quality of infrared dried apple slices

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Quality of Infrared Dried AppleSlicesDorota Nowak a & Piotr P. Lewicki aa Department of Food Engineering and ProcessManagement, Warsaw Agricultural University(SGGW), Warszawa, PolandPublished online: 06 Feb 2007.

To cite this article: Dorota Nowak & Piotr P. Lewicki (2005) Quality of Infrared DriedApple Slices, Drying Technology: An International Journal, 23:4, 831-846

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Quality of Infrared Dried Apple Slices

Dorota Nowak and Piotr P. Lewicki*Department of Food Engineering and Process Management,Warsaw Agricultural University (SGGW), Warszawa, Poland

Abstract: The aim of this work was to compare quality of apple slices dried bynear infrared heating and convection in such parameters in which final materialtemperature in both methods was similar. The infrared drying was done at the dis-tance between the emitters (with total power of 7.875 kW=m2) and heated surfaceequal to 10, 20, and 30 cm. Flow of ambient air was set at 0.5, 1.0, and 1.5m=s.Convective drying was done in the same dryer using hot air at 65 and 75�C flow-ing with velocity 1.5m=s. Quality attributes measured in this work included:color, kinetics of water adsorption, mechanical properties, and microstructure.It was stated that the changes in chromaticity coefficients are not dependent onthe mode of heat supply, but are related to the final temperature of the driedmaterial. Luminance of dried apple slices was affected by temperature as well.Final material temperature, not the way heat is supplied, could be responsiblefor the differences in the ability of dry apple slices to adsorb water. The similarcorrelation was stated for mechanical properties: slope of initial part of the defor-mation curve (crispness), breaking force (hardness or crispness), and work ofbreaking were all related to the final material temperature. Microstructure of con-vective and infrared dried apple were different but it seems that the drying ratecan be responsible for observed differences.

Keywords: Color; Adsorption; Mechanical properties; Crispness; Work of defor-mation; Convective drying; Microstructure

INTRODUCTION

Heating with the use of infrared energy becomes more and more interest-ing to the food industry due to the progress in construction of infraredemitters. High emissivity and specified dominating wavelength characterizepresent infrared emitters.

�Correspondence: Piotr P. Lewicki, Department of Food Engineering and Pro-cess Management, Warsaw Agricultural University (SGGW), Nowoursynowska159c, Warszawa 02-776, Poland; E-mail: [email protected]

Drying Technology, 23: 831–846, 2005Copyright Q 2005 Taylor & Francis, Inc.ISSN: 0737-3937 print/1532-2300 onlineDOI: 10.1080/DRT-200054206

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Infrared heating has some advantages in comparison to convectiveheat transfer. There is no resistance to heat transfer, and the inertia ofheating system is low. Hence, cost of energy can be lowered, the timeof heating can be shortened,[1–3] and control of product temperaturecan be easily done. Application of infrared energy in drying reduces theneed for a high velocity of air, thus the contact of product undergoingdrying with oxygen is cut down.

Economic and technical advantages of infrared heating must beaccompanied by the quality of final product, which fulfils consumerexpectation. Mogi et al., cited by Sakai,[4] reported that using infraredemitters and hot drying air a product of high quality was obtained. Fuand Lien[5] dried shrimp using combined infrared-convective heating.The degree of oxidation of unsaturated fatty acids was assayed, and sat-isfactory results were obtained at optimized drying parameters. Infraredheating applied to dry coffee resulted in shortening of the time of processand high quality of the product.[6] In China, infrared heating was used todry herbs containing volatiles. It was found that quality expressed as anamount of volatiles after drying of infrared dry herbs was much betterthan those dried by convection.[7]

Itoh (cited in Sakai[4]) investigated quality of Welsh onion dried bythree different methods, that is drying by far infrared heating accompaniedby air flow, drying by far infrared heating under vacuum, and drying byconvection. The least loses of chlorophyll occurred during infrared-convec-tive drying. Mongporaneet et al.[8] optimized drying of Welsh onion takingcontent of chlorophyll as a measure of quality. They stated that the totalchlorophyll content was highest at radiation intensity of 90W.

Namiki et al.[9] investigated quality of dried carrot. Freeze-dried andinfrared heated materials were compared. Both methods from carotenecontent in dried material point of view were considered as equivalent.However, microscopic examination of dried material showed thatfreeze-dried carrot contains fissures and cracks while infrared dried tissuewas similar to the fresh one.

