influence of whey protein composite coatings on plum fruit quality

Influence of Whey Protein Composite Coatingson Plum (Prunus Domestica L.) Fruit Quality

Elsy Reinoso & Gauri S. Mittal & Loong-Tak Lim

Received: 31 May 2007 /Accepted: 1 August 2007 /Published online: 14 September 2007# Springer Science + Business Media, LLC 2007

Abstract This study evaluated the quality of plums(Prunus domestica L.) coated with whey protein isolate(WPI) and WPI composite coatings containing 5 or 10%(w/w) flaxseed oil blended with beeswax. WPI and 10%lipid composite coatings were less susceptible to crack,flake, and blister defects during the 15 days storage at 5°Ccompared to the 5% lipid formulation. The firmness ofplums, determined by the penetration force using a 10-mmprobe, was not significantly affected by the coating typesexcept for the WPI-coated samples, which showed asignificantly higher penetration force because of the higherstrength for WPI film. Mass loss of plums during storagewas substantially reduced because of coating, especiallywhen coatings of higher lipid content were used. This wasconsistent with the water permeability for the standalonefilms, which decreased considerably when flaxseed andbeeswax were added. The incorporation of lipid phase toWPI also significantly weakened oxygen barrier andmechanical properties. Migration of plasticizer and lipidphase to the film surface was observed during water vaporpermeability tests, especially when the films were exposedto elevated humidity conditions. Overall, sensory evalua-tion showed that the coated plums were more acceptablethan the uncoated controls.

Keywords Whey protein isolate films . Lipid compositefilms . Beeswax . Flaxseed oil . Barrier and mechanicalproperties . Plum . Edible coatings

Introduction

An edible film is a continuous thin layer of edible polymer,which can be applied as a coating or used as a standalonemembrane for improving food quality. It is often used topreclude unwanted mass transports (e.g., moisture, oxygen,aroma, and flavor), enhance visual attributes (e.g., gloss andcolor), and function as a carrier to deliver active materials(e.g., antimicrobial agents and nutraceuticals). Over the pastdecade, edible films have been subjected to intensiveresearch because of their potential for reducing the use ofsynthetic thermoplastics in food packaging.

Edible films can be derived from proteins, polysacchar-ides, lipids, and resins. By and large, proteins have receivedthe most attention in edible film research because of theirabundance as agricultural by-products and food processingresiduals. The presence of reactive amino acid residualsalso enable protein to be modified and crosslinked throughphysical and chemical treatments to produce novel poly-meric structures (Gennadios 2002; Lundblad 2005). Wheyproteins from bovine milk have been studied to a greatextent because of their ability to form transparent andflexible films, which exhibit good barrier and mechanicalproperties (Krochta 2002). Whey proteins are globularproteins, which remain soluble after precipitation of caseinat pH 4.6 during cheese making. In bovine milk, thesethermal-labile proteins consist of mainly α-lactalbumin, β-lactoglobulin, and other proteins present in smaller fractions(e.g., bovine serum albumin, immunoglobulins, protease-peptones). Whey proteins are commercially available as

Food Bioprocess Technol (2008) 1:314–325DOI 10.1007/s11947-007-0014-1

E. ReinosoIovate Health Sciences Research Inc,5100 Spectrum Way, Mississauga, ON L4W 5S2, Canada

G. S. MittalSchool of Engineering, University of Guelph,Guelph, ON N1G 2W1, Canada

L.-T. Lim (*)Department of Food Science, University of Guelph,Guelph, ON N1G 2W1, Canadae-mail: [email protected]

whey protein isolates (WPI) or whey protein concentrates(WPC), which have protein contents of >90 and 20–85%,respectively (Khwaldia et al. 2004).

Studies on whey protein films have been mainly focusedon improving their barrier and mechanical propertiesthrough optimization of protein denaturation conditions(e.g., temperature, pH, time, and protein content), additionof cross-linking agents, and judicious use of plasticizers andadditives. Perez-Gago and Krochta (2001) studied the effectof denaturation conditions and concluded that denaturationof WPI at 90°C for 30 min provided stronger films interms of tensile properties than films cast from solutionsdenaturized at lower temperatures and for shorter heatingtime. Shaw et al. (2002b) compared the effect of glycerol,xylitol, and sorbitol plasticizers for forming WPI films andfound that glycerol imparted the best stress-strain propertiesto the film. Because of the hydrophilic nature of wheyprotein, various lipids have also been added to the film-forming solution to reduce the water-sensitivity of the films(McHugh 2000; Shaw et al. 2002a). Materials such asbeeswax, candelilla wax (McHugh 2000; Shellhammer andKrochta 1997), and soy oil (Shaw et al. 2002a) wereinvestigated, among which beeswax is most effective forreducing the water vapor permeation rate.

