influence of whey protein coatings in plum

8/10/2019 Influence of Whey Protein Coatings in Plum

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Influence of Whey Protein Composite Coatings

on Plum (Prunus DomesticaL.) 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 5C

compared to the 5% lipid formulation. The firmness of

plums, determined by the penetration force using a 10-mm

probe, was not significantly affected by the coating types

except for the WPI-coated samples, which showed a

significantly higher penetration force because of the higher

strength for WPI film. Mass loss of plums during storage

was substantially reduced because of coating, especially

when coatings of higher lipid content were used. This was

consistent with the water permeability for the standalone

films, which decreased considerably when flaxseed and

beeswax were added. The incorporation of lipid phase to

WPI also significantly weakened oxygen barrier and

mechanical properties. Migration of plasticizer and lipid

phase to the film surface was observed during water vapor

permeability tests, especially when the films were exposed

to elevated humidity conditions. Overall, sensory evalua-

tion showed that the coated plums were more acceptable

than the uncoated controls.

Keywords Whey protein isolate films . Lipid composite

films . 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 standalone

membrane for improving food quality. It is often used to

preclude unwanted mass transports (e.g., moisture, oxygen,

aroma, and flavor), enhance visual attributes (e.g., gloss and

color), and function as a carrier to deliver active materials

(e.g., antimicrobial agents and nutraceuticals). Over the past

decade, edible films have been subjected to intensive

research because of their potential for reducing the use of

synthetic thermoplastics in food packaging.

Edible films can be derived from proteins, polysacchar-

ides, lipids, and resins. By and large, proteins have received

the most attention in edible film research because of their

abundance as agricultural by-products and food processing

residuals. The presence of reactive amino acid residuals

also enable protein to be modified and crosslinked through

physical and chemical treatments to produce novel poly-

meric structures (Gennadios 2002; Lundblad 2005). Whey

proteins from bovine milk have been studied to a great

extent because of their ability to form transparent and

flexible films, which exhibit good barrier and mechanical

properties (Krochta 2002). Whey proteins are globular

proteins, which remain soluble after precipitation of casein

at pH 4.6 during cheese making. In bovine milk, these

thermal-labile proteins consist of mainly -lactalbumin, -

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) 1:314325

DOI 10.1007/s11947-007-0014-1

E. Reinoso

Iovate Health Sciences Research Inc,

5100 Spectrum Way, Mississauga, ON L4W 5S2, Canada

G. S. Mittal

School of Engineering, University of Guelph,

Guelph, ON N1G 2W1, Canada

L.-T. Lim (*)

Department of Food Science, University of Guelph,

Guelph, ON N1G 2W1, Canada

e-mail: [email protected]


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whey protein isolates (WPI) or whey protein concentrates

(WPC), which have protein contents of >90 and 2085%,

respectively (Khwaldia et al. 2004).

Studies on whey protein films have been mainly focused

on improving their barrier and mechanical properties

through optimization of protein denaturation conditions

(e.g., temperature, pH, time, and protein content), addition

of cross-linking agents, and judicious use of plasticizers andadditives. Perez-Gago and Krochta (2001) studied the effect

of denaturation conditions and concluded that denaturation

of WPI at 90C for 30 min provided stronger films in

terms of tensile properties than films cast from solutions

denaturized at lower temperatures and for shorter heating

time. Shaw et al. (2002b) compared the effect of glycerol,

xylitol, and sorbitol plasticizers for forming WPI films and

found that glycerol imparted the best stress-strain properties

to the film. Because of the hydrophilic nature of whey

protein, 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 (McHugh2000; Shellhammer and

Krochta 1997), and soy oil (Shaw et al. 2002a) were

investigated, among which beeswax is most effective for

reducing the water vapor permeation rate.

To date, end use applications of whey protein films are

mainly targeted for use as carriers for antimicrobial agents

(Min et al.2005,2006; Min and Krochta2005; Seydim and

Sarikus2006) and as protective barrier coatings to increase

the shelf-life of food products (Cisneros-Zevallos and

Krochta 2003; Martin-Diana et al. 2006; Moldao-Martins

et al. 2003). Several studies concluded that whey protein

coatings, when applied to fresh fruits, vegetables, nuts, and

meat, are capable of extending their shelf life (Le Tien et al.