The ability to imbibe water is also a measure of dried product qual-ity. Rehydration of dried Welsh onion showed that drying by far infraredheating under vacuum was the best method. However, the color of onionwas best preserved when far infrared heating accompanied by flow of airwas applied. Drying of parsley by far infrared heating yielded productwith the least losses of vitamin C, volatiles, and changes of taste.[4] Para-meters of infrared heating of parboiled rice decided about the productcolor, percent gelatinized kernel, and head rice yield.[10] There are someindications that infrared heating affects enzymes in food and that itcan be beneficial to the flavor of the product.[11]

Published results considering quality of infrared-heated products arescarce and not conclusive, especially when different methods of heating

832 Nowak and Lewicki

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are compared. This is because the quality attributes are not correlatedwith temperature of the material, the variable that pronouncedly affectsquality of biological materials.

The aim of this work is to compare quality of selected food productdried by infrared heating and convection at such parameters at whichfinal material temperature in both methods is alike.

MATERIAL AND METHODS

Apple (Malusdomestica) v. Idared cut into slices of thickness 5.5� 0.1mmand diameter around 8 cm was used in this work. The skin was notremoved from the apple, and slices were not pretreated before drying.

Drying was done in a prototype dryer described in details in publi-cation by Nowak and Lewicki.[12] The dryer could be used for infraredand convective drying as well. The emitters with total power of7.875 kW=m2 and dominated wavelength 1.2 mm were mounted onadjustable rack. The drying was done at the distance between the emittersand heated surface equal to 10, 20, and 30 cm. The flow of ambient airover the heated surface was set at 0.5, 1.0, and 1.5 m=s. Mass of the dry-ing material and its temperature were recorded continuously. Tempera-ture of three slices placed in different locations on the wire tray wasmeasured with thermocouples inserted into the material from that side,which was not illuminated with infrared energy. Drying was done untilconstant mass was reached.

Convective drying was done in the same dryer using hot air at 65 and75�C. The air flow was set at 1.5m=s. Quality attributes measured in thiswork included color, kinetics of water adsorption, mechanical properties,and microstructure.

Color of apple slices after drying and stored for 6 months was mea-sured with chromameter CR-300 (Minolta). Measurements were done in15 replications, and color was expressed in the CIE system Y, x, y.

Kinetics of water vapor adsorption was measured by placing appleslices in environment with RH ¼ 100% and continuous recordingincrease in mass of the material. Apple slices prior to measurement werestored for 3 months in a desiccator with anhydrous CaCl2 in order toobtain the same initial water content. Adsorption of water vapor wasfollowed for 40 h at 25�C.

Mechanical properties of dried apple slices were measured usingTexture Analyser TA-XT2 (Stable Micro System Ltd., Godalming,Surrey, U.K.). Material subjected to breaking test was prepared the sameway as that used in water vapor adsorption measurement. A three-pointbend rig HD=3PB was used with cross-head speed of 0.1mm=s. Measure-ments were done in 15 replicates.

Infrared Dried Apple Slices 833

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Microstructure of dried apple slices was observed in scanning elec-tron microscope JSM-35R (JOEL, Japan Electron Optics LaboratoryCo., Ltd., Tokyo, Japan). Material preparation and analysis of micro-structure were done in the Laboratory of Electron Microscopy ofWarsaw Agricultural University.

RESULTS AND DISCUSSION

Color of Dried Apple Slices

Slices of raw apple were characterized by luminance Y ¼ 61.82� 2.34and chromaticity coefficients: x ¼ 0.3387� 0.0025 and y ¼ 0.3569�0.0034. This corresponds to a pale yellowish color of apple flesh.

Drying done under different conditions affected color of apple slices,and moves chromaticity coefficients toward more saturated yellow color(Fig. 1). Analysis of data presented in Fig. 1 shows that the changes inchromaticity coefficients are not dependent on the mode of heat supply,but are related to the temperature of dried material at the final stages of

Figure 1. Influence of drying on chromaticity coefficients of apple slices. .:1 ¼ 10 cm, v ¼ 0:5m=s; t ¼ 95:7�C; �: 1 ¼ 30 cm, v ¼ 0:5m=s, t ¼ 74:7�C;

4

:1 ¼ 10 cm, v ¼ 1:0m=s, t ¼ 81:6�C; !: 1 ¼ 30 cm, v ¼ 1:0m=s, t ¼ 63:0�C; &:1 ¼ 10 cm, v ¼ 1:5m=s, t ¼ 72:0�C; &: 1 ¼ 30 cm, v ¼ 1:5m=s, t ¼ 53:2�C; �:1 ¼ 20 cm, v ¼ 0:5m=s, t ¼ 87:1�C; ^: conv65, t ¼ 63:2�C; ~: 1 ¼ 20 cm,v ¼ 1:0m=s, t ¼ 72:7�C; 4: conv75, t ¼ 73�C; e: 1 ¼ 20 cm, v ¼ 1:5m=s,t ¼ 59:9�C; �þ : raw apple.