To date, end use applications of whey protein films aremainly targeted for use as carriers for antimicrobial agents(Min et al. 2005, 2006; Min and Krochta 2005; Seydim andSarikus 2006) and as protective barrier coatings to increasethe shelf-life of food products (Cisneros-Zevallos andKrochta 2003; Martin-Diana et al. 2006; Moldao-Martinset al. 2003). Several studies concluded that whey proteincoatings, when applied to fresh fruits, vegetables, nuts, andmeat, are capable of extending their shelf life (Le Tien et al.2001; Lee et al. 2003; Lee and Krochta 2002; Martin-Dianaet al. 2006; Perez-Gago et al. 2003b).

Because whey protein films are edible, they are idealcarriers for nutraceuticals to enhance the nutritional valueof the coated food product. Properties of calcium caseinateand WPI films, added with vitamin E (α-tocopherylacetate) and a mixture of calcium lactate and calciumgluconate, were investigated by Mei and Zhao (2003). Theyreported that high nutraceutical contents can compromisetensile and permeability properties of the film; however,these properties are mainly influenced by the nature of thecoating materials and the type and concentration ofnutraceuticals incorporated into the coating formulations.In another study, fresh and frozen strawberries andraspberries were coated with chitosan added with calciumand Vitamin E to increase the nutritional values of thesefruits (Han et al. 2004). These authors showed that chitosancoatings extended the shelf-life of fresh strawberries andraspberries by reducing the weight loss. Furthermore,changes in properties such as acidity and color were

delayed during storage. Edible coatings also have beenused as oxygen barriers and carriers for antibrowningagents to delay the browning of apple and potato slices(Le Tien et al. 2001; Lee et al. 2003).

In this study, the main objective was to develop an edibleWPI coating for plum (P. domestica L) to improve itsquality and nutritional value by incorporating flaxseed oilto the coating formulation. Plum was chosen for this studyover other fruits because: (1) its skin can be consumedtogether with the flesh and therefore provides an opportu-nity for carrying nutraceuticals; (2) it has a relatively shortshelf life, and (3) its dull skin appearance can be improvedto enhance consumer appeals. Flaxseed oil was used in thisstudy because it has been recognized as a significant sourceof omega-3 fatty acids, which is important for stroke andheart disease prevention (Health Canada 2006). Further-more, the lipid phase also reduces the hydrophilicity of thecoating to enhance its water barrier properties.

Materials and Methods

Materials

Plums were obtained from a local supermarket from thesame batch. Visual inspection was conducted to ensureconsistent ripeness and absence of major defects or physicaldamages that could interfere with the experiments. Plumswere stored in a refrigerator at 5°C. WPI was supplied byDavisco International, Inc. (BiPro, Le Seur, MN, USA)with a protein content of 93.2%. USP grade glycerol wassupplied by Fisher Scientific (Nepean, ON, Canada).Flaxseed oil was supplied by Heartland (Toronto, ON,Canada) as received without further purification. Polyoxy-ethylene sorbitan monostearate was supplied by SomersetCompany (Tween 60, Renton, WA, USA) with a purity>95%. Beeswax was supplied by Somerset Company(Renton, WA, USA) with a purity >98%.

Coating Preparation

The film-forming solutions were prepared after the proce-dure of Banerjee and Chen (1995), which allowed a quickand uniform dispersion of WPI powder in water. Theprocess involved adding WPI to ice-water (1:1 w/w)mixture that had been preblended for 1 s using a Braunhand-held blender (Model MR400, Kronberg, Germany).The protein and ice-water mixture was blended in theblender for 15 s, followed by manual mixing for 30 s, andfurther blended for 15 s. The solution was then stirred usinga magnetic stirrer for 8 min to ensure the formation of auniform solution. To denature the WPI, the solution washeated at 90°C for 30 min in a Cole-Parmer water bath

Food Bioprocess Technol (2008) 1:314–325 315315

(Model 12501-01, Montreal, QC, Canada) and then cooleddown to room temperature. At this point, glycerol was addedat a ratio of 1 g of glycerol per g of protein. To prepare thesolution for lipid composite film, the WPI/glycerol solutionwas heated to 70°C and Tween 60 emulsifier was added.Beeswax was melted separately and added to flaxseed oil at70°C. The resulting lipid blend was then added to the WPI/glycerol while providing mixing with a blender. Theformulations of the coating are shown in Table 1.

Coating Procedure

Plums were previously washed using tap water and thenallowed to dry at room temperature. Coating was applied bydipping each plum into the solution for 5 min and drained.After the coating process, plums were dried using a fan(Airworks Model FFH2, Markham, ON, Canada) at an airvelocity of 1.5 m/s for 30 min at 21±2°C. Coated plumswere stored at 5°C for 15 days.

Film Casting

Film-forming solutions were cast on Plexiglas plates of17×30 cm. The total solids content per area was maintainedat 0.028 g/cm2 to minimize thickness variations. Castsolutions were allowed to dry on a leveled granite surfacefor 48 h at 23±3°C. Dried films were then peeled intact.

