effect of pe/ag2o nano-packaging on the quality of apple slices

EFFECT OF PE/AG2O NANO-PACKAGING ON THE QUALITY OFAPPLE SLICESjfq_385 171..176

LING ZHOU1, SINING LV1, GUIPING HE2, QIANG HE1,3 and BI SHI2,3

Departments of 1Food Science and Engineering and 2Biomass Chemistry and Engineering, Sichuan University, Chengdu 610065, China

3Corresponding author. TEL:+86-28-85405508; FAX: +86-28-85400356;EMAIL: [email protected] (Q. He),[email protected] (B. Shi)

Received for Publication November 1, 2009Accepted for Publication January 20, 2011

doi:10.1111/j.1745-4557.2011.00385.x

ABSTRACT

Effects of low-density polyethylene (LDPE) packaging and nano-structured silver-polyethylene (PE/Ag2O) packaging on the overall sensory quality, color, weight andsolute loss of fresh-cut apples stored at 5 and 15C were studied in this research.Results showed that, compared with normal LDPE packaging, the PE/Ag2O packag-ing helped to maintain the freshness of apple slices. Nano-structured PE/Ag2O bagdelayed apple browning and decreased the weight loss of apple slices during storage.In addition, it can also prevent apple slices from microbial spoilage. As a result, thequality of apple slices saved at 5C in PE/Ag2O bag was acceptable in 12 days, whilethose held in LDPE bag showed some deterioration in 6 days. Furthermore, consid-ering the potential health risk resulting from silver leak, the safety of PE/Ag2O bagwas evaluated, and the result indicated an acceptable safety for food packaging.

PRACTICAL APPLICATIONS

Types of nano-structured bags are recently commercialized for food packaging inmany countries, but few documents report their performance in quality mainte-nance. This study indicates that nano-structured PE/Ag2O bag is effective in main-taining the freshness of apple slices with extended shelf-life. Results of this work maybe useful for promoting the acceptance and application of similar nano-packaging inthe quality maintenance of fresh-cut products.

INTRODUCTION

Over the past two decades, fresh-cut apples have emerged as apopular snack in food service companies, school lunch pro-grams and for family consumption, as a result of consumerdemands for fresh, nutritious and convenient foods.However, because of active metabolism of apple tissues,damage during peeling and slicing, and exposure of cut sur-faces to external factors, fresh-cut apple slices usually haveshort shelf life with quality deterioration like browning,decay, softening and off-flavors (Mark Abbott et al. 2004;Hodges and Toivonen 2008). For these reasons, much atten-tion had been attached to extend the shelf life of fresh-cutapple, including modified atmosphere (MA) packaging,storage at low temperature and chemical means (Rupasingheet al. 2006; Chung and Moon 2009). Although these methodspromise good quality preservation, the maintenance of appleslices with sensory and nutritional fresh-like characteristics isstill a challenge during storage.

MA packaging with sealed polymeric films or trays is ofprime importance for fresh-cut apple produce. The gas com-position within the package is modified through the respira-tion of apple tissues, which depends on both storagetemperature and gas permeability of the packaging film.Many plastic materials are commercialized for food packag-ing. Among them, polyethylene (PE) is the most commontype for food surface contact because of its chemical resis-tance and inertness and high water barrier property (Peyches-Bach et al. 2009). Considering the susceptibility of fresh-cutapple slices to microorganisms, some antimicrobial agents areintroduced into the polymer film (Del Nobile et al. 2004). Inaddition to serving as gas and solute barrier, such active pack-aging films can also prevent apple slices from microbial spoil-age, and consequently improve the overall quality of productwith extended shelf life.

