See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/43271391 Sequential Infrared Radiation and Freeze-Drying Method for Producing Crispy Strawberries ARTICLE · JANUARY 2008 DOI: 10.13031/2013.24205 · Source: OAI CITATIONS 21 READS 131 5 AUTHORS, INCLUDING: Zhongli Pan University of California, Davis 147 PUBLICATIONS 2,044 CITATIONS SEE PROFILE Tara Mchugh United States Department of Agriculture 119 PUBLICATIONS 4,003 CITATIONS SEE PROFILE Delilah Wood United States Department of Agriculture 88 PUBLICATIONS 1,614 CITATIONS SEE PROFILE Available from: Delilah Wood Retrieved on: 04 February 2016
Vol. 51(1): 205-216 2008 American Society of Agricultural and Biological Engineers ISSN 0001-2351 205
SEQUENTIAL INFRARED RADIATION AND FREEZE‐DRYING
METHOD FOR PRODUCING CRISPY STRAWBERRIES
C. Shih, Z. Pan, T. McHugh, D. Wood, E. Hirschberg
ABSTRACT. Sequential infrared and freeze‐drying (SIRFD) as a new processing method was studied for producing high‐qualitycrispy fruit pieces at reduced cost. This research investigated the drying characteristics of strawberry slices and the qualityof the finished products under SIRFD. The 4 mm thick strawberry slices were pre‐dehydrated to 30%, 40%, and 50% levelsof weight reduction with infrared (IR) heating at each of the three different intensities (3000, 4000, and 5000 W m‐2). Thepre‐dehydrated samples were then further freeze‐dried to achieve a final moisture content of about 5%. For comparison, theslices were also pre‐dehydrated with hot‐air drying (62.8�C) followed by freeze‐drying (SHAFD) and dried with regularfreeze‐drying without pre‐dehydration. The drying kinetics of strawberry slices under IR, hot‐air, and freeze‐drying weredetermined and modeled. The color, shrinkage, rehydration ratio, and crispness of finished products were measured. The IRradiation heating had a much higher drying rate than hot‐air during the pre‐dehydration. The product produced with SIRFDhad more desirable color, higher crispness, and more shrinkage, but a lower rehydration ratio than regular freeze‐drying,which, however, did not produce a high‐crispness product. The microstructure characteristics of the dried products explainedthe differences in quality produced with the different methods. IR pre‐dehydration to a 40% weight reduction level reducedrequired freeze time by 42%, indicating a great energy saving potential for SIRFD, since the energy efficiency of freeze‐dryingis very low. It has been concluded that SIRFD could be a desirable method for producing high‐crispness strawberry pieces.
mericans are being urged to eat more fruits andvegetables to improve their health and fightobesity. An important aid in this fight will be newprocessing technologies that produce more
varieties of high‐quality products at reduced cost. Amongsuch products, crispy fruit pieces have become significant asadd‐ins in breakfast cereals and snacks. Crispy strawberrypieces are an example. They are typically produced byfreeze‐drying, which requires long drying time and usessignificant amounts of energy. This study explored the use ofsequential infrared and freeze‐drying (SIRFD) to producehigh‐quality crispy strawberry slices, with reducedprocessing time that should significantly reduce energy use.
Freeze‐drying (FD), used as a single process or incombination with other techniques to minimize the adversequality changes associated with dried products, has been
Submitted for review in September 2007 as manuscript number FPE7193; approved for publication by the Food & Process EngineeringInstitute Division of ASABE in December 2007.
The authors are Connie Shih, Former Graduate Student, Department ofBiological and Agricultural Engineering, University of California, Davis,California; Zhongli Pan, ASABE Member Engineer, Research Engineer,Processed Foods Research Unit, USDA‐ARS Western Regional ResearchCenter, Albany, California, and Associate Adjunct Professor, Departmentof Biological and Agricultural Engineering, University of California,Davis, California; Tara H. McHugh, Food Technologist, Processed FoodsResearch Unit, and Delilah Wood, Chemist, Bioproduct Chemistry andEngineering Research Unit, USDA‐ARS Western Regional ResearchCenter, Albany, California; and Edward Hirschberg, President,Innovative Foods, Inc., South San Francisco, California. Correspondingauthor: Zhongli Pan, Processed Foods Research Unit, USDA‐ARSWestern Regional Research Center, 800 Buchanan St., Albany, CA 94710;phone: 510‐559‐5861; fax: 510‐559‐5851; e‐mail: [email protected].
studied (Hammami and Rene, 1997; Lin et al., 1998;Shishehgarha et al., 2002; Lin et al., 2005). Duringfreeze‐drying, the product structure stiffens, whichsubsequently prevents solute and liquid motion (Levine andSlade, 1989). Ice crystals grow and create a uniform networkthroughout the product, yielding a dense, evenly spread, andhomogeneous porous matrix after sublimation. Therefore,the freeze‐dried product should be much crisper than theproduct from hot‐air drying. Despite this ability to provide ahigh‐quality product, freeze‐drying is expensive, whichlimits its use in the food industry. It has normally beenrestricted to high‐value products such as coffee, crispy fruitsand vegetables, ingredients for ready‐to‐eat foods, and somearomatic herbs.