Tests on Coated Plums

Coating Stability

The purpose of this test was to evaluate the visual quality ofthe coatings during storage at 5°C. Three main defectsoccurred were identified: (1) flaking—presence of flakes,which can be dislodged readily from the plum surface; (2)blistering—local loss of adhesion and lifting of coatingfrom the plum skin; and (3) cracking—breaks or splits inthe coating surface. These defects were visually evaluatedafter 2, 6, 9, 12, and 15 days of storage using the criteriagiven in Table 2.

Firmness

Puncture tests were conducted for the coated and controlsamples to determine the effects of coating on fruit firmnessat the end of 15 days storage at 5°C. Firmness wasmeasured using an Instron Universal Testing Machine(Instron Corp., Model 1122, Canton, MA, USA) equippedwith a 10-mm diameter cylinder probe, which was set topenetrate the sample for 20 mm. The maximal forcerequired to penetrate the fruit was measured for 15 plumsfrom each treatment. The test was repeated twice bypuncturing once on each opposite face of the fruits. Toevaluate the effect of coating on the firmness of fruit, theplums coated with WPI were divided into two groups. Onegroup was tested with the intact coating (WPI), whereas theother group was tested with the coating peeled off (WPI-P).

Mass Loss

Sample sizes of 18 fruits per treatment, including thecontrol samples, were used for mass loss evaluation. Theplums were weighed at 0, 2, 6, 9, 12, and 15 days duringstorage. The mass loss was expressed as percentage loss ofinitial mass. The precision balance used for weightmeasurement had an accuracy of 0.1 mg.

Table 2 Criteria to evaluate film defects on plums applied with WPIand WPI-FS coatings

Rank Flaking Cracking Blistering

None No visualevidence offlaking

No visualevidence ofcracking

No visualevidence ofblistering

Slight One spot isaffected with anarea less than10% of theplum surfacearea

Presence of asmall crack

Presence of asmall blister(<1 mm)

Moderate More than onespot whereflakingoccurred butaffects less than20% of theplum surfacearea

Small cracksthat affect lessthan 20% ofthe plumsurface area

Presence ofblisters thataffect less than20% of theplum surfacearea

High More than onespot whereflakingoccurred thataffects morethan 20% of theplum surfacearea

Cracks affectmore than20% of theplum surfacearea

Presence ofblisters thataffect morethan 20% of theplum surfacearea

Table 1 Coating formulations used for coating plums

Formulation WPI (%) WPI-5FS(%)

WPI-10FS(%)

WPI 10 9.2 8.7Protein 9.1 8.6 8.1GLY 9 9 8FS 0.0 5.0 10.0Emulsifier (Tween 60) 0.20 0.20 0.20BW 0.00 0.50 1.00

WPI Whey protein isolate, GLY glycerol, FS flaxseed oil, BW beeswax

316 Food Bioprocess Technol (2008) 1:314–325

Sensory Analysis

Sensory evaluation of the coated and control samples wasconducted on the 15th day of storage using a sensory panelconsisted of 29 individuals with age ranging from 20 to45 years old. Panel members were previously screened forallergic reactions to the ingredients used in the coatings.Samples were removed from refrigeration 1 h before thesensory evaluation to allow them to equilibrate to roomtemperature. Coded samples were presented to the panelistwho was asked to score appearance, overall impression,flavor, sweetness, and firmness using a hedonic scale of 0–10 (0—dislike; 10—like). The panel was also asked if theuse of edible films and the incorporation of nutraceuticalsin coating would influence their purchase decision.

Tests on Films

Tensile Properties

Films were cut into dumbbell shape specimens (24×110 mm) using a sharp razor blade to introduce a stressconcentration in the middle section. The narrow section wasused to calculate the cross-sectional area. The specimenswere conditioned at 23±2°C and 50±5% relative humidity(RH) for 48 h before testing.

Tensile test was conducted in accordance with ASTMmethod D882-02 (ASTM 2002). The film strips wereattached to the pneumatically actuated grips of an Instrontester. The crosshead speed was set at 200 mm/min, and theinitial gauge length was 70 mm. The test was performed at23±2°C and 50±5% RH. Seven samples per film type weretested, and the average values were reported.

Water Vapor Transmission Rate

Water vapor transmission rate (WVTR) was determinedbased on ASTM E96/E96M-05 (ASTM 2005b) usinganhydrous calcium sulfate as a desiccant. Twenty gramsof the desiccant were placed in each aluminum dish withflanged rim, on which the test films were sealed usingmolten wax. Samples were placed in controlled environ-ment chambers set at 23±2°C and RH of 30, 43, 60, and75%. Samples were weighed within an accuracy of0.0001 g at 0, 6, 14, 20, 24, 38, 44, 62, 68, and 86 h.Three replicates for each film were tested, and the meanvalues were reported.