2001; Lee et al.2003; Lee and Krochta2002; Martin-Diana

et al. 2006; Perez-Gago et al. 2003b).

Because whey protein films are edible, they are ideal

carriers for nutraceuticals to enhance the nutritional value

of the coated food product. Properties of calcium caseinate

and WPI films, added with vitamin E (-tocopheryl

acetate) and a mixture of calcium lactate and calcium

gluconate, were investigated by Mei and Zhao (2003). They

reported that high nutraceutical contents can compromise

tensile and permeability properties of the film; however,

these properties are mainly influenced by the nature of the

coating materials and the type and concentration of

nutraceuticals incorporated into the coating formulations.

In another study, fresh and frozen strawberries and

raspberries were coated with chitosan added with calcium

and Vitamin E to increase the nutritional values of these

fruits (Han et al.2004). These authors showed that chitosan

coatings extended the shelf-life of fresh strawberries and

raspberries by reducing the weight loss. Furthermore,

changes in properties such as acidity and color were

delayed during storage. Edible coatings also have been

used as oxygen barriers and carriers for antibrowning

agents 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 edible

WPI coating for plum (P. domestica L) to improve its

quality and nutritional value by incorporating flaxseed oil

to the coating formulation. Plum was chosen for this studyover other fruits because: (1) its skin can be consumed

together with the flesh and therefore provides an opportu-

nity for carrying nutraceuticals; (2) it has a relatively short

shelf life, and (3) its dull skin appearance can be improved

to enhance consumer appeals. Flaxseed oil was used in this

study because it has been recognized as a significant source

of omega-3 fatty acids, which is important for stroke and

heart disease prevention (Health Canada 2006). Further-

more, the lipid phase also reduces the hydrophilicity of the

coating to enhance its water barrier properties.

Materials and Methods

Materials

Plums were obtained from a local supermarket from the

same batch. Visual inspection was conducted to ensure

consistent ripeness and absence of major defects or physical

damages that could interfere with the experiments. Plums

were stored in a refrigerator at 5C. WPI was supplied by

Davisco International, Inc. (BiPro, Le Seur, MN, USA)

with a protein content of 93.2%. USP grade glycerol was

supplied 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 Somerset

Company (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 quick

and uniform dispersion of WPI powder in water. The

process involved adding WPI to ice-water (1:1 w/w)

mixture that had been preblended for 1 s using a Braun

hand-held blender (Model MR400, Kronberg, Germany).

The protein and ice-water mixture was blended in the

blender for 15 s, followed by manual mixing for 30 s, and

further blended for 15 s. The solution was then stirred using

a magnetic stirrer for 8 min to ensure the formation of a

uniform solution. To denature the WPI, the solution was

heated at 90C for 30 min in a Cole-Parmer water bath

Food Bioprocess Technol (2008) 1:314325 315315


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(Model 12501-01, Montreal, QC, Canada) and then cooled

down to room temperature. At this point, glycerol was added

at a ratio of 1 g of glycerol per g of protein. To prepare the

solution for lipid composite film, the WPI/glycerol solution

was heated to 70C and Tween 60 emulsifier was added.

Beeswax was melted separately and added to flaxseed oil at

70C. The resulting lipid blend was then added to the WPI/

glycerol while providing mixing with a blender. Theformulations of the coating are shown in Table1.

Coating Procedure

Plums were previously washed using tap water and then

allowed to dry at room temperature. Coating was applied by

dipping 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 air

velocity of 1.5 m/s for 30 min at 212C. Coated plums

were stored at 5C for 15 days.

Film Casting

Film-forming solutions were cast on Plexiglas plates of

1730 cm. The total solids content per area was maintained

at 0.028 g/cm2 to minimize thickness variations. Cast

solutions were allowed to dry on a leveled granite surface

for 48 h at 233C. Dried films were then peeled intact.

Tests on Coated Plums

Coating Stability

The purpose of this test was to evaluate the visual quality of

the coatings during storage at 5C. Three main defects

occurred were identified: (1) flakingpresence of flakes,

which can be dislodged readily from the plum surface; (2)

blisteringlocal loss of adhesion and lifting of coating

from the plum skin; and (3) crackingbreaks or splits in

the coating surface. These defects were visually evaluated

after 2, 6, 9, 12, and 15 days of storage using the criteria

given in Table2.

