Ultrasound-Assisted Infrared Drying of Pear Slices: Textural Issues

ULTRASOUND-ASSISTED INFRARED DRYING OF PEAR SLICES:TEXTURAL ISSUESFILIP DUJMIC1, MLADEN BRNCIC, SVEN KARLOVIC, TOMISLAV BOSILJKOV, DAMIR JEŽEK,BRANKO TRIPALO and IRENA MOFARDIN

Faculty of Food Technology and Biotechnology, Department of Food Engineering, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia

1Corresponding author.TEL: +385-1-4605-222;FAX: +385-1-4605-200;EMAIL: [email protected]

Received for Publication June 27, 2012Accepted for Publication October 16, 2012

doi:10.1111/jfpe.12006

ABSTRACT

The aim of this research was to investigate the impact of high-intensity ultrasoundas pretreatment on the duration of drying and texture characteristics of infrared-dried pear slices using different amplitudes. Ultrasound device with a powercapacity of 400 W, working at a frequency of 24 kHz and amplitudes of 25, 50, 75and 100%, was used for pretreatment. Drying was performed in an infrared dryerat 70C. The results obtained showed how application of ultrasound with variousamplitudes influenced the shortening of the drying time and allows the elimina-tion of more water from the pear slices. The results showed that hardness ofsamples gradually decreased (from untreated sample [104.72 N], 50% of ampli-tude [93.461 N] to 100% amplitude [62.206 N]) with an increase in ultrasoundintensity and that all process parameters had significant influence on the texturalproperties of pear slices (hardness, elasticity and work required for chewing).

PRACTICAL APPLICATIONS

Drying in food technology is usually based on the use of conventional methodssuch as heat exchange by conduction or convection, which may have negativeeffects on textural issues of final products. In order to achieve final products ofhigh quality, new and sophisticated techniques for food treatment are required.One of such methods is the power ultrasound-aided drying. This method isproven efficient for preserving main characteristics and quality of products, andadditionally leads to shortened drying time.

INTRODUCTION

Fruit plays an important role in human nutrition because ofits nutritional properties. Drying of fruits and vegetables isone of the oldest methods of food preservation. Althoughdrying is energy-intensive and consequently cost-intensivebecause it is a simultaneous heat and mass transfer processaccompanied by phase change, it is still indispensable infood industry (Barbanti et al. 1994). Although dryingextends shelf life, it also leads to undesirable changes incolor and texture, and causes partial loss of nutritive com-ponents (Bankole et al. 2005; Brncic et al. 2010; Mothibeet al. 2011). Therefore, to maintain quality, it is necessary toimprove existing traditional methods and explore new tech-nologies for the food industry.

The conventional ways of drying are still present in theindustry but their disadvantage lies in quality loss of the

final product. Dried foods could be stored for long periodswithout the demand for cooled transport and specificstorage as well (Ježek et al. 2006; Nacheva et al. 2007). Thisis achieved with the decrease of available moisture or wateractivity within the material, but, on the other hand, it alsocauses shrinkage of the fruit and loss cell wall integrity,which can cause flaccid tissues with a rubber texture (Hillsand Remigereau 1997).

Infrared (IR) Drying

IR drying has been studied for achieving fast drying andreducing quality loss of fruits and vegetables (Baysal et al.2004; Brncic et al. 2010). Compared with hot air drying, IRheating offers many advantages such as greater energy effi-ciency, heat transfer rate and heat flux, which results inreduced drying time and higher drying rate. It has been

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Journal of Food Process Engineering ISSN 1745-4530

397Journal of Food Process Engineering 36 (2013) 397–406 © 2012 Wiley Periodicals, Inc.

investigated as a potential method for increasing heatingefficiency and obtaining high-quality dried food stuffs incomparison to conventional way of drying, includingpeaches (Wang and Sheng 2006), carrots (Togrul 2006),onions (Wang 2002), rice (Afzal and Abe 1997), apples(Brncic et al. 2010), and many other fruits and vegetables(Volonchuck and Shornikova 1998).

