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Available online at www.sciencedirect.com

Postharvest Biology and Technology 49 (2008) 171–179

Quality and internal characteristics of Huanghua pears (Pyrus pyrifoliaNakai, cv. Huanghua) treated with different kinds of coatings during storage

Ran Zhou a,b, Yun Mo b, Yunfei Li b,∗, Yanyun Zhao c, Guixiang Zhang d, Yunsheng Hu d

a Institute of Refrigeration & Cryogenic Engineering, Shanghai Jiao Tong University, Shanghai, Chinab Department of Food Science & Technology, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 201101, China

c Department of Food Science & Technology, Oregon State University, Corvallis, USAd Department of Radiology, Shanghai Jiao Tong University Affiliated First People’s Hospital, Shanghai, China

Received 4 June 2007; accepted 15 December 2007

bstract

We studied the quality and internal changes in Huanghua pears (Pyrus pyrifolia Nakai, cv. Huanghua) coated with shellac, SemperfreshTM

sucrose polyester base coating), and carboxymethyl chitosan (CMC) during cold storage (4 ◦C). The changes in respiration rate, weight loss, cellembrane permeability and texture profile analysis (TPA) such as hardness, brittleness and chewiness were recorded periodically for up to 60 days

fter harvest to compare the effects of the applied coatings. Magnetic resonance (MR) imaging was used to inspect the internal characteristics of

ears after storage. The soluble solids contents (SSCs), titratable acidity (TA), ascorbic acid concentration, and taste scores were also measured.ur data suggest that shellac coating was more effective in reducing the respiration rate and weight loss and in maintaining the quality of pears

han Semperfresh and CMC coatings.2007 Elsevier B.V. All rights reserved.

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eywords: Huanghua pears; Coatings; Cold storage; Quality; TPA; Magnetic r

. Introduction

Huanghua pear (Pyrus pyrifolia Nakai cv. Huanghua) is anmportant subtropical fruit in China and is widely planted inouthern China (Lin et al., 2003). Because of its thin peel,risp flesh, rich juice, and good taste, it is popular among con-umers. However, rapid postharvest physiological changes inuanghua pears are responsible for a short ripening period and

apid senescence and pose a challenge for their marketing (Lint al., 2003). Similar to other fruit, Huanghua pears are suscepti-le to deterioration accompanied with shrivelling, softening, andecay under improper storage conditions. Therefore, alternativeovel practices are required for the preservation of Huanghuaears.

Edible coatings are alternative storage methods for fresh

roducts and have attracted increasing attention because ofnvironmental considerations and the trends toward the use ofonvenience foods (Ozden and Bayindirli, 2002). These bio-

∗ Corresponding author. Tel.: +86 21 34206918; fax: +86 21 34206918.E-mail address: yfli@sjtu.edu.cn (Y. Li).

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925-5214/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.postharvbio.2007.12.004

nce imaging

ased coatings have been known to prevent fruit and vegetablesrom deterioration by inhibiting respiration, reducing dehy-ration, maintaining textural quality, retaining volatile flavor,nd decreasing microbial growth (Han et al., 2004). Edibleoatings can preserve fruit as efficiently as modified atmo-phere storage and maintain their quality during storage (Park,999). However, in some cases, edible coatings were not suc-essful and have even degraded fruit quality (Hagenmaier,005).

The occurrence of physiological disorders such as core flushflesh breakdown induced by improper coatings (Park, 1999) –

long with the lack of external symptoms, suggests that magneticesonance (MR) imaging is appropriate for the internal inspec-ion of fruit. MR imaging, a non-invasive and non-destructive

ethod, can provide visualization of even minute localized mor-hological changes in the interior portions and water distributionn intact plant tissues (Clark et al., 1997, 2003; Zhou and Li,007) and is more sensitive for detecting these physiological dis-

rders in fruit (Gonzalez et al., 2001; Lammertyn et al., 2003).owever, there are few reports on the application of MR imaging

o the monitoring of physiological changes in Huanghua pearsreated with different kinds of coatings during storage.

