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Thermochimica Acta 525 (2011) 56– 61

Contents lists available at ScienceDirect

Thermochimica Acta

journa l h o me page: www.elsev ier .com/ locate / tca

hermal transitions of pulp and cuticle of blueberries

aulo Díaz, Olivia Henríquez, Javier Enrione, Silvia Matiacevich ∗

epartamento de Ciencia y Tecnología de los Alimentos, Facultad Tecnológica, Universidad de Santiago de Chile, Av. Libertador Bernardo O’Higgins N◦ 3363,stación Central, 9170022 Santiago, Chile

r t i c l e i n f o

rticle history:eceived 2 March 2011eceived in revised form 1 July 2011ccepted 22 July 2011vailable online 30 July 2011

a b s t r a c t

The thermal transitions of two blueberries cultivars (Duke and Briggitte) were studied by differentialscanning calorimetry as part of comprehensive study of blueberry conservation. Thermal transitions andtexture (hardness) were determined in fresh and frozen fruits freezed at 5 ◦C/min and 40 ◦C/min. Glasstransition temperatures were successfully detected in pulp and cuticle, which was associated to thesoluble sugar fraction present in the fruit. Transitions associated to water crystallization and melting

eywords:SClueberriesuticleulphermal transitions

showed differences between both cultivars, which were explained by their soluble solids concentration.An endothermic transition at 50 ◦C was associated to the melting of epicuticular wax present in the cuti-cles. The cuticle was less affected by ice formation, which is an important feature to fruit protection at lowtemperatures. Hardness showed that cooling rate had not a significant effect on this quality parameter.Despite the complexity in the structure of blueberries, thermal characterization by DSC was successful.

© 2011 Elsevier B.V. All rights reserved.

tructure

. Introduction

Blueberries represent a large group of high sugar content fruitsor which storage stability is important. The consumption of blue-erries has increased worldwide due its well known health benefits1] by the presence of anticancer and anti-oxidants compoundsnd low calories associated to its components. Therefore blueber-ies have begun to be considered as an important component of aealthy diet [2,3]. Blueberries are little fruits, spherical in shape,lue in color, from the genus Vaccinium with high nutritional valuend with potential anti-diseases effects but with very limited shelf-ife. The opening of new markets and the retention of current onesepends on the ability to deliver a consistently high-quality prod-ct to geographically distant destinations (e.g., from south to northemisphere). The color range of blueberry varies from light blueo deep black and depends on the cultivar and the presence of anpicuticular wax on their skin, which gives to fruit an attractiveppearance [4]. Literature mentions that main quality indicatorsn blueberry are fruit appearance (color, size and shape), firmnesss a measure of fruit’s structure integrity, flavor (soluble solids,ritrable acidity and pH) and nutritive value (vitamins A, C andntioxidants) [5]. Changes on those indicators have a profound

ffect on consumer’s acceptability. Clearly, the retention of theseuality parameters is paramount to successfully compete in theresh fruit international market.

∗ Corresponding author. Tel.: +56 2 7184517; fax: +56 2 7764089.E-mail address: silvia.matiacevich@usach.cl (S. Matiacevich).

040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.tca.2011.07.021

Under refrigerated conditions at temperatures near 0 ◦C, shelf-life of blueberry is about 14–20 days [6,7]. Therefore, conservationmethods such as freezing are required, extending the fruit shelf-lifeup to 18 months. An important preservation parameter is the cool-ing or freezing rates prior storage, which can produce an impact onthe glass transition temperature and therefore on its stability [8,9]and microstructure [9]. During the freezing process, quality dete-rioration can occur at three well established stages: (i) nucleation,(ii) crystallization, (iii) crystal growth and recrystallization. Hence,the freezing method used for blueberries determines the possiblepattern of ice formation (nucleation) in the fruit. The nucleation ofice and the number and size of ice crystals formed are dependenton the freezing rate: slower cooling rates will produce fewer nucle-ation, larger ice crystals and more dislocation (migration) of watermolecules. Faster cooling rates will produce more nucleation, butsmaller ice crystals [10,11]. Standard freezing of wild blueberriesproduces a significant juice loss (drip) after thawing, which appearsto be caused by ice crystal damage to cell vesicles [8,9]. It is believedthat frozen foods are stable when stored at −18 ◦C, but literaturesuggests that this is not entirely true due to micro-structural andquality changes can occur at this stage, related mainly to ice for-mation [9]. Indeed, the assessment of thermal parameters such asglass transition temperature, end point of freezing and maximalfreeze-concentration conditions are considered to be relevant andimportant in food processing (i.e., freezing and drying) and stabil-