Firmness

Puncture tests were conducted for the coated and control

samples to determine the effects of coating on fruit firmness

at the end of 15 days storage at 5C. Firmness was

measured using an Instron Universal Testing Machine

(Instron Corp., Model 1122, Canton, MA, USA) equipped

with a 10-mm diameter cylinder probe, which was set to

penetrate the sample for 20 mm. The maximal force

required to penetrate the fruit was measured for 15 plums

from each treatment. The test was repeated twice by

puncturing once on each opposite face of the fruits. To

evaluate the effect of coating on the firmness of fruit, the

plums coated with WPI were divided into two groups. One

group was tested with the intact coating (WPI), whereas the

other group was tested with the coating peeled off (WPI-P).

Mass Loss

Sample sizes of 18 fruits per treatment, including the

control samples, were used for mass loss evaluation. The

plums were weighed at 0, 2, 6, 9, 12, and 15 days during

storage. The mass loss was expressed as percentage loss of

initial mass. The precision balance used for weight

measurement had an accuracy of 0.1 mg.

Table 2 Criteria to evaluate film defects on plums applied with WPI

and WPI-FS coatings

Rank Flaking Cracking Blistering

None No visual

evidence of

flaking

No visual

evidence of

cracking

No visual

evidence of

blistering

Slight One spot is

affected with an

area less than

10% of the

plum surface

area

Presence of a

small crack

Presence of a

small blister

(


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Sensory Analysis

Sensory evaluation of the coated and control samples was

conducted on the 15th day of storage using a sensory panel

consisted of 29 individuals with age ranging from 20 to

45 years old. Panel members were previously screened for

allergic reactions to the ingredients used in the coatings.

Samples were removed from refrigeration 1 h before thesensory evaluation to allow them to equilibrate to room

temperature. Coded samples were presented to the panelist

who was asked to score appearance, overall impression,

flavor, sweetness, and firmness using a hedonic scale of 0

10 (0dislike; 10like). The panel was also asked if the

use of edible films and the incorporation of nutraceuticals

in 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 stress

concentration in the middle section. The narrow section was

used to calculate the cross-sectional area. The specimens

were conditioned at 232C and 505% relative humidity

(RH) for 48 h before testing.

Tensile test was conducted in accordance with ASTM

method D882-02 (ASTM 2002). The film strips were

attached to the pneumatically actuated grips of an Instron

tester. The crosshead speed was set at 200 mm/min, and the

initial gauge length was 70 mm. The test was performed at

232C and 505% RH. Seven samples per film type were

tested, and the average values were reported.

Water Vapor Transmission Rate

Water vapor transmission rate (WVTR) was determined

based on ASTM E96/E96M-05 (ASTM 2005b) using

anhydrous calcium sulfate as a desiccant. Twenty grams

of the desiccant were placed in each aluminum dish with

flanged rim, on which the test films were sealed using

molten wax. Samples were placed in controlled environ-

ment chambers set at 232C and RH of 30, 43, 60, and

75%. Samples were weighed within an accuracy of

0.0001 g at 0, 6, 14, 20, 24, 38, 44, 62, 68, and 86 h.

Three replicates for each film were tested, and the mean

values were reported.

Oxygen Transmission Rate

Oxygen transmission rate (OTR) values were determined in

accordance with ASTM D3985-05 (ASTM 2005a) using

Ox-Tran 2/20 modular system (Modern Control Inc.,

Minneapolis, MN, USA). The films were previously

conditioned at 45% RH before testing. To prevent the

coulometric sensor from overloading, the films were

masked with aluminum foil to reduce the exposed area to

24.6 cm2. The permeation test was conducted isostatically

by sandwiching the films in the permeability cell, where the

upstream side of the film was exposed to a continuous flow

of air (21% oxygen) and the downstream side to a carriergas of 2% H2/98%N2 gas mixture. The test was conducted

at 50% RH, and two replicates for each film type were

evaluated. Oxygen permeability was calculated as follows:

PO2 OTR

p :t

where OTR is in cm3/(m2 d),Pis partial pressure of oxygen

(21.3 kPa), and t is film thickness in mm (average of five

readings).

Statistical Analysis

Statistical analysis was performed using SPSS Release 14.0

software (Chicago, IL, USA). Analysis of variance and

multiple comparison test (Duncan) were applied to analyze

the data for coating stability, sensory analysis, mass loss,

firmness, oxygen transmission, and tensile properties.