Infrared (IR) heating offers many advantages overconventional hot‐air drying. When IR is used to heat or dryfruits, the radiation penetrates (depth depending on thecomposition of the fruit and wavelength of the radiation) andis converted to heat by molecular vibration (Ginzburg, 1969).The wave energy is absorbed directly by the fruit, and energyloss is low. It has been reported that the drying rate for foodmaterials using IR heating is higher than for conventionalhot‐air drying and increases with increased power supply toa far‐infrared emitter (Gabel et al., 2004; Masamura et al.,1988). IR heats fruits more uniformly, resulting in betterquality than with other drying methods (Sakai and Hanzawa,1994; Nowak and Lewicki, 2004).
Combining IR radiation with convection heating and/orvacuum has also been studied (Abe and Afzal, 1997;Mongpraneet et al., 2002; Hebbar et al., 2004; Kumar et al.,2005). The combination provides a synergistic effect and isconsidered more efficient than radiation or hot‐air heatingalone. Afzal et al. (1999) reported that combining
A
206 TRANSACTIONS OF THE ASABE
far‐infrared radiation and hot air resulted in faster drying andused considerably less energy than hot‐air drying alone. Acombination of IR and freeze‐drying was also studied fordrying sweet potato (Lin et al., 2005).
To take advantage of IR drying, IR drying may be used forpre‐dehydration before freeze‐drying (Hirschberg et al.,2006). SIRFD, a two‐step process of infraredpre‐dehydration followed by freeze‐drying, was created byHirschberg et al. (2006). IR drying first removed a significantamount of moisture from sliced strawberries, and thenfreeze‐drying was used to dry the product to low moisturecontent. The process aimed at taking advantage of thefast‐drying benefits of IR and the high product quality offreeze‐drying. Since IR itself is a very energy‐efficientdrying method, it is expected that SIRFD can save energy byreducing drying time compared to the current freeze‐dryingpractice. SIRFD could also reduce the energy used forfreezing before freeze‐drying because less water needs to befrozen.
The objectives of the study were to (1) study the dryingcharacteristics of strawberry slices under IR heating, (2)determine the required freeze‐drying time of pre‐dehydratedstrawberry slices, and (3) evaluate the quality of strawberryslices produced with SIRFD.
MATERIALS AND METHODSMATERIALS
Fresh strawberries (variety Camarosa) obtained fromFrozsun Foods, Inc. (Oxnard, Cal.) were used in this study.They were washed with water, destemmed, and then slicedinto pieces 4.10 ±0.10 mm thick using a food processor(model FP 200, Hobart Corp., Troy, Ohio). The slices werethen dried using various methods including IR, hot air, andfreeze‐drying, as specified in the Experimental Designsection. The moisture content of the fresh strawberriesranged between 89.9% and 91.0% wet basis (w.b.).
To determine the moisture content of the fresh and driedberries, 10 to 15 g samples were placed in pre‐weighedaluminum weighing dishes and dried according to AOACmethods (AOAC, 1994) in a vacuum oven (model V01218A,Lindberg/Blue, Asheville, N.C.). The dishes were removedand weighed using a balance with an accuracy of 0.01 g(model 602, Denver Instrument Co., Arvada, Colo.). Threesamples from each trial were used. The average moisturecontent of the three replicates is reported on both wet and drybases.
EXPERIMENTAL DESIGN AND DRYING PROCEDURES
To determine the drying characteristics and productquality, sliced strawberries were pre‐dehydrated to remove30%, 40%, and 50% of their initial weight using each of thethree IR intensities (3000, 4000, and 5000 W m‐2). Thecorresponding target moisture contents were 86%, 83%, and80%, respectively, after the pre‐dehydration. The weightchanges were measured every minute during the dryingprocess using a digital balance. For comparison, sampleswith similar weight removals were also produced usinghot‐air drying. Then both sets of samples were frozen at‐18°C before being further dried using freeze‐drying to a finalmoisture content of about 5% (w.b.) for quality evaluation.When hot air is used for pre‐dehydration before freeze‐
drying, the process is called sequential hot‐air freeze‐drying(SHAFD). The dried products were evaluated for color,thickness shrinkage, rehydration ratio, crispness, andfirmness. The samples were also dried for different durationsto determine the drying rate of freeze‐drying. All dryingexperiments were conducted in triplicate, and the reporteddata are average values.