ity under various storage conditions of temperature and moisturecontent [12]. Thus, a product stored at a temperature (Ts) belowthe glass transition temperature of the maximal freeze concen-trated fraction (Tg’), it may be expected to be composed of ice and a

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reeze-concentrated phase in a glassy state and thus a long-termtability may be anticipated [13]. Moreover, only limited infor-ation is available looking a thermal transitions and stability of

ystems such the aforementioned fruits.Changes in structure, solute concentration associated with the

ruit ripening, as well as fruit water status can be analyzed simul-aneously by DSC [14]. Thermal transitions of fruits determinedy this technique have been focused mainly in water sorption,lass transition temperature and state diagram of freeze-driedn fruits such as raspberry [15], grapefruit [16], kiwifruit [17,18],reeze-dried or spray-dried juices of persimmon [19], apple [20]nd tomato [21]. However, little information has been publishedooking at the thermal characterization of fresh fruits. Literatureescribes only research on unfrozen fraction of water in onion,rape, strawberry [22], apples [20] and grapefruit [16]. Meanwhile,hermal transitions of cuticles have been only partially studied, par-icularly in isolated cuticle (enzymatical isolation) of apple [23,24],omato [24] and extracted cuticular waxes from grape berry [25].he fruit cuticle is chemically heterogeneous in nature, basicallyonsisting of a waxy fraction, soluble in organic solvents, andn insoluble cuticular matrix, the cutin, that forms the structureramework of the cuticle. Cuticular waxes constitute the main bar-ier limiting the transport across the plant–atmosphere interface,ontrolling transpiration [26], foliar uptake of xenobiotics [27,28]nd the resistance against fungi [29]. This is modulated by their par-ial arrangement in crystalline regions, where the middle portionsf the long aliphatic chains of the wax are regularly aligned, and inmorphous regions where short-chain aliphatics and cyclic com-ounds form clusters outside crystalline regions [30]. Those waxesan, in fact form crystalline oriented parallel to the surface of theuticular membrane [31].

The objective of this work was to analyze thermal transitionsn both cuticles and pulp of two blueberry cultivars using differen-ial scanning calorimetry (DSC), as part of comprehensive study oflueberry conservation.

. Materials and methods

.1. Plant material

This study was carried out during the growing season of theummer of 2010 at the central valley of the Region Metropolitan inhile (Curacaví, Hortifrut S.A.). Two common blueberries cultivarsBrigitte and Duke) from Highbush variety were used. Cvs. Briggittend Duke were hand harvested and immediately packed in 230 mLlastic “clam-shell” containers (industry fruit standard containers)nd transported to laboratory at the Universidad de Santiago dehile for testing.

Prior the experimental work, fruits were selected manually, dis-arding those visually damaged, without peduncle or flower restnd red (unripe) fruit.

The cuticles were obtained from blueberry using a sharp scalpeleparating it from the pulp of the fruit, then scraped lightly in ordero remove any residual pericarp tissue [32–34]. Five fruits repli-ates were used for this purpose. The pulp obtained was groundnd homogenized.