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drying. Apple slices heated to temperature below 65�C are not statisticallydifferent as far as chromaticity coefficients are concerned. However, theyare statistically different from those of raw apple. Increase of materialtemperature at final stages of drying changes the color, and observed dif-ferences are statistically significant. It is characteristic that temperatureaffected more coefficient x than y.

The chromaticity coefficients were not affected by storage in darkfor 6 months. Observed differences (Table 1) were not statistically sig-nificant. Similar results were published by Nowak et al.,[13] who showedthat apples dried by convection and stored in darkness did not changecolor during storage. Onion dried by convection and with the use ofinfrared energy did not differ in color after drying or after prolongedstorage.[14]

Luminance of dried apple slices was affected by temperature as well(Fig. 2). At final temperature of material undergoing drying not exceed-ing 70�C the luminance was not influenced by drying temperature andmode of heating supply. At temperatures above 70�C luminancedecreased with temperature and at 95�C it was close to 50. The decreasewas almost linear with temperature. Storage for 6 months did not affectsubstantially the luminance of apple slices. However, some lightening ofapple slices dried at temperature exceeding 70�C was observed.

Luminance of apple slices dried at temperature below 70�C washigher than that of raw apple. This may be due to the substitution ofwater by air and formation of porous structure. Light absorption and

Table 1. Chromaticity coefficients of dried apple

Drying modetemperature, �C

After drying After storage

x SD y SD x SD y SD

Infrared95.7 0.3797 0.0136 0.3760 0.0064 0.3706 0.0118 0.3729 0.006387.1 0.3725 0.0125 0.3704 0.0066 0.3664 0.0093 0.3725 0.003881.6 0.3644 0.0054 0.3687 0.0047 0.3656 0.0085 0.3658 0.005374.4 0.3614 0.0058 0.3654 0.0030 0.3657 0.0094 0.3656 0.005872.7 0.3615 0.0086 0.3637 0.0051 0.3647 0.0084 0.3643 0.006372.0 0.3599 0.0088 0.3651 0.0071 0.3623 0.0084 0.3628 0.005463.0 0.3503 0.0047 0.3600 0.0057 0.3514 0.0047 0.3571 0.003759.9 0.3527 0.0092 0.3606 0.0063 0.3546 0.0069 0.3633 0.004453.2 0.3520 0.0059 0.3580 0.0053 0.3530 0.0051 0.3559 0.0033

Conv.73.0 0.3554 0.0091 0.3623 0.0059 0.3540 0.0051 0.3582 0.003563.2 0.3481 0.0043 0.3561 0.0043 0.3549 0.0053 0.3577 0.0039


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scattering by porous body is different than that by moist surface, hencethe material looks lighter. Research done by Lewicki and Duszczyk[15]

shows that color changes of vegetables were predominantly due to struc-ture alterations caused by the drying process. Drying at temperatureexceeding 70�C can promote nonenzymatic browning and luminance ofthe material decreases. At the same time, chromaticity coefficients aremoved toward more yellow color. Storage of this material seems to causelightening of apple slices. It can be either due to physical changes of thestored material[14] or to oxidation of brown pigments formed duringdrying. Similar changes of luminance of onion dried by convection orby infrared energy were observed by Lewicki et al.[14]

Analysis of chromaticity coefficients and luminance of dried appleslices shows no distinct effect of the mode of heat supply on the colorof the material and its stability during storage. Nevertheless, there is apronounced effect of the final material temperature on its color. Theeffect of temperature below 70�C, is small and observed changes in colorof apple slices are mostly due to alterations of structure and concentra-tions of solids caused by drying. Temperature of material higher than70�C decreases luminance and moves chromaticity coefficients toward

Figure 2. Influence of final temperature of material undergoing drying and sto-rage on luminosity of apple slices. .: 1 ¼ 10 cm, v ¼ 0:5m=s; t ¼ 95:7�C; �,1 ¼ 30 cm, v ¼ 0:5m=s, t ¼ 74:7�C;