Analysis of covariance was applied to water transmission

rate data to determine difference between slopes. All

statistical analyses were conducted at a significant level of

p=0.05.

Results and Discussion

Coating Stability

During the 15-day storage at 5C, WPI-5FS coating

exhibited the highest incidence of defects, whereas WPI

coating produced the least (Figs. 1,2, and3). We observed

that cracking and flaking tended to occur concurrently

because the formation of cracks also resulted in a poor

adhesion between the coating and the plum skin, which led

to flaking (Fig. 4). In contrast, the presence of blisters did

not correlate with the formation of cracks (Fig. 5).

Nevertheless, blistering could lead to flaking, and there

was a significant interaction between cracking and flaking.

Cracks and flakes not only substantially diminished the

visual appeal of the fruit but also compromised the ability

of the coating to protect the fruit.

The poor stability of WPI-5FS coating during storage at

5C may be attributed to the migration of oil to the coating

surface. This created voids in the protein matrix and led to

material shrinkage, causing the coating to dislodge from the

skin surface (blistering) and also promote cracking. The



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stability of the film-forming emulsion was also important

for achieving a uniform coating during the dipping and

drying processes. Emulsions that were unstable couldundergo phase separation, which introduced considerable

coating thickness variation when applied to the plum

surface. The progressive development of coating defects

during storage for the WPI-5FS-coated plums appeared to

support these hypotheses.

The better coating stability of WPI-10FS than WPI-5FS

can be attributed to the higher content of the lipid phase in

the former formulation. Beeswax is made up of mainly wax

acid esters, wax acids, and hydrocarbon, the viscous

properties of which are primarily because of free fatty acid

fraction (Shellhammer et al. 1997). It has been shown that

beeswax is capable of plasticizing whey protein film, which

is hypothesized to be caused by the free fatty acids fraction

in beeswax (Talens and Krochta 2005). We speculated that

the enhanced plasticization caused by the additional

beeswax, along with the increased hydrophobic interaction

of the lipid phase with the cuticle layer of the skin, might

have contributed to the better coating stability of WPI-10FScompared to WPI-5FS.

Fruit Firmness

After 15 days of storage at 5C, the effect of coating on

fruit firmness was insignificant, except that plums with the

intact WPI coating exhibited a significant higher firmness

value (Table3). This can be attributed to the higher tensile

strength of WPI films in comparison to other films

(Tensile Properties). This observation agreed with a study

conducted by Perez-Gago et al. (2003a) in which plums

were coated with hydroxypropyl methylcellulose-lipid

edible composite films. They reported that the firmness

was similar between the coated and control fruits after

6 weeks of storage at 1C. This effect was attributed to the

Fig. 2 Frequency of film flak-

ing during 15 days storage at

5C for plums coated with WPI,

WPI-FS, and WPI-10FS formu-

lations. Refer to Table 1 for

formulation codes

Fig. 1 Frequency of film crack-

ing during 15 days storage at 5C

for plums coated with WPI,

WPI-FS, and WPI-10FS formu-

lations. Refer to Table1for

formulation codes

318 Food Bioprocess Technol (2008) 1:314325


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were to be extended and the temperature increased. This

has commercial significance because mass loss is one of the

critical quality degradation factors for fresh produce.

Tensile Properties

The tensile strength of WPI film was around three times

higher than the other two composite films (Table 4). The

addition of the lipid phase dramatically weakened the film

strength, which is in agreement with the results reported

previously (Banerjee and Chen 1995; Cisneros-Zevallos

and Krochta 2003; Mei and Zhao 2003). The dispersed

flaxseed oil-beeswax in WPI matrix and the crystalline

nature of beeswax might have contributed to the reduced

film strength because of the heterogeneous structure and the

lack of interfacial interaction between the dispersed lipid

and continuous WPI phases.

In this study, no significant difference was observed on

tensile strength of films containing 5% (WPI-5FS) and 10%

(WPI-10FS) flaxseed oil, although the elongation values

were significantly different among the three formulations

(Table 4). Overall, WPI-10FS films showed the lowest

mechanical properties, which appeared to be inconsistent

with the coating stability data. The relatively higher

temperature used during film testing compared to the

storage temperature for the coated plums may be another

reason for this inconsistency. In addition, the tensile

properties of the standalone films determined here were

likely not adequate for reflecting the mechanical properties

of the coating because of filmskin interaction for the latter.