Epicuticular wax extraction from blueberries was achieved byoaking ten fruits (replicates) in chloroform for 30 min, where thepicuticular wax was completely solubilized [35]. Then, the solventas evaporated at room temperature in order to obtain wax in theSC pans for thermal measurements.

.2. Model system

Glucose and fructose are sugars present in blueberries at signif-cant concentrations [36]. These compounds are considered to be

Acta 525 (2011) 56– 61 57

major vitrifying agents in plants [37,38]. Therefore, in order to iden-tify the Tg in blueberries associated to sugar–water fraction, a modelsystem (SMS) based on glucose and fructose with well known Tg

was prepared at a concentration similar to the reported for blue-berries in literature [36]. SMS solution concentration was 16% oftotal solids using glucose (Riedel-de Häen) and fructose (Merck) inequal ratio (1:1).

2.3. Storage conditions

Blueberries were stored at three different conditions: (i) freshblueberries were stored at 5 ◦C for 22 days. Then the mechanicaland thermal properties were measured by a texture analyzer andby differential scanning calorimeter (DSC), respectively. (ii) Frozenat slow rate (5 ◦C/min) in trays 6.0 cm deep in a storage room withslowly moving air stream at −18 ◦C. Fruits were stored there for 18months. Blueberries were analyzed by mechanical analysis. For theassessment of thermal transitions, fresh blueberries (cuticle andpulp) were cooled at 5 ◦C/min in the DSC, taking samples of cuticleor pulp and sealing them in aluminum pans immediately beforethe analysis. (iii) Frozen at intermediate cooling rate (40 ◦C/min)by an IQF (individually quick frozen) industrial process (personalcommunication, Hortifrut SA, Chile) and stored at −18 ◦C for 18months. After that time blueberries were analyzed by mechanicalanalysis.

2.4. Fruit quality indicators

The following parameters were used to assess fruit qualitybefore storage conditions:

Water content: It was determined gravimetrically using an ana-lytical balance (Mettler Toledo, Switzerland). Twelve blueberrieswere dried in an oven (Wiseven, Korea) at 105 ◦C for 24 h untilreach constant weight [39]. The analysis was performed in trip-licate. Water content was expressed in % wet basis (g water/100 gwet sample)

Fruit size: Equatorial and polar diameter was measured usinga digital calliper (Bull Tools, USA) and roundness index (RI) wasdetermined by the ratio polar diameter/equatorial diameter. Amean and standard deviation of 30 measurements were reported.

Total soluble solids content: It was determined by placing 1 mL ofhomogenised pulp obtained from the milling of 5 blueberries on aportable refractometer (0-32◦Brix, RHB-32ATC). The analysis wasperformed in triplicate. The mean and standard deviation for thereplicates were recorded and expressed as ◦Brix.

Hardness: It was defined as the fruit’s stiffness; it was measuredby compression test using a Texturometer (DOFBO.5TS, Zwick,Germany). Data were collected using Texture Expert Version 1.22software (Stable Micro System Ltda.). A 12 mm diameter ball com-pressed each individual fruit (n = 30) perpendicular to its axis(equatorial height) at a rate of 1 mm/min to a threshold force of10%. The hardness of each blueberry was measured as the Maxi-mum Force (N) necessary to compress 10% of the initial equatorialheight of the fruit. Therefore, the hardness value (N) mean of the 30replicates was reported with their corresponding standard devia-tion.

2.5. Differential scanning calorimetry (DSC)

10–20 mg of the pulp, cuticle and extracted epicuticular waxwere separately weighed, loaded into aluminum pans of 30 �Land subjected to thermal scans using a DSC (Pyris Diamond DSC,

Perkin Elmer, USA). Prior the measurements, the DSC was cali-brated using indium (melting onset temperature 156.6 ± 1.56 ◦C,�H = 28.6 ± 1 J/g). The reference used during the analysis was anempty pan. All experiments were performed in triplicate using the

