, solid gain (SG) wascalculated from the equation (Kowalska et al., 2008):

SG ¼ sf �mf � si �mi

si �mið7Þ

where m – sample mass (g), i – initial, f – final.In the ensuing statistical analysis Statgrafics Plus v. 3.0. (Micro-

soft), Excel 2000 (Microsoft), Table Curve 2D v. 3 (Jadel) computersoftware was used. For the obtained averaged results, correspond-ing standard deviations (SD) were calculated. Statistical compari-son for kinetic curves was performed with the use of Statistica5.0 (StatSoft) software package. In the course of analysis, Fisher’sF-test for verification of the hypothesis of equality of means foranalysed coefficients in the measured samples was used, and Pear-son correlation coefficient was computed. The least significant dif-ference (LSD) between mean values was calculated for analysedtechnological coefficients considering pairs of investigated sam-ples, in relation to the applied variable using F-test (multiple rangetest). For the purpose of analyses, significance level of 0.05 wasassumed.

3. Results and discussion

3.1. Influence of osmotic dehydration on rehydration properties offreeze-dried strawberries

As a result of the analysis, statistically significant influence ofosmotic dehydration (IA-IC) on rehydration of freeze-dried straw-berries in comparison to freeze-dried strawberries without osmo-tic dehydration (I) was discovered (Fig. 1). Freeze-dried fruit afterprevious osmotic pre-treatment were characterised by lower watercontents after 120 min of rehydration than fruit not subjected toosmotic dehydration. It was also noted that osmotic dehydrationin starch syrup (IC) caused a significant difference in rehydrationin relation to the analogical process in conducted sucrose (IA)and glucose (IB) solution. Between the two latter solutions beingno statistically significant difference in this respect (Fig. 1).

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 20 40 60 80 100 120 140time t [min]

wat

er c

onte

nt u

[g

H2O

/g d

.m.]

IA

IB

IC

I

significance level 0.05

Fig. 1. Influence of osmotic dehydration and the type of osmotic solution on watercontent (u) as a function of rehydration time, for freeze-dried strawberriesosmotically dehydrated. Type of osmotic solution: IA – sucrose, IB – glucose, IC –starch syrup, I – without osmotic dehydration.

C. Agnieszka, L. Andrzej / Journal of Food Engineering 97 (2010) 267–274 269

Rehydration of dried fruit, previously osmotically dehydrated insugar solution is a complex process dependent on several factors.In the first stage of rehydration, the surface layer of sugar is dis-solved, which makes water adsorption inside capillary-porousmaterial difficult. As a result, in this stage water is kept on the sur-face mainly by adsorption forces. These forces, incidentally, arelower than in the case of dried fruit without osmotic pre-treat-ment. Later, in the course of the continued rehydration, as a resultof leakage of a certain amount of sugar from inside the fruit, it ismainly the structure of the fruit that determines the degree ofrehydration (Lenart, 1991).

Rehydration properties are inherently linked with structuralfeatures. Tzee Lee et al. (2006) confirmed dependence of rehydra-tion capacity on plant material structure. They found that freeze-dried potatoes and avocados during 3 min of rehydration showeda high degree of water adsorption, whereas in the case of bananasthis degree was five times smaller, in spite of the same conditionsof freeze-drying. Erle and Schubert (2001) who investigated appleswhich were osmotically dehydrated in sucrose solution beforemicrowave drying, proved the protective effect of osmotic sub-stances on the structure of dried material, as well as their benefi-cial influence on volume and shape behaviour on tissue level.The phenomenon of structure strengthening of strawberries beforefreezing as an effect of osmotic dehydration in sucrose solutionwas also confirmed by Suutairenen et al. (2000). Specifically, osmo-tic dehydration strengthened the structure of freeze-dried straw-berries, which influenced, for instance, their sorption andrehydration properties. After osmotic pre-treatment surface layersof the fruit were saturated by sugars, which made difficult for themto absorb water and water vapour (Ciurzynska and Lenart, 2008).