For many decades, silver nanoparticles have beenrecognized as an effective antimicrobial agent due to theirbroad-spectrum inhibitory activities. When silver (Ag)

Journal of Food Quality ISSN 1745-4557

171Journal of Food Quality 34 (2011) 171–176 © 2011 Wiley Periodicals, Inc.

nanoparticles are incorporated into polymer like PE, the par-ticles disperse evenly and keep nano-size into the film. SuchAg-incorporated film may have changed gas permeability andshow antibacterial activity,which is potentially useful in fresh-cut food packaging. A former study (Sanpui et al. 2008) dem-onstrated the effectiveness of Ag-containing nanocompositesagainst different microorganisms. Del Nobile et al. (2004)developed an Ag-containing nano-packaging system, andfound encouraging results in extending the shelf life of applejuice. An et al. (2008) extended the shelf life of asparaguscoated with silver nanoparticles dispersed in polyvinylpyrroli-done. Moreover, Li and Gao (2002) found that Ag-containingnano-packaging could induce the decomposition of ethyleneemitted from fruit metabolism, so fruit ripening caused by thegas was retarded with the extension of product shelf-life.Although many silver-based nanotechnologies showed posi-tive effect in the preservation of fruits and vegetables, fewstudies concerned the application of nano-packaging inquality maintenance of fresh-cut products. The present studywas aimed to compare and evaluate the total quality of fresh-cut apple slices packaged in a type of commercialized nano-structured silver-polyethylene (PE/Ag2O) bag and in normallow-density polyethylene (LDPE) bag.

MATERIALS AND METHODS

Materials and Treatment

“Fuji” apple was chosen for this study because of its extensiveuse and the rapid browning of tissue after cutting.Apples withuniform size, appearance and maturity were purchased froma local market, and they were washed and stored at 5C prior tothe cutting operation. Two types of commercially availablebags (self-sealed, 25 ¥ 35 cm2), authorized for food packag-ing, were used to pack fresh-cut apple slices. One is LDPE bag(thickness 100 mm, density 0.924 g/cm3, GB9687-88, YiGuo,Nanjing, China), and the other is nano-structured PE/Ag2Obag (ISO9001/KS A9001, Cleanaid, Goyang-si Gyeonggi-do,Korea).

Apples were processed in batches of eight fruits to mini-mize excessive exposure to air. Usually, a dip treatment withbrowning inhibitor like calcium ascorbate can help to main-tain the freshness of apple slice. However, in this research, wefocus on investigating the sole effect of nano-packaging, so no

dip treatment was applied. In one batch of treatment, eightapples were wedge-cut into eight equal slices per apple, andthen packaged in four LDPE bags and four PE/Ag2O bags. Ineach bag, there are eight slices, respectively, coming from theeight apples. The eight bags of apple slices were stored at5 � 1C and 15 � 1C in darkness, and then employed for theanalyses on one sampling day. Seven batches of apples wereprocessed in total and their analyses were carried out after 2,4, 6, 8, 12, 16 and 24 days of storage, respectively.

Sensory Evaluation

Overall sensory quality of fresh-cut apple slices was evaluatedimmediately after opening the packages, based on the senso-rial attributes of color, texture, off-odor and decay. A 9-pointhedonic scale, as shown in Table 1, was used for the evalua-tion. Samples for evaluation were coded with random three-digit numbers to mask the treatment identity, in order tominimize test subjectivity and to ensure test accuracy. Thequality evaluation was performed by a four-member expertpanel that was trained to recognize and score prior to test. Theoverall sensory quality of packaged apple slices was expressedas the average of sum from the four panelists, and a score of 7was considered the limit of quality acceptability.

Weight Loss Determination

Weight loss of fresh-cut apple slices was measured at the endof indicated storage period. The results were expressed as thepercentage loss of initial weight.