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ollowing thermal profile: samples were cooled at a cooling ratef 5 ◦C/min and 40 ◦C/min as slow freezing and intermediate (IQF)reezing rate, respectively. Then samples were heated from −70 ◦Co 95 ◦C at a heating rate of 10 ◦C/min. Two temperature scansere performed to each sample. The glass transition temperature

Tg), water crystallization temperature (Tc), ice melting tempera-ure (Tm), and ice melting enthalpy (�Hm) were calculated by theSC software (Pyris Software v 9.0.2. USA). Tg values were obtained

rom the second scan and defined as the midpoint of the change ineat capacity. The transition temperatures related to crystalliza-ion and melting of water were calculated as the onset of the peakt each corresponding transition. The heat of fusion defined as thehange in enthalpy (�H) was calculated from the area under theirorresponding endothermic peak. The relationship between �H’sEI), an index of differences in the amount of water crystallizedy the different freezing methods (slow and IQF cooling rate), wasbtained from following equation:

I =(

1 − �HIQF

�Hslow

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All experiments were performed in triplicates following theame protocol. Water content of the samples was determined gravi-etrically after each DSC analysis. Pans were punctured before

ried in an oven at 100 ◦C for 7 days. Thus, the change in heat capac-ty and enthalpy were expressed in terms of dry matter (d.w.b.) (J/C

d.w.b.).

.6. Statistical analysis

Preliminary analysis of variance indicated no differencesetween the replicates for each analytical measurement. Conse-uently, the combined data were analyzed reporting the mean andtandard deviation. Differences between the reported data werenalyzed by ANOVA and t-test using Prism GraphPad, v4 software.idth of the melting peak associated to water obtained by DSC was

tted with Lorentz equation (R2 > 0.95) using Origin v8 software.

. Results and discussions

The results obtained comparing both blueberry cultivars priortorage showed the following,

(i) Water content of both cultivars was 84 ± 2% in wet basis(w.w.b.)

(ii) The equatorial diameter (1.6 ± 0.2 cm) and polar diameter(1.2 ± 0.2 cm) did not showed significant differences (p > 0.05)between cultivars, however cv. Briggitte had a greater round-ness index (RI) (0.74 ± 0.01) than cv. Duke (0.72 ± 0.01)

iii) Significant differences (p < 0.05) in soluble solids content wereobtained, with 15.2 ± 0.8 ◦Brix and 10.7 ± 0.7 ◦Brix for cv. Brig-gitte and Duke, respectively.

ig. 2. Blueberry cv. Duke: (a) fresh; (b) thawed blueberry freezed at slow cooling rate (5imilar behavior was observed in cv. Briggitte.

berries, for fresh, stored at 5 ◦C during 22 days, frozen at −18 ◦C at 5 ◦C/min coolingrate (slow rate) and frozen at intermediate cooling rate (IQF) at 40 ◦C/min stored for18 months.

The hardness of the blueberries was assessed by compressionmechanical test, as it provides information of structure integrityof the fruit during storage. Moreover, mechanical testing couldbe used as good predictor of sensorial attributes as suggested forvarious fresh fruits such as apple [40], canned peaches [41] andblueberries [42–44]. However, in the case of small fruits, the tex-ture analysis has a disadvantage related with high variation of themechanical test; hence experimental data exhibit an appreciablestandard deviation. In order to reduce this variability, 30 repli-cations (30 fruits) were measured [42–44] obtaining a variationcoefficient of data (defined as SEM/mean × 100, VC) in the range of5–10%. An explanation for this variation in the experimental datacould be related to the inherent heterogeneity between fruits. Itwas observed a reduction in VC (∼5%) in fresh blueberries due theirsimilar hardness. The highest VC (∼10%) was observed in frozenblueberries due to greater deterioration in their internal structure.The results in hardness are summarized in Fig. 1. The results showthat both, slow and intermediate cooling rate, had not a significanteffect (p < 0.05) on the maximum force obtained comparing to freshfruit (Fig. 1). Despite visual differences may be stated betweencooling rates (Fig. 2) and that bigger fruits had lower values of hard-ness [42,43], literature show that firmness for blueberries is notconsidered a critical quality factor since it remain constant duringstorage and do not fall during the postharvest storage period [45].Indeed, correlation between compression firmness and sensoryscores for texture were not associated with intensity of skin tough-ness [44]. However results show that internal structure damageobserved in blueberries would be independent of cooling rate.