It was confirmed that osmotic dehydration of strawberries(Fig. 2b and d) caused structure strengthening of freeze-driedstrawberries in comparison to fruit without osmotic pre-treatment(I) (Fig. 2a). As a consequence of tissue impregnation with sucrose,cellular walls became bulky, while cells closest to the dried surfaceof material sustained substantial damage (Fig. 2b). Sucrose pene-trated deep into cells, and during the crystallization destroyedthe cellular walls, which explains the surface layer damage (Ciu-rzynska and Lenart, 2008).

As a matter of fact, glucose also strengthened the structure offreeze-dried strawberries (IB) (Fig. 2c) in comparison to fruit notsubjected to osmotic pre-treatment (I) (Fig. 2a). Freeze-driedstrawberries osmotically dehydrated in glucose solution (IB) wereuniformly impregnated with the sugar, this phenomenon being re-lated to smaller molecular mass of glucose in comparison to su-crose. For the same reason, cells which are close to the surface ofdried material were deformed to a smaller degree. Structure offreeze-dried strawberries was investigated by means of birefrin-gent interferometry, which allowed to discern sugar crystals asshining objects. Shining glucose crystals were present in the sur-

face layer of osmotically dehydrated freeze-dried strawberries(IB), and also in inner parts of dried material. As well as that, fi-brous structures of glucose were observed, such as those typicalfor creamed honey (Bakier, 2004). This particular form of glucosecrystals indicates that freeze-dried strawberries osmotically dehy-drated in glucose solution (IB) are characterised by a greater plas-ticity in comparison to fruit osmotically dehydrated in sucrosesolution (IA) or starch syrup (IC) (Ciurzynska and Lenart, 2008).Similar sugar crystals were observed in strawberries not subjectedto any osmotic pre-treatment (I), which suggests that both kinds offreeze-dried fruit have similar sorption properties.

Finally, starch syrup as well strengthened the structure of thedried material (IC) (Fig. 2d) in comparison to fruit without osmoticdehydration (I) (Fig. 2a). The syrup filled the cells of freeze-driedstrawberries, creating a small number of empty spaces (pores).Furthermore, starch syrup penetrated the tissue to a greater extentthan sucrose, but to a smaller extent than glucose. The surfacelayer was saturated the most heavily, though the central cells sus-tained considerable damage too (Ciurzynska and Lenart, 2008).

3.2. Influence of osmotic dehydration on sorption isotherms of freeze-dried strawberries

Sorption isotherms of freeze-dried strawberries have a sigmoi-dal shape, characteristic to most food products (Fig. 3). On this ba-sis sorption isotherms can be classified as type I isotherms(Brunauer et al., 1940). They reflect the mechanism of water bind-ing and properties of the material. Incidentally, Palou et al. (1997)worked out similar classification for vacuum-dried cookies andcrisps, while Swami et al. (2005) did the same for convective-driednuggets. Also, Moraga et al. (2004) obtained for freeze-dried straw-berries a sorption isotherm course typical for products with highsugar content. This fact can be related to slow changes in the watercontent balance at low water activity, and its rapid increase abovewater activity 0.5. At that water activity level interactions betweenthe solvent (water) and the soluble substance are linked with sugardissolution.

Actually, only osmotic dehydration in glucose solution (IB) in astatistically significant way influenced water vapour sorption infreeze-dried strawberries in relation to the fruit without osmoticdehydration, considering the water activity range of 0.113–0.903(Fig. 3). Osmotic dehydration in sucrose solution (IA) and starchsyrup (IC) caused water vapour sorption capacity decrease in wateractivity aw range (0.113–0.648) in relation to freeze-dried straw-berries without osmotic pre-treatment (I), but the differences werenot statistically significant. Moreover, within the discussed aw

range, strawberries osmotically dehydrated in sucrose solution(IA) and starch syrup (IC) exhibited lower water content in relationto the fruit without osmotic dehydration, whereas for fruit osmot-ically dehydrated in glucose solution (IB) at water activity above0.328, refraction in sorption isotherm curves and decrease in watervapour sorption capacity were observed (Fig. 3).