IR and Hot‐Air Pre‐Dehydration
An infrared dryer/dehydrator equipped with two catalyticIR emitters powered by natural gas (Catalytic DryingTechnologies LLC, Independence, Kansas) was used in thisresearch. The catalytic IR dryer consists of infrared emitters30 × 60 cm in area, with a waveguard around the emitters toprevent infrared radiation from escaping and to keep heatinguniform. With the waveguard, the heating area was 34 ×64.5 cm. The average IR intensities were measured with anOphir FL205A thermal excimer absorber head (OphirOptronics, Inc., Wilmington, Mass.) with ±3% accuracy.The drying tray was placed between the two IR emitters in aposition parallel to the emitter face. Strawberry slices wereheated from both top and bottom. An automatic dataacquisition and control system developed in our laboratorycontrolled and recorded various operation parameters. Aschematic diagram of the equipment is shown in figure 1. TheIR dryer was operated in a continuous heating mode, withnatural gas continuously supplied to the emitters. Continuousheating delivered high heat to the product in a relatively shorttime for quick drying. Strawberry slices were arranged in asingle layer on the drying tray (metal screen), which wassprayed with PAM cooking spray (ConAgra Foods, Inc.,Omaha, Neb.) to prevent the slices from sticking to the tray.Slices were placed within the confines of the waveguard at aloading of approximately 1.33 kg m‐2 (or about 240 g for eachbatch). The change in sample weight during the dryingprocess was measured using a digital balance until the targetweight reduction was reached. Type‐T thermocouples (0.15 sresponse time) placed at the center of the strawberry sliceswere used to measure the product temperature.
For comparison, a hot‐air cabinet dryer (product code 062,Proctor and Schwartz, Inc., Horsham, Pa.) was also used todry the samples, to obtain a drying curve and for qualityevaluation. The dryer was set at 62.8°C based on common
Gas flowregulator valve
Gas in
Balance
Drying tray
Product
Computer
Figure 1. Schematic diagram of catalytic infrared dryer setup.
207Vol. 51(1): 205-216
industrial practice, and the sample weight changes were alsomeasured using a digital balance during drying. The dryingrates of IR and hot‐air drying were calculated based on theweight change and expressed as g water / g initial weight ×min. After the IR or hot‐air pre‐dehydration, the slices weretransferred to wax paper by flipping the drying tray, and thentransported to a large‐scale air‐blast freezing system with atemperature of ‐18°C.
Freeze‐Drying
The frozen pre‐dehydrated samples and the controlsamples were removed from the freezer after they werecompletely frozen. They were then placed as a single layerin a pilot‐scale Ultra\VirTual Series EL freeze‐dryer (VirTisCo., Gardiner, N.Y.). The freeze‐dryer was operated inshelf‐driven mode, which was controlled based on shelftemperature, and run with programmed procedures.
To determine the drying characteristics of the strawberryslices during freeze‐drying, the pre‐dehydrated and controlsamples were dried for various times (0.0167, 1, 2, 4, 6, 8, 16,22, and 29 h). The control samples were also dried for 50 h.The samples were weighed at the end of each drying period,and the moisture contents were calculated and reported. Allsamples used for quality evaluation had about 5% moisturecontent (w.b.).
DRYING KINETICS
Drying Models of Freeze‐Drying
Modeling the drying process is important forcharacterizing the processes of different drying methods andconditions. Two models, Page (Lin et al., 2005; Doymaz,2004; Page, 1949) and Henderson‐Pabis (Doymaz, 2004;Henderson and Pabis, 1961), were selected to describe thefreeze‐drying process since they have been widely used inmodeling thin‐layer drying processes, including the drying ofvegetables and fruits (Abe and Afzal, 1997). These twomodels were selected for this study because of theirsimplicity, high correlation to most drying data, and commonuse in the literature. Model curves were fitted to theexperimental data, and the performances of the models weredetermined by the correlation of determination (R2). Agreater R2 indicates a better fitting of the model.
For the Page model:
[ ]n
e
e ktMM
MMMR −=
−
−= exp
0
(1)
where MR is the moisture ratio, M is the moisture content (%w.b.) at any given time during drying, M0 is the initialmoisture content (% w.b.), Me is the equilibrium moisturecontent (% w.b.), k is the drying constant (h‐1), t is time (h),and n is an exponent.
Drying constant k and exponent n can be calculated byusing experimental data. When the MR was plotted onsemi‐logarithmic axis versus the time (h), the slope of thefitting line was the constant k.
For the Henderson‐Pabis model:
( )ktaMM
MMMR
e
e −=−
−= exp*
0
(2)
where a is the drying constant. The values of k and a can alsobe calculated based on experimental data. All modelingcalculations were done using SigmaPlot, version 3 (JandelCorp., San Rafael, Cal.).
QUALITY EVALUATION
Thickness Shrinkage Measurement
The thicknesses of the unprocessed samples and driedsamples were measured using a Cen‐tech electronic digitalcaliper (Harbor Freight Tools, Camarillo, Cal.). Shrinkagewas determined based on the difference between initial andfinal thickness and reported as a percentage based on initialthickness. Ten sample pieces from each drying treatmentwere measured, and the average shrinkage is reported.