Similar results related to textural properties have been observed ingreen peas [46], strawberries [47] and microstructural changes onblueberries [9]. The intermediate cooling rate will produce smallerice crystals that will not rupture cell membranes, resulting in a

◦C/min); (c) (b) thawed blueberry freezed at intermediate cooling rate (40 ◦C/min).

P. Díaz et al. / Thermochimica Acta 525 (2011) 56– 61 59

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observed between pulp and cuticle �Cp values.As it has been claimed previously, blueberries (pulp and cuti-

cle) contain high water content (>79%wb), thus DSC thermograms

ig. 3. DSC thermogram for cv. Duke. Similar thermogram was observed in cv. Briggransition temperature (Tg’). Onset of water melting and crystallization temperatur

rmer product than slow cooling rate. However, not significantifferences (p > 0.05) were observed in this study comparing thelow and intermediate cooling rate. Indeed the highest values inardness were obtained for the fresh blueberries (p < 0.01).

Fig. 2 shows the changes in structure observed after thawinghe fruits frozen at intermediate and slow cooling rate. The innertructure of fresh blueberry is presented in Fig. 2a. Fig. 2b showshe deterioration observed in the inner structure and variationsn color of the blueberry frozen by IQF (40 ◦C/min) whilst Fig. 2chows mayor fruit deterioration with loss in structure and dramaticrowning when frozen at slow freezing rate (5 ◦C/min).

Thermal properties of a complex system such as blueber-ies were analyzed by DSC. Thermal transitions associated to thearbohydrates content in the fruit (Tg), water present (water crys-allization and ice melting) and epicuticular wax in cuticles wereuccessfully determined.

DSC thermograms were similar to those reported in the liter-ture for other freeze-dried fruits equilibrated to an equivalentange of relative humidity [15,17,22]. Experimental thermogramsor both pulp and cuticle from cv. Duke showed similar thermalrofiles (Fig. 3). A similar result was observed in cv. Briggitte butith differences in the thermal profiles related to water transi-

ions. The values of the thermal transitions (Fig. 3) from both pulpnd cuticle did not show significant differences (p > 0.05) with: (i)g (onset) at −44 ± 2 ◦C, according to high water content of bothresh cultivars before storage (84 ± 2%wb). However, Tg of the fruittored at 5 ◦C for 22 days was −40.9 ± 0.3 ◦C, which was associatedo lower water content (79.1 ± 0.5%wb) compared to the fresh blue-erries (84 ± 2%wb); (ii) the temperature (onset) of endothermicransitions related to the melting of ice crystals (on heating) were4.7 ± 1 ◦C and −7.0 ± 1.0 ◦C for cv. Duke and Briggitte, respectively;

iii) the temperature (onset) of exothermic transitions associ-ted to water crystallization (on cooling) were −14.0 ± 0.9 ◦C and16.7 ± 0.7 ◦C for cv. Duke and Briggitte, respectively; (iv) width of

he peak of the endothermic transition associated to ice meltingor both cultivars (on heating) was 5.1 ± 1.3 ◦C and 7.5 ± 1.5 ◦C forntermediate and slow cooling rate, respectively.

The Tg values obtained in this study for cvs. Duke and Briggitteere in accordance to those reported previously. Kader et al. [48]

eported a Tg of −43.9 ◦C in blueberry juice from a pool of cultivarsf Vaccinum corymbosus, meanwhile Allan-Wojtas et al. [9] reported

n onset Tg of −45.7 ◦C in wild blueberries juice from Nova Scotia.