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The successful application of edible coatings to Huanghuaears depends mainly on the selection of appropriate coat-ngs that can provide a suitable barrier for decreasing theespiration rate and establishing a desirable internal gas com-osition (Baldwin et al., 1995; Han et al., 2004). Shellac waxnd sucrose polyester are widely used as edible coatings forruit, with successful uses reported in oranges (Baldwin et al.,995), Ankara pears (Sumnu, 2000), pomegranates (Nanda etl., 2001), apples (Bai et al., 2002), and cherries (Yaman andayo�nd�rl�, 2002). Chitosan has also been widely used in pre-

erving fruit (Agullo et al., 2003), but its application is restrictedecause it is essentially insoluble in water at neutral pH (Gend Luo, 2005). Carboxymethyl chitosan (CMC), a chitosanerivative, is an alternative that is water-soluble besides havingther advantages (Wang and Gao, 2002; Ge and Luo, 2005).owever, to our knowledge, there is no available scientific lit-

rature regarding the effect of shellac wax, sucrose polyester,nd CMC on maintaining the quality of Huanghua pears duringtorage.

In this study, we selected three kinds of coatings, such as shel-ac wax, SemperfreshTM (sucrose polyester base coating), andMC, that comprised a wide range of barriers to gas exchange.he aim of the current study was to compare the effects ofhellac coating, Semperfresh coating, and CMC coating on theostharvest quality of Huanghua pears and to determine the mostppropriate edible coating; MR images of the coated fruit werelso analyzed to compare the quality changes in pears treatedith different coatings after storage.

. Materials and methods

.1. Fruit and coating treatments

Huanghua pears (P. pyrifolia Nakai, cv. Huanghua) wereand-harvested at commercial maturity, which was determinedased on fruit skin color and the harvest date, from an orchardn Fengxian, Shanghai, China; they were transported to our lab-ratory on the same day. Fruit were selected for uniformity ofize and appearance. All harvested pears were chosen withoutiseases, bruises, or injuries.

The following three kinds of experimental coatings were usedn this study. For shellac coating, we used refined, dewaxed,leached, food-grade shellac (KFull-060715, Kunming Kfulliotechnology Co. Ltd., Kunming, China; 14.3 g/100 mLater), NH3 (0.8 g/100 mL water), and food-grade poly-imethylsiloxane antifoam (XP010, Runqi Food Technologyo. Ltd., Shanghai, China; 0.01 g/100 mL water). Semperfreshoating (SemperfreshTM; AgriCoat Industries Co. Ltd., Eng-and; distributed by HongYuanXinDa Co. Ltd., Beijing, China)as obtained in liquid concentrate form (50 g/100 mL water)

ontaining sucrose esters and glycerides of fatty acids, andarboxymethyl cellulose. The raw liquor was diluted with dis-illed water to obtain the appropriate concentration for pears

1.0 g/100 mL water), which is recommended by the prod-ct specification of the Semperfresh raw liquor. For CMCoating, the following were used: N,O-caboxymethyl chitosanSJJ-060611, food-grade, water-soluble, odorless, and taste-

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Technology 49 (2008) 171–179

ess powder, 90% deacylation; Nantong Xincheng Biologicalndustrial Co. Ltd., Nantong, China; 2 g/100 mL water), glyc-rol (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China;.5 g/100 mL water), and Tween 80 (Sinopharm Chemicaleagent Co. Ltd., Shanghai, China; 1.2 g/100 mL water), dl-�-

ocopheryl acetate (VE; Sinopharm Chemical Reagent Co. Ltd.,hanghai, China; 0.4 g/100 mL water). All other chemicals weref analytical grade.

Each coating was applied by dipping pears in its respectiveolution for 15 s. The fruit were suspended on stainless steelhelves by fastening their stems, and dried under fans. The con-rol pears were dipped in distilled water by following the samerocedure. Subsequently, the pears were stored at 4 ◦C and anpproximately 95% relative humidity (RH), which is recom-ended as the optimum storage conditions for Huanghua pears

Lin et al., 1997).

