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QUALITY AND DRYING CHARACTERISTICS OF APPLE CUBESSUBJECTED TO COMBINED DRYING (FD PRE-DRYING ANDHAD FINISH-DRYING)TAMÁS ANTAL1,3, BENEDEK KEREKES1, LÁSZLÓ SIKOLYA1 and MOHAMED TAREK2

1Department of Vehicle and Agricultural Engineering, Institute of Engineering and Agricultural Sciences, 2Agricultural and Molecular Service and

Research Institute, College of Nyíregyháza, Kótaji str. 9-11., H-4400 Nyíregyháza, Hungary

3Corresponding author.TEL: +36 42 599 400;FAX: +36 42 402 485;EMAIL: [email protected]

Received for Publication March 24, 2014Accepted for Publication June 2, 2014

doi:10.1111/jfpp.12313

ABSTRACT

The drying methods tested were hot-air drying (HAD) and freeze-drying (FD), aswell as a combination of blanching (100C, 1 min) prior to FD pre-drying andHAD finish drying. The effect of drying methods on moisture content, tempera-ture, energy consumption, duration of drying, rehydration properties, color,texture and sensory were determined. Apple’s HAD and FD kinetics was describedby an exponential and third-degree polynomial model, while the combineddrying-blanching prior to freeze pre-drying and hot-air finish-drying (BFD-HAD) kinetics consisted of two periods: polynomial until a converting point andexponential beyond that point. Among the products, the quality of apple samplesprocessed by FD was the best, followed by BFD-HAD, whereas the products madeusing HAD were the worst. The drying times obtained for HAD and BFD-HADwere shorter than the drying times obtained for FD. The best BFD-HAD process,from quality and energy consumption considerations, was found to a moisturecontent of 30% w.b. (BFD time: 12 h), followed by 3 h of HAD. It was observedthat BFD-HAD can reduce by about 8 h the time needed for FD.

PRACTICAL APPLICATIONS

Apple (Malus domestica L.) is one of the most widely grown fruit crops in theworld. Available in fresh and processed forms, apples are rich source of vitamins,organic acids, polyphenols, anthocyanins and minerals. Currently, dehydrating is afrequent practice since dried apples are included in numerous processed foodssuch as snack products, integral breakfast foods, etc. The aim of this article is toexamine the effectiveness of freeze-drying enhanced with two-stage vacuum freezeand hot-air drying. It can be suggested to apply the blanching as pretreatmentsand combined drying (FD-HAD) for reduction of operational cost. Our researchwork confirmed that BFD-HAD is a good alternative instead of the FD. The devel-opment of new drying techniques possibility for apple with good physical, chemi-cal properties may be of interest in order to the market supply (food supplement,functional food, etc.).

INTRODUCTION

Freeze-drying (FD), also known as lyophilization, is a sepa-ration process widely used in biotechnology, fine chemicals,food and pharmaceutical industries (Liapis and Bruttini1995). FD accomplishes water removal by sublimation atvery low temperatures and pressures, which ensures betterquality of the dehydrated products (Zhang et al. 2011).

Freeze-dried products achieve a high rating for physical,chemical properties: rapid rehydration, little shrinkage, highretention of color, low bulk density, high chemical content,low water activity and superior taste, flavor, aroma.However, FD is very costly because of high investment andoperational costs (long process time), limit its applicationto industrial scale. Long drying times are caused by a highheat transfer resistance that especially exist in the final

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drying period and substantially increases operating costs(Lombrana et al. 2001). Furthermore, compared with con-ventional drying techniques, additional energy is needed forfreezing and thawing of the material and the operation andrecovery of freeze condensers. An acceleration and produc-tivity increase for FD processes would raise their competi-tiveness (Rother et al. 2011).

Energy consumption and quality of dried products andother critical parameters are very important in the selectionof a drying process. Combination drying is considered thebest way to reduce energy uptake and improve quality(Raghavan et al. 2005). Therefore, cost-effective alternativesystems such as combination drying should be promoted toreap the advantages of advanced drying systems withminimum cost and simple technologies (Dev and Raghavan2012). Combination drying with an initial conventionaldrying process followed by a freeze finish process or reversehas proven to reduce drying time while maintain productquality and minimizing energy requirements. According toearlier reports, hot-air (HAD) pre-drying can reduce byabout half the drying time needed for traditional FD(Kumar et al. 2007).

Convective drying (HAD) is still most popular methodapplied to reduce the moisture content of fruits and veg-etables. This method is most commonly used to dry apple.However, this method has several disadvantages and limita-tions, for instance, it requires relatively long times and hightemperatures. The hot air (70–90C) causes degradation ofimportant flavour compounds and nutritional substances,as well as color alteration. Another disadvantage of HAD isshrinkage, hard surface, which results on tissue collapse(Szumny et al. 2010).

Mathematical modeling can play an important role in thedesign and control of process parameters during drying,and performing simulations using accurate kinetics modelscan contribute to the optimization of the process(Khraisheh et al. 2000). Process models validated by experi-ments ought to predict the influence of most relevantparameters on drying time, and help to improve the energyefficiency of combinations of drying processes (Zhang et al.2006).

The apples are consumed in the form of various pro-cessed such as juice, jam, marmalade, dried apples and babyfood (Mandala et al. 2005; Doymaz 2009). Dried apples aresignificant raw materials for many food products (Mandalaet al. 2005). Apples (Malus domestica) are one of the mostwidely grown and economically important fruit crops allover the world. These naturally occurring antioxidative sec-ondary plant products belong to flavonoids, a group ofpolyphenols with cardio and cancer protective effects(Schulze et al. 2014).