Oxygen Transmission Rate

Oxygen transmission rate (OTR) values were determined inaccordance with ASTM D3985-05 (ASTM 2005a) usingOx-Tran 2/20 modular system (Modern Control Inc.,

Minneapolis, MN, USA). The films were previouslyconditioned at 45% RH before testing. To prevent thecoulometric sensor from overloading, the films weremasked with aluminum foil to reduce the exposed area to24.6 cm2. The permeation test was conducted isostaticallyby sandwiching the films in the permeability cell, where theupstream side of the film was exposed to a continuous flowof air (21% oxygen) and the downstream side to a carriergas of 2% H2/98%N2 gas mixture. The test was conductedat 50% RH, and two replicates for each film type wereevaluated. Oxygen permeability was calculated as follows:

PO2 ¼ OTR

p:t

where OTR is in cm3/(m2 d), P is partial pressure of oxygen(21.3 kPa), and t is film thickness in mm (average of fivereadings).

Statistical Analysis

Statistical analysis was performed using SPSS Release 14.0software (Chicago, IL, USA). Analysis of variance andmultiple comparison test (Duncan) were applied to analyzethe data for coating stability, sensory analysis, mass loss,firmness, oxygen transmission, and tensile properties.Analysis of covariance was applied to water transmissionrate data to determine difference between slopes. Allstatistical analyses were conducted at a significant level ofp=0.05.

Results and Discussion

Coating Stability

During the 15-day storage at 5°C, WPI-5FS coatingexhibited the highest incidence of defects, whereas WPIcoating produced the least (Figs. 1, 2, and 3). We observedthat cracking and flaking tended to occur concurrentlybecause the formation of cracks also resulted in a pooradhesion between the coating and the plum skin, which ledto flaking (Fig. 4). In contrast, the presence of blisters didnot correlate with the formation of cracks (Fig. 5).Nevertheless, blistering could lead to flaking, and therewas a significant interaction between cracking and flaking.Cracks and flakes not only substantially diminished thevisual appeal of the fruit but also compromised the abilityof the coating to protect the fruit.

The poor stability of WPI-5FS coating during storage at5°C may be attributed to the migration of oil to the coatingsurface. This created voids in the protein matrix and led tomaterial shrinkage, causing the coating to dislodge from theskin surface (blistering) and also promote cracking. The


stability of the film-forming emulsion was also importantfor achieving a uniform coating during the dipping anddrying processes. Emulsions that were unstable couldundergo phase separation, which introduced considerablecoating thickness variation when applied to the plumsurface. The progressive development of coating defectsduring storage for the WPI-5FS-coated plums appeared tosupport these hypotheses.

The better coating stability of WPI-10FS than WPI-5FScan be attributed to the higher content of the lipid phase inthe former formulation. Beeswax is made up of mainly waxacid esters, wax acids, and hydrocarbon, the viscousproperties of which are primarily because of free fatty acidfraction (Shellhammer et al. 1997). It has been shown thatbeeswax is capable of plasticizing whey protein film, whichis hypothesized to be caused by the free fatty acids fractionin beeswax (Talens and Krochta 2005). We speculated thatthe enhanced plasticization caused by the additional

beeswax, along with the increased hydrophobic interactionof the lipid phase with the cuticle layer of the skin, mighthave contributed to the better coating stability of WPI-10FScompared to WPI-5FS.

Fruit Firmness

After 15 days of storage at 5°C, the effect of coating onfruit firmness was insignificant, except that plums with theintact WPI coating exhibited a significant higher firmnessvalue (Table 3). This can be attributed to the higher tensilestrength of WPI films in comparison to other films(“Tensile Properties”). This observation agreed with a studyconducted by Perez-Gago et al. (2003a) in which plumswere coated with hydroxypropyl methylcellulose-lipidedible composite films. They reported that the firmnesswas similar between the coated and control fruits after6 weeks of storage at 1°C. This effect was attributed to the

Fig. 2 Frequency of film flak-ing during 15 days storage at5°C for plums coated with WPI,WPI-FS, and WPI-10FS formu-lations. Refer to Table 1 forformulation codes

Fig. 1 Frequency of film crack-ing during 15 days storage at 5°Cfor plums coated with WPI,WPI-FS, and WPI-10FS formu-lations. Refer to Table 1 forformulation codes


reduced respiration rate of the fruit under cold conditionand that the natural waxes present on the skin wereeffective in preventing the moisture loss. However, whenplums were exposed to 20°C, the coated fruits weresignificantly much firmer than the uncoated control, whichwas attributed to low O2 modified atmosphere created bythe edible film. Under the modified atmosphere condition,respiration rate of the fruit was reduced, and consequently,plum softening decelerated (Perez-Gago et al. 2003a).Thus, the effects of edible coating on plum firmness areexpected to be more pronounced during prolonged storage,especially at elevated temperatures.