The achieved results can be compared to investigations carriedout for convectively dried apples (Lenart and Lewicki, 1988). Intheir case, osmotic pre-treatment changed the shape of sorptioncurves in aw range from 0.3 to 0.7. With increasing osmotic dehy-dration degree, sorption isotherms have a flatter course. Moreover,Gondek and Lewicki (2005), who investigated water vapour sorp-tion for raisins and dried mango, pineapple, papaya and apricot,showed that in water activity between 0.5–0.7 there is a curveinflexion, while with further aw increase, water content in fruitrises. As for strawberries osmotically dehydrated in sucrose solu-tion, their water content increases for aw above 0.648, and the iso-therm inflection may be related to sugar transformation. With theincrease of water content in the product, its crystalline sugar dis-solves, which can lead to the creation of a solution of concentration

Fig. 2. Microstructure of freeze-dried strawberries. Type of osmotic solution: (a) I – without osmotic dehydration, (b) IA – sucrose solution, (c) IB – glucose solution,(d) IC – starch syrup. Zoom 50�. Scanning microscope FEI Company, type Quanta 200.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1water activity aw

wat

er c

onte

nt u

[g

H2O

/g d

.m.]

IA

IB

IC

I

significance level 0.05

Fig. 3. Influence of osmotic dehydration and the type of osmotic solution on watercontent (u) as a function of water activity (aw), for freeze-dried strawberriesosmotically dehydrated. Type of osmotic solution: IA – sucrose, IB – glucose,IC – starch syrup, I – without osmotic dehydration.

270 C. Agnieszka, L. Andrzej / Journal of Food Engineering 97 (2010) 267–274

close to dilution. Then the amount of absorbed water increases sig-nificantly, which may be linked to a tendency to reach a thermody-namic equilibrium within the environment.

An attempt at choosing a mathematical model best fitting forthe description of sorption isotherms was made. The five followingmodels were analysed: Oswin, Halsey (Akanbi et al., 2006), Igle-sias–Chrife (Johnson and Brennan, 2000), Peleg (Lewicki, 1998),and Lewicki (Lewicki, 1998) (Table 1). An attempt was also under-taken to apply the GAB model (Lewicki, 1997), but the parametersof the GAB equation did not satisfy the conditions stipulatedby Lewicki (1997) so as to be eligible for experimental datadescription.

All in all, it was resolved to choose Peleg’s empirical equation(Lewicki, 1998) on the basis of minimum MRE, RSS, SEE, RMS value,

and the highest R2 value (Table 2). Peleg developed a semi-empir-ical four-parameter model to describe sigmoid moisture sorptionisotherms. His equation turned out to be well-fitted for equilib-rium moisture sorption data for ten different products at wateractivity up to about 0.85–0.95 (Lewicki, 1998). Fig. 4 shows samplegraphic points obtained from Peleg’s model adjusted to experimen-tal water vapour sorption isotherm data for freeze-dried strawber-ries without osmotic dehydration (I) (30 �C), and for thoseosmotically dehydrated in glucose solution (IB). Also, Lewicki(1998) applied Peleg’s model to describe sorption isotherms for27 products, GAB model (Lewicki, 1997) for 23 products, and hisown model for 28 products. He demonstrated that the highestprobability of fitting experimental data with the minimum meanrelative error is guaranteed by Peleg’s model. In a similar vein, Pa-lou et al. (1997) surmised that Peleg’s model was best suited fordescription of isotherms for cookies and chips, because the ob-tained relative standard deviation was lower than 7%. Kowalskaet al. (2005) affirmed that the characteristic sigmoidal shape of ob-served isotherms is related to the occurrence of a monomolecularsorption range in the milieu of water activity aw < 0.3, a multi-layersorption for 0.3 < aw < 0.65, and capillary condensation at aw > 0.65.It has to be noted that this type of isotherm is often encounteredwith food products. The demonstrated differences in water contentequilibrium may depend on chemical composition, type of compo-nents, and structure of investigated materials.