4

: 1 ¼ 10 cm, v ¼ 1:0m=s, t ¼ 81:6�C; &:1 ¼ 10 cm, v ¼ 1:5m=s, t ¼ 72:0�C; &: 1 ¼ 30 cm, v ¼ 1:5m=s, t ¼ 53:2�C; �:1 ¼ 20 cm, v ¼ 0:5m=s, t ¼ 87:1�C; ^: conv65, t ¼ 63:2�C; ~: 1 ¼ 20 cm,v ¼ 1:0m=s, t ¼ 72:7�C; 4: conv75, t ¼ 73�C; e: 1 ¼ 20 cm, v ¼ 1:5m=s,t ¼ 59:9�C.


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more saturated yellow color. Nonenzymatic browning probably occursand causes changes in color of dried apple slices. The correlation betweenchanges in color and the amount of infrared energy supplied to the sam-ples of potato and pineapple (temperature of the material during drying)was noted by Tan et al.[16]

Kinetics of Water Vapor Adsorption

Water content 1.022 to 1.127 g=g d.m. was reached in dried apple sliceskept 40 h over distilled water. The difference is close to 10% and showsthat the ability of dried material to adsorb water vapor is little dependenton the drying variables.

Rate of adsorption is the highest during the first hour of the process.During that time, 10% of water is adsorbed, but the rate of adsorptiondecreases about 2-fold. The highest rate of adsorption during the firsthour of the process shows the infrared dried material, whose final tem-perature was 53.2�C. The lowest rate was observed for infrared driedmaterial, whose final temperature was 95.7�C. The difference after onehour of adsorption was about 25%. The difference decreased with timeof adsorption.

The above data suggest that the final material temperature mayaffect the ability of dried apple slices to adsorb water vapor. The relation-ship between the time to adsorb one gram of water and temperature(Fig. 3) is statistically significant, indeed. The determination coefficientis 0.70 at 9 degrees of freedom. The result for convective drying is withinthe confidence limits. Hence, it can be concluded that the mode of heatsupply does not affect ability of dry material to adsorb water vapor.

Drying causes many changes in plant material[17] and they can bepronounced by the ability of dry product to imbibe water.[18] It wasshown by Tsami et al.[19] that mode of drying affects the course of watersorption isotherms. However, published results relate observed differ-ences to the way material is dried, and not to its temperature. Results col-lected in this work show that final material temperature could beresponsible for the differences in the ability of dry apple slices to adsorbwater. Considering the way heat is supplied to the material undergoingdrying, results of this work show that convective and infrared heatingaffect ability of apple slices to adsorb water to the same degree.

Rheological Properties of Dried Apple Slices

Dried apple slices can be consumed directly, hence their crispness orcrunchiness becomes important quality attribute. Results published byVickers and Bourne[20] and Katz and Labuza[21] showed that there is a


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correlation between mechanical properties measured instrumentally andsensory assessed quality. On that basis, rheological properties measuredin this work were also treated as assessment of quality of dried apple slices.

Apple slices subjected to breaking test were equilibrated over anhy-drous CaCl2. The water activity was from 0.058 to 0.068 for all the testedsamples.

An example of the relationship between force and distance is pre-sented in Fig. 4. The course of deformation is similar to that reportedby Stanley[22] for cornstarch extrudates, and Hsieh et al.[23] for puffed ricecakes, products with low moisture content and highly porous.

Dried apple slices are very heterogeneous from the point of view ofmechanical properties. This is mainly due to the natural heterogeneous-ness of plant material and variability of drying conditions in the spaceof drying chamber. The infrared energy field is not uniform in the dryingchamber and each slice is dried under slightly different conditions. Thesame applies to convective drying, since temperature and humidity ofhot air change along the drying tray.

Figure 3. Influence of final temperature of material undergoing drying on timeneeded to reach the water content 1 g=gd.m by apple slices. Dotted lines indicateconfidence limits at p ¼ 0:95. .: 1 ¼ 10 cm, v ¼ 0:5m=s; t ¼ 95:7�C; �,1 ¼ 30 cm, v ¼ 0:5m=s, t ¼ 74:7�C;

4: 1 ¼ 10 cm, v ¼ 1:0m=s, t ¼ 81:6�C; !:

1 ¼ 30 cm, v ¼ 1:0m=s, t ¼ 63:0�C; &: 1 ¼ 10 cm, v ¼ 1:5m=s, t ¼ 72:0�C; &:1 ¼ 30 cm, v ¼ 1:5m=s, t ¼ 53:2�C; �: 1 ¼ 20 cm, v ¼ 0:5m=s, t ¼ 87:1�C; ^:conv65, t ¼ 63:2�C; ~: 1 ¼ 20 cm, v ¼ 1:0m=s, t ¼ 72:7�C; 4: conv75, t ¼ 73�C;

e: 1 ¼ 20 cm, v ¼ 1:5m=s, t ¼ 59:9�C.