In the present research, the objectives were to comparethe drying kinetics of HAD, FD and combined drying-

blanching prior to freeze pre-drying and hot-air finish-drying (BFD-HAD), material temperature under dryingand energy consumption. Moreover, our aim was to investi-gate the effects of BFD-HAD on FD and HAD characteris-tics of Idared apple cubes and the key quality attributes, viz.color, texture, rehydration ratio (RR) and sensory assess-ment of the dried products. There is currently little litera-ture available about this combined drying.

MATERIALS AND METHODS

Material

Ripe Idared apples were picked from the orchard nearNyíregyháza and stored in a refrigerator (5C) until use. Theapples were cored with a household tool, washed with tapwater and cut into cubes with 10 mm thickness. Thesamples were divided into eight groups, each group ofsamples weighed 200 g. The initial mass of apple cubes wasmeasured using a balance (model JKH-500, Jadever Co.,Taipei, Taiwan).

Drying Equipment and Procedure

The apple cubes were dried by different drying methodswith the optimal drying technology until final moisturecontent (5–6%, wet basis: w.b.). The moisture loss wasrecorded by a digital balance (model JKH-500, Jadever Co.)at 1-h intervals during drying for determination of dehy-dration curves. These experiments were repeated threetimes and the average of three results for each treatment wasused in this paper.(1) Convective drying was carried out in a hot-air dryer(model LP306, LaborMIM, Budapest, Hungary) at 75C withan air flow rate of 1 m/s. Air humidity was regulated at15–20%. The samples (200 g) were spread uniformly, insingle layer on the trays of dryer. After 1 h, the trays weretaken out of the equipment, weighed and then put back inthe dryer. During the drying process, the weight of the applecubes was recorded to construct a drying curve, and thetemperature (material and air), air velocity, air humiditywas measured using a Testo 4510 type meter (Testo GmbH,Lenzkirch, Germany). The mass was measured on an ana-lytical balance (model JKH-500, Jadever Co.) with a preci-sion of ±0.1 g. The apples were dehydrated until theyreached the final moisture content (5%, w.b.).(2) FD was performed in a laboratory-scale Armfield FT-33freeze-dryer (Armfield, Ltd., Ringwood, England). Theapple samples (200 g) were frozen at −23C in a freezing/heating chamber and freeze-dried to a moisture content of5–6% (w.b.) at an absolute pressure of 70–85 Pa with achamber temperature of 20C and a condenser temperature

APPLE DRYING T. ANTAL ET AL.

Journal of Food Processing and Preservation •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.2

of −50C. Thermocouples (four pieces) of freeze drier wereinserted into the apple cubes. The weight loss of the sampleswas followed by a data logger and a RS-232 attached to apersonal computer, acquired the data readings from plat-form cell, which is placed within the sample chamber.(3) To reduce the drying time and energy consumption indrying Idared apple, BFD-HAD can be used to replace thetraditional FD method. Apple cubes were dried by BFDfirst, HAD process initiated when moisture content ofsamples reach to 75% ± 1%, 60% ± 1%, 40% ± 1%,30% ± 1% and 20% ± 1% w.b., respectively. Otherwise, theapple cubes were removed from the freeze dryer at a specifictime (4, 7, 10, 12 and 14 h) and the drying process wereresumed in hot-air dryer.

The drying parameters are in agreement with the above-mentioned. Hot water blanching (100C, 1 min) was used as apretreatment before FD. The samples were dehydrated to afinal moisture content of ∼5% (w.b.). The dehydrated applesamples were packaged into polyethylene bags with silica gel.

Moisture Content

Moisture content was determined by the gravimetricmethod (model LP306, LaborMIM). At regular time inter-vals during the drying process, samples were taken out anddried for 8 h at 105C until constant weight. Weighing wasperformed on a digital balance (model JKH-500, JadeverCo.) and then moisture content (w.b.) was calculated. Theinitial moisture content of the apple was found to be 85.9%(w.b.), 6.09 kg H2O/kg dry matter (dry basis: d.b.). The testswere performed in triplicate.

Hot Water Blanching

Hot water blanching was carried out by immersing applecubes into 100C for 1 min distilled water in a stainless steelpan heated by an induction cooking plate. The weight ratioof apple cubes and distilled water was 1:2. After the pre-treatment, the samples were cooled using a fan until therewas no visible water on the surface of the material. Theblanching pretreatment was carried out in triplicate.

Modeling of Drying Kinetics

There are several empirical approaches for modeling thedrying kinetics. Henderson and Pabis (exponential) andthird-degree polynomial models were used to fit the dryingcurves (MR versus drying time) in this study, based onreported works on drying thin-layer fruits (Doymaz andIsmail 2011; Antal et al. 2012). Selected thin-layer dryingmodels, Henderson-Pabis and polynomial models, aredetailed in the following (Eqs. 1 and 2):

MR a e k t= ⋅ − ⋅ (1)

where MR is the dimensionless moisture ratio, a is thedrying coefficient, k is the drying constant and t is thedrying time (h).

MR a t b t c t= ⋅ + ⋅ + ⋅ +3 2 1 (2)

where, MR is the dimensionless moisture ratio, a, b, c are thedrying coefficient and t is the drying time (h).

The moisture content of samples is defined by (Eq. 3):

Mm m

mt

t f

f

= −(3)

where, Mt is the moisture content at time t on dry basis (kgH2O/kg dm), mt is the weight of material at specific t (kg)and mf is the dry matter weight of the material (kg).

The dimensionless moisture ratio (MR) was calculated as(Eq. 4):

MRM M

M M= −

−t e

e0

(4)

where, Me is the equilibrium moisture content (kg H2O/kgdm) and M0 is the initial moisture content (kg H2O/kg dm).