.2. Measurement of respiration rate and weight loss

Five fruit per treatment were randomly selected and placedn individual 1.45-L jars. Each jar contained one pear (a totalf 20 jars). A small hole had been made on the lid of each jar,nd high-strength adhesive tape covered the hole of each lid.ubsequently, the jars containing the pears were left at 4 ◦Cnd 95% RH for 24 h. A gas analyzer (CheckMate 9900; PBIansensor, Denmark) was used to measure the respiration rate of

he pears by sampling the gas through the tape and analyzing theO2 concentration in each jar. The measurements were repeated

hree times. The respiration rates of the pears were expressed inmol CO2 per kilogram per hour.

Five pears per treatment were randomly selected and usedo determine weight loss by using a scale with an accuracyf 0.01 g (JA2002; Shanghai Jingtian Electronic Apparatus Co.td., China). The measurement was performed after the coat-

ng treatments and then after storage and results expressed asercentage of the initial weight.

.3. Permeability analysis of pear cell membranes

Five pears per treatment were randomly selected and rinsedith double-distilled deionized water and then lightly wipedith filter paper before examination. Using a cork borer, we

ollected a piece of pear flesh with a diameter of 14.5 mm fromhe equatorial region and cut it into slices of 2 mm thickness. Thelectrical conductivity of the slice samples was measured usingdigital conductometer (DDB-6200; Shanghai Leici Appara-

us, Shanghai, China) with a DJS-1 conductivity immersionlectrode. The assay was performed according to the methodescribed by Feng et al. (2005). Slice samples weighing 5 gere rinsed with double-distilled deionized water three times

nd immersed in 100 mL double-distilled deionized water forh. Subsequently, the initial electrolyte leakage was measured.he samples were then boiled for 5 min, and the total electrolyte

eakage was assayed after the sample reached room temperature.ach pear was examined repeatedly three times. The membraneermeability was expressed by the relative leakage rate, whichas calculated as the percentage of the total electrolyte leakage.

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.4. Texture profile analysis (TPA) of the samples

TPA was measured using a TA-XT2i Texture Analyzer (Sta-le Microsystems Ltd., UK) with a 40 kg load cell, using atandard compression platen (SMS P/100). Five pears per treat-ent were measured during storage. The fruit were peeled, and

sing a cork borer, we collected cylindrical plugs with a diam-ter of 10 mm from the equatorial region and cut them intolices 10 mm thick. Four cylindrical plugs were obtained fromour opposite sides of each fruit at the equatorial region. Theest was then performed with a pre-test and post-test speed ofmm/s, a test speed of 1 mm/s, a 5 g auto-trigger force, and 70%eformation from the initial sample height of 10 mm. A typicalPA force–time curve of the pears with three peaks is shown

n Fig. 1. The terminologies of hardness and chewiness wereefined according to the methods of Alvarez et al. (2002). Hard-ess was defined as the second peak, expressed as N. Chewinessas calculated as hardness × cohesiveness × springiness. Cohe-

iveness was measured as the ratio of the positive force areauring the second compression to the positive area during therst compression (Alvarez et al., 2002). Springiness was defineds the ratio of the time from the start of the second area up to theecond probe reversal over the time to that between the start ofhe first area and the first probe reversal (Alvarez et al., 2002).ohesiveness and springiness are dimensionless, and the unit forhewiness is N. Brittleness was measured as the distance whichhe probe traveled to reach the first peak in the first cycle (Khin etl., 2007). The unit for brittleness is millimeters. Huanghua pearsan easily be soft during storage (Lin et al., 2003), so the TPAarameters of hardness, brittleness and chewiness which reflecthe characteristics related to fruit softening were selected to com-are the effects of the three coatings on reducing fruit softening.