Mass Loss

Mass loss was substantially reduced during storage for thecoated plums (Fig. 6). Moreover, the mass loss rates (slope

of the plots) were different. Uncoated plums and plumscoated with WPI exhibited the highest mass loss rate of∼0.0074 g/day. On the other hand, plums coated with WPI-5FS and WPI-10FS showed lower mass loss rates of 0.0061and 0.0055 g/day, respectively. The reduced mass loss ratefor the composite coatings was attributable to the reducedmoisture loss because of the increased film hydrophilicityimparted by flaxseed oil and beeswax. This hypothesis wassupported by the lowering of water vapor transmissionvalues when flaxseed oil and beeswax were added to thecoating formulation (“Water Vapor Permeability”).

After 15 days of storage at 5°C, control samples had amass loss of 3.7% compared to plums coated with WPI-10FS, which lost 2.7% of their original mass. Although thedifference in mass loss was relatively small under theconditions tested, the mass loss is expected to become moreprominent for the uncoated plums if the storage duration

Fig. 4 Photo showing cracking and flaking of WPI-5FS coatingapplied to a plum after 15 days of storage at 5°C. Refer to Table 1 forformulation codes

Fig. 3 Frequency of film blis-tering during 15 days storage at5°C for plums coated with WPI,WPI-FS, and WPI-10FS formu-lations. Refer to Table 1 forformulation codes

Fig. 5 Photo showing blistering of WPI-5FS coating applied to aplum after 15 days of storage at 5°C. Refer to Table 1 for coatingformulation codes


were to be extended and the temperature increased. Thishas commercial significance because mass loss is one of thecritical quality degradation factors for fresh produce.

Tensile Properties

The tensile strength of WPI film was around three timeshigher than the other two composite films (Table 4). Theaddition of the lipid phase dramatically weakened the filmstrength, which is in agreement with the results reportedpreviously (Banerjee and Chen 1995; Cisneros-Zevallosand Krochta 2003; Mei and Zhao 2003). The dispersedflaxseed oil-beeswax in WPI matrix and the crystallinenature of beeswax might have contributed to the reducedfilm strength because of the heterogeneous structure and thelack of interfacial interaction between the dispersed lipidand continuous WPI phases.

In this study, no significant difference was observed ontensile strength of films containing 5% (WPI-5FS) and 10%

(WPI-10FS) flaxseed oil, although the elongation valueswere significantly different among the three formulations(Table 4). Overall, WPI-10FS films showed the lowestmechanical properties, which appeared to be inconsistentwith the coating stability data. The relatively highertemperature used during film testing compared to thestorage temperature for the coated plums may be anotherreason for this inconsistency. In addition, the tensileproperties of the standalone films determined here werelikely not adequate for reflecting the mechanical propertiesof the coating because of film–skin interaction for the latter.

Water Vapor Permeability

The addition of lipid phase significantly depressed theWater vapor permeability (WVP) values (Fig. 7). Forexample, at 30% RH, the average WVP values for WPIfilm decreased from 0.91 to 0.75 and 0.29 g·mm/h·m2·kPawhen 5 and 10% flaxseed oil fractions were added,respectively. Similar trends were observed for 43 and 60%RH. Overall, the WVP values for the WPI and WPIcomposite films were in the same order of magnitude as

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 2 4 6 8 10 12 14 16

Time [days]

Mas

s lo

ss [

%]

Control

WPI

WPI-5FS

WPI-10FS

Fig. 6 Mass loss of uncoatedand coated plums during a 15-day storage at 5°C

Table 3 Firmness of uncoated and coated plums measured bypuncture test

Sample Force (N)

Uncoated plums 6.5±0.8bWPI-Pa 6.0±1.4bWPI 8.5±2.0aWPI-5FS 6.5±1.1bWPI-10FS 6.0±0.9b

Means with the same letter are not significantly different.a Plums coated with WPI where film was removed for the test

Table 4 Tensile properties of WPI and WPI-FS films

Film Thickness(mm)

Tensile strength(MPa)

Elongation atbreak (%)

WPI 0.229 6.10±0.18a 19.49±1.2cWPI-5FS 0.280 1.84±0.24b 12.04±2.8dWPI-10FS 0.266 1.83±0.51b 7.50±1.2e

Means with the same letters in a column are not significantly different.


those reported by Talens and Krochta (2005; Table 5). Asshown, the water barrier properties of the resultingcomposite films were much weaker than the typicalpetroleum-based polymers.

It is noteworthy that when exposed to 75% RH, WPI andWPI-5FS films exhibited similar WVP values, indicatingthat the protein phase for these films was plasticizedextensively, which created free volumes large enough forwater to diffuse rather easily. In comparison, when a greaterflaxseed oil fraction was used, the WPI-10FS films resistedthe plasticization effect of water more effectively at thesame RH, suggesting that the existence of protein–lipidinteraction that enhanced water barrier properties. Qualita-tively, during the WVP tests at 75% RH, the WPI filmswere plasticized and swelled considerably and becamewavy in appearance (Fig. 8a). WPI-FS films also exhibited

a similar behavior but to a much lesser extent, especiallywhen the lipid phase content was increased (Fig. 8b and c).The plasticization of the protein films also increased thediffusivity of glycerol and the lipid phase in the film matrix,resulting in greasy and hazy surfaces. The absorbed water,which increased the polarity of the film matrix, may haveexpelled the lipid phase out of the film. This hypothesiswas supported by the fact that these structural defects werenot observed when the films were tested at 30% RH(Fig. 8a–c).