The Me values were a bit lower than final moisturecontent of dried apple samples. Such low values of Me had anegligible effect on MR, which depended mainly on thevalues of Mt and M0 (Calín-Sánchez et al. 2013).

The goodness of fit between the experimental and pre-dicted data was determined using the correlation coefficient(R2) and root mean square error (RMSE), as describedbelow (Eq. 5):

RMSE =−( )

=∑ MR MR

N

i pre i

i

N

exp, ,2

1 (5)

where N is the number of observations, exp is the experi-mental and pre is the predicted.

Therefore, the best model was chosen as one with thehighest coefficient of correlation (R2) and the least RMSE.

Measuring of Energy Consumption

Energy used in the drying and heating process is importantfor production processes in the industrial and householdsectors. However, the price of energy is extremely expensive;therefore, there are a strong incentive to invent processesthat will use energy efficiently. Currently, widely useddrying processes are complicated and inefficient; moreover,it is generally damaging to the environment. What is neededis a simplified, lower cost approach to this process one that

T. ANTAL ET AL. APPLE DRYING


will be replicable in a range of situations (Jindarat et al.2011). The total energy consumption (E, kWh) during FD,HAD and BFD-HAD was measured by an energy-cost-checker (model EKM 265, Conrad Electronic GmbH,Hirschau, Germany).

Color

The color is one of the most important appearance attributeof food materials since it influences consumer acceptability(Maskan 2001). The color measurements can be used in anindirect way to estimate color change of foods since it issimpler and faster than chemical analysis. Hunter colorparameters (L, a, b) have previously proved valuable indescribing visual color deterioration.

The color of raw and dried samples was measured using aspectrophotometer (model ColorLite sph900, ColorLiteGmbH, Katlenburg-Lindau, Germany). The instrument wasstandardized each time with a white ceramic plate. Theresults are expressed as Hunter L*, a*and b*, respectively,where L* is the degree of lightness, a* the degree of redness(+) and greenness (−), and b* the degree of yellowness (+)and blueness (−). The apple samples were scanned at fivedifferent locations to determine the average L*, a*and b*values as the average of the five measurements.

The spectrophotometer supplied with special adapter.MA38 adapter converts the scanning spot from 3.5 to38 mm. This device can be used to measure differentpowders and small samples. The apple cubes were cut intosmall portion with knife and put into adapter for colormeasuring.

The total color differences (TCD) were estimated fromthe coordinates of the color by applying the following equa-tion (Eq. 6):

TCD * * *= − + − + −( ) ( ) ( )L L a a b b02

02

02 (6)

where, L0* is the degree of lightness at control, a0* is thedegree of redness and greenness at control and b0* is thedegree of yellowness and blueness at control.

Firmness Measurement

According to Shewfelt and Prussia (1993), firmness is theprimary textural attribute measured in fruits and vegetablesand a combination of cell structure integrity and tissueturgor. During dehydration, the cell wall structure, as a sub-stantial factor for fruit firmness, is partly modified due towater loss (Moreno et al. 2004).

Textural attributes of the apple cubes were measuredusing a texture analyser CT3-4500 (Brookfield EngineeringLaboratories, Middleboro, UK). Compression test wascarried out to generate a plot of force (N) versus time (s).

The probe had a diameter of 4 mm, and was adjusted for atravel distance of 20 mm at test speed of 1 mm/s. Themaximum depth of penetration was 2 mm and trigger forcewas 10 g. The mean value of firmness for each treatmentwas the calculated and expressed in Newtons (N). Averagevalues of firmness from five measurements were calculatedfor each experimental condition.

Rehydration Process

Dried food products are usually rehydrated before consump-tion. Rehydration is a complex process intended to restore theproperties of the fresh product by contacting dehydratedproducts with a liquid phase. The process is composed ofthree simultaneous steps: (1) water absorption into the drymaterial; (2) swelling of the rehydrated product; and (3) lossor diffusion of soluble components. The water temperature isthe most important factor influencing the rehydration andgenerally more rapid rehydration is obtained at higher watertemperatures. Treatments such as drying and rehydrationproduce changes in the structure and composition of producttissues (Cox et al. 2012).

The dried apple samples were soaked in controlled tem-perature of 75C distilled water for 90 min and then put onthe blotting paper to remove surplus water from the surfaceof samples. The weight used in each experiment was1 ± 0.1 g of dehydrated apple samples. The weighingwas performed on a digital balance. A dried apple cube wasadded to 150 mL of distilled water. The RR was evaluated asfollows (Eq. 7):

RR r

d

= m

m(7)

where md is the weight of apple after drying (kg) and mr isthe weight of apple after rehydration (kg).

The sample weighing was performed in triplicate.

Sensory Assessment

The sensory qualities of dried apple samples were analyzedin terms of appearance (1–4 points), texture (1–3 points)and flavor (1–3 points). The apple cubes were tested by apanel of 10 panelists, ages 25–50 years (five females and fivemales, all members of the college), with sensory evaluationof foods and vegetables experience. Sum score (total of 10points) of the three characteristics was used to rank the finalproducts. The apple samples with at least 7 points wereaccepted as good products (Huang et al. 2011). The panel-ists relied on their training experience to score products andapple samples were presented in plastic beakers with lids,which stored in room temperature for 30 min prior toanalyses. Dried samples were provided in random order in



beakers to the evaluators. The judges were asked to givetheir remarks about each of the samples. The evaluation wasperformed twice.

Statistical Analysis

Data analyses were determined using the PASW Statistics 18software (IBM Corp., New York, NY), and analyses of vari-ance (ANOVAs) were conducted by ANOVA procedure,Duncan test. Mean values were considered significantly dif-ferent when P < 0.05.