Color
Color values L, a, and b were measured using a MinoltaCR‐200 reflectance colorimeter (Minolta, Japan). Thecolorimeter (illuminant D65, 2° observer) was calibratedagainst a standard ceramic white tile (Y = 94.4, x = 0.3159,y = 0.3333). Because the color varied on the surface of thestrawberry slices, the dried samples were ground to powderusing a small‐scale blender to obtain representative colors. A1 g sample of strawberry powder was put in a 5 cm diameterplastic Petri dish. The lens of the colorimeter, covered withplastic wrap, was placed directly on the strawberry powderto measure the color values. Three measurements were madeof each sample, and the average value is reported. The L colorvalue indicates the degree of brightness or whiteness of theproduct. The a and b color values measure the degree ofredness and yellowness, respectively. The b parameter wasnot reported since it is not closely related to the color qualityof strawberries. It was used to calculate the hue angle, anindicator of browning color, with the following equation(Karabulut et al., 2006):
Hue angle = tan‐1(b/a) (3)
where hue angle is expressed in degrees (0°). Preferred colorsof strawberry slices are red or dark red.
Rehydration Ratio
Since dried product such as this may be used in cereals, arehydration test was performed. Five samples of driedstrawberry from each drying trial were placed in whole milkfor 3 min. They were then removed from the milk, gentlydried with blotting paper, and reweighed. The rehydrationratio was calculated by dividing the final weight by theoriginal weight (Lin et al., 1998). The reported value is theaverage of three measurements for each of the samples.
Crispness
Crispness was evaluated using a TA.XT2 texture analyzer(Texture Technologies Corp., Scarsdale, N.Y.). Driedstrawberry samples were tested using a 6.4 mm (0.25 in.)diameter ball probe and its accompanying chip/crackerfixture (TA‐101). A “pipe” cylinder with an outside diameterof 25 mm and an inside diameter of 18 mm was mounted onthe plate component of the TA‐101 to support a strawberrypiece for the test. The values of initial slope (crispness, g
208 TRANSACTIONS OF THE ASABE
mm‐1) of the force curve were measured and calculated. Thetest was performed in five replicates for each sample.
SCANNING ELECTRONIC MICROSCOPY (SEM)To evaluate the structural change of slices dried with
different methods and to understand the mechanism of watertransport during drying, SEM studies of cross‐sections of thedried slices were performed. Selected samples were carefullycut using sharp razor blades (Ted Pella, Inc., Redding, Cal.)to expose a cross‐section surface. Specimens were mountedon aluminum stubs using double‐coated carbon tabs (TedPella, Inc., Redding, Cal.), sputter‐coated withgold‐palladium using a Denton Desk II sputter‐coating unit(Denton Vacuum, Moorestown, N.J.), and photographed in aHitachi S‐4700 field emission scanning electron microscope(Hitachi, Japan) at 2 kV. Digital images were captured at1280 × 960 pixel resolution with 40× magnification.
STATISTICAL ANALYSIS
Analysis of variance (ANOVA) and the test of meancomparison according to Tukey's honest significantdifference (HSD) were conducted with a level of significanceof 0.05. The statistical software SAS System for Windows,version 9.0 (SAS Institute, Inc., Cary, N.C.), was used for theanalysis.
RESULTS AND DISCUSSIONPRODUCT TEMPERATURE AND DRYING RATES OF IR AND
HOT‐AIR DRYINGThe temperature change of the strawberry slices was
closely related to the IR heating intensity (fig. 2). Highradiation intensity raised product temperature much faster.Under low radiation intensity (3000 W m‐2), the heating rateremained very low throughout the drying. For example, withIR intensity of 5000 W m‐2, it took about 2.5 min for thetemperature to reach about 80°C, compared to 19.5 min at3000 W m‐2. Clearly, if fast heating is desired, it is essentialto use high IR intensity.
The IR drying tests showed much higher drying ratesthroughout the course of drying than the hot‐air drying, dueto the high heat delivery of IR (fig. 3). The tests also showedthat increasing the radiation intensity in the infrared dryingtrials improved the drying rate. This is consistent with thereported research of Afzal and Abe (1998) on onion dryingcharacteristics using far‐infrared.
Drying rates for IR varied with the radiation intensity, asexpected, and there was an absence of, or very briefappearance of, a constant‐rate period. This could be becauseof the quick drying on the surface of products at hightemperature. The hot‐air drying tests, on the other hand,showed a more distinct constant‐rate period at much lowervalue compared to IR drying.
Figure 2. Temperature of strawberry slices dried under different radiation intensities.
0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
0.080
0.090
0.100
0 1 2 3 4 5 6 7 8 9 10
Moisture Content (d.b.)
Dry
ing
Rat
e (g
/g in
itia
l wei
gh
t x
min
)
RI = 3000
RI = 4000
RI = 5000
Hot air
Figure 3. Drying rates of different drying methods and radiation intensity (RI, W m‐2) of strawberry slices.
209Vol. 51(1): 205-216
Table 1. Time needed to obtain various weight reductions for catalyticIR and hot‐air drying (min), and actual weight
reductions achieved in the tests (%).
Targeted WeightReduction
IR Intensity (W m‐2)
Hot Air3000 4000 5000
30% 8 min,29.68%
6 min,31.86%
4 min,27.89%
18 min,30.49%
40% 11 min,40.21%
8 min,43.07%
6 min,43.88%
24 min,41.08%
50% 14 min,50.81%
10 min,52.18%
7 min,51.77%
30 min,51.18%
Table 2. Time reduction (%) for catalytic IR drying compared to hot‐air drying.