Color Measurement

Color measurements were made periodically with a Minoltacolorimeter (X-rite 8200, light source: pulsed xenon,port size:4.0/8.0/19.0 mm, standard observer: 10°, Hong Kong, China).A standard white reflector plate was used to calibrate theequipment.Measurementsweremadeimmediatelyafterappleslices were removed out from packaging bag.The residual coretissue of apple slices may brown intensely and unevenly,whichcan not reflect the real effect resulting from packaging. So, thecore tissue was avoided and the wedge surface center was intouch with the lens of colorimeter when taking L* readings.Each slice was read on two sides, giving a total of 16 values per

TABLE 1. HEDONIC SCALE FOR SENSORY EVALUATION OF PACKAGED FRESH-CUT APPLE SLICES

Sensory appearance Hedonic scale Score

Excellent, essentially free from defects Like extremely 9Fresh, slight browning, no off-odor Like moderately 7Firm texture, moderate browning, slight off-odor Neither like nor dislike 5Soft texture, strong browning, off-odor, mildew spots Dislike moderately 3Strong browning and off-odor, decay, extremely poor Dislike extremely 1

EFFECT OF PE/AG2O PACKAGING ON APPLE SLICES L. ZHOU ET AL.

172 Journal of Food Quality 34 (2011) 171–176 © 2011 Wiley Periodicals, Inc.

bag.Average of the L-values was used to characterize the light-ness and the browning extent of packaged apple slices.

Soluble Solid (SS) on the Surface ofApple Slice

The concentration of SS on the surface of apple slice was peri-odically determined during storage. It was conducted byadding 20 mL of distilled water into each bag of apple slices,followed by slight rinse and filtration treatment withcommon quantitative filter paper. Then, the SS content (%) ofthe filtrate was determined with a hand refractometer (RHB-32, HongHan, ShenZhen, China), and the value determinedwas employed to reflect the SS accumulated on the surface ofapple slices.

Silver Release Test

Silver leakage from PE/Ag2O bag was investigated to evaluatethe potential health-risk for food packaging. The filtrate col-lected for the determination of SS on apple slice surface, asdescribed above, was also employed for silver release test. Inorder to investigate the transfer of silver into apple tissue, therinsed apple slices were placed into a stomacher bag (SewardLimited, London, U.K.) and homogenized in distilled waterwith a stomacher (Seward Stomacher 400, London, U.K.).The homogenized apple slurry was subsequently digested atpH 3.0 and 90C for 2 h, and the digestion solution was usedfor silver analysis. All silver analyses were performed by usingan inductively coupled plasma spectrometer (ICP, Perkin-Elmer Optima 2100DV, Washington DC).

Statistical Analysis

Experimental data were analyzed by analysis of variance usingstatistical software SAS (Edition 8.0). The experiment wascarried three times, and the means and standard deviationswere the average of repetitions. Significance of differences wasdefined at P � 0.05.

RESULTS AND DISCUSSION

Sensory Quality of Packaged Apple Slices

The overall sensory quality of packaged apple slices wasevaluated to reflect the acceptability of the product and the

willingness-to-eat attitude of consumer. As shown in Table 2,there was a decline in quality over storage period, regardless ofpackaging materials and storage temperature, which wasmainly due to the respiration, browning, water loss andmicrobial spoilage of apple slices (Watada and Qi 1999;Hodges and Toivonen 2008). In fact, browning was found tobe a crucial sensory defect limiting the shelf-life of apple slicesduring storage, which should be targeted to improve thequality. The apple slices stored at 5C, in LDPE or PE/Ag2Obags, generally maintained freshness with higher qualityscores than those stored at 15C. Storage temperature is knownto be the most important factor influencing the quality offresh-cut products, and its effect on quality maintenance hasbeen well documented (Kader 2002). In general, low tem-perature may decrease the respiration rate and enzyme activ-ity of fresh-cut apple slices and therefore slow downphysiological deterioration. Reducing temperature may alsoreduce the growth of microorganisms and microbial-relatedspoilage of apple slices.