RESULTS AND DISCUSSION

In order to assess the advantage of the two-stage drying(BFD-HAD), the drying kinetics, drying time, temperature,energy consumption, color, texture, rehydration and sensorybetween FD, HAD products and BFD-HAD products werecompared. The FD was used as a control.

Drying Characteristics

The dimensionless moisture contents (MR) versus dryingtime at different drying processes and the conversion pointsare shown in Fig. 1. As shown and seen from the Fig. 1, thethin-layer drying models can be used for the prediction ofdrying behaviors of apples. It was found that the tempera-ture increased quickly at BFD-HAD1–3 than that in thesublimation phase – constant rate period of FD curve (toturn of curvature at FD curve: 12.5 h). This was becausemost of free water had been removed, resulting a decrease indrying time (39–61%), compared with FD (Table 1). Simi-larly, in the desorption phase at FD curve – second stage ofdrying (falling rate period) – the temperature rose morequickly (BFD-HAD4–5). Therefore, the drying rateincreased when HAD was applied at BFD-HAD method.The improvement in drying kinetics when hot-air is appliedduring drying has already been reported for strawberries(Xu et al. 2006), carrots (Kumar et al. 2007), pumpkin

FIG. 1. EXPERIMENTAL DATA (SYMBOLS)AND MODELS FITTED (LINE) MOISTURECONTENTS OF APPLE CUBES AT DIFFERENTDRYING METHODS AND CONDITIONS

TABLE 1. EFFECT OF DRYING METHODS ONMOISTURE CONTENT, DRYING TIME OFDEHYDRATED APPLE

Drying methodFinal moisturecontent (%, w.b.) Total drying time (h) Reduction in FD time (%)

HAD 5.0 6a –FD 6.1 23g –BFD 5.8 18f 21.74f

BFD-HAD1 5.1 9b 60.87a

BFD-HAD2 5.2 12c 47.83b

BFD-HAD3 5.3 14d 39.13c

BFD-HAD4 5.5 15de 34.78cd

BFD-HAD5 5.3 16e 30.43de

a,b,c Mean separation between rows based on probability of significant difference (P < 0.05).Values with the same superscript show no significant difference within the drying methods.BFD, blanching prior to freeze-drying; FD, freeze-drying; HAD, hot-air drying.



(Kumar et al. 2007) and bamboo shoots (Xu et al. 2005).The drying period at combination BFD pre-drying (4 h)and HAD finish-drying (5 h) was 2.55 times shorter thanthe drying period in single-stage FD treatment. Similarly, amarked reduction occurs in drying time at all two-stagedrying compared with FD dehydration (Table 1).

Figure 1 shows that the traditional FD process requiredthe longest drying time (23 h). This is because FD, undervacuum conditions, supplies the sublimation heat by con-duction. The rate of heat transfer is slow and thus dryingtakes a long time (Duan et al. 2012).

Blanching in hot water prior to FD reduced the dryingtime up to 21%. Acevedo et al. (2008) have argued thatblanching causes disruption of cell membranes and a con-comitant faster and more complete drying.

It was found that for Idared apple, the moisture contentsof HAD, BFD, FD, BFD-HAD1–5 dried samples whendrying was finished amounting to 0.103, 0.152, 0.166, 0.109,0.116, 0.128, 0.134, 0.122 kg H2O/kg dm., respectively.

It was found that no differences between HAD and BFD-HAD moisture content of dried samples. The final moisturecontent of FD and BFD apple cubes slightly higher thanthose of HAD ones (Table 1).

The statistical results from models are summarized inTable 2. The best model describing the thin-layer dryingcharacteristics of apple cubes was chosen as the one withhighest R2 values and the lowest RMSE values. Obviously, agood agreement exist between the experimental data andthe mathematical models, which is confirmed by the highvalues of the R2 (0.9813–0.9998) in all runs. In all cases, theRMSE values ranged from 0.013067 to 0.162545, respec-tively. The mathematical models (Eqs. 1 and 2) adequatelyrepresent the drying processes. This is especially true for thepolynomial model. The best coefficient of determination(R2) at FD application was 0.9998 within all dryingmethods.

According to Table 2, the k value increases as the HADdrying time (from 5 to 2 h) decreased during the combineddrying (BFD-HAD2–5) process.

Material Temperature

Temperature time history in the FD process is important asit reflects the general drying performance (Duan et al.2010). The material temperature curves at different dryingprocess are given in Fig. 2. From Fig. 2, it was found that theFD and BFD process could be divided three phases: phase oftemperature dropping (to −23C, it was not indicated intemperature profile), phase of sublimation and phase ofdesorption. Referring to Fig. 1, sublimation processoccurred in the first 11 and 12.5 h of drying and the BFDand FD Idared samples temperature profile in Fig. 2 showsthat after 11 and 12.5 h of sublimation process, drying tem-perature of samples change to 0C. Most of the free waterwas eliminated in this phase. In order to obtain excellentproduct quality, the temperature in the sublimation stageshould be lower than co-melting temperature (Duan et al.2007), but temperature of apple samples at combineddrying (BFD-HAD) was above the melting temperature. Asa result, higher material temperature (61–68C) could beapplied in this phase to improve the drying rate. Boundwater is removed during third phases of FD. According toLiapis and Bruttini (2007), bound water exist because ofphysical adsorption, chemical adsorption. This water isremoved by heating from the samples under vacuum. Themass transfer resistance increases with increasing dried layerthickness; this phenomenon can slow the drying rate in FDand BFD. Compared with traditional FD, the duration ofthe second stage of drying is reduced greatly in BFD. Whenmaterial temperature (constant state) is close to the tem-perature of the heating plate (20C), the drying process iscomplete. Unlike the BFD-HAD1–3 process, phase of subli-mation was relatively short and the temperature rosequickly. In the second stage of drying at BFD-HAD4–5, thetemperature also rose more quickly. The increase in materialtemperature and time of heat holding to 61–68C decreasedthe drying time at BFD-HAD compared with duration ofFD. It can be seen from Fig. 2 that the heating rate duringBFD-HAD1–5 was faster than that of FD process.