.5. MR imaging experiments

The MR imaging technique was used to monitor the processf ripening in the pears that had been coated with different mate-ials at the end of the cold storage period. Five pears randomly

Fig. 1. A typical texture profile analysis (TPA) curve of Huanghua pears.

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Technology 49 (2008) 171–179 173

elected from each treatment were inspected. The experimentsere performed on a whole-body 1.5 T MRI scanner (Generallectric Medical Systems, Waukesha, WI) using a conventionalnee coil in the First Hospital of Shanghai Jiao Tong University.

cod-liver oil capsule was placed on the equatorial region ofach fruit and fixed using adhesive tape. The following parame-ers for T2-weighted sequences were used: repetition time (TR),000 ms; echo time (TE), 80 ms; number of acquisitions, 4; fieldf view (FOV), 10 cm; slice thickness, 2 mm; imaging matrix,56 × 256 pixels. We obtained five images of the slices with4 mm interslice gap. At each imaging session, the image of

he slice from the equatorial region of each pear in the trans-erse plane was selected for further study. Then, four regionsf interest (ROI) in the equatorial MR image of each pear werearefully located on four opposite sides within the area of theruit flesh. Each ROI had 2500 pixels (each pear area had about6,000–39,000 pixels depending on the fruit size) and the aver-ge of the pixel intensities of the pear images was analyzed.

.6. Quality evaluations

Five pears per treatment were used to determine solubleolids contents (SSCs), titratable acidity (TA), and ascorbiccid, initially and at the end of the cold storage period. SSCsere measured individually from the pressed juice of each

ruit by means of a hand-held sugar refractometer (WYT-; Chendu Optical Apparatus Co. Ltd., China). A previouslyescribed method was used to measure TA, and the resultsere expressed as percentage malic acid (Sumnu, 2000). The,6-dichloroindophenol titrimetric method was used to deter-ine the ascorbic acid contents (AOAC, 1990). The results were

xpressed in milligrams of ascorbic acid per 100 g fruit weightFW).

For taste analysis (acceptability), five fruit per treatment aspool removed from cold storage were equilibrated at room

emperature (22 ◦C) for 1 h before evaluation. They were thenleaned, peeled and cut into wedges for sensory evaluation. 12anelists (6 men and 6 women; aged 18–50 years) familiar withensory evaluation procedures, and consuming fresh pears ateast twice a month, were trained using a triangle test (Deng etl., 2005). All possible combinations were used for each pan-list. Taste analysis (acceptability) was subsequently performed,nd the taste was scored on a 9-point scale (1, extremely poor; 3,oor; 5, acceptable, limit of marketability; 7, good; and 9, excel-ent) (Deng et al., 2005). Three different pooled flesh samples ofears of each treatment were presented to panelists in randomrder. Each sample was identified with a three-digit number.anelists rated samples in individual rooms under incandescent

ight. Palates were cleansed with bottled water between samples.

.7. Statistical analysis

Statistical analyses were performed using SAS 8.0. Vari-

nce analysis was conducted for the results obtained from eachreatment. Duncan’s range test was performed for a statisticalomparison including both the effect of coating and the effectf storage time, with significance determined at the 5% level.

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. Results and discussion

.1. Respiration

The main characteristics of the respiration rates of theuanghua pears treated with different kinds of coatings are

hown in Fig. 2. According to the results, throughout the stor-ge period, the respiration rates of coated pears significantlyecreased (p < 0.05). These values were only 59.9–79.1% ofhose of the control samples at the beginning of the cold storageeriod. By day 60, the respiration rates of the control samplesere 1.25–1.36 times higher than those of the coated fruit. Pre-ious studies indicated that the gas exchange between fruit andhe atmosphere occurs partly by diffusion through open poresstomates, lenticels, stem, and blossom scars) and partly by per-eation through fruit skin (Bai et al., 2002), and that it occursainly through pores (Amarante et al., 2001). The data shown

n Fig. 2 indicated that all the coating treatments decreased theespiration rates of pears, which could be due to the partial oromplete blockage of pores.