Oxygen Gas Transmission

Oxygen permeability values were significantly differentbetween the three films (Table 6). WPI films have thelowest permeability values but increased considerably as

0

0.5

1

1.5

2

2.5

3

3.5

4

25 35 45 55 65 75

Relative Humidity [%]

Wa

ter

va

po

ur p

erm

ea

bil

ity

[g

.mm

/h.m

2.k

Pa

] WPI

WPI-5FS

WPI-10FS

Fig. 7 Water vapor permeabili-ty values for WPI and WPI-FSfilms at 23°C and under 30, 43,60, and 73% RH. Refer toTable 1 for film formation codes

Table 5 Comparison of water vapor permeability values of WPI and synthetic films

Film Test conditions Permeability (g mm/m2

h kPa)Source

WPI:GLY (1:1) 23°C, 43% RH 1.960±0.024a Current studyWPI:GLY:FS:BW: (1:1:0.6:0.06) 23°C, 43% RH 1.210±0.094bWPI:GLY:FS:BW: (1:1:1.24:0.124) 23°C, 43% RH 0.570±0.056cWPI:GLY 1:1 23°C, 40–20% 2.570±0.34 Talens and Krochta (2005)WPI:BW:GLY 1:1:1 23°C, 40–20% 1.510±0.27WPI:BW:GLY 3:3:1 23°C, 40–20% 0.130±0.09Beeswax 26°C, 100% RH 0.004 Shellhammer and Krochta (1997)PVC 26°C, 100% RH 0.026LDPE 26°C, 100% RH 0.001

Current study values are means of three replicates. Means with the different letters are significantly different at 5% level.WPI Whey protein isolate, GLY glycerol, FS flaxseed oil, BW beeswax


the lipid content increased. The dispersed lipid material inthe protein might have resulted a heterogeneous structurethat facilitated oxygen transmission. In addition, themigration of oil to film surfaces can also create holes thatreduced the resistance to oxygen transmission. As shown inTable 6, WPI films exhibited a reasonable oxygen barrierproperty under low to intermediate RH. They also com-pared favorably with other synthetic thermoplastics, such asLDPE and HDPE films. This characteristic makes WPI a

promising carrier for protecting flaxseed oil againstoxidation. However, the issue of oil migration to thesurface still needs to be resolved, especially when thepolymer is exposed to elevated RH conditions.

Sensory Analysis

Before coating, the surface finish of plums was dull (picturenot shown). Treating the fruit with WPI coating enhancedconsiderably its visual appearance by imparting a glossyfinish (Fig. 9). Although the addition of flaxseed oil andbeeswax decreased the gloss produced by the WPI coating,the appearance scores for plums coated with these com-posite films were still higher than the uncoated controls.The coated plums also had higher average flavor andoverall impression scores than the uncoated samples. Incontrast, the effects of coating on sweetness and firmnesswere less pronounced. It is noteworthy that, whereas thepuncture test detected a higher penetration force for theWPI coated plums (“Fruit Firmness”), the panelist did notperceive any difference in firmness among all plumsamples. This appeared to imply that the texture of theflesh played a more important role than the toughness of theskin in determining the perception of fruit firmness.Overall, this survey showed that the coatings tested didnot have a negative impact on consumer perception. Thisfinding, along with the improved appearance, has animportant implication on the marketability for the coatedproducts because consumers tend to buy fruits with goodvisual attributes.

Table 7 summarizes the responses of three questions toevaluate the effects of coating on the purchase decision ofconsumer. Although, the majority of the surveyed consum-ers (85%) bought plums either “occasionally,” “only inseason,” or “once in a while,” 73% of them consideredplums with longer shelf life a benefit. Only 31% indicatedthat their tendency to purchase would increase if the fruitswere added with nutritional substances. Interestingly, 77%of the surveyed consumers favored the incorporation ofnatural ingredient for coating plums to extend their shelflife. Based on this survey, it appears that consumers may be

a

b

c

Fig. 8 Appearance of films after water vapor transmission test. a WPIfilms, b WPI-5FS films, and c WPI-10FS films. Samples on the leftand right were exposed to 75 and 30% RH for 86 h, respectively.Refer to Table 1 for formulation codes

Film Test conditions Permeability coefficient(cm3 μm/m2 d kPa]

Source

WPI:GLY(1:1) 23°C; 50% RH 120.0±10.0a Current studyWPI:GLY:FS:BW: (1:1:0.6:0.06) 23°C; 50% RH 278.4±10.1bWPI:GLY:FS:BW: (1:1:1.24:0.124) 23°C; 50% RH 525.7±40.3cWPI:Gly (2.3:1) 23°C; 50% RH 76.1 Miller and Krochta