Water Vapor Permeability

The addition of lipid phase significantly depressed the

Water vapor permeability (WVP) values (Fig. 7). For

example, at 30% RH, the average WVP values for WPI

film decreased from 0.91 to 0.75 and 0.29 gmm/hm2kPa

when 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 WPI

composite 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]

Massloss[%]

Control

WPI

WPI-5FS

WPI-10FS

Fig. 6 Mass loss of uncoated

and coated plums during a 15-

day storage at 5C

Table 3 Firmness of uncoated and coated plums measured by

puncture test

Sample Force (N)

Uncoated plums 6.50.8b

WPI-Pa 6.01.4b

WPI 8.52.0a

WPI-5FS 6.51.1b

WPI-10FS 6.00.9b

Means with the same letter are not significantly different.aPlums 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 at

break (%)

WPI 0.229 6.100.18a 19.491.2c

WPI-5FS 0.280 1.84 0.24b 12.042.8d

WPI-10FS 0.266 1.83 0.51b 7.50 1.2e

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



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those reported by Talens and Krochta (2005; Table 5). As

shown, the water barrier properties of the resulting

composite films were much weaker than the typical

petroleum-based polymers.

It is noteworthy that when exposed to 75% RH, WPI and

WPI-5FS films exhibited similar WVP values, indicating

that the protein phase for these films was plasticized

extensively, which created free volumes large enough for

water to diffuse rather easily. In comparison, when a greater

flaxseed oil fraction was used, the WPI-10FS films resistedthe plasticization effect of water more effectively at the

same RH, suggesting that the existence of proteinlipid

interaction that enhanced water barrier properties. Qualita-

tively, during the WVP tests at 75% RH, the WPI films

were plasticized and swelled considerably and became

wavy in appearance (Fig. 8a). WPI-FS films also exhibited

a similar behavior but to a much lesser extent, especially

when the lipid phase content was increased (Fig. 8b and c).

The plasticization of the protein films also increased the

diffusivity 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 have

expelled the lipid phase out of the film. This hypothesis

was supported by the fact that these structural defects were

not observed when the films were tested at 30% RH

(Fig. 8ac).

Oxygen Gas Transmission

Oxygen permeability values were significantly different

between the three films (Table 6). WPI films have the

lowest 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 [%]

Watervapourpermeability[

g.mm/h.m

2.k

Pa] WPI

WPI-5FS

WPI-10FS

Fig. 7 Water vapor permeabili-

ty values for WPI and WPI-FS

films at 23C and under 30, 43,

60, and 73% RH. Refer to

Table1for 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) 23C, 43% RH 1.9600.024a Current studyWPI:GLY:FS:BW: (1:1:0.6:0.06) 23C, 43% RH 1.210 0.094b

WPI:GLY:FS:BW: (1:1:1.24:0.124) 23C, 43% RH 0.5700.056c

WPI:GLY 1:1 23C, 4020% 2.5700.34 Talens and Krochta (2005)

WPI:BW:GLY 1:1:1 23C, 4020% 1.5100.27

WPI:BW:GLY 3:3:1 23C, 4020% 0.1300.09

Beeswax 26C, 100% RH 0.004 Shellhammer and Krochta (1997)

PVC 26C, 100% RH 0.026

LDPE 26C, 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, GLYglycerol, FS flaxseed oil, BWbeeswax



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the lipid content increased. The dispersed lipid material in

the protein might have resulted a heterogeneous structure

that facilitated oxygen transmission. In addition, the

migration of oil to film surfaces can also create holes that

reduced the resistance to oxygen transmission. As shown in

Table 6, WPI films exhibited a reasonable oxygen barrier

property under low to intermediate RH. They also com-

pared favorably with other synthetic thermoplastics, such as

LDPE and HDPE films. This characteristic makes WPI a

promising carrier for protecting flaxseed oil against

oxidation. However, the issue of oil migration to the

surface still needs to be resolved, especially when the

polymer 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 enhanced

considerably its visual appearance by imparting a glossy

finish (Fig. 9). Although the addition of flaxseed oil and

beeswax 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 and

overall impression scores than the uncoated samples. In

contrast, the effects of coating on sweetness and firmness

were less pronounced. It is noteworthy that, whereas the

puncture test detected a higher penetration force for the

WPI coated plums (Fruit Firmness), the panelist did notperceive any difference in firmness among all plum

samples. This appeared to imply that the texture of the

flesh played a more important role than the toughness of the

skin in determining the perception of fruit firmness.