Nonthermal Technologies inFood Technology

In search of effective ways of preserving the macro- andmicronutrients, new methods of food preservation are cur-rently being developed. Modern methods of drying in theindustry seek to preserve as many natural properties of foodas possible, increasing shelf life and market value. Preserv-ing the chemical composition of fruits and vegetables isessential in maintaining the original quality of the followingattributes: color, flavor, texture and appearance, as these arethe first sensory characteristics of food that a consumerregisters (Brncic et al. 2008a; Adejumo and Oje 2009).Increased need of novel food processing technologies intoday’s food industry has led to the development of non-thermal technologies, such as high hydrostatic pressure,high-intensity ultrasound, pulsed electric fields, oscillatingmagnetic fields, pulsed light and cold plasma (Knorr 1993;Mason 1998; Fuente-Blanco et al. 2006; Leadley and Will-iams 2006; Xiao 2008; Brncic et al. 2010).

Ultrasound in Food Technology

High-intensity ultrasound is considered as one of the men-tioned nonthermal technologies with various advantagesover standard food processing equipment (Knorr 2004;Barbosa-Canovas et al. 2005; Balachandran et al. 2006;Cucheval and Chow 2008; Patist and Bates 2008). Applica-tions of high-intensity ultrasound in food engineeringcould be used to improve the operations of extraction(Chemat et al. 2011), homogenization and emulsification(Canselier et al. 2002; Bosiljkov et al. 2009), defoaming,screening, inactivation of microorganisms (Pitt and Rodd2003), waste water treatment, filtration, ultrasonic-assistedfreezing (Zheng and Sun 2005), sonocrystallization (Luquede Castro and Priego-Capote 2007) and drying (Cañizares-Macías et al. 2004; Fuente-Blanco et al. 2006; Fernandeset al. 2008; Mothibe et al. 2011). Moreover, propagation ofultrasonic wave or sonication could be used for improvedblanching (Cruz et al. 2010), meat processing (Chang et al.2012), cleaning (Fuchs 1999) and enhanced membraneseparation processes (Muthkumaran et al. 2006). This tech-nology could also be used as a procedure that is performedprior to other operations such as pretreatment beforedrying of fruits. Ultrasonic pretreatment could ensure many

positive effects on the quality of the final dried product, andthe advantage of using ultrasound is that the process can becarried out at ambient temperature and no heating isrequired, thus reducing the probability of food degradation(Mason 1998).

The high-intensity ultrasound or power ultrasound isdefined with low frequencies that start from 18 kHz up to2 MHz (Brncic et al. 2009; Guan et al. 2011). High powerultrasound provides enough energy for splitting the inter-molecular bond, while intensity above 10 W/cm2 createscavitation, which is the main reason for the changes in thephysical properties during sonication and occurrences ofvarious chemical reactions (Mason 1998; Bosiljkov et al.2011). Cavitation is the formation of micro-scale bubblesfilled with gas that are created within the liquid, which ispropagated by ultrasonic wave (Wambura and Yang 2010).When the cavitation bubble reaches its critical size andwhen the processes within the bubble are critical, it col-lapses by spreading the energy throughout the environment.The temperature inside the bubble could rise up to 5500 Kand pressure of 50 MPa.

Ultrasonic-Assisted Drying

The ultrasonic pretreatment is one of the promising appli-cations of ultrasound within food drying technology(Fuente-Blanco et al. 2006). This procedure involves immer-sion of the fruit in water or in a hypertonic aqueous solu-tion, to which ultrasound is applied. Mothibe et al. (2011)investigated the use of ultrasound as a pretreatment prior toair drying. The combined process (ultrasound and airdrying) minimized the total processing time. Removal ofwater from the products, which is obtained by drying ripefruits and vegetables, is performed to a degree that ensuresextending the shelf life. Fruits are often dried up to 16–25%(maximum 27%) and vegetables have 12–14% watercontent after drying. A pretreatment with ultrasound can beused to reduce the initial water content or to modify thefruit tissue structure combined with shorter time of treat-ment (drying). This kind of pretreatment has an importantimpact on dried foodstuffs and advantages in physico-chemical and textural properties as well (Carlos and DeMichelis 2011). Loss of water, which occurs during drying,results in structural changes, which lead to textural andsensory characteristics being different from the freshproduct.

Texture of Foods

Texture as a parameter of sensory quality has the largestinfluence on the taste modulation release effect. These tastecomponents must be released from the food matrix to thesuitable receptors of the tongue. Release of the taste is

ULTRASOUND-ASSISTED INFRARED DRYING OF PEAR SLICES: TEXTURAL ISSUES DUJMIC ET AL.