3.3. Influence of osmotic dehydration on water vapour sorptionkinetics of freeze-dried strawberries

Statistical analysis of sorption curves showed that osmoticdehydration in sucrose solution (IA) and in starch syrup (IC) causeda decrease in water vapour sorption in relation to strawberrieswithout osmotic dehydration, while glucose solution (IB) applica-tion resulted in the highest water content after 20 h of the process

Table 1Models of sorption isotherms chosen to describe experimental data.

Model Equation Nomenclature

Oswin (Akanbi et al., 2006)u ¼ A � aw

1�aw

� �B A, B – constant of equationaw – water activityu – water content [g H2O/g d.m.]

Halsey (Akanbi et al., 2006) aw ¼ exp � � AuB

� �A, B – constant of equationaw – water activityu – water content [g H2O/g d.m.]

GAB (Lewicki, 1997) u ¼ u0�k�c�awð1�awÞ�½1þðc�1Þ�k�aw �

k ¼ exp � dRT

� �

c ¼ c0 � exp � dRT

� �aw – water activityu – water content [g H2O/g d.m.]d – heat of sorption of monomolecular layer of water [K]R – universal gas constant = 8.314 [J/mol/K]T – absolute temperature [K]

Iglesias–Chirife (Johnson and Brennan, 2000) ln �ðuþ ðu2 þ u0:5Þ0:5 ¼ Aþ B � aw A, B – constant of equationaw – water activityu – water content [g H2O/g d.m.]u0.5 – water content when aw = 0.5 [g H2O/g d.m.]

Peleg (Lewicki, 1998) u ¼ A � aBw þ c � aD

w A, B, C, D – constant of equationaw – water activityu – water content [g H2O/g d.m.]

Lewicki (Lewicki, 1998) u ¼ Fð1�awÞG

� F1þaH

wF, G, H – constant of equationaw – water activityu – water content [g H2O/g d.m.]

Table 2Parameters of fitting water vapour sorption models for freeze-dried strawberriesosmotically dehydrated. Type of osmotic solution: IA – sucrose, IB – glucose, IC –starch syrup, I – without osmotic dehydration.

Coefficients for selected model.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1water activity aw

wat

er c

onte

nt u

[g

H2O

/g d

.m.]

experimentalpoints I

Peleg's model Iexperimentalpoints IB

Peleg's modelIB

significance level 0.05

Fig. 4. Fitting of Peleg’s model to describe water vapour sorption isotherms forfreeze-dried strawberries without osmotic dehydration (I) and osmotically dehy-drated in glucose solution (IB).

C. Agnieszka, L. Andrzej / Journal of Food Engineering 97 (2010) 267–274 271

(Fig. 5). Observed differences were statistically significant in com-parison to fruit not subjected to osmotic pre-treatment.

The achieved results confirm the hypothesis of a significantinfluence of surface sucrose layer on the decrease in adsorbedwater vapour amount, which can be related to lower hygroscopic-ity of osmotically dehydrated dried material (Janowicz et al., 2007).

On the basis of the carried out investigations it was found thatosmotic dehydration in sucrose and glucose solution resulted inthe highest growth of dry mass content. Osmotic dehydration atthe temperature of 30 �C for frozen strawberries resulted in solid

gain (SG) in the range from 0.34 to 0.67 g H2O/g d.m. dependingon the type of osmotic solution (Table 3). Strawberries osmoticallydehydrated in starch syrup (C) achieved statistically significantlower solid gain value (SG) (about 50%) in comparison to the fruitosmotically dehydrated in glucose (B) and sucrose solution (A), lar-gely due to the higher molecular mass of starch. Differences be-tween samples osmotically dehydrated in glucose and sucrosesolutions were not statistically significant.

As well as that, it was shown that strawberries osmoticallydehydrated in sucrose, glucose solutions and starch syrup exhibitstatistically insignificant differences in water content and wateractivity (Table 3).