TABLE 2. CURVE FITTING CRITERIA FOR THEVARIOUS MATHEMATICAL MODELS AND THEMODEL PARAMETERSDrying methods

Model parameters Statistics

k a b c R2 RMSE

FD – 0.0001 −0.0045 −0.0086 0.9998 0.013067BFD – 0.0003 −0.0072 −0.0096 0.9997 0.016773HAD 0.643 2.3053 – – 0.9852 0.153862BFD-HAD1 0.79 1.8784 – – 0.9947 0.087525BFD-HAD2 0.714 1.3484 – – 0.9977 0.065113BFD-HAD3 0.757 0.9096 – – 0.9973 0.073328BFD-HAD4 0.83 0.6198 – – 0.9981 0.045897BFD-HAD5 0.944 0.3145 – – 0.9813 0.162545

BFD, blanching prior to freeze-drying; FD, freeze-drying; HAD, hot-air drying; RMSE, root meansquare error.



Electricity Energy Uptake ofDrying Processes

Figure 3 shows the energy consumption (E) of drying pro-cesses, which was measured by an energy-cost-checker.Energy consumption was 12.42, 9.72, 1.53, 3.43–8.07 forFD, BFD, HAD and BFD-HAD1–5, respectively.

The energy consumption (E) of FD was 12.42 kWh; thislevel is significantly higher than those other dryingmethods. Figure 3 shows that the water removed from thesamples amount to little energy uptake in case of combineddrying. The BFD-HAD1 resulted in a 72.38% decrease in Ewith respect to traditional FD, whereas HAD produced a87.68% decrease. Similar to our results, with a decrease in

time of HAD (at FD-HAD), the energy consumptionincreased (Xu et al. 2005). There was no significant differ-ence between BFD-HAD4 and BFD-HAD5 methods. Simi-larly, no significant differences were observed betweenBFD-HAD4 and BFD-HAD3 procedure.

Color of Apple Cubes Associated withDifferent Drying Methods

The drying causes many changes in color of apple samples.TCD for all drying treatments with respect to the control(FD) is reported in Table 3. Chromatic coordinates for rawapples were 74.2, 0.25 and 13.62 for L*, a* and b*, respec-tively. It was observed that the color of convective dried

FIG. 2. THE EFFECT OF COMBINEDMETHODS ON THE TEMPERATURE OF APPLESAMPLES

FIG. 3. ENERGY CONSUMPTION OFFD, BFD, HAD AND BFD-HADa,b,c Different letters indicate a significantdifference (P < 0.05) in a column.



(HAD) apple was significantly darker (L value) in compari-son with FD (control) raw samples. Moreover, the a* and b*value increased. The apple cubes dried using the HAD pro-cedure produced dehydrated apples with golden browncolor at a high value of b* amounting to 19.82 (Chong et al.2013). The TCD value of HAD dried apple was 23.59.Similar results were obtained by Kutyla-Olesiuk et al.(2013), where HAD of Idared apples at 70C caused adecrease of L* value from 81.9 to 77.7, increase of a* valuefrom −5.7 to 4.2 and b* value from 22.7 to 28.8. Changes inthe color of HAD apple were associated with long-lastinghigh temperature during drying process (the high tempera-ture lead to Maillard reaction; Mandala et al. 2005).

The use of FD and BFD caused an increase of L* param-eter. The FD and BFD samples were characterized by a

higher value of the parameter L* in comparison with freshapple, which represented the decrease in the degree ofbrowning discoloration, probably because of the low tem-perature in the chamber and blanching. The lowest changein a* and b* were noticed for FD apple cubes (TCD = 8.27).The FD and BFD drying of apple samples at mild tempera-ture (20C) produced treated apples with relatively smallcolor changes (TCD).

The experimental results in Table 3 show the effect ofHAD drying time on color change in case of BFD-HAD.Moreover, in BFD-HAD also, statistically significant changein the L*, a* and b* values was observed, which is indicatesthe browning reaction during drying. A decrease in L* valueand increase in a* value indicated the increase of browningin other words, evolved the Maillard reaction (Deng andZhao 2008). The TCD of BFD-HAD1–3 dried appleamounted to 22.69, 15.96 and 13.43 and were significantlyhigher compared with the samples dehydrated by BFD-HAD4–5 methods. The results of the color determinationreveal that combined BFD-HAD4–5 drying techniques pro-duced dehydrated apple with low TCD, which was 11.24 and10.42, respectively. The low color change was achieved atrelatively low values of parameter a* (4.22 and 2.18) andhigh values of parameters L* (64.94 and 64.66) and b* (8.64and 9.88). No significant differences in TCD values wereobserved between BFD and BFD-HAD4–5 samples. Accord-ing to Xu et al. (2006), the total value of color difference(TCD) decreased as time of FD is prolonged at FD-HAD.

Firmness Changes

The firmness (texture) of dried apple samples was evaluatedby a compression test, and the results can be seen in Fig. 4.