WeightReduction
IR Intensity (W m 2)
3000 4000 5000
30% 55.6% 66.7% 77.8%
40% 54.2% 66.7% 75.0%
50% 53.3% 66.7% 76.7%
Table 1 summarizes the times needed to obtainapproximately 30%, 40%, and 50% weight reductions withdifferent IR intensities and hot‐air drying. Catalytic IR dryinggenerally reduced weight faster than did hot‐air drying. IRdrying times shortened with increases in IR intensity, but ofcourse lengthened for greater levels of weight reduction.Compared to hot‐air drying, the time saving of IR dryingranged from 53.3% to 77.8%, and the saving increased withthe increase in IR intensity (table 2). However, time savingswere not significantly affected by the level of weightreduction.
FREEZE‐DRYING CHARACTERISTICS
Since hot air had a much lower drying rate than IR heating,the samples prepared with hot air were only used forproducing dried products and their freeze‐drying rate was notmeasured. The moisture results of freeze‐drying showed thatthe IR pre‐dehydrated strawberry slices took less time toreach a specific moisture content than the slices withoutpre‐dehydration (regular freeze‐drying or control) (table 3).The samples with 40% weight reduction pre‐dehydrated at IRintensity of 4000 W m‐2 took only 29 h to achieve the finalmoisture content of approximately 5% (w.b.), compared to50 h for the control. The SIRFD method saved about 42% offreeze‐drying time. This indicates a significant energy savingpotential, since the energy efficiency of freeze‐drying is verylow.
The results also showed that the moisture did not changemuch at the early stage of the freeze‐drying. This was due tothe freeze‐drying principle and operation procedures. Thefreeze‐drying process involves three stages: (1) the freezingstage, (2) the primary drying stage, and (3) the secondarydrying stage. In the freezing stage, the temperature of thestrawberry slices was lowered to ‐40°C from about ‐18°Cwith a setting of 2 h. Drying of the foodstuff took place in theprimary drying stage when the drying chamber wasevacuated and its pressure was reduced to a value that wouldallow frozen water to sublimate. Therefore, moisture loss didnot take place until after the 2 h freezing stage. After thefreezing stage, decrease in moisture content occurred in thefreeze‐drying process.
It was found that the pre‐dehydration resulted in fasterdrying during freeze‐drying, which may be related to amountof the water in the product. Based on statistical analysisresults, IR intensity did not have a significant effect on thefreeze‐drying (p > 0.05), but the level of weight reductionduring pre‐dehydration did have a significant effect (p <0.05). For example, at the end of a 29 h freeze‐drying period,the moisture contents of SIRFD samples dried at radiationintensity of 5000 W m‐2 were 5.76%, 4.98%, and 4.39%,respectively, for 30%, 40%, and 50% weight reductionduring catalytic IR drying. This showed that a high level ofpre‐dehydration can significantly reduce the freeze‐dryingtime. However, the quality of final products needs to beconsidered in determining an appropriate pre‐dehydrationlevel.
DRYING MODELING OF FREEZE‐DRYING PROCESSBecause IR intensity during pre‐heating did not
significantly affect the freeze‐drying process, the averagemoisture content of samples dehydrated with different IRintensities was used for modeling of freeze‐drying. Inaddition, since drying did not take place until the primarydrying stage (2 h after the beginning of the freezing stage),only the weight reduction data during the primary andsecondary drying stages were used to calculate the moistureratio for the modeling.
The parameters of the Page and Henderson‐Pabis modelsfor the drying trials are summarized in table 4. The R2 valuesindicate that both models fit reasonably well with theexperimental data. Although the Page model showed a higherR2, the predicted data did not fit well with the measured dataat the middle and end of the drying process. The Page modelunderpredicted the moisture content values of the IR‐treatedsamples and overpredicted the moisture content values of the
Table 3. Average moisture contents of samples with different treatments during freeze‐drying.
control samples. Especially, the model did not perform wellfor the control samples. The Henderson‐Pabis modelperformed better than the Page model when the moistureratio was low and gave better‐predicted data. Therefore, only
the predicted moisture ratio data from the Henderson‐Pabismodel under different drying conditions are illustrated infigure 4. It is recommended that the Henderson‐Pabis modelbe used to predict the drying characteristics of strawberryslices.
THICKNESS SHRINKAGEShrinkage was evident for all drying methods and
conditions used in this study, but the extent was dependent onthe drying methods (fig. 5). In general, SIRFD samples hadslightly higher shrinkage in thickness than samples dried withregular FD; however, they had much less shrinkage thansamples dried with SHAFD. Since regular FD samples werenot pre‐dehydrated, structural rigidity was created during thefreeze stage of the freeze‐drying process. This structuralrigidity prevented collapse of the solid matrix during drying
0 10 20 30 40 50
Time (h)
MR
1.20
1.00
0.80
0.60
0.40
0.20
0.00
Figure 4. Measured and predicted moisture ratio (MR) from Henderson‐Pabis model for IR pretreated and control samples under freeze‐drying (WR= weight reduction).