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Although the dried slices differ substantially, each from the others,doing measurements in 15 replications it was assumed that same averageinformation about mechanical properties of the material is obtained. Thefollowing variables were calculated:

. Slope of initial part of the deformation curve

. Breaking force

. Work of breaking calculated as the area under the deformation curve

The slope of initial straight part of the deformation curve is treated as themeasure of crispness,[21,24] and is expressed in N=mm. The data in Fig. 5show that the slope is a function of the final material temperature. Therelationship shows also that the higher the final material temperature,the crisper the dried apple slices. The spread of points is large and thedetermination coefficient is 0.58 but the relationship is statistically signifi-cant. It is worth noting that crispness of apple slices dried by convectionfalls within the confidence limits of the relationship between slope andtemperature. It means that there is no difference between slices driedby the two investigated methods as far as the crispness is concerned.

The breaking force is treated as the measure of hardness[23] or crisp-ness.[21] Data collected in Table 2 shows that standard deviation of

Figure 4. Relationship between force and deformation for replicate measure-ments of infrared dried apple slices, which final temperature was 87.1�C.


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Figure 5. Relationship between crispness of dried apple slices and final materialtemperature. Dotted lines indicate confidence limits at p ¼ 0:95. .: 1 ¼ 10 cm,v ¼ 0:5m=s, t ¼ 95:7�C; �: 1 ¼ 30 cm, v ¼ 0:5m=s, t ¼ 74:7�C;

4

: 1 ¼ 10 cm,v ¼ 1:0m=s, t ¼ 81:6�C; !: 1 ¼ 30 cm, v ¼ 1:0m=s, t ¼ 63:0�C; &: 1 ¼ 10 cm,v ¼ 1:5m=s, t ¼ 72:0�C; &: 1 ¼ 30 cm, v ¼ 1:5m=s, t ¼ 53:2�C; �: 1 ¼ 20 cm,v ¼ 0:5m=s, t ¼ 87:1�C; ^: conv65, t ¼ 63:2�C; ~: 1 ¼ 20 cm, v ¼ 1:0m=s,t ¼ 72:7�C; 4: conv75, t ¼ 73�C; e: 1 ¼ 20 cm, v ¼ 1:5m=s, t ¼ 59:9�C.

Table 2. Hardness of dried apple slices

Temperature,�C

Maximum force,N

Standard deviation(SD)

Infrared drying 95.7 7.52 1.5787.1 8.42 2.8581.6 7.85 2.4574.4 9.07 2.7172.7 6.43 1.9372.0 6.71 1.7363.0 8.71 2.3659.9 6.89 1.0453.2 6.77 2.27

Convective drying 73.0 7.94 2.2663.2 8.89 1.63


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measured values is large and can be as large as 34%. Because of this largevariability, there is no statistically significant relationship betweenthe breaking force and either mode of drying or final temperature ofthe drying material.

Work needed to break the dried apple slices depends on the dryingconditions (Fig. 6). The variability of the calculated values is even largerthan that observed for breaking force. Standard deviation reaches even70% of the measurement value. There is an inverse relationship betweenthe work and the final temperature of material undergoing drying. Thedetermination coefficient is 0.59 and at 9 degrees of freedom it is statisti-cally significant. The work needed for breaking of apple slices dried byconvection does not differ significantly from that calculated for infrareddried material.

Increase of the slope of the initial part of the deformation curvewith temperature accompanied by the decrease of breaking work charac-terizes well mechanical properties of the material. High final materialtemperature results in fast water evaporation and short drying time.