The sugar composition in Highbush blueberries has beeneported to be a mixture of glucose and fructose in equal propor-ion (1:1) [36]. As shown in Fig. 4, the Tg value obtained from the

otted arrow and zoomed graph insertion indicated the midpoint of apparent glassre indicated for each cultivar.

model system based on glucose and fructose was −42.4 ◦C ± 0.5 ◦C,which was similar to the value obtained for the fresh blueberries(−44 ◦C ± 2 ◦C) in this study. This result is in accordance to the valuereported by Rizzolo et al. [36], where they correlated the observedTg (−43.9 ◦C) in a mixture of glucose and fructose in a same ratio(1:1) added to blueberry juice with a measured Tg of −42.6 ◦C. Sya-maladevi et al. [15] associated the exhibited Tg to the amorphoussoluble compounds (glucose and fructose) in raspberry samples.Therefore, the glass transition temperature obtained in this studywas attributed to sugar components present in the blueberries andmay be identified also as an apparent glass transition temperatureof the maximally freeze concentrated blueberry (Tg’). However fur-ther studies are necessaries to determine the effective value of Tg’and Tm’. Rahman et al. [12,49] and Díaz et al. [50] have recently pro-posed methodologies based on annealing between apparent valuesof Tg’ and Tm’ in order to determine the effective value of Tg’ and Tm’by DSC. Indeed this methodology should be used in further studiesin order to obtain the state diagram of blueberries.

The change of heat capacity (�Cp) from glass transitiontemperature was mainly affected between pulp and cuticle,being significantly (p < 0.05) lower in cuticle (0.17 ± 0.01) thanpulp (0.23 ± 0.02), but not significant differences were observedbetween cooling rates and between cultivars. Likely the higheramount of sugar in pulp than cuticle could generate the differences

Fig. 4. Glass transition temperature (Tg) for the model system (glu-cose:fructose = 1:1). Arrow indicates the onset of the transition.

60 P. Díaz et al. / Thermochimica Acta 525 (2011) 56– 61

Table 1Normalized enthalpy values (�H) obtained of both cultivars of fresh blueberries frozen at two different cooling rates stored at 5 ◦C during 22 days. Variation coefficient (VC)was lower than 10% in all cases. The relationship between �H’s (EI) was obtained following Eq. (1).

Fruit Cooling rate cv. Duke cv. Briggitte

�H (J/g dry sample) EI �H (J/g dry sample) EI

Pulp 5 ◦C/min (slow rate) 2722 43 940 48.340 ◦C/min (IQF rate) 1556 486

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Cuticle 5 ◦C/min 211840 ◦C/min 489

lso provide information about ice melting endotherms, indicatinghe proportion of frozen water in the samples. The mean value ofhe onset temperature of ice melting endothermic peak was con-idered as Tm (onset of ice crystals melting) resulting in values of4.7 ± 1 ◦C and −7.0 ± 1.0 ◦C for cv. Duke and Briggitte, respectively.oth values of Tg and Tm were obtained from the same experi-ent. However, some differences were obtained for fresh (84%wb)

nd stored (79%wb) blueberries, which were attributed to experi-ental variability. Some authors have also noted a little difference

∼3 ◦C) in Tg at different water contents [15,51,52]. For instance,ai et al. [51] observed a Tg of −61.6 ◦C and −58.4 ◦C in apple withater contents of 85.6% and 73.2% (wb), respectively. Syamaladevi

t al. [52] also observed Tg’ in raspberry of −57.4 ◦C and −55.8 ◦Cor samples with water contents of ∼70% and ∼60% (wb), respec-ively. Syamaladevi et al. [15] also reported a Tg of −63.5 ± 5 ◦C inaspberry with moisture content between 75% and 86% (wb).