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
Reg FD 3000
SIRFD,30%
4000
SIRFD,30%
5000
SIRFD,30%
SHAFD,30%
3000
SIRFD,40%
4000
SIRFD,40%
5000
SIRFD,40%
SHAFD,40%
3000
SIRFD,50%
4000
SIRFD,50%
5000
SIRFD,50%
SHAFD,50%
Strawberry Samples
Th
ickn
ess
Sh
rin
kag
e (%
)
W m-2 W m-2 W m-2 W m-2 W m-2 W m-2 W m-2 W m-2 W m-2
Figure 5. Thickness shrinkage of dehydrated strawberry slices dried with different methods and conditions (Reg FD = regular freeze‐drying, SIRFD= sequential infrared radiation and freeze‐drying, and SHAFD = sequential hot‐air and freeze‐drying).
211Vol. 51(1): 205-216
(Mujumdar, 1995). The thickness shrinkages were 5.0% forregular FD samples. For SIRFD, more shrinkage wasobserved with higher weight reduction level inpre‐dehydration. For example, the samples pre‐dehydratedunder 5000 W m‐2 had 11.6%, 19.14%, and 20.8% thicknessshrinkages for the 30%, 40% and 50% weight reductions,respectively. This is because more moisture was removed andevaporated from the sample during the pre‐dehydration stageand also because of the longer drying time required to obtain50% weight reduction than to obtain 30% or 40% reductions.Ketelaars et al. (1992) found that shrinkage during drying isattributed to moisture removal and to stresses developed inthe cell structure during drying.
Product shrinkage also depended on radiation intensity:thickness shrinkage decreased as radiation intensityincreased. For instance, shrinkage decreased from 16.6% fora sample dried under 3000 W m‐2 to 11.6% for a sample driedunder 5000 W m‐2 with 30% weight reduction. The extent ofchange in the thickness of the slices can be explained by thedrying rate. Since drying at higher radiation intensityrequired a shorter time to achieve the target weight reduction,the heat exposure time for the slices was shorter compared todrying at lower radiation intensity. Therefore, deteriorationin cell structure and matrix was less.
Compared to SHAFD samples, SIRFD samplesexperienced much less thickness shrinkage in the driedproduct. The results showed that with 50% weight reduction,a SHAFD sample had 34.13% thickness shrinkage ascompared to 20.85% for the SIRFD sample dried under5000 W m‐2. This might be due to the longer hot‐air dryingtime causing more cells to collapse. For a product with lessshrinkage, it is recommended that higher radiation intensity,such as 5000 W m‐2, be used during pre‐dehydration.
COLORFigure 6 shows the L values of fresh and dried strawberry
samples. In general, the drying treatment resulted insignificantly increased whiteness (increased L value).However, the lightness of regular FD and SHAFD samples
was greater than the SIRFD samples, resulting in a lightercolor tone of dried products. The increased whiteness wassimilar to results found in the study of Li and Ma (2003),where brightness/whiteness of sliced strawberries increasedafter freeze‐drying. In fact, the water content of the fresh anddried products also affects their appearance.
For the SIRFD‐processed samples, L values decreased asweight reduction increased, given the same radiationintensity. This could be due to the extended IR drying timecausing darkening in the strawberry slices. The resultsindicated that darker color developed at high weightreduction levels with IR heating.
The color measurements showed that the redness ofSIRFD samples was generally stronger than the redness offresh, regular FD, and SHAFD strawberry samples, resultingin a dark‐red color in the dried products (fig. 7). Thisphenomenon could be attributed to the water‐reduction effectand as a result of increased concentration of the red pigments(anthocyanins) brought about by water removal in the driedproduct (Hammami and René, 1997). Compared to SHAFD,the samples of SIRFD experienced a higher dryingtemperature in pre‐dehydration, which led to greater avalues. This is because the non‐enzymatic browning reactionwas accelerated by temperature (Jamradloedluk et al., 2007).The change is ascribed to the fact that in IR pre‐dehydration,the product temperature of the strawberry slice increasedfaster than in hot‐air drying. Therefore, SIRFD strawberrieswere obviously more reddish than those dried by SHAFD.
Color measurement results were significantly different (p< 0.05) for all drying methods and conditions. Theweight‐reduction level in pre‐dehydration had moreinfluence on the L and a values of strawberry slices than theother process variables.
The SIRFD and SHAFD samples had lower hue anglevalues than fresh samples, but higher than regularfreeze‐dried samples. Based on the statistical analysisresults, the hue angles were significantly different among allsamples (p < 0.05) (fig. 8). The desired color of finished
0
10
20
30
40
50
60
70
80
FreshSB
Reg FD 3000
SIRFD,30%
4000
SIRFD,30%
5000
SIRFD,30%
SHAFD,30%
3000
SIRFD,40%
4000
SIRFD,40%
5000
SIRFD,40%
SHAFD,40%
3000
SIRFD,50%
4000
SIRFD,50%
5000
SIRFD,50%
SHAFD,50%
Strawberry Samples
L
W m-2 W m-2 W m -2 W m -2 W m -2 W m -2 W m-2 W m-2 W m -2
i
a
b c cd cef hg fg de efg h h
b
Figure 6. Color value L of strawberry samples with different drying methods (samples with different letters are significantly different at p < 0.05).