Figure 6. Relationship between work of breaking of dried apple slices and finalmaterial temperature. Dotted lines indicate confidence limits at p ¼ 0:95. .:1 ¼ 10 cm, v ¼ 0:5m=s; t ¼ 95:7�C; �, 1 ¼ 30 cm, v ¼ 0:5m=s, t ¼ 74:7�C;

4

:1 ¼ 10 cm, v ¼ 1:0m=s, t ¼ 81:6�C; !: 1 ¼ 30 cm, v ¼ 1:0m=s, t ¼ 63:0�C; &:1 ¼ 10 cm, v ¼ 1:5m=s, t ¼ 72:0�C; &: 1 ¼ 30 cm, v ¼ 1:5m=s, t ¼ 53:2�C; �:1 ¼ 20 cm, v ¼ 0:5m=s, t ¼ 87:1�C; ^: conv65, t ¼ 63:2�C; ~: 1 ¼ 20 cm,v ¼ 1:0m=s, t ¼ 72:7�C; 4: conv75, t ¼ 73�C; e: 1 ¼ 20 cm, v ¼ 1:5m=s,t ¼ 59:9�C.


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Under these conditions, large shrinkage stresses are created and injuryincurred to the tissue structure can be more extensive than that observedin samples dried slowly. Moreover, fast drying facilitates formation ofamorphous domains, which are brittle and not as mechanically resistantas crystalline ones.[25] On the other hand, fast evaporation of water canaffect the crystallinity of cell walls. Increase of cellulose crystallinitycaused by dehydration was observed by Sterling.[26] Increased cross-linking of proteins and structural changes in enzyme proteins were alsoobserved. Hardening effects of hot drying air temperature on apple tissuewas observed by Lewicki and Jakubczyk.[27] At temperature lower than70�C, mechanical properties of dried apple were not pronouncedly affec-ted by hot air temperature. But increase of temperature to 80�C increasedresistance of dried material to deformation in comparison to that dried at70�C. In conclusion, it can be stated that drying causes changes in biopo-lymers and tissue structure. These changes are temperature dependent.The higher the material temperature, the larger force is needed to initiatedisintegration. Once the disintegration is initiated, it propagates easilyand less work is needed to break material into pieces. From the sensorypoint of view, dried material is crispier. At the same time, there is no evi-dence to assign observed changes in mechanical properties of dried appleslices to the mode of heat supply.

Microstructure of Dried Apple Slices

Apple slices dried by convection at hot air temperature of 65�C (finalmaterial temperature 63.2�C) and infrared dried slices at final materialtemperature 53.2�C and 95.7�C were examined in scanning electronmicroscope. Pictures of the tissue structure are presented in Fig. 7.

Microstructure of convective dried apple slices is pretty homogeneousand cells are smaller than those observed in infrared dried material. Thecells are spread uniformly in the cross-section of the slice. Structure ofapple tissue infrared dried at the final material temperature close to 96�Cis variable. Large cells are present mostly in the surface layer absorbinginfrared energy. On average, the cavities are much larger than those occur-ring in the convective dried apple slices. It is worth noticing that the voidsin infrared dried apple slices at final material temperature close to 53�C arealso larger than those in the convective dried slices. Hence, there is no cor-relation between the structure and final material temperature.

Time of drying by convection was 250min, while infrared dryingtook 192min and 104min at the final material temperature 53.2�C and

Figure 7. Microphotographs of internal structure of apple slices dried: a) byconvection at 65�C, b) infrared at final temperature 95.7�C, c) infrared at finaltemperature 53.2�C.

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95.7�C, respectively. It suggests that the drying rate can be responsible forthe changes in structure of the material. At high drying rate, large shrink-age stresses are created and the damage of the tissue structure is muchlarger than that at low drying rates. These observations correlate wellwith the influence of the material temperature on the mechanical proper-ties of dried apple slices. Barret and Peleg[28] showed that there is arelationship between mechanical resistance and the size of air cells inporous materials. The larger the cells, the less resistant the material.

CONCLUSIONS

Results presented in this work show that such material properties ascolor, ability to adsorb water, and mechanical resistance to breakageare not dependent on the way the heat is supplied to the material under-going drying, as far as convective and infrared drying are concerned. Itseems that two variables are most important; that is, material finaltemperature and the rate of drying.

Rate of drying is probably responsible for the tissue structure. Highdrying rate damages tissue and the material becomes fragile. The work ofdisintegration is smaller than that needed for samples dried at low dryingrates. Drying rate also affects phase changes in the material. Formationof amorphous and crystalline domains affects mechanical properties ofthe dried material.

Material temperature, especially at the final stages of the drying,causes chemical changes in the apple slices. Some browning is observedand color coefficients move toward more saturated color. Temperaturecan also affect phase changes in the material due to its influence on vis-cosity. Hence, formation of glassy or crystalline states can be related tothe final material temperature and result in different mechanical proper-ties of the dried material.

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quality of infrared dried apple slices

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