The respiration rate is a good index for the quality of fruit dur-ng storage. Edible coatings lead to high carbon dioxide and lowxygen internal gas concentrations in coated fruit by loweringespiration rates, which contributes to longer shelf-life of fruitOzden and Bayindirli, 2002; Hagenmaier, 2005). High levels ofarbon dioxide in fruit restrict succinic dehydrogenase activitynd induce the accumulation of succinic acid, which finally leadso the inhibition of the Krebs cycle (Ozden and Bayindirli, 2002;eng et al., 2006). Additionally, low levels of oxygen suppress

ytochrome oxidase activity and play a role in the inhibition ofhe activities of oxidases such as ascorbic acid oxidase, polyphe-

ol oxidase, and glycolic acid oxidase (Ozden and Bayindirli,002). Clearly, these coating treatments contribute to the reduc-ion of vital activities, thus maintaining the quality of Huanghuaears during storage.

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ig. 2. Changes in the respiration rates of pears treated with different coatings duringears coated by shellac; “Semperfresh”, those coated by Semperfresh; and “CMC”,bove them for all treatments are not significantly different (p > 0.05).

Technology 49 (2008) 171–179

The Huanghua pear is a climacteric fruit (Lin et al., 2003). Ashown in Fig. 2, there was a climacteric rise in the respirationates of the control samples by day 60. However, the climac-eric rises of the coated pears were not obvious at the end oftorage. The reason for this phenomenon was that the coatingreatments restricted the respiration rates and decreased the vitalctivities of pears, which in turn induced the delay of the respi-atory climacterics of the coated fruit. Shellac coating is moreffective in reducing the respiration rates of fruit although theifference between the three coating treatments was not signif-cant (p > 0.05). This could be because shellac coating is morefficient in restricting the gas exchange between fruit and thetmosphere during storage.

.2. Weight loss

Compared with the control samples, the coated pears showedsignificantly (p < 0.05) reduced weight loss during storage

Fig. 3). After 60 days of storage, the fruit coated with shellacnd Semperfresh showed 5.82 and 6.94% weight loss, respec-ively, as compared to 8.47 and 7.84% weight loss in control fruitnd CMC-coated fruit; this difference was significant (p < 0.05).revious studies indicated that approximately 3–5% weight loss

eads to shrivelling in apples and approximately 5% weight lossas the normal acceptable limit for grapes (Bai et al., 2002; Deng

t al., 2006). However, in this study, all coatings reduced weightoss in pears during storage, and no shrivelling was observed inny treatment, including in the control samples.

Weight loss is caused by respiratory weight loss and evap-ration of water from the fruit (Amarante et al., 2001). Theain mechanism contributing to weight loss is the evaporation

f water activated by a gradient of water vapor pressure at dif-erent locations in fruit (Yaman and Bayo�nd�rl�, 2002). Wateriffuses preferentially through a liquid aqueous phase in theuticle, where water conductance is considerably higher, rather

60 days of storage at 4 ◦C. “Control” denotes the control pears; “shellac”, thethose coated by carboxymethyl chitosan (CMC). Values with the same letter

R. Zhou et al. / Postharvest Biology and Technology 49 (2008) 171–179 175

Fig. 3. Changes in the weight loss of pears treated with different coatings dur-ing 60 days of storage at 4 ◦C. “Control” denotes the control pears; “shellac”,t“l

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Fig. 4. Changes in the electrolyte leakages of flesh for pears treated with dif-ferent coatings during 60 days of storage at 4 ◦C. “Control” denotes the controlpears; “shellac”, the pears coated by shellac; “Semperfresh”, those coated bySVd

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he pears coated by shellac; “Semperfresh”, those coated by Semperfresh; andCMC”, those coated by carboxymethyl chitosan (CMC). Values with the sameetter above them for all treatments are not significantly different (p > 0.05).

han through pores (Ben-Yehoshua et al., 1985; Amarante etl., 2001). Generally, coatings which exhibit good water vaporarrier properties are highly hydrophobic in nature (Park et al.,004). In this study, the CMC and Semperfresh coatings inducedreater weight loss because they are more hydrophilic than thehellac coating. Water loss can cause flesh softening, fruit ripen-ng, and senescence by ethylene production and other metaboliceactions (Bai et al., 2002). Clearly, relatively lower weight lossn shellac-coated pears contributed to maintaining better qualityf fruit during cold storage.