Total sugar content for strawberries of the Senga Sengana vari-ety after 3 h of osmotic pre-treatment increased almost threetimes. Glucose, having about half the mass weight of sucrose, canmore easily penetrate into tissue through existing pores and freespaces (Fig. 2c). Strawberries osmotically dehydrated in glucosesolution displayed a water vapour sorption kinetics course similarto fruit without osmotic pre-treatment, and the discrepancies inthe process rate were not statistically significant. Probably the di-rectly reducing sugars, which appear naturally in the fruit, changechemical composition to a lesser degree than osmotic dehydrationin sucrose solution and starch syrup. This is in accord with inves-

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25

time t [h]

wat

er c

onte

nt u

[g

H2O

/g d

.m.]

IA

IB

IC

I

IA

IB

IC

I

predicted

experimantal

significance level 0.05

Fig. 5. Influence of osmotic dehydration and the type of osmotic solution on water content (u) as a function of time, for freeze-dried strawberries osmotically dehydrated.Type of osmotic solution: IA – sucrose, IB – glucose, IC – starch syrup, I – without osmotic dehydration. aw of environment – 0.648.

Table 3The effect of pre-treatment on solid gain (SG), water content (u) and water activity(aw) in strawberries osmotically dehydrated in sugar solutions. Type of osmoticsolution: A – sucrose, B – glucose, C – starch syrup.

Sugar solutions SG [g s.s./g d.m.] u [g H2O/g d.m.] aw

A 0.67 2.24 0.976B 0.60 2.26 0.979C 0.34 2.57 0.975

272 C. Agnieszka, L. Andrzej / Journal of Food Engineering 97 (2010) 267–274

tigation made by Kowalska et al. (2008), who confirmed the differ-ences in the influence on the effective diffusion coefficient forwater and solids by using various osmoactive substances. For in-stance, when osmotic dehydration of pumpkin was carried out inglucose solution, the effective diffusion coefficient of water andsolids was the highest; likewise, it was similar and lower in thecase of sucrose and starch syrup solutions.

Moreover, statistically significant differences were noted inwater content between osmotically dehydrated freeze-driedstrawberries depending on the type of osmotic solution used(Fig. 5). The lowest value after 20 h of water vapour sorption pro-cess was obtained for freeze-dried strawberries osmotically dehy-drated in sucrose solution (IA), while the highest for strawberriesosmotically dehydrated in glucose solution (IB).

For the mathematical description of the relationships betweenwater content in freeze-dried strawberries and sorption time rangean exponential equation was chosen (1) (Fig. 5, Table 4). The fol-lowing factors had substantial influence on the choice of Eq. (1)for modelling sorption of water vapour curves in the investigatedtime range (Fig. 5): high correlation coefficient (R2) for freeze-driedstrawberries, and comparatively low for most of the relations meanrelative error (MRE) for both experimental and predicted data con-cerning the water content at the beginning and after 20 h of watervapour sorption.

Table 4Parameters of fitting exponential equation u ¼ aþ b � 1� expð�c�sÞ� �

to describe water vapwater content, u20 – water content after 20 h. Type of osmotic solution: IA – sucrose, IB –

Freeze-driedstrawberries

Coefficients of equation R2 MRE [%] Experiment

a b c u0

IA 0.045 0.134 0.087 0.998 8.32 0.039IB 0.057 0.178 0.186 0.999 9.37 0.048IC 0.031 0.132 0.219 0.999 4.18 0.028I 0.043 0.154 0.283 0.998 13.74 0.034

With reference to freeze-dried strawberries without osmoticdehydration (Fig. 5), the correlation coefficient (R2) of the chosenexponential Eq. (1) was in the range of 0.998–0.999. For osmoti-cally dehydrated fruit, the value of mean relative error (MRE)was in the range of 4.18–9.37%, while for freeze-dried strawberries(I) not subjected to osmotic pre-treatment it was higher (13.7%).The observed considerable MRE value increase resulted from dis-crepancies between experimental and predicted initial water con-tent for freeze-dried fruit. Dried strawberries were marked bysimilar experimental and calculated water contents after 20 h ofthe sorption process; therefore, it was decided to apply an expo-nential equation for the description of sorption kinetics.

