Edible in Fruit

7/29/2019 Edible in Fruit

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Development ofadvanced edible

coatings for fruits

Hyun Jin Park*Graduate School of Biotechnology, Korea University,

5-Ka, Anam-Dong, Sungbuk-Ku, Seoul 136-701,South Korea (tel: 82-2-3290-3450; fax: 82-2-927-9028;

e-mail: [email protected])

Edible coatings can provide an additional protective coating

for fresh products and can also give the same eect as

modied atmosphere storage in modifying internal gas

composition. Recently, several edible coatings for preserving

fruits such as oranges, apples, and grapefruits were suc-

cessfully applied. But, in some cases, edible coatings werenot successful. In fact, fruit quality was worse. The success

of edible coatings for fresh products totally depends on the

control of internal gas composition. This article is designed

to help develop a systematic means of selecting edible

coatings to maximize quality and shelf life of fresh fruits and

vegetables. Methods will be introduced to select edible

coatings based on their gas permeation properties relative

to controlling internal gas composition of target products.

# 2000 Published by Elsevier Science Ltd. All rights reserved.

Major losses in quality and quantity of fresh fruitsoccur between harvest and consumption [1]. Savings

obtained through reduction of postharvest fruit losses is

regarded as ``a hidden harvest'' [2]. Through a better

understanding of the respiration process of fresh fruits,

several techniques have been developed that are suc-

cessful in extending shelf life. Controlled atmosphere

storage and modied atmosphere storage have been

used for preserving fruits by reducing their quality

changes and quantity losses during storage. Edible

coatings on fresh fruit can provide an alternative to

modied atmosphere storage by reducing quality chan-

ges and quantity losses through modication and con-

trol of the internal atmosphere of the individual fruits.

A historical view of edible coatingsWax was the rst edible coating used on fruits. The

Chinese applied wax coatings to oranges and lemons in

the 12th and 13th centuries [3]. Although the Chinese

did not realize that the full function of edible coatings

was to slow down respiratory gas exchange, they foundthat wax-coated fruits could be stored longer than non-

waxed fruits. In the 1930s hot-melt paran waxes

became commercially available as edible coatings of

fresh fruits such as apples and pears. Erbil and Muftugil

[4] reported that coating of peach surfaces with wax

emulsions decreased water vapor and oxygen transmis-

sion, thus diminishing respiration rate and increasing

shelf life of the fruit. Nisperos-Carriedo et al. [5] and

Baldwin et al. [6] observed that oils or waxes and cellu-

lose had similar eects of preventing spoilage and

retaining fresh-picked quality for tropical fruits.

Several attempts have been made to develop other

materials that could be used to coat, produce and mod-ify internal gas composition for short-term storage. El

Ghaouth et al. [7] and Zhang and Quantick [8] sug-

gested that chitin and chitosan (deacetylated chitin)

from marine invertebrates could be used for making a

transparent lm for application as an edible coating on

fruits and vegetables. In 1982, Lowings and Cutts

reported on an edible coating material that is non-

phytotoxic, tasteless, odorless, and eective in preser-

ving fruits. This coating material is a mixture of sucrose

fatty acid esters (SFAE), sodium carboxymethyl cellu-

lose, and mono- and di-glycerides. SFAE was originally

developed as an emulsier. However, it has been estab-lished that the ripening of fruits be retarded by a coating

of SFAE. SFAE mixtures have been commercially

available since the 1980s, for coating fruits and vege-

tables, under the trade names `TAL Pro-long' and

`Semperfresh' [915]. Park et al. [16,17] applied zein

coating on the surface of tomatoes and reported that the

lm coating delayed color change, weight loss and

maintained rmness during storage.

Problems associated with edible coatings to beovercome

Even though some edible coatings have been success-

fully applied to fresh produce, other applications have

0924-2244/00/$ - see front matter Copyright # 2000 Published by Elsevier Science Ltd. All rights reserved.P I I : S0924-2244(00)00003-0

Trends in Food Science & Technology 10 (1999) 254260

*H.J. Park is also with the Department of Packaging Science,Clemson University, Clemson, SC 29634-0370, USA.

Review


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adversely aected quality. Modication of internal

atmospheres by the use of edible coatings can increase

disorders associated with high carbon dioxide or low

oxygen concentration [18]. Smock [19] indicated that

waxing apples and pears inhibited normal ripening rate

and if sucient wax was applied respiration was greatlyinhibited and alcoholic avors were developed by anae-

robic fermentation. Smith and Stow [11] reported that

apples (cv. Cox's Orange Pippin) coated with sucrose

fatty acid ester did have reduced detrimental changes in

terms of fruit rmness, yellowing and weight loss, but

also had increased core ush incidence. Park et al. [17]

reported that tomatoes coated with 2.6 mm zein lm

produced alcohol and o-avors internally which were

attributable to an internal gas composition too low in

oxygen and too high in carbon dioxide. Smith et al. [20]

summarized the eects on physiological disorders asso-

ciated with modication of internal atmosphere by useof coatings, as core ush, esh breakdown, and accu-

mulation of ethanol and alcoholic o-avors.

Wax and SFAE mixtures are the most widely used

edible coatings for fruits and vegetables. But, they are

not equally eective on all produce. Another problem is

that consumers tend to be wary of waxy coatings.

Therefore, development of alternate edible coatings that

do not impart a waxy taste are desirable. The eects of

edible coatings on internal gas composition and their

interactions on quality parameters must be determined

for coated fresh produce. For example, color change

and rmness are very important quality parameters in

fruits. As Shewfelt et al. [21] stated, color change,rmness loss, ethanol fermentation, decay ratio and

weight loss of edible lm coated fruits are all important

quality parameters for some products. Success of edible

coatings for fruits depends mainly on selecting lms or

coatings that can give a desirable internal gas composi-

tion that is appropriate for a specic product. Also, if a

coating is too thick detrimental eects can result due to

an internal oxygen concentration below a desirable and

benecial level and an associated increased carbon

dioxide concentration above a critical tolerable level.

Such a condition leads to anaerobic fermentation. This

is to accomplished by: (a) developing several ediblecoatings, (b) measuring gas permeation properties of

selected coatings, (c) measuring diusion properties of

skin and esh of selected fruits, (d) predicting internal

gas compositions for the fruits coated with edible lms,

and (e) observing coating eects on the quality changes

of fruits.

Gas permeation properties of edible coatingsThere are several possible edible coatings for fruits,

such as cellulose, casein, zein, soy protein, and chitosan.

These were chosen since they have the desirable char-

acteristics of generally being odorless, tasteless and

transparent. It is not easy to measure the gