W pracy, do oceny czystości powierzchni, zastosowano dwie metody

Moisture content;Wilgotność [%]Moisture content;Wilgotność [kgw.kg -1 solid]Gulnara GRDZELISHVILI, Pavel HOFFMANwhere a view factor (sometimes called a configuration factor)F m,v = 1, because we determine the heat flow by radiationprovided that the sample is completely surrounded bythe environment with a temperature equal to the ambientair temperature.The radiative heat transfer coefficient α E could be calculatedby the equation (4) (Hemzal 2007).The convective heat transfer coefficient α C could be determinedfor both horizontal and vertical surfaces separatelyby means of the appropriate criterial correlations forNusselt criterion (Rieger, Šesták 1993; Lin et al. 2009;Hemzal 2007).According to the equations 2 and 3, the rate of moisturevaporization n w [kgm –2 s –1 ] is equal to (5).Based on the drying experiments of apple slices the averagerate of moisture vaporization was determined by the equation(6).The deviation of the calculated average rate of moisturevaporization from the experimentally determined averagerate of moisture vaporization could be defined according tothe equation (7).Results and discussion: effect of blanchingNIR drying of blanched and unblanched apple slices was doneat the density of radiation approximately 5000 Wm –2 , atwavelength peak of 1 µm, at air temperature 23°C and airvelocity 0.5 ms –1 . The distance between emitter and driedproducts was 100 mm. The drying curves are shown in figure3. The drying curves are typical ones for similar fruits andvegetables. The moisture content decreased exponentiallywith elapsed duration of drying.The moisture content of slices a little increased during blanchingprocedure because of taking water but the drying durationof blanched samples was shorter by 74 minutes in case of10% final moisture content. That is caused by changes in thestructure, in the chemical composition and in the colloidalproperties. It could be considered that drying of the blanchedapple slices is more effective and more practical.,(3)(4)(5)(6)(7)10090807060504030201000 50 100 150 200Time [min];Time [min]Fig. 3. Drying curves of blanched and unblanched apple slices (qray = 5000W·m –2 , λ = 1 µm, l = 100 mm, v = 0.5 ms –1 )Rys. 3. Krzywe suszenia blanszowanych i nieblanszowanych plastrówjabłka (qray = 5000 W·m –2 , λ = 1 µm, l = 100 mm, v = 0,5 ms –1 )Effects of air velocity and of air temperatureThe data of loss of the moisture content with elapsed durationof drying were analyzed to study the effect of different airvelocities and different air temperatures for apple slices.There were executed under either the natural or the forcedconvection with air velocity 0.25 ms –1 . The air temperatureswere about 23 and 28°C during experiments.The air velocity influences the drying duration of apple slicesas shown in figure 4. The decrease of drying air velocity reducesthe drying duration in case of infrared drying (also seeNowak, Lewicki 2004). That means that at the natural convection(v = 0 ms –1 ), because of lower cooling effect on the driedmaterial surface, the time needed for drying is shorter then incase of forced convection. The difference was about 13 min incase of 20% final moisture content.In a similar way the lower air temperature causes the higherheat transfer from dried sample but also causes the lowermass transfer from apple slices to surrounding environment,and that makes the duration of drying longer. At air temperatures23 and 28°C the difference was 15 minutes for approximately17% final moisture content (Fig. 5). The higher heattransfer is caused by higher difference between material andsurrounding air in case of 23°C then in case of 28°C. This isalso confirmed by equation 3. The lower mass transfer shouldbe caused by influence of cooling effect on the surface of driedmaterial.pobrano z www.ips.wm.tu.koszalin.pl765432100 50 100 150 200Time [min];Czas [min]14 Inżynieria Przetwórstwa Spożywczego 1/4–2013(5)