TABLE 3. COLOR VALUES AND COLOR CHANGE OF APPLE SAMPLESAFTER DRYING PROCESSES

Drying methods

Color parameters

L a b TCD

HAD 53.13f 8.87g 19.82e 23.59e

FD (control) 82.25a 1.05a 11.89a 8.27a

BFD 84.9ab 0.86a 14.42b 10.75b

BFD-HAD1 52.48f 6.45f 15.77bc 22.69e

BFD-HAD2 59.53e 5.22e 17.48d 15.96d

BFD-HAD3 63.11cd 3.04bc 6.57d 13.43c

BFD-HAD4 64.94c 4.22d 8.64bc 11.24b

BFD-HAD5 64.66c 2.18b 9.88b 10.42b

a,b,c Mean separation between rows based on probability of significantdifference (P < 0.05).BFD, blanching prior to freeze-drying; FD, freeze-drying; HAD, hot-airdrying; TCD, total color differences.

FIG. 4. TEXTURE OF DRIED APPLEASSOCIATED WITH DIFFERENT DRYINGMETHODSa,b,c Different letters indicate a significantdifference (P < 0.05) in a column.



The fresh apple presented a firmness of 4.52 N, which ishigh compared with the hardness values of FD, BFD andBFD-HAD3–5 treatments. BFD-HAD4-5 resulted in a21.88% and 26.31% decrease in hardness with respect toFD. Hot-dry air would cause a substantial increase in firm-ness of the Idared apple cubes. Figure 4 shows that the hard-ness decreased with the HAD drying duration from 5 to 2 hat combined drying methods. These results indicate that intreatments HAD and BFD-HAD1, higher tissue hardeningoccurred, which is in accordance with the increase of thestrength of cellular adhesion (Vega-Gálvez et al. 2012).There was no significant difference among them. Accordingto Lewicki and Jakubczyk (2004), the convection drying(50–80C) caused substantial changes in cell size and cell sizedistribution in Idared apple.

It is observed that the hardness of BFD and BFD-HAD4–5 samples were significantly lower than for the FD(control) samples. During blanching, there was rapid loss ofturgor and membrane integrity, which caused loosening ofthe cell walls (Canet et al. 2005). The porous structureresults a relatively slower drying rate during FD.

According to Deng and Zhao (2008), more porous spacein FD apple samples resulted in softer texture and higherrehydration capacity. The firmness of the BFD-HAD4samples was equal to the BFD-HAD5, and no significantdifference between them.

Effect of Different Drying Methods on RR ofApple Samples

The values of the RR for the different drying methods driedapple sample are detailed in Fig. 5. The rehydration capaci-ties of samples dried by FD and combined BFD-HAD4–5

were the same, and this can be explained by the fact that thesurface micro-holes and capillaries (Cui et al. 2008). The RRwas slightly higher, affected by BFD-HAD4, showing a 5.8%increase, correlate with FD. The BFD samples exhibited thehighest RR than the other three drying methods. The obser-vations are in agreement with our previous work (Antalet al. 2013). This may be due to the partial gelatinization,loss of soluble solids and cellular modification of the planttissue (Cunningham et al. 2008). The presented results showno statistically significant differences in the RR between theBFD-HAD4 and the BFD-HAD5 apple cubes. Similarly, nosignificant differences were observed between BFD-HAD2and BFD-HAD3 samples.

Similar to our results, with a decrease in time of HAD (atFD-HAD), the rehydration capacity of bamboo shootsimproved (Xu et al. 2005). The Idared apples dried by HADhad significantly lower RR than those other samples. Thisconsolidated, rigid structure leads to the absence of path-ways for water access.

Result of Sensory Analysis

In general, the drying methods significantly affected theintensities of the main sensory attributes of dried material(Calín-Sánchez et al. 2013). Results of sensory assessment ofthe finished products are listed in Table 4. It can be seen thatthe analysis for Idared apple cubes dehydrated by FD is thehighest followed by BFD-HAD5, and there was no signifi-cant difference among them. Similar results were obtainedby Huang et al. (2011) after FD, where FD apple chips hadrelatively high sensory score (7.7 points). However, appleprocessed by HAD and BFD-HAD1 have the lowest sensory

FIG. 5. REHYDRATION RATIO OF DRIEDAPPLE CUBESa,b,c Different letters in the same columnindicate a significant difference (P < 0.05),Duncan test.



values (5.4 and 5.1 points). It was found that there were noobvious differences in the sensory evaluation between thetwo drying methods.

The reason is that relative high temperature at HAD maylead to browning, shrinkage, rigid surface in productsduring the end stage of BFD-HAD, which results to adecrease in product quality. In fact, the previous results ontexture, color and rehydration had indicated the low scoreof HAD and BFD-HAD1–2 apple samples. Only applesdried with HAD and BFD-HAD1–3 were characterized bydarker yellow color. However, the most rigid apple surfacewas observed in the case of HAD. Strange flavor was notnoticeable in all samples. The BFD material was character-ized by significant less intense flavor. It is noted that FDsamples gave the highest value in appearance and texture,whereas the BFD-HAD5 exhibited the highest flavor degree.The sensory score for apple cubes dried by FD, BFD-HAD3–5 exceeds 7 points, which means that these appleproducts are generally acceptable to the consumers.