398 Journal of Food Process Engineering 36 (2013) 397–406 © 2012 Wiley Periodicals, Inc.

narrowly tied with the way food is broken up in the mouth.Consequently, it is also tied with the initial texture andchange of the texture during chewing (Brncic et al. 2006;Makri 2009a). Texture is a set of parameters of quality thatare closely related to the structural and mechanical proper-ties of foodstuffs (Burubai et al. 2007; Brncic et al. 2008b;Makri 2009b). Force/deformation relationship is time-dependent, while force (F), deformation (D) and time (t)represent three basic variables that are used in the study ofthe mechanical and textural properties of various foods (Luand Abbott 2004).

Pears

Pears are accepted by consumers due to their high nutri-tional and low energy values, as well as good taste. Pears area source of dietary fiber and contain significant amounts ofpotassium, but the relatively short shelf life of raw fruitcompared with the processed one is the limiting factor.Pears contain the largest portion of carbohydrates, sugars(60–70%), proteins (1.4%), minerals (1.5–3.5%) and crudefiber (7%). Part of the vitamin destroyed (about 50% ofvitamins A and C) in its entirety can be stored as B-complexvitamins (Chen et al. 2007; Tanriöven and Eksi 2005).

MATERIALS AND METHODS

Preparation of Samples

Fresh pears (Williams) were obtained from a local market.Before each experiment, fruits were washed, hand-peeledand cut in rectangular-shaped slices with dimensions of30 ¥ 30 ¥ 5 mm using a very sharp stainless steel knife. Dueto the statistical analysis and uniformity in weight duringdrying, the samples had a total weight of 16 � 1 g for eachof the four samples.

Ultrasonic Pretreatment

The ultrasonic pretreatments were carried out in a 250-mLlaboratory glass for each sample. The sample was placedinto a glass and then poured over with 100 mL of distilledwater. Titanium probe with a diameter of 22 mm wasmounted on the ultrasonic device, UP400S (Hielscher,Teltow, Germany), with declared maximal output power of400 W. Ultrasonic frequency was 24 kHz. Amplitudes usedin research were 25, 50, 75 and 100% of the nominal outputpower. Cycle of ultrasound was set to 1, which is the full-time period. Sonication was performed for 5 min. Theexperiments were carried out under ambient temperature(25C). Each assay was carried out in triplicate. The controlsample was not treated with ultrasound and was placeddirectly into the dryer.

Drying

Drying of the samples was carried out in IR moistureanalyzer/dryer LJ16 (Mettler-Toledo, Greifensee, Switzer-land). Temperature of drying was adjusted at 70C and achange in mass of the samples was read off every 1 min.Drying was conducted until constant mass of the samplewas achieved. Three slices were dried inside the device; thatis, three parallel measurements for each treatment.

Texture Analysis

Hardness, elasticity and work were measured with TA.HD-Plus texture analyzer (Stable Micro Systems, Godalming,U.K.) and the obtained textural profile curves were analyzedwith the software “Texture exponent (Stable Micro Systems,Ltd.).” The blade set setup was used and the probe speedduring the test was programmed at 5 mm/s, with penetra-tion depth set to 20 mm. The sample was placed on the baseand was cut into two slices to obtain force–time and force–distance curves, from which the achieved values were readoff. Work is calculated as the total area under force–distancecurve and it is expressed in mJ. This is equivalent to workneeded for the first bite through the sample during sensoryanalysis.

RESULTS AND DISCUSSION

The aim of this research was to investigate the influence ofultrasonic pretreatment on drying rate and textural proper-ties of pear slices. Influences of ultrasound treatment andamplitude on the textural properties of pear slices were ana-lyzed with Statistica v.8 software (Stat Soft, Inc., Tulsa,OK) and presented using adequate diagrams. Performedanalysis included analysis of variance (ANOVA) and posthoc analysis (Tukey’s test), by which the influence of ampli-tudes on drying time, hardness, elasticity and work requiredfor chewing the pear slices is expressed (Fernandes andRodrigues 2007; Falade and Shogaolu 2010; Day et al. 2012).