212 TRANSACTIONS OF THE ASABE
0
5
10
15
20
25
30
35
40
FreshSB
Reg FD 3000
SIRFD,30%
4000
SIRFD,30%
5000
SIRFD,30%
SHAFD,30%
3000
SIRFD,40%
4000
SIRFD,40%
5000
SIRFD,40%
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a
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e bcdbc bc bc
cd
aba
abcbcd
bcd abcbcd
de
Figure 7. Color value a of strawberry samples with different drying methods (samples with different letters are significantly different at p < 0.05).
product may have a hue angle value similar to the freshsample, which has a hue of 22.3° (orange‐red color). Two ofthe SIRFD samples (3000 W m‐2 with 50% weight reductionlevel, and 4000 W m‐2 with 30% weight reduction level)resembled the fresh strawberry product. The SHAFDsamples also had hue angle values closer to those of the freshsamples. However, with increased L value, the samples wereobserved to have a light orange‐reddish color. The regular FDsample, on the other hand, had the lowest hue angle value(19.3°), but with the greater increase in L value and smallera value, the sample appeared pinkish. However, Baysal et al.(2003) found that hue angles were not significantly differentamong raw, hot air, microwave, and infrared dried samples intheir study of drying carrot and garlic.
For the SIRFD samples, hue angle values changed withradiation intensity and weight reduction. When the radiationintensity was low, increased weight reduction level increasedhue angle value. However, when radiation intensity was high,
increased weight reduction decreased hue angle value. Basedon the statistical analysis results, radiation intensity had amore significant effect on hue angle value than weightreduction level.
Although the darkening of the red color in SIRFD samplemay be a result of non‐enzymatic browning andconcentration increase of anthocyanins, from visualobservation, the SIRFD samples had a darker red color tone,which is more desirable than the light pinkish color tone seenin the regular FD sample and the light orange‐reddish colortone seen in the SHAFD sample (fig. 9).
MICROSTRUCTURE OF STRAWBERRY SLICE CROSS‐SECTIONThe microstructure of a cross section of strawberry slices
dried under different drying methods (regular FD, SIRFD,and SHAFD) was investigated by scanning electronmicroscopy (SEM). During drying, water in the berry couldbe transported though several possible pathways (Tyree,
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bcd bcd
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bc
Figure 8. Hue angle of strawberry samples from all drying tests (samples with different letters are significantly different at p < 0.05).
213Vol. 51(1): 205-216
(a)
(b)
(c)
Figure 9. Effect of drying method on appearance of dried strawberries: (a) regular FD strawberry samples, (b) SIRFD strawberry samples, and(c) SHAFD strawberry samples.
1970). In the first pathway, water passes from one cell to thenext via cytoplasmic strands (plasmodesmata). In the second,water alternately enters and leaves successive cells along itspathway by passing through plasmalemma membraneboundaries. But the most important pathway for watermovement through plant tissues is through the cell wall.
It was evident that the regular FD strawberry (fig. 10a) haduniform and small porous structure, and little or no damageor disruption of cell walls at the slice surface. By contrast, theSIRFD sample (fig. 10b) showed collapsed cells in thesurface layer, with a dense layer or crust on the surfaces, butporous structure in the slices. This is as expected since thesurfaces of strawberry slices were exposed to heat duringdrying, experiencing rapid increase of surface temperatureduring catalytic IR drying, which caused surface collapse ata rate comparable to or higher than the rate at which moisturemoved from the interior to the surface. Consequently, a denselayer formed at the surface of the SIRFD sample. Large pores(intercellular spaces) were seen in the center region of thestrawberry slice, which could be due to water vapor createdduring catalytic IR drying. When a strawberry slice is driedby infrared, it heats rapidly; water vapor expands the cellularwalls and develops large pores within the material(Jamradloedluk et al., 2007). This unique microstructurecould enhance crunchiness or crispness.
Unlike with catalytic IR heating, the temperature ofsamples dried with hot air increased gradually from ambienttemperature to the drying temperature. As the temperature
slowly increased, the moisture in the materials was released,making the vapor pressure caused by internal evaporation ofmoisture less than in the case of catalytic IR drying.Therefore, the SHAFD sample (fig. 10c) revealed severestructural damage of the cell walls. In particular, the cellwalls of the center region were completely collapsed. Thelong drying time during hot‐air drying also contributed to thestructure collapse in the SHAFD samples. As a result, thehot‐air dried samples were not as crispy as the SIRFD driedsamples.
REHYDRATION RATIO
The differences in rehydration capacity of strawberriesdried by different methods at different conditions are shownin figure 11. In general, the SIRFD samples had a lowerrehydration ratio than the regular FD strawberry samples;however, the SIRFD samples had higher rehydration ratiothan the SHAFD samples during soaking. Since strawberrychips are to be consumed as snacks or with breakfast cereal,a relatively lower rehydration ratio may be preferred.However, strawberry chips that do not rehydrate are also notacceptable because low rehydration capacity indicatessevere disruption in the strawberry structure.