Analysing the shape of curves of water vapour sorption rate inthe function of water content in the common water content range0.05–0.15 g H2O/g d.m., it was discovered that osmotic dehydra-tion in sucrose solution (IA) and starch syrup (IC) resulted in statis-tically significantly lower rates of water vapour sorption in relationto strawberries without osmotic dehydration (I) (Fig. 6). At thesame time, osmotic pre-treatment in glucose solution (IB) did notcause any statistically significant difference in the shape of curvesin relation to fruit without osmotic dehydration (I).

Furthermore, statistically significant differences in water va-pour sorption rate with regard to the type of osmotic solution werefound. Osmotic dehydration in sucrose solution (IA) caused a sta-tistically significant decrease in the rate of water vapour sorptionin comparison to strawberries osmotically dehydrated in starchsyrup (IC) and glucose solution (IB), while osmotic pre-treatmentin glucose solution (IB) resulted in obtaining the highest rate ofwater vapour sorption (Fig. 6).

Also Lenart (1990) showed that osmotic dehydration decreaseswater vapour sorption rate in convectively dried apples. As a resultof osmotic pre-treatment, the qualitative and quantitative charac-ter of water content in the function of time changes. Independently

our sorption kinetics for freeze-dried strawberries osmotically dehydrated. u0 – initialglucose, IC – starch syrup, I – without osmotic dehydration.

al water content [g H2O/g d.m.] Predicted water content [g H2O/g d.m.]

u20 u0 u20

0.158 0.045 0.1560.232 0.057 0.2320.163 0.030 0.1610.199 0.043 0.197

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 0.05 0.1 0.15 0.2 0.25water content u [gH2O/g d.m.]

sorp

tion

rate

du/

dt [

g H

2O/g

d.m

.*h] IA

IB

ICI

significance level 0.05

Fig. 6. Influence of osmotic dehydration and the type of osmotic solution on rate ofwater vapour sorption (du/dt) as a function of water content (u), for freeze-driedstrawberries osmotically dehydrated. Type of osmotic solution: IA – sucrose, IB –glucose, IC – starch syrup, I – without osmotic dehydration. aw of environment –0.648.

C. Agnieszka, L. Andrzej / Journal of Food Engineering 97 (2010) 267–274 273

of the difference in dehydration degree, dried apples with osmoticpre-treatment were also characterised by lower water content inrelation to dried apples without osmotic pre-treatment. Their rateof water vapour sorption was also lower (Janowicz et al., 2007).

4. Conclusions

Overall, for osmotically dehydrated freeze-dried strawberries adecrease of rehydration and sorption capacity in relation to freeze-dried fruit not subjected to osmotic dehydration was noticed.Rehydration and sorption properties were related to structuralchanges in freeze-dried strawberries during osmotic dehydrationand freeze-drying processes.

Sorption isotherms for freeze-dried strawberries both osmoti-cally dehydrated and those without osmotic dehydration have asigmoidal shape, characteristic for most food products. Peleg’smodel was selected as the most appropriate for the mathematicaldescription of the sorption isotherms. Osmotic dehydration in glu-cose solution caused a fall in the curves’ course in comparison tostrawberries osmotically dehydrated in sucrose solution and starchsyrup.

Strawberries osmotically dehydrated in glucose solution weresaturated to a higher degree, which is related to small molecularmass of the sugar. Glucose is a directly reducing sugar naturallyoccurring in fruit tissue; thus, strawberries osmotically dehydratedin glucose solution absorb water vapour to a similar degree as fruitnot subjected to osmotic dehydration.

Osmotic pre-treatment in sucrose solution and starch syrup de-creased the rate of water vapour sorption for freeze-dried straw-berries in relation to fruit without osmotic pre-treatment, whileusing glucose solution for osmotic dehydration resulted in obtain-ing the highest rate of water vapour sorption as well as the highestrate of water vapour sorption, comparable with strawberries notsubjected to osmotic pre-treatment.

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