Drying Time

The influence of ultrasound amplitude on drying time wasanalyzed using ANOVA and showed a significant influenceof ultrasonic pretreatment (amplitude) on shortening ofdrying time (P < 0.05). Cavitation effect on the pear cellularstructure is directly linked to this as implosion of bubblescreates small channels and micro pores within and in thesurface of pear. This effectively enhances diffusion of waterto the surface and shortens drying time, which is in accor-dance with the previous research (Brncic et al. 2010;Mothibe et al. 2011).

DUJMIC ET AL. ULTRASOUND-ASSISTED INFRARED DRYING OF PEAR SLICES: TEXTURAL ISSUES


Interdependence of changeable amplitude and ultrasonictreatment as pretreatment for drying on the drying time ofpear slices is shown in Fig. 1, while post hoc statisticalanalysis for the influence of ultrasonic pretreatment on thedrying time of pear slices is presented in Table 1. Results ofpost hoc analysis indicate that drying time of the pear slicesis significantly shortened with an increase of ultrasonicamplitude. All applied amplitudes of ultrasound had signifi-cant influence on decreasing the drying time compared withuntreated pear samples. Applied maximal amplitude of100% had much stronger influence in relation to 25%amplitude. This is visible from the data presented in Table 1.Comparison of P values of post hoc analysis between ampli-

tudes shows that a significant reduction already occurs at25% amplitude, with p(A25%) = 0.000176, and the dryingtime is further decreased by increasing the amplitude to100%. With p(A100%) = 0.000176, it is evident that the onethat has the greatest influence on the drying time is themaximum amplitude.

Higher amplitude, and consequently, higher intensitiesof ultrasound had considerable larger effect on tied mois-ture in samples, while for the same time of drying,increased diffusion of water, throughout the samples, wasenabled. This channel enables the relieved release of mois-ture from the samples during drying, but also improvesthe final texture.

FIG. 1. INFLUENCE OF AMPLITUDE ONDRYING TIME (MIN) OF PEAR SLICES

TABLE 1. POST HOC ANALYSIS (TUKEY’STEST) ON THE INFLUENCE OF ULTRASONICAMPLITUDE ON PEAR SLICES DRYING TIMECOMPARED WITH UNTREATED PEAR SLICES(P < 0.05*)

CellNo.

P value

Amplitude(A, %)

(1) (2) (3) (4) (5)233.33 198.33 180.00 171.67 160.00

1 0 0.000176 0.000176 0.000176 0.0001762 25 0.000176 0.000375 0.000178 0.0001763 50 0.000176 0.000375 0.055280 0.0002584 75 0.000176 0.000178 0.055280 0.0077565 100 0.000176 0.000176 0.000258 0.007756

* Statistical significance (P < 0.05).1, mean value of drying time (min) for unsonicated pear slices; 2, mean value of drying time (min)for ultrasonically treated pear slices at 25% amplitude; 3, mean value of drying time (min) forultrasonically treated pear slices at 50% amplitude; 4, mean value of drying time (min) for ultra-sonically treated pear slices at 75% amplitude; 5, mean value of drying time (min) for ultrasoni-cally treated pear slices at 100% amplitude.



Textural Properties

ANOVA showed a significant influence of ultrasonic pre-treatment (amplitude) on all analyzed textural properties(hardness, elasticity and work required for chewing). Theinfluence on the textural properties is presented in Table 2by correlation of hardness, elasticity and the work requiredfor chewing and applied amplitude.

Hardness. Interdependence of changeable amplitude andultrasonic treatment as pretreatment for drying on hardnessof pear slices is shown in Fig. 2, while post hoc statisticalanalysis for pear slices hardness after ultrasonic pretreat-ment is presented in Table 3. Significant influence ondecreasing the force is noticed for all applied amplitudes ofultrasound except for p(A25%) in comparison to untreatedsample. Moreover, a comparison of P values obtained usingpost hoc analysis between amplitudes (p[A0%] > p[A100%]) ofhardness is in accordance with decreasing force needed forbreaking down the sample. While P values of post hocanalysis between amplitudes (p[A50%] and p[A75%]) and(p[A0%] and p[A25%]) are greater than 0.05 and do not have

significant influence on hardness, but in these cases, theforce needed for breaking down the sample is in accordancewith the trend of decreasing the hardness of the sampleswith higher amplitude.