Moisture content;Wilgotność [%]Moisture content;Wilgotność [%]Moisture content;Wilgotność [kg w. kg -1 solid]Moisture content;Wilgotność [kg w. kg-1 solid]Rehydration ratio;Współczynnik rehydracjiRelative mass;Masa [%]ARTYKUŁ NAUKOWY RECENZOWANY765432100 50 100 150Time [min];Czas [min]10090807060504030201000 50 100 150Time [min];Czas [min]Fig. 4. Drying curves of blanched apple slices at different air velocities: 0 ms –1 ;0.25 ms –1 (qray = 5000 Wm –2 , λ = 1 µm, l = 100mm, Ta = 301 K)Rys. 4. Krzywe suszenia blanszowanych plastrów jabłka w zależności od prędkościprzepływu powietrza: 0 ms –1 ; 0,25 ms –1 (qray = 5000 Wm –2 , λ = 1 µm,l = 100mm, Ta = 301 K)765432100 50 100 150Time [min];Czas [min]10090807060504030201000 50Time [min];100 150Czas [min]Fig. 5. Drying curves of blanched apple slices at different air temperatures:pobrano z www.ips.wm.tu.koszalin.pl28°C; 23°C (qray = 5000 Wm –2 , λ = 1 µm, l = 100 mm, v = 0 ms –1 )Rys. 5. Krzywe suszenia blanszowanych plastrów jabłka w zależności od różnychtemperatur powietrza: 28°C; 23°C (qray = 5000 Wm –2 , λ = 1 µm, l = 100 mm,v = 0 ms –1 )Rehydration ratioThe rehydration ratio was considered for the dried appleslices as one of the important quality attribute. The rehydrationratio values of dried samples were estimated as it wasdescribed in earlier section (see equation 1). Relationshipbetween the rehydration ratio and the rehydration duration ofblanched and unblanched dried slices is presented on figure 6.3,53,02,52,01,51,00,50,0Fig. 6. Relationship between the rehydration ratio and the duration of rehydrationof blanched and unblanched dried apple slices (for the range 0 – 135 minutes)Rys. 6. Zależność pomiędzy współczynnikiem rehydratacji, a czasem trwaniarehydratacji blanszowanych i nieblanszowanych suszonych plastrów jabłka (dlazakresu 0 – 135 minut)The maximum of water uptake ability (after 24 hours rehydration)was for blanched dried samples 3.96 and for unblancheddried slices 3.46 on the average.The degree of restoration during rehydration is dependenton different drying conditions and on the final moisturecontent. At the beginning the rehydration rate was muchfaster for the samples that had lower final moisture contentas is shown on figure 7.The rehydration degree of unblanched dried samples waslower than for blanched apple slices.807060504030201000 50Time [min];100 150Czas [min]Blanched; 0.5 m/sUnblanched; 0.5 m/s0 15 30 45 60 75 90 105 120 135Fig. 7. Influence of the different drying conditions and the final moisture contenton the ability of restoration of apple slicesRys. 7. Wpływ różnych warunków suszenia i końcowej wilgotności na zdolnośćprzywrócenia plastrów jabłkaRate of moisture vaporizationBlanchedDuration of rehydration [min];Czas rehydratacji [min]UnblanchedThe rates of moisture vaporization by calculation and by experimentaldata (Fig. 8.) were determined according to theequations 5 and 6.Inżynieria Przetwórstwa Spożywczego 1/4–2013(5) 15

Mass;Masa [g]Moisture vaporization;Odparowanie wilgoci [kgm -2 s -1 ]Temperature;Temperatura [°C]Gulnara GRDZELISHVILI, Pavel HOFFMANFig. 8. The rate of moisture vaporization by calculation and by experimental dataRys. 8. Szybkość parowania wilgoci uzyskana na podstawie danych teoretycznychi danych doświadczalnychExperimental data are particularly given in nomenclature andpart of them is presented in table 1 and on figure 9 and 10.Table 1. Some experimental data and the deviations of moisture vaporizationTabela 1. Dane doświadczalne i odchylenia parowania wilgociTm=(Tm1+ Tm2)/2 (K) Δ Tm= Tm– Ta0 301 0 –10 312.6 11.6 11.9320 318 17 7.4230 321.25 20.25 0.1840 324.23 23.23 10.9450 328.1 27.1 13.7760 331.8 30.8 21.1170 336 35 25.3780 340.1 39.1 19.0290 344.5 43.5 19.05100 350.3 49.3 8.93The average rate of moisture vaporization n w ex (kgm –2 s –1 )was defined for every 10 minutes sections from 6 to 105minutes of IR drying experiment, that data are shown byfigure 9.The changes of sample diameter D and thickness H wasdetermined experimentally too.4,03,53,02,52,01,51,00,50,00,00070,00060,00050,00040,00030,00020,00010by experimental databy calculation0 20 40 60 80 100 120Time [min];Czas [min]0 20 40 60 80 100 120Time [min];Czas [min]Fig. 9. Drying curve of an apple sample with the initial weight 3.7 g and thefinal moisture content 17.1%pobrano z www.ips.wm.tu.koszalin.plRys. 