.3. Relative electrical conductivity

Electrolyte leakage is an important indicator of plasma mem-rane integrity of flesh cells, which is related to the quality ofruit (Deng et al., 2005). According to Fig. 4, the relative electri-al conductivities of all the samples rose, corresponding to theecrease in plasma membrane integrity of flesh cells in pears dur-ng storage. This phenomenon was probably observed becausehe plasma membrane of fruit cells would tend to be unstablend consequently lose its integrity during storage (Feng et al.,005).

As shown in Fig. 4, the control samples generally had higherevels of electrolyte leakage than the coated pears. After 60 daystorage, the leakage rate of flesh cells reached 63.1% for the con-rol samples and 41.7–49.8% for the coated fruit. Furthermore,ompared to the other two coating treatments, shellac coatingas more effective in reducing the changes in the electrolyte

eakage of pear cells during storage. Kou et al. (2002) indicatedhat lipoxygenase, an important factor related to over-ripeningf fruit during storage, could accelerate tissue electrolyte leak-ge. The activity of lipoxygenase is positively correlated with

he respiratory intensity of Chinese jujubes (Kou et al., 2002).herefore, in this study, the lower respiratory rates caused byifferent kinds of coatings might have suppressed lipoxygenasectivity, reducing the changes in electrolyte leakage in pear cells.

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emperfresh; and “CMC”, those coated by carboxymethyl chitosan (CMC).alues with the same letter above them for all treatments are not significantlyifferent (p > 0.05).

urther, as shown in Fig. 4, shellac coating was more efficientn reducing changes in the plasma membrane of the pear cellsuring storage.

.4. Texture profile analysis

The TPA results of hardness, brittleness and chewinessecreased with increased storage time to varying degreesFig. 5). The hardness of Huanghua pears decreased signifi-antly, reaching 12.0–19.7% for all samples after 60 days oftorage. Pears exposed to long storage time lost 6.45–14.5% ofheir original brittleness at the end of the cold storage period.

similar phenomenon was also observed with the chewinessf pears during storage. Previous research indicated that theonsequence of disassembly of primary cell wall and middleamella structures of fruit flesh could contribute to changes inruit texture during storage (Yang et al., 2007). Hardness duringipening in climacteric fruit, such as Huanghua pears, is gener-lly attributed to degradation of the cell wall and loss of turgorressure in the cells reduced by water loss (Lin et al., 2003;ohani et al., 2004; Khin et al., 2007). The data indicated thatoating treatments may maintain hardness by inhibiting wateross (Fig. 5). Coating may also inhibit the activities of pectin-egrading enzymes closely related to fruit softening by reducinghe rate of metabolic processes during senescence (Conforti andinck, 2002; Zhou et al., 2007), which also contributed to fruitardness maintenance. Previous studies have reported a similarerformance of delaying softening by shellac coating in applesBai et al., 2002), by Semperfresh coating in quinces (Yurdugul,005), and by chitosan coating in citrus fruit (Chien et al., 2007).he decrease in brittleness shows that less distance was needed

o break the samples’ structure, which was mainly due to loss ofell turgor (Khin et al., 2007) and degradation of the cell wall.hewiness reflecting the continuing resistance of fruit flesh tohewing is closely related to structure and composition of cell

176 R. Zhou et al. / Postharvest Biology and

Fig. 5. Changes in the TPA results of hardness (a), brittleness (b), and chewi-ness (c) for pears treated with different coatings during 60 days of storage at4 ◦C. “Control” denotes the control pears; “shellac”, the pears coated by shel-lct

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ac; “Semperfresh”, those coated by Semperfresh; and CMC, those coated byarboxymethyl chitosan (CMC). Values with the same letter above them for allreatments are not significantly different (p > 0.05).

all in fruit (Pan and Tu, 2005; Yang et al., 2007). Accordingo the results (Fig. 5), the reduction of vital activities and wateross of coating treatments contributed to maintenance of the TPAarameters of pears during storage, even though the effects wereot obvious in the first 30 days storage.