(1997)LDPE 23°C; 50% RH 1870.0HDPE 23°C; 50% RH 427.0EVOH (70% VOH) 23°C; 50% RH 12.0PVDC-Based films 23°C; 50% RH 0.4–5.1

Table 6 Comparison ofoxygen permeability values ofWPI and synthetic films

Current study values are meansof two replicates. Means withthe different letters are signifi-cantly different at 5% level.WPI Whey protein isolate, GLYglycerin, FS flaxseed oil, BWbeeswax


Fig. 9 Results of sensory evaluation of plums coated with WPI and WPI-FS formulations after 15 days of storage at 5°C. Average scores arepresented in the box located in each figure. Refer to Table 1 for formulation codes

Table 7 Responses for surveyed questions related to coated plums

Questions

How often do you buy plums? Often(15%)

Occasionally(27%)

Only inseason(27%)

Once in awhile(31%)

Never(0%)

When buying plums, would it be a benefit to have a longer refrigerated life? Yes,increase(73%)

No (12%) Do notcare(15%)

Would it affect your buying decision to know that the plums were addedwith a nutritional substance?

Yes,increase(31%)


Would it affect your purchasing decision if you knew in advance that theplums were coated with natural ingredients to extend shelf-life?

Yes,increase(77%)



more willing to buy coated plums with an extended shelflife than plums added with a nutraceutical in the coating.One reason could be due to the consumer’s perception offruits as natural products; the addition of nutritionalsubstances might be viewed as a lost of “naturalness” ofthe fruit.

Conclusions

The addition of flaxseed oil and beeswax in WPI filmreduced mass loss from plums by providing a better watervapor barrier. Shelf life of plums can be extended becausephenomena associated with mass loss such as shrivelingand firmness loss were delayed. Coating of plums with WPIfilms enhanced appearance of plums by creating glossysurfaces. Although the presence of beeswax decreasedglossiness, a better appearance was observed in comparisonwith the uncoated plums. Overall, coating of plum did notimpact the perceived flavor and firmness of the plums.Plums coated with WPI showed least significant coatingdefects over a storage period of 15 days at 5°C. WPI-10FSformulation also exhibited a low incidence of defects,making this coating stable as well. The addition of a lipidphase consisting of 5% flaxseed oil and 0.5% beeswax tothe WPI matrix compromised the film physical stability bythe formation of defects that weakens its barrier properties.

This study confirmed the feasibility of applying ediblefilms containing flaxseed oil to enhance the nutritionalvalue of plums; however, flaxseed oil migration to thesurface pose important stability issues that need to beaddressed. Future studies should take this into considerationto determine the ability of the WPI films to preserve thestability of the added nutraceutical.

References

ASTM (2002). D882-02 Standard test methods for tensile properties ofthin plastic sheeting. In Annual book of ASTM Standards AmericanSociety for Testing and Materials. Philadelphia, PA: ASTM.

ASTM (2005a). D3985-05 Standard test method for oxygen gastransmission rate through plastic film and sheeting using acoulometric sensor. In Annual book of ASTM StandardsAmerican Society for Testing and Materials. Philadelphia, PA:ASTM.

ASTM (2005b). E96/E96M-05 Standard test methods for water vaportransmission of materials. In Annual book of ASTM StandardsAmerican Society for Testing and Materials. Philadelphia, PA:ASTM.

Banerjee, R., & Chen, H. (1995). Functional properties of edible filmsusing whey protein concentrate. Journal of Dairy Science, 78,1673–1683.

Cisneros-Zevallos, L., & Krochta, J. M. (2003). Whey proteincoatings for fresh fruits and relative humidity. Journal of FoodScience, 68(1), 176–181.

Gennadios, A. (Ed.) (2002). Protein-based films and coatings. BocaRaton, FL: CRC.

Han, C., Zhao, Y., Leonard, S. W., & Traber, M. G. (2004). Ediblecoatings to improve storability and enhance nutritional value offresh and frozen strawberries (Fragaria ananassa) and raspberries(Rubus ideaus). Postharvest Biology and Technology, 33, 67–78.

Health Canada (2006). Transforming the food supply—Report of thetrans fat task force submitted to the Minister of Health June 2006.Available at: http://www.hc-sc.gc.ca. Accessed 23 May 2007.

Khwaldia, K., Perez, C., Banon, S., Desobry, S., & Hardy, J. (2004).Milk proteins for edible films and coatings. Critical Reviews inFood Science and Nutrition, 44, 239–251.

Krochta, J. M. (2002). Proteins as raw materials for films andcoatings: Definitions, current status and opportunity. In A.Gennadios (Ed.), Protein-based films and coatings (pp. 1–42).Boca Raton, FL: CRC.