CONCLUSIONS

Idared apple cubes were dried using three different dryingmethods, i.e., hot-air (HAD), freeze-dried (FD) and theso-called two-stage blanching prior to freeze pre-drying andhot-air finish-drying (BFD-HAD). The two-stage drying(BFD-HAD) methods pronouncedly affected the dryingtime, energy uptake and physical-mechanical properties ofapple cubes. The drying kinetics was estimated usingHenderson–Pabis and third-order polynomial models.From the statistical test results and correlation, it is under-stood that thin-layer models have been successful in pre-dicting MR of drying apples. Single-stage FD could lead tothe best quality (e.g., the color of the samples was close tofresh apple), but had the longest drying time. An increase ofthe HAD drying time at BFD-HAD methods resulted in a

decrease of the quality (color, firmness and rehydrationcapacity). According to research results of Xu et al. (2006),the moisture content after FD stage (conversion point) wasan important factor affecting the final dehydrated productquality. It was found in this study that the TCD of applecubes dried using two-stage methods (BFD-HAD4–5) wereequal and lower compared with the BFD and FD methods.The rehydrated BFD-HAD4 apple samples was higher RR,than those of FD ones. In case of organoleptic evaluation,also, good results were provided by BFD-HAD5; the sensoryvalues were closer to the ideal ones (FD). This studyrevealed that the best quality of the combined dried productin terms of color, hardness, rehydration and sensory couldbe achieved by the BFD-HAD4–5. It can also be concludedthat BFD-HAD1 has the potential to decrease the dryingcost (more than 70%). The optimum combined drying wasBFD-HAD4 (12 h of BFD pre-drying followed by 3 h ofHAD finish-drying) take account of physical-mechanicalproperties, at which the energy consumption was 7.24 kWh,with a drying period of 15 h. Future work should focus onthe optimization of conversion point of drying curves andthe quality evaluation (microstructure, chemical compo-nents) of suitable combined treatments.

ACKNOWLEDGMENTS

This research was supported by the European Union andthe State of Hungary, co-financed by the European SocialFund in the framework of TÁMOP 4.2.4. A/2-11-1-2012-0001 “National Excellence Program.”

REFERENCES

ACEVEDO, N.C., BRIONES, V., BUERA, P. and AGUILERA,J.M. 2008. Microstructure affects the rate of chemical, physicaland color changes during storage of dried apple discs. J. FoodEng. 85, 222–231.

ANTAL, T., KEREKES, B. and SIKOLYA, L. 2012. Evaluation ofalkaline-ionized water pre-treatment for quality of driedapple varieties. In Proceedings of International DryingSymposium, Xiamen, China, November 11–15, 2012.

ANTAL, T., KEREKES, B. and SIKOLYA, L. 2013. Evaluation ofvarious chemical pre-treatments for physical properties offreeze dried apple. In Proceedings of InternationalMultidisciplinary Conference, Nyíregyháza, Hungary,May 22–24, 15–20.

CALÍN-SÁNCHEZ, Á., FIGIEL, A., LECH, K., SZUMNY, A. andCARBONELL-BARRACHINA, Á.A. 2013. Effects of dryingmethods on the composition of thyme (Thymus vulgaris L.)essential oil. Drying Technol. 31, 224–235.

CANET, W., ALVAREZ, M.D., LUNA, P., FERNANDEZ, C. andTORTOSA, M.E. 2005. Blanching effects on chemistry, qualityand structure of green beans (cv. Moncayo). Eur. Food Res.Technol. 22, 421–430.

TABLE 4. SENSORY EVALUATION OF APPLES DRIED BY DIFFERENTMETHODS

Drying methods

Sensory values

Appearance Texture Flavor Total scores

HAD 1.8 1.3 2.3 5.4ef

FD (control) 3.9 2.8 2.0 8.7a

BFD 3.4 2.1 1.4 6.9d

BFD-HAD1 1.7 1.5 1.9 5.1f

BFD-HAD2 1.9 1.7 2.1 5.7e

BFD-HAD3 3.1 1.9 2.4 7.4c

BFD-HAD4 3.5 1.9 2.3 7.7bc

BFD-HAD5 3.6 2.1 2.5 8.2ab

a,b,c Values followed by the same letter, within the same row, are notsignificantly different (P < 0.05), Duncan test.BFD, blanching prior to freeze-drying; FD, freeze-drying; HAD, hot-airdrying.



CHONG, C.H., LAW, C.L., FIGIEL, A., WOJDYLO, A. andOZIEMBLOWSKI, M. 2013. Colour, phenolic content andantioxidant capacity of some fruits dehydrated by acombination of different methods. Food Chem. 141,3889–3896.

COX, S., GUPTA, S. and ABU-GHANNAM, N. 2012. Effect ofdifferent rehydration temperatures on the moisture, contentof phenolic compounds, antioxidant capacity and texturalproperties of edible Irish brown seaweed. LWT – Food Sci.Technol. 47, 300–307.

CUI, Z.W., LI, C.Y., SONG, C.F. and SONG, Y. 2008. Combinedmicrowave-vacuum and freeze drying of carrot and applechips. Drying Technol. 26, 1517–1523.

CUNNINGHAM, S.E., MCMINN, W.A.M., MAGEE, T.R.A. andRICHARDSON, P.S. 2008. Experimental study of rehydrationkinetics of potato cylinders. Food Bioprod. Process. 86,15–24.

DENG, Y. and ZHAO, Y. 2008. Effect of pulsed vacuum andultrasound osmopretreatments on glass transitiontemperature, texture, microstructure and calciumpenetration of dried apples (Fuji). LWT – Food Sci. Technol.41, 1575–1585.

DEV, S.R.S. and RAGHAVAN, G.S.V. 2012. Advancements indrying techniques for food, fiber and fuel. Drying Technol. 30,1147–1159.

DOYMAZ, I. 2009. An experimental study on drying of greenapples. Drying Technol. 27, 478–485.

DOYMAZ, I. and ISMAIL, O. 2011. Drying characteristics ofsweet cherry. Food Bioprod. Process. 89, 31–38.

DUAN, X., ZHANG, M. and MUJUMDAR, A.S. 2007. Studieson the microwave freeze drying technique and sterilizationcharacteristics of cabbage. Drying Technol. 25, 1725–1731.