The fact that SIRFD strawberry slices had lowerrehydration capacity than the regular FD products could beexplained by the crust formation seen in the SIRFD samples.The crust on the surfaces of the SIRFD samples could haveslowed down the milk penetrating into the dried sample
(a)
(b)
Surface
(c)
Figure 10. SEM of cross‐section of strawberry slices dried under different drying methods: (a) regular FD, (b) SIRFD, and (c) SHAFD.
214 TRANSACTIONS OF THE ASABE
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W m-2 W m-2 W m-2W m-2 W m-2 W m-2 W m-2 W m-2 W m-2
a
a
a
b b b bb
b bb b
b
Figure 11. Rehydration ratio of strawberry slices dried with different drying methods after 3 min soaking (samples with different letters aresignificantly different at p < 0.05).
during the rehydration process. On the other hand, the moreporous structure in the regular FD sample facilitated rapidrehydration when the sample was added to the milk. It isgenerally believed that the degree of rehydration isdependent on the degree of cellular and structural disruption(McMinn and Magee, 1997). Since the regular FD sample didnot experience high‐temperature heating, the cell structurewas not damaged and structural rigidity was maintained,creating a porous structure. As for the SHAFD samples, thecellular tissues were completely collapsed due to severeheating, making the SHAFD samples more difficult to swellcompared to the SIRFD and regular FD samples.
When comparing both pre‐dehydrated samples, theSIRFD samples had much better rehydration capability(1.0308 to 1.7138) than the SHAFD samples (0.9174 to1.0649). The lower rehydration values of the strawberrysamples were evidence of shrinkage in the product and the
collapse of cellular tissues caused by severe heating and/orprolonged drying, with the resulting irreversiblephysico‐chemical changes. Thus, it was more difficult torehydrate the SHAFD samples because there was moreshrinkage and damage in their cellular tissues, as mentionedearlier. In a review by Sakai and Hanzawa (1994), therehydration capability of Welsh onions dried with far‐IRradiation under vacuum was greater than for those dried withhot air. Similar results were also reported by Kumar et al.(2005) for infrared and hot‐air drying of onions.
The rehydration ratio of the SIRFD samples decreased asthe IR‐drying weight reduction increased. For example, therehydration ratio of the 5000 W m‐2 SIRFD sample decreasedfrom 1.71 to 1.03 as the weight reduction increased from 30%to 50% during soaking for 3 min. This is expected, since moreshrinkage was observed with higher weight reduction. As aresult, the strawberry slices could not easily soak up milk.
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-1
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Figure 12. Crispness comparison of strawberry slices dried with different drying methods (samples with different letters are significantly different atp < 0.05).
215Vol. 51(1): 205-216
CRISPNESSTexture is an important sensory attribute for many
cereal‐based foods. A crisp food should be firm and snapeasily when deformed, emitting a crunchy sound.Mechanical tests such as compression tests have been used tocorrelate crispness to a physical parameter in aforce‐deformation curve (Krokida et al., 2001).
The crispness of strawberry slices dried with differentmethods is shown in figure 12. Statistical analysis indicatedthat drying method had a significant effect (p < 0.05) on thecrispness of the final products. The samples processed withSIRFD had higher crispness than those processed with eitherthe regular FD or SHAFD method. Crispness was mainlyrelated to the crust/dense layer formation and structuralchange. The SIRFD sample had a modest crust and largeporous structure in the central region, resulting in ahigh‐crispness product. As mentioned earlier, the pores in thecentral region of the SHAFD sample collapsed. Furthermore,because the membranes of all the cells were completelydisrupted in the SHAFD sample, the middle lamellapractically disappeared, indicating breakdown of pectins inthe middle lamella of the cell walls and lose of binding forcebetween cells (Alvarez et al., 1995). Thus, the SHAFDsample was a less crisp product.
The SIRFD samples dried with radiation intensity of5000 W m‐2 were crisper than others dried at lower intensity.Drying temperature may contribute to the effect of crispness;increased temperature at higher radiation intensity wouldremove moisture faster in the strawberry slices.
CONCLUSIONSequential infrared and freeze‐drying (SIRFD) can be
used as an alternative method for producing crispystrawberry chips. IR drying had a much higher drying ratethan hot‐air drying, and the rate increased remarkably withincreases in radiation intensity. Strawberry chips dried withSIRFD had a much crisper texture and darker red color thanthe regular freeze‐dried or SHAFD products. IRpre‐dehydration reduced the required time during thesubsequent freeze‐drying process. The products from SIRFDhad more shrinkage than regular freeze‐dried products, butless shrinkage than SHAFD products.
ACKNOWLEDGEMENTS
The authors thank Don Olson, USDA‐ARS WesternRegional Research Center, for his support in the experimentsand partial financial support from Innovative Foods, Inc.,under USDA‐ARS CRADA No. 58‐3K95‐5‐1089.
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