Elasticity. Interdependence of changeable amplitude andultrasonic treatment as pretreatment for drying on elasticityof pear slices is shown in Fig. 3, and post hoc statisticalanalysis for pear slices elasticity after ultrasonic pretreat-ment is presented in Table 4. There is significant influenceof all amplitude of ultrasonic treatment on elasticity ofultrasonically treated pear slices compared with untreatedpear slices. Based on the obtained results for textural prop-erties, changes in elasticity were the largest at 100% ampli-tude; thus, changes in cellular structure were evident. Basedon the post hoc analysis presented in Table 1 and the depen-dence of elasticity on ultrasonic amplitude presented inTable 4, it is evident that optimal processing parameters forthis type of processor are 50% of the amplitude for 5 min.At those parameters, there is 11% loss of elasticity, which isstill acceptable based on the consumer’s perspective. Furtherincrease of time or amplitude leads to an even more short-

TABLE 2. ANALYSIS OF VARIANCE –INFLUENCE OF PROCESS PARAMETERS ONHARDNESS, ELASTICITY AND WORKREQUIRED FOR CHEWING PEAR SLICES(P < 0.05*)

Processparameter

P value

Hardness (N) Elasticity (mm) Work (mJ)

A (%) 0.0000011288455 0.0000000141029 0.0000001815784

* Statistical significance (P < 0.05).

FIG. 2. INFLUENCE OF AMPLITUDE ON PEARSLICES HARDNESS (N)



ening of drying time, at the expense of elasticity. Theseparameters significantly shorten the drying time, keepingelasticity in the acceptable range. It can also be observedfrom the results of post hoc analysis how possible increaseof amplitude (by stronger ultrasonic devices) would influ-ence the decrease in elasticity of the samples.

Work. Figure 4 represents the interdependence of change-able amplitude and time of ultrasonic treatment as pretreat-ment for drying on work required for chewing of pearslices. Likewise, post hoc statistical analysis for pear sliceswork required for chewing after ultrasonic pretreatment ispresented in Table 5.

As work is directly linked to other texture parameterssuch as hardness and elasticity, all applied amplitudes ofultrasound had significant influence on work required forchewing compared with untreated pear samples. A com-parison of P values of post hoc analysis between amplitudes(p[A50%], p[A75%] and p[A100%]) show a significant influenceon work required for chewing compared between theseamplitudes.

Nevertheless, works required for chewing these samplesare in accordance with increasing value of work requiredfor chewing the sample. This could lead to further (in thefuture) increase of ultrasonic amplitude in different deviceswith larger nominal output power (400 W). The conducted

TABLE 3. POST HOC ANALYSIS (TUKEY’STEST) – INFLUENCE OF ULTRASONICAMPLITUDE ON PEAR SLICES HARDNESSCOMPARED WITH UNTREATED PEAR SLICES(P < 0.05)*

Cellno.

P value

Amplitude(A, %)

(1) (2) (3) (4) (5)104.72 106.07 93.461 85.122 62.206

1 0 0.988700 0.019922 0.000493 0.0001762 25 0.988700 0.009836 0.000341 0.0001763 50 0.019922 0.009836 0.094262 0.0001774 75 0.000493 0.000341 0.094262 0.0002415 100 0.000176 0.000176 0.000177 0.000241

* Statistical significance (P < 0.05).1, mean value of hardness (N) for unsonicated pear slices; 2, mean value of hardness (N) for ultra-sonically treated pear slices at 25% amplitude; 3, mean value of hardness (N) for ultrasonicallytreated pear slices at 50% amplitude; 4, mean value of hardness (N) for ultrasonically treatedpear slices at 75% amplitude; 5, mean value of hardness (N) for ultrasonically treated pear slicesat 100% amplitude.

FIG. 3. INFLUENCE OF AMPLITUDE ON PEARSLICES ELASTICITY (MM)



texture analyses indicate that ultrasonic pretreatment influ-enced the sample textural properties, which is in accordancewith previous research (Brncic et al. 2010). Hence, byincreasing the ultrasonic amplitudes and intensities, thehardness of the samples decreases, which is a consequenceof pear tissue structural composition. A decrease in hard-ness is not a positive attribute that leads to improved crispi-ness of dried fruit slices, which is one of the main factors forthe final consumer judgments. Exposure of foods to hightemperatures during drying may lower the texture, colorand nutritional properties (Nacheva et al. 2007; Ježek et al.2008; Jimoh et al. 2009). In this case, ultrasonic pretreat-ment could have a positive effect on the textural properties

of pear and therefore make it more acceptable to the finalconsumer (Brncic et al. 2010; Mothibe et al. 2011).