The onset temperatures of crystallization and ice melting wereot significantly different (p > 0.05) between cooling rates (slownd intermediate) and between cuticle (78%, wb) and pulp (68%,b). These results can be explained by the dependency of the tem-eratures of crystallization and ice melting on the concentrationf soluble solids (colligative properties) rather than their waterontent [7]. Significant differences (p < 0.05) in aforementionedhermal parameters were detected between cultivars, being lowern cv. Briggitte compared to cv. Duke, in accordance to the higheroluble solids concentration (◦Brix) in cv. Briggitte.

The width of the peak associated to ice melting was slightlyigher at slow than intermediate cooling rate, which was relatedo greater distribution of ice crystal size. Lower cooling rates willroduce less nucleation, larger ice crystals and more dislocationmigration) of water molecules in comparison with faster coolingates, which will produce more nucleation and smaller ice crystals.

The thermal transition of cuticle presented an endothermicransition with an onset temperature at ∼50 ◦C, which was alsobserved in the DSC re-run indicating a reversible nature of theransition, being associated to the fusion of a lipid based orderedtructure. The thermal event observed in the cuticle is attributedo the melting of waxy components presents in the fruit exter-al structure. It has been reported in grape berry cuticle at 72 ◦C53], in tomato cuticle at 30 ◦C [54] and 30–45 ◦C [25], but it hasot been reported in blueberries. Therefore, in order to confirmhis finding, the waxy component was extracted from the cuticle,ried by solvent evaporation and analyzed by DSC. The resultedhermograms resemble the thermal transition observed in cuticleFig. 5), confirming that the thermal transition observed at ∼50 ◦Can be attributed to the melting of waxy components. Moreover,he epicuticular wax content was calculated based on the obtainednthalpy values (�H) giving ∼70 �g/mg of cuticle.

Table 1 shows the values of normalized enthalpy (based on thery weight of the sample) of ice melting obtained at 5 ◦C/min (slowate) and 40 ◦C/min (IQF), in pulp and cuticle from both fresh culti-

ars. A comparison between the melting enthalpies of the ice fusion,howed lower endothermic energies in both cultivars frozen at highooling rate, with a reduction of 43% and 48.3% for pulp and 77%nd 80.7% for the cuticle for cv. Duke and Briggitte, respectively

Fig. 5. DSC thermograms of cuticle and extracted epicuticular wax (lipids).

compared to low cooling rate. It is important to take into accountthat a higher enthalpy implies higher water content in frozen state,which may affect the internal structure of fruit (Fig. 2a and b). Thusthe cuticle was less affected by ice formation, which is very impor-tant for frozen fruit protection. Although IQF cooling rate is actuallywidely used in the food industry, the internal structure of blueber-ries was also affected by ice formation (Fig. 2c). Allan-Wojtas et al.[9] reported that the formation of large ice crystals in the area justbelow the fruit skin, where the pigment is located, could cause suffi-cient structural damage to compromise the compartmentalizationof the anthocyanin in the cell and cause its subsequent “bleeding”(Fig. 2b and c).

4. Conclusions

Transitions associated to water (crystallization, melting) in bothblueberry cultivars showed differences associated to their solublesolids concentration. Tg values were related directly to sugar com-position (glucose and fructose) of the fruit. Endothermic transitionat 50 ◦C was associated to the melting of epicuticular wax present inthe cuticles. Fast cooling rates promoted ice nucleation with smallerice crystals allowing lower structural deterioration compared toslow cooling rates in accordance with results previously reportedin literature. The cuticle was less affected by ice formation, which isan important feature to fruit protection at low temperatures. Tex-tural attribute such as hardness showed that cooling rate had not asignificant effect on this quality parameter. Despite the complexityin the structure of blueberries, thermal characterization by DSC itwas possible.

The authors acknowledge financial support from FONDECYT No.11100209, FONDECYT No. 1110607, CONICYT Project PBCT-PSD-62,DGT No. 091071-M and DICYT from VRID-Universidad de Santiagode Chile.

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