DUAN, X., ZHANG, M., MUJUMDAR, A.S. and WANG, S.2010. Microwave freeze drying of sea cucumber (Stichopusjaponicus). J. Food Eng. 96, 491–497.

DUAN, X., REN, G.Y. and ZHU, W.X. 2012. Microwave freezedrying of apple slices based on the dielectric properties.Drying Technol. 30, 535–541.

HUANG, L.L., ZHANG, M., MUJUMDAR, A.S. and LIM, R.X.2011. Comparison of four drying methods for re-structuredmixed potato with apple chips. J. Food Eng. 103, 279–284.

JINDARAT, W., RATTANADECHO, P. andVONGPRADUBCHAI, S. 2011. Analysis of energyconsumption in microwave and convective drying process ofmulti-layered porous material inside a rectangular wave guide.Exp. Therm. Fluid Sci. 35, 728–737.

KHRAISHEH, M., MCMINN, M. and MAGEE, T. 2000.A multiple regression approach to the combined microwaveand air drying process. J. Food Eng. 43, 243–250.

KUMAR, H.S.P., RADHAKRISHNA, K., NAGARAJU, P.K. andRAO, D.V. 2007. Effect of combination drying on thephysico-chemical characteristics of carrot and pumpkin.J. Food Process. Preserv. 25, 447–460.

KUTYLA-OLESIUK, A., NOWACKA, M., WESOLY, M. andCIOSEK, P. 2013. Evaluation of organoleptic and texture

properties of dried apples by hybrid electronic tongue. Sens.Actuators B Chem. 187, 234–240.

LEWICKI, P.P. and JAKUBCZYK, E. 2004. Effect of hot airtemperature on mechanical properties of dried apples. J. FoodEng. 64, 307–314.

LIAPIS, A.I. and BRUTTINI, R. 1995. Freeze-drying ofpharmaceutical crystalline and amorphous solutes in vials:Dynamic multi-dimensional models of the primary andsecondary drying stages and qualitative features of themoving interface. Drying Technol. 13, 43–72.

LIAPIS, A.I. and BRUTTINI, R. 2007. Freeze drying.In Handbook of Industrial Drying (A.S. Mujumdar, ed.)CRC press, New York, NY.

LOMBRANA, J.I., ZUAZO, I. and IKARA, J. 2001. Moisturediffusivity behavior during freeze drying under microwaveheating power application. Drying Technol. 19, 1613–1627.

MANDALA, I.G., ANAGNOSTARAS, E.F. and OIKONOMOU,C.K. 2005. Influence of osmotic dehydration conditions onapple air-drying kinetics and their quality characteristics.J. Food Eng. 69, 307–316.

MASKAN, M. 2001. Kinetics of color change of kiwifruitsduring hot air and microwave drying. J. Food Eng. 48,169–175.

MORENO, J., BUGUEÑO, G., VELASCO, V., PETZOLD, V. andTABILO-MUNIZAGA, G. 2004. Osmotic dehydration andvacuum impregnation on physicochemical properties ofChilean papaya (Carica candamarcensis). J. Food Sci. 69,102–106.

RAGHAVAN, G.S.V., RENNIE, T.J., SUNJKA, P.S., ORSAT, V.,PHAPHUANGWITTAYAKUL, W. and TERDTOON, P. 2005.Overview of new techniques for drying biological materialswith emphasis on energy aspects. Braz. J. Chem. Eng. 22,195–201.

ROTHER, M., STEIME, P., GAUKEL, V. and SCHUCHMANN,H.P. 2011. How to meet the freeze drying standard incombined drying processes: Pre and finish drying of carrotdice. Drying Technol. 29, 266–277.

SCHULZE, B., HUBBERMANN, E.M. and SCHWARZ, K. 2014.Stability of quercetin derivatives in vacuum impregnatedapple slices after drying (microwave vacuum drying, airdrying, freeze drying) and storage. LWT – Food Sci. Technol.57, 426–433.

SHEWFELT, R.L. and PRUSSIA, S.E. 1993. Postharvest Handling:A Systems Approach, Academic Press Inc., New York, NY.

SZUMNY, A., FIGIEL, A., GUTIÉRREZ-ORTÍZ, A. andCARBONELL-BARRACHINA, Á.A. 2010. Composition ofrosemary essential oil (Rosmarinus officinalis) as affected bydrying method. J. Food Eng. 97, 253–260.

VEGA-GÁLVEZ, A., AH-HEN, K., CHACANA, M., VERGARA,J., MARTÍNEZ-MONZÓ, J., GARCÍA-SEGOVIA, P.,LEMUS-MONDACA, R. and DI SCALA, K. 2012. Effect oftemperature and air velocity on drying kinetics, antioxidantcapacity, total phenolic content, colour, texture andmicrostructure of apple (var. Granny Smith) slices.Food Chem. 132, 51–59.



XU, Y.Y., ZHANG, M., TU, D.Y., SUN, J.C., ZHOU, L.Q. andMUJUMDAR, A.S. 2005. A two-stage convective air andvacuum freeze-drying technique for bamboo shoots. Int. J.Food Sci. 40, 589–595.

XU, Y.Y., ZHANG, M., MUJUMDAR, A.S., DUAN, X. and SUN,J.C. 2006. A two-stage vacuum freeze and convective airdrying method for strawberries. Drying Technol. 24,1019–1023.

ZHANG, F., ZHANG, M. and MUJUMDAR, A.S. 2011. Dryingcharacteristics and quality of restructured wild cabbage chipsprocessed using different drying methods. Drying Technol. 29,682–688. 3.

ZHANG, M., TANG, J., MUJUMDAR, A.S. and WANG, S. 2006.Trends in microwave related drying of fruits and vegetables.Trends Food Sci. Technol. 17, 524–534.