However, treatments with maximum applied amplitudeadd up to an oversized decrease of hardness and thereforeare not recommended for pretreatment before drying underthe mentioned setup of the device. This could be observedin Table 5, where the work required for chewing is pre-sented. Values for this property significantly arise with anincrease in amplitude, while for the maximum (100%) ofthis parameter, the required work is 32% higher whencompared with ultrasonically untreated samples. As ampli-tudes of 50, 75 and 100% have no significant influence onwork when compared, thus, 50% amplitude is the most

TABLE 4. POST HOC ANALYSIS (TUKEY’STEST) – INFLUENCE OF ULTRASONICAMPLITUDE ON PEAR SLICES ELASTICITY(MM) COMPARED WITH UNTREATED PEARSLICES (P < 0.05)*

CellNo.

P value

Amplitude(A, %)

(1) (2) (3) (4) (5)8.5457 7.8077 7.5470 7.0913 5.7723

1 0 0.000176 0.000176 0.000176 0.0001762 25 0.000176 0.014932 0.000177 0.0001763 50 0.000176 0.014932 0.000367 0.0001764 75 0.000176 0.000177 0.000367 0.0001765 100 0.000176 0.000176 0.000176 0.000176

* Statistical significance (P < 0.05).1, mean value of elasticity (mm) for unsonicated pear slices; 2, mean value of elasticity (mm) forultrasonically treated pear slices at 25% amplitude; 3, mean value of elasticity (mm) for ultrasoni-cally treated pear slices at 50% amplitude; 4, mean value of elasticity (mm) for ultrasonicallytreated pear slices at 75% amplitude; 5, mean value of elasticity (mm) for ultrasonically treatedpear slices at 100% amplitude.

FIG. 4. INFLUENCE OF AMPLITUDE ONWORK (mJ) REQUIRED FOR CHEWINGPEAR SLICES



applicable for pretreatment, while all benefits are taken intoconsideration.

CONCLUSIONS

Ultrasonic treatment of pear slices before drying enablesshorter time of drying. An increase in ultrasonic intensityadditionally leads to much shortening of the drying time.An increase in ultrasonic intensity leads to a decrease inhardness of the dried samples, despite removal of higherwater content due to the creation of micro channels withinand at the surface of pear slices formed by cavitation, whichmay be due to the structural composition of pear tissue.Amplitude of 50% was shown to achieve adequate resultsfor ultrasonic pretreated pear slices. Maximum ultrasonicamplitude resulted in too low hardness of the sampleswhere these parameters of pretreatment were not suitable.Elasticity of the samples was reduced for all amplitudes ofultrasound as compared with the untreated samples. Ultra-sonic pretreatment assured drying process with shorter timeduration, which led to a reduced quality loss of the driedsamples.

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TABLE 5. POST HOC ANALYSIS (TUKEY’STEST) – INFLUENCE OF ULTRASONICAMPLITUDE ON WORK (mJ) REQUIRED FORCHEWING PEAR SLICES COMPARED WITHUNTREATED PEAR SLICES (P < 0.05)*

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1 0 0.004312 0.000176 0.000176 0.0001762 25 0.004312 0.000208 0.000189 0.0001763 50 0.000176 0.000208 0.960213 0.0653234 75 0.000176 0.000189 0.960213 0.1773155 100 0.000176 0.000176 0.065323 0.177315

* Statistical significance (P < 0.05).1, mean value of work (mJ) required for chewing pear slices for unsonicated pear slices; 2, meanvalue of work (mJ) required for chewing pear slices for ultrasonically treated pear slices at 25%amplitude; 3, mean value of work (mJ) required for chewing pear slices for ultrasonically treatedpear slices at 50% amplitude; 4, mean value of work (mJ) required for chewing pear slices forultrasonically treated pear slices at 75% amplitude; 5, mean value of work (mJ) required forchewing pear slices for ultrasonically treated pear slices at 100% amplitude.



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Ultrasound-Assisted Infrared Drying of Pear Slices: Textural Issues

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