Although all the apple slices tested showed similar trend ofchange in overall sensory quality during storage, the productssaved in PE/Ag2O bag exhibited slower decrease of score thanthose in LDPE bag. The apple slices were visually acceptable(score higher than 7) for up to 12 days when held in PE/Ag2Obag at 5C. A visible change in color was observed in the case ofLDPE packaging, where a few browning spots could be foundon parts of apple slices when held at 5C for 6 days, though a7.1 of quality score was recorded meanwhile for the products.After 12 days of storage at 5C, the apple slices in LDPE bagappeared with a wide area of browning and produced a slightoff-odor, coinciding with an unacceptable quality for con-sumption. These observations indicate that the nano-structured PE/Ag2O packaging tested showed a betterperformance on maintaining the freshness of apple slicesthan normal LDPE packaging did, which can effectivelyextend product shelf-life when combined with low-temperature storage.

Weight Loss of Packaged Apple Slices

As discussed above, the overall sensory quality of fresh-cutapple slices depends on several quality sub-indices, amongwhich moisture and solute migration, respiration rate andbrowning reaction are the most important. Weight loss is a

TABLE 2. SENSORY QUALITY OF APPLE SLICES AS AFFECTED BY PACKAGING AND STORAGE TEMPERATURE

Packagingmaterial

Temperature(C)

Storage time (days)

0 2 4 6 8 12 16

PE/Ag2O bag 15 9 7.6 � 0.3 7.2 � 0.2 7 � 0.1 6.5 � 0.35 9 8.2 � 0.1 7.8 � 0.2 7.4 � 0.1 7.4 � 0.1 7.3 � 0.2 6.8 � 0.1

LDPE bag 15 9 7.3 � 0.2 7 � 0.1 6.8 � 0.35 9 7.3 � 0.1 7.3 � 0.2 7.1 � 0.1 7 � 0.2 6.6 � 0.2

L. ZHOU ET AL. EFFECT OF PE/AG2O PACKAGING ON APPLE SLICES


physiological event in fresh-cut fruits that can be limited byusing appropriate packaging and storage temperature. Theweight loss of packaged apple slices may be mainly attributedto two reasons. In the first case, moisture migration leads towater loss and secondly, the respiration of living tissue con-sumes some nutrients. Fresh-cut apple slices have high wateractivity and reducing water loss is most required to maintaintheir freshness. Therefore, the packaging material of appleslices should have suitable permeability to respiration gaseslike O2 and CO2 but very low permeability to water vapor.

Figure 1 shows the weight loss of apple slices packaged inLDPE and PE/Ag2O bags. Generally, both packaging materi-als behaved as good barrier to water vapor, which the weightloss tested kept at low values (<2%) even the loss increasedwith storage time. For both packaging materials, the appleslices stored at 5C had lower weight loss than those at 15C,because low temperature may reduce product respiration rateand slow down the permeation of water vapor. In comparisonwith LDPE bag, a significant (P < 0.05) decrease of weightloss occurred in PE/Ag2O bag in any case of storage tempera-ture. After 12 days of storage at 5C, the weight loss of appleslices in LDPE and PE/Ag2O bags was 0.26 and 0.05%,although the weight loss occurred at 15C was 0.78 and 0.46%,respectively. Afterwards, the weight loss of apple slices inLDPE bag kept increasing, whereas no substantial change inweight loss was found in PE/Ag2O bag after 16 days of storageat 5C and 15C.

Besides acting as a barrier to water vapor, a proper sealedpackaging for apple slices is also expected to generate a MAcapable of achieving a reasonable extension of shelf-life byslowing down tissue respiration (Jacxsens et al. 2000; Soliva-Fortuny et al. 2004). As two types of commercialized packag-

ing materials, LDPE and nano-structured PE/Ag2O bagsshowed varied performance in reducing the weight loss ofapple slices during storage, which might be attributed to theirdifferent properties such as polarity, permeability and barrierproperties (Dirim et al. 2004; Peyches-Bach et al. 2009). Allthe observations above suggest that in comparison withnormal LDPE packaging, the PE/Ag2O packaging testedmight behave better on maintaining the freshness of appleslices by decreasing the water loss and respiration rate ofapple.