9. Krzywa suszenia próbki jabłka z początkową masą 3,7 g i końcowąwilgotnością rzędu 17,1%9080706050403020100Fig. 10. Average material temperatures and temperature differences betweenair and materialRys. 10. Średnie temperatury materiału i różnice temperatur pomiędzypowietrzem i materiałemThe deviations of the calculated average rate of moisturevaporization from the experimentally determined averagerate of moisture vaporization were established accordingto the equation 7 and are presented in table 1.The deviation of the calculated average rate of moisturevaporization from the experimentally determined averagerate according to equation 7 is shown in table 1. Certaindeviation of the calculated data (defined by equation 5)from the experimental ones (defined by equation 6) couldbe explained by the complexity of determining the exactvalues of the radiation density of the IR lamp (due to unevendistribution of the radiation on the irradiated surface),of the absorptivity and of the samples sizes (because theyare constantly changeable). Therefore in all cases it is betterto calculate the rate of moisture vaporization in successivelysmall sections of drying.Conclusions0 20 40 60 80 100 120Time [min];Czas [min]The infrared–convective drying of apple slices is quite effective.The test of the effect of blanching confirms that the dryingtime of blanched apple slices is much shorter than forunblanched raw materials slices. Decrease of air velocity reducesthe drying time. The lower air temperature makes theduration of drying longer.The rehydration ratio was found higher for blanched driedsamples. The maximum of water uptake ability was 3.96 forblanched apple samples and it was equal to 3.46 forunblanched dried slices on the average. The lower final moisturecontent influenced the faster restoration at the beginningof rehydration process.Results of calculation of the rate of moisture vaporizationseem to be in a reasonable agreement with experimental data.AcknowledgementsWe wish to thank for the support from grant SGS2012 at ČVUTin Prague (SGS12/057/OHK2/1T/12) and the research programMŠMT ČR (6840770035), which enabled us to make anexperimental verification the drying hypothesis.t mΔT16 Inżynieria Przetwórstwa Spożywczego 1/4–2013(5)

ARTYKUŁ NAUKOWY RECENZOWANYReferences1. Ginzburg A. S. 1969. Infrared radiation in food industry(in Polish). Wydawnictwo Naukowo–Techniczne, Warszawa.2. Hemzal K. 2007. Transport phenomena in environmentaltechnology (in Czech). Publishing House of Czech TechnicalUniversity in Prague.3. Kiseljova T. F. 2007. Drying technology (in Russian).KTIPP, Kemerovo.4. Kocabiyik H., Tezer D. 2009. Drying of carrot slices usinginfrared radiation. International Journal of Food Scienceand Technology, 44, 953–959.5. Lin Y. L., Li S. J., Zhu Y., Bingol G., McHugh Z., Tara H.2009. Heat and mass transfer modeling of apple slices undersimultaneous infrared dry blanching and dehydration process.Drying Technology, 27, 10, 1051–1059.6. Mayor L., Sereno A. M. 2004. Modeling shrinkage duringconvective drying of food materials: a review. Journal ofFood Engineering, 61, 373–386.7. Nowak D., Lewicki P. 2004. Infrared drying of apple slices.Innovative Food Science and Emerging Technologies,5, 353–360.8. SzePheng O., Chung Lim L. 2011. Drying kinetics andantioxidant phytochemicals retention of salak fruit underdifferent drying and pretreatment conditions. DryingTechnology, 29, 429–441.9. Rahman M., Shafiur A. 2005. Dried food properties: challengesahead. Drying Technology, 23, 695–715.10. Ratti C., Mujumdar A. S. 1995. Infrared drying. Handbookof industrial drying, New York, 742.11. Rieger F., Šesták, J. 1993. Momentum, heat and masstransport (in Czech). Publishing House of Czech TechnicalUniversity in Prague, 299.12. Sharma G. P., Verma R. C., Pathare P. B. 2005. Thin–layerinfrared radiation drying of onion slices. Journal of FoodEngineering, 67, 361–366.Gulnara Grdzelishvili, Pavel HoffmanCzech Technical University in PragueDepartment of Process Engineeringg.gulnara@yahoo.compavel.hoffman@fs.cvut.czivan.fort@fs.cvut.czpobrano z www.ips.wm.tu.koszalin.plInżynieria Przetwórstwa Spożywczego 1/4–2013(5) 17