In the current study, compared with the Semperfresh coat-

ng, shellac coating and CMC coating were more efficient ineducing changes in the TPA values of pear flesh during storagelthough the CMC coating induced greater weight loss than thehellac coating (Fig. 5). This may have been due to changes in

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Technology 49 (2008) 171–179

he cell wall structure and composition playing more importantoles in changes in the fruit texture than turgor loss due to theeduced moisture content during storage. However, the mech-nism behind this phenomenon is not well understood and isorth further investigation.

.5. MR imaging

Changes in the structure and chemical composition of fruitissue during storage are reflected in the changes in water char-cteristics in the fruit tissue (Letal et al., 2003). T2-weighted MRmages revealed the physical properties of water molecules andater distribution in fruit tissue. The T2-weighted MR imagesf Huanghua pears treated with different coatings after 60 daysf cold storage are shown in Fig. 6. The mesocarp tissue of pearsas clearly shown as regions with uniform gray intensity inter-

persed with darker strands that denoted vascular tissue. Thendocarp of five locules radiated from the central pear core. MRmaging also has a high sensitivity for detecting incipient brown-ng and flesh breakdown of fruit (Hernandez-Sanchez et al.,007), which could be caused by improper coatings (Park, 1999).ccording to the figure, no obvious physiological disorders werebserved in any treatment; this supports the idea that the stor-ge conditions and the coating treatments were appropriate foraintaining the quality of pears during storage.However, the image of the control sample (Fig. 6A) is brighter

han the images of other treatments (Fig. 6B–D). This haslso been shown by the data of the average intensity levelsf the pear images. The average of the pixel intensities ofhe control samples was 1.45 × 103, which was significantlyigher than 1.36 × 103 for shellac-coated pears, 1.39 × 103 foremperfresh-coated pears and 1.37 × 103 for CMC-coated pearsp < 0.05), and that there were no significant differences in theverage intensity levels between the three coated treatmentsp > 0.05). Furthermore, around the core of the control pear, thereere obvious small dark regions (Fig. 6A). It has been reported

hat the ripening process of fruit during storage often resultsn an increase in the amount of free water in regions of active

etabolism (Chen et al., 1989; Galed et al., 2004). Previoustudies indicated that the deterioration of the plasma membranentegrity of flesh cells induces high levels of free water in theruit tissue, resulting in higher proton densities that correspondo relatively brighter images of fruit (Lammertyn et al., 2003;ernandez-Sanchez et al., 2007). Free water gradually diffuses

o the fruit skin and evaporates, resulting in dry and brownpaces with lower intensities on the MR images (Lammertyn etl., 2003). Clearly, higher respiration rates and a greater extentf the loss of cell membrane integrity, which induced activeetabolism, led to higher levels of free water and a brighter MR

mage of the control sample. Higher weight loss was the reasonor darker regions around the core of the control pear (Fig. 6).

According to the figure, there were some differences betweenhe images of the coated pears (Fig. 6B–D). The MR images

f the shellac-coated and CMC-coated pears were darker thanhose of the Semperfresh-coated pears even though the differ-nce in the average intensity levels was not significant differentp > 0.05). The difference in the images between the shellac- and

R. Zhou et al. / Postharvest Biology and Technology 49 (2008) 171–179 177

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ig. 6. Comparison of the equatorial transverse T2-weighted images of pearsil capsules used to mark positions. (A) Control pear; (B) pear coated by shellaCMC).