Color Changes in Packaged Apple Slices

Discoloration usually occurs on the cut-surface of apple slicesduring storage, which degrades sensory property and nutri-tional value and discourages the consumer purchase of prod-ucts. Enzymatic browning is a major problem leading to colorchange of apple slices due to the reaction of phenolic com-pounds with oxygen, catalyzed by endogenous polyphenoloxidase (PPO) (He and Luo 2007; He et al. 2008). In addition,microbial spoilage also has unfavorable color effects, becauseit may result in the formation and accumulation of pigmenton the cut-surface of apple slices. In this study, parameter L*was applied to investigate changes in color of apple slicespackaged in LDPE and PE/Ag2O bags over storage time, and adecrease in L* value means an increase in darkness.

All apple slices tested showed a trend of increase in dark-ness, as the L* values decreased over time (Fig. 2). The differ-ent color appearances of apple slices at 5 and 15C indicate thatthe low temperature induced a significant (P < 0.05) slow-down of discoloration due to the inhibition of PPO activityand microbial-related darkening. Apple slices were found to

0 5 10 15 20 25

0.0

0.5

1.0

1.5

2.0

Wei

ght l

oss

(%)

Time (d)

FIG. 1. WEIGHT LOSS OF PACKAGED APPLE SLICES(�: PE/Ag2O film, 5C; �: LDPE film, 5C; �: PE/Ag2O film, 15C; �: LDPEfilm, 15C).

0 4 8 12 2470

72

74

76

78

80

L v

alue

Time (d)

FIG. 2. LUMINOSITY (L*) VALUES OF PACKAGED APPLE SLICES(�: PE/Ag2O film, 5C; �: LDPE film, 5C; �: PE/Ag2O film, 15C; �: LDPEfilm, 15C).



brown quickly when presented without any packaging (datanot shown). In the experiment, LDPE and PE/Ag2O bagshelped to retard the discoloration of apple slices, and thePE/Ag2O packaging worked more effectively both at 5 and15C. As a result of apple respiration, the O2 in package wouldbe consumed together with the production of some othergases like CO2 and ethylene. Oxygen may speed up the respi-ration and ethylene is related to the ripening process andtissue injury. Thus, PE/Ag2O packaging may reduce applebrowning by acting as O2 barrier. Ethylene may induce a risein phenylalanine ammonia lyase activity in apple tissues,which would facilitate the synthesis of phenolic compounds,thus resulting in increased browning (Raybaudi-Massiliaet al. 2007). For this reason, the Ag-containing nano-packaging can also reduce apple browning by inducing thedecomposition of ethylene produced in package (Li and Gao2002). In addition, as both silver nanoparticles andAg-containing nanocomposites exhibited broad-spectrummicroorganism inhibitory activities (Del Nobile et al. 2004;Fernandez et al. 2009), thus packaging with nano-structuredPE/Ag2O bag may also retard microbial-related discolorationof apple slices. Considering the fact that browning inhibitorslike calcium ascorbate are widely used in fresh-cut fruits (Heand Luo 2007), the PE/Ag2O bag tested is supposed to keep thecolor of apple slices more effectively when it works withbrowning inhibitors.

Soluble Solid (SS) Accumulation on theSurface of Packaged Apple Slices

Cutting treatment of apple may release cellular fluids andinduce SS to accumulate on the surface of apple slices. Thisrelease means solute/nutrient loss, and also provides mediaupon which microorganisms, if present, may survive andgrow. Thus, low content of SS is expected on the surface ofapple slices in fresh-cut produce. SS accumulation on appleslice surface varies with many factors during storage, such astissue respiration, SS oxidation degradation and microbegrowth, as affected by temperature and packaging conditions.