Le Tien, C., Vachon, C., Mateescu, M. A., & Lacroix, M. (2001). Milkprotein coatings prevent oxidative browning of apples andpotatoes. Journal of Food Science, 66(4), 512–516.

Lee, S. Y., & Krochta, J. M. (2002). Accelerated shelf-life testing ofwhey-protein-coated peanuts analyzed by static headspace gaschromatography. Journal of Agricultural and Food Chemistry,50, 2022–2028.

Lee, J. Y., Park, H. J., Lee, C. Y., & Choi, W. Y. (2003). Extendingshelf-life of minimally processed apples with edible coatings andantibrowning agents. Lebensmitte—Wissenschaft und- Technolo-gie, 36(3), 323–329.

Lundblad, R. L. (Ed.) (2005). Chemical reagents fr protein modifica-tion (3rd Ed.). Boca Raton, FL: CRC.

Martin-Diana, A. B., Rico, D., Frias, J., Mulcahy, J., Henehan, G. T. M.,& Barry-Ryan, C. (2006). Whey permeate as a bio-preservative forshelf-life maintenance of fresh-cut vegetables. Innovative FoodScience and Emerging Technologies, 7, 112–123.

McHugh, T. H. (2000). Protein–lipid interactions in edible films andcoatings. Nahrung, 44(3), 148–151.

Mei, Y., & Zhao, Y. (2003). Barrier and mechanical properties of milkprotein-based edible films containing nutraceuticals. Journal ofAgricultural and Food Chemistry, 51, 1914–1918.

Miller, K., & Krochta, J. M. (1997). Oxygen and aroma barrierproperties of edible films: A review. Trends in Food Science &Technology, 81, 228–237.

Min, S., Harris, L., & Krochta, J. M. (2005). Antimicrobial effects oflactoferrin, lysozyme, and the lactoperoxidase system and ediblewhey protein films incorporating the lactoperoxidase systemagainst Salmonella enterica and Escherichia coli O157:H7.Journal of Food Science, 70, M332–M338.

Min, S., Harris, L. J., & Krochta, J. M. (2006). Inhibition of Salmonellaenterica and Escherichia coli O157:H7 on roasted turkey by ediblewhey protein coatings incorporating the lactoperoxidase system.Journal of Food Protection, 69(4), 784–793.

Min, S., & Krochta, J. M. (2005). Inhibition of Penicillium communeby edible whey protein films incorporating lactoferrin, lactoferrinhydrolysate, and lactoperoxidase systems. Journal of FoodScience, 70, M87–M94.

Moldao-Martins, M., Beirao-da-Costa, S. M., & Beirao-da-Costa, M.L. (2003). The effects of edible coatings on postharvest quality ofthe “Bravo de Esmolfe” apple. European Food Research andTechnology, 217, 325–328.

Perez-Gago, M. B., & Krochta, J. M. (2001). Denaturation time andtemperature effects on solubility, tensile properties, and oxygenpermeability of whey protein edible films. Journal of FoodScience, 66(5), 705–710.

Perez-Gago, M. B., Rojas, C., & Del Rio, M. A. (2003a). Effect ofhydroxypropyl methylcellulose-lipid edible composite coatingson plum (cv. Autumn giant) quality during storage. Journal ofFood Science, 68(3), 879–883.


http://www.hc-sc.gc.ca

Perez-Gago, M. B., Serra, M., Alonso, M., Mateos, M., & Del Rio, M.A. (2003b). Effect of solid content and lipid content of wheyprotein isolate-beeswax edible coatings on color change of fresh-cut apples. Journal of Food Science, 68(7), 2186–2191.

Seydim, A. C., & Sarikus, G. (2006). Antimicrobial activity of wheyprotein based edible films incorporated with oregano, rosemary andgarlic essential oils. Food Research International, 39, 639–644.

Shaw, N. B., Monahan, F. J., O’Riordan, E. D., & O’Sullivan, M.(2002a). Effect of soya oil and glycerol on physical properties ofcomposite WPI films. Journal of Food Engineering, 51, 299–304.

Shaw, N. B., Monahan, F. J., O’Riordan, E. D., & O’Sullivan, M.(2002b). Physical properties of WPI films plasticized with

glycerol, xylitol, or sorbitol. Journal of Food Science, 67, 164–167.

Shellhammer, T. H., & Krochta, J. M. (1997). Whey protein emulsionfilm performance as affected by lipid type and amount. Journalof Food Science, 62(2), 390–394.

Shellhammer, T. H., Rumsey, T. R., & Krochta, J. M. (1997).Viscoelastic properties of edible lipids. Journal of FoodEngineering, 33, 305–320.

Talens, P., & Krochta, J. M. (2005). Plasticizing effects of beeswaxand carnauba wax on tensile and water vapor permeabilityproperties of whey protein films. Journal of Food Science, 70(3),E239–E243.


influence of whey protein composite coatings on plum fruit quality

Documents