MC-coated pears was not clear. Moreover, the vascular tissuef the Semperfresh-coated pears was not very clearly visiblen the image. These data support the idea that shellac coatingnd CMC coating are more effective in decreasing the free waterontent of pears during storage. The lack of free water resulted inower proton densities, which corresponded to darker images ofhe shellac- and CMC-coated pears after 60 days of cold storage.

.6. SSC, TA, ascorbic acid, and taste evaluation

The SSC and the TA and ascorbic acid levels in Huanghua

ears decreased in all treatments after 60 days of storageTable 1). Compared with the control samples, the shellac-oated pears had higher SSCs and TA and ascorbic acid levels,nd the difference was significant (p < 0.05). Soluble solids and

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able 1ffects of different coatings on the quality attributes of Huanghua pears initially and

reatments SSC (g/100 g) TA (g malic acid/1

arvest date 12.23 a 0.25 a

fter 60 days of storageControl 11.20 c 0.16 cShellac 11.73 b 0.22 abSemperfresh 11.43 bc 0.19 bcCMC 11.49 bc 0.18 bc

ean values within the same column followed by the same letter are not significantloated by shellac; “Semperfresh”, those coated by Semperfresh; and “CMC”, those c

0 days of storage at 4 ◦C. The circular features adjacent to fruit are cod-liver) pear coated by Semperfresh; and (D) pear coated by carboxymethyl chitosan

rganic acids of fruit are substrates that are consumed by res-iration during storage (Ozden and Bayindirli, 2002; Yamannd Bayo�nd�rl�, 2002). Ascorbic acid is primarily regulatedy ascorbic acid oxidase and phenoloxidase, whose activitiesre influenced by the oxygen contents in the storage conditionYaman and Bayo�nd�rl�, 2002). In this study, shellac coatingas more effective in the retention of SSCs and the TA and ascor-ic acid levels because of the lower gas permeability of shellacoating that inhibited the respiratory rates and retarded the over-ll metabolic activities of pears during storage. The decreases inaste scores of Huanghua pears were observed in all treatments

y day 60 (Table 1); the taste score was generally higher forhe coated pears than for the control samples, but there was noignificant difference in taste scores between the three coatingreatments (p > 0.05).

at the end of the cold storage period at 4 ◦C

00 g) Vc (mg/100 g FW) Taste score (1–9)

5.13 a 8.25 a

1.41 c 5.07 c2.27 b 7.25 b1.90 bc 7.00 b1.91 bc 6.67 b

y different (p > 0.05). “Control” denotes the control pears; “shellac”, the pearsoated by carboxymethyl chitosan (CMC).

1 gy and

4

plsacblcctarH

A

oTiYt

R

A

A

A

A

B

B

B

C

C

C

C

C

D

D

F

G

G

G

H

H

H

K

K

L

L

L

L

L

N

O

P

P

P

S

W

78 R. Zhou et al. / Postharvest Biolo

. Conclusion

Compared with the uncoated Huanghua pears, all the coatedears showed significantly reduced respiration rates and weightoss (p < 0.05) and delayed changes in the ripening parametersuch as permeability of the cell membrane, TPA, SSC, TA andscorbic acid levels, and taste (p < 0.05). However, Semperfreshoating was less effective in reducing the changes in cell mem-rane permeability than shellac coating, and CMC coating wasess effective in decreasing weight loss than shellac coating. Inomparison with Semperfresh coating and CMC coating, shellacoating was more effective in inhibiting metabolism and main-aining the quality of Huanghua pears during storage, which waslso supported by the MR images. On the basis of these data, weecommend the use of shellac coating to maintain the quality ofuanghua pears during long-term cold storage.

cknowledgements

This work is the main part of the project ‘Research and Devel-pment of Fresh Produce Modern Logistics Technology andrading Demonstration’ (2004BA527B) financed by the Min-

stry of Science and Technology of China. The authors thank L.P.an, M.S. Lin, X. Jin and H.R. Qiu for assistance in performing

he experiments.

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