As presented in Fig. 3, the four curves showed similar SSchange trend. Generally, SS content on the surface of appleslices increased at the first phase of storage (Fig. 3) due to cel-lular fluid release resulting from fresh-cut processing. Then,the SS content gradually decreased to a stable value, whichmay be ascribed to two effects. First, the self-healing ofdamaged tissues slowed down the release of cellular fluids,and second, the SS that accumulated on the surface of appleslices might be consumed because of enzymatic (or oxidative)degradation or microbial utilization. Between LDPE andPE/Ag2O packaging, there is no substantial difference in SScontent on the surface of apple slices during storage.However, after 8 days of storage, the apple slices held at 15Cappeared with lower surface SS content than those held at 5C,

suggesting that more degradation or microbial utilization ofSS may occur at higher temperature on the surface of appleslices.

Silver Leakage from PE/Ag2O Bag

The World Health Organization (WHO) guidelines fordrinking-water quality indicate that high levels of silver, up to0.1 mg/L, can be tolerated without risk to health (WHO2003). The US Environmental Protection Agency (USEPA)also proposed that the secondary maximum contaminantlevel of silver ion in drinking water must be less than0.10 mg/L (USEPA 2006). Many data are available concerningthe safety of silver, and in fact different silver nanotechnolo-gies (i.e., AlphaSan, AgION, Zeomic, etc.) are approved by theEuropean Food Safety Authority and Food and Drug Admin-istration, and are being employed in aseptic surfaces. In thiswork, the silver leakage of PE/Ag2O bag tested was analyzed toevaluate its potential health risk for food packaging. Thehighest concentration of silver ion, released from the nano-structured PE/Ag2O bag, was determined to be less than10 mg/L, in accordance with the requirement of WHO andUSEPA. In addition, apple slices were digested for Ag analysisin order to investigate the transfer of silver into inner tissue,and the value was found to be lower than 2 ¥ 10-4 mg per gramof apple, suggesting a negligible presence of silver in the innertissue of apple slices. All observations indicate that thePE/Ag2O bag can be used for food packaging with acceptablesafety.

CONCLUSIONS

Unlike intact fruits, fresh-cut apple slices usually have shortshelf life because of tissue damage and exposure of cut

0 2 4 6 8 10 12 141.0

1.5

2.0

2.5

3.0

SS

(%

)

Time (d)

FIG. 3. SOLUBLE SOLID CONTENT ON THE SURFACE OF PACKAGEDAPPLE SLICES(�: PE/Ag2O film, 5C; �: LDPE film, 5C; �: PE/Ag2O film, 15C; �: LDPEfilm, 15C).

L. ZHOU ET AL. EFFECT OF PE/AG2O PACKAGING ON APPLE SLICES


surface. Apple slices are prone to deterioration like browning,decay and off-flavors during storage, thus its quality mainte-nance with sensory and nutritional fresh-like characteristicsis a challenge in fresh-cut produce. This study shows that theapple slices packaged in nano-structured PE/Ag2O bagappeared with better overall sensory quality, lower weight lossand slower discoloration during storage period, in compari-son with those in normal LDPE bag. Further, ICP analysisconfirmed the safety of the PE/Ag2O bag for food packaging.Then, the conclusion can be drawn that the nano-structuredPE/Ag2O bag is acceptable for fresh-cut packaging and it canhelp to maintain the freshness of apple slices. The PE/Ag2Obag may be effective to extend the shelf-life of apple sliceswhen it works with browning inhibitors.

ACKNOWLEDGMENTS

This research was financially supported by the SpecialFund for Agro-scientific Research in the Public Interest(hykj200803033), the Program for New Century ExcellentTalents in University (NCET080367) and Sichuan YouthSci-tech Fund (2010JQ0009). We also give our thanks toDr Yaguang Luo (Produce Quality and Safety Lab, USDA)for kindly providing PE/Ag2O bags.

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effect of pe/ag2o nano-packaging on the quality of apple slices

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