Overall, this survey showed that the coatings tested did

not have a negative impact on consumer perception. This

finding, along with the improved appearance, has an

important implication on the marketability for the coated

products because consumers tend to buy fruits with good

visual attributes.

Table 7 summarizes the responses of three questions to

evaluate the effects of coating on the purchase decision of

consumer. Although, the majority of the surveyed consum-

ers (85%) bought plums either occasionally, only in

season, or once in a while, 73% of them considered

plums with longer shelf life a benefit. Only 31% indicated

that their tendency to purchase would increase if the fruits

were added with nutritional substances. Interestingly, 77%

of the surveyed consumers favored the incorporation of

natural ingredient for coating plums to extend their shelf

life. Based on this survey, it appears that consumers may be

a

b

c

Fig. 8 Appearance of films after water vapor transmission test.a WPI

films, b WPI-5FS films, and c WPI-10FS films. Samples on the left

and right were exposed to 75 and 30% RH for 86 h, respectively.

Refer to Table1 for formulation codes

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

Source

WPI:GLY(1:1) 23C; 50% RH 120.0 10.0a Current study

WPI:GLY:FS:BW: (1:1:0.6:0.06) 23C; 50% RH 278.410.1b

WPI:GLY:FS:BW: (1:1:1.24:0.124) 23C; 50% RH 525.740.3c

WPI:Gly (2.3:1) 23C; 50% RH 76.1 Miller and Krochta

(1997)LDPE 23C; 50% RH 1870.0

HDPE 23C; 50% RH 427.0

EVOH (70% VOH) 23C; 50% RH 12.0

PVDC-Based films 23C; 50% RH 0.45.1

Table 6 Comparison of

oxygen permeability values ofWPI and synthetic films

Current study values are means

of two replicates. Means with

the different letters are signifi-

cantly different at 5% level.

WPIWhey protein isolate,GLY

glycerin, FS flaxseed oil, BW

beeswax



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Fig. 9 Results of sensory evaluation of plums coated with WPI and WPI-FS formulations after 15 days of storage at 5C. Average scores are

presented in the box located in each figure. Refer to Table1 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 not

care

(15%)

Would it affect your buying decision to know that the plums were added

with a nutritional substance?

Yes,

increase

(31%)

No (38%) Do not

care

(27%)

Would it affect your purchasing decision if you knew in advance that the

plums were coated with natural ingredients to extend shelf-life?

Yes,

increase

(77%)

No (12%) Do not

care

(12%)



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more willing to buy coated plums with an extended shelf

life than plums added with a nutraceutical in the coating.

One reason could be due to the consumers perception of

fruits as natural products; the addition of nutritional

substances might be viewed as a lost of naturalness of

the fruit.

Conclusions

The addition of flaxseed oil and beeswax in WPI film

reduced mass loss from plums by providing a better water

vapor barrier. Shelf life of plums can be extended because

phenomena associated with mass loss such as shriveling

and firmness loss were delayed. Coating of plums with WPI

films enhanced appearance of plums by creating glossy

surfaces. Although the presence of beeswax decreased

glossiness, a better appearance was observed in comparison

with the uncoated plums. Overall, coating of plum did not

impact the perceived flavor and firmness of the plums.Plums coated with WPI showed least significant coating

defects over a storage period of 15 days at 5C. WPI-10FS

formulation also exhibited a low incidence of defects,

making this coating stable as well. The addition of a lipid

phase consisting of 5% flaxseed oil and 0.5% beeswax to

the WPI matrix compromised the film physical stability by

the formation of defects that weakens its barrier properties.

This study confirmed the feasibility of applying edible

films containing flaxseed oil to enhance the nutritional

value of plums; however, flaxseed oil migration to the

surface pose important stability issues that need to be

addressed. Future studies should take this into consideration

to determine the ability of the WPI films to preserve the

stability of the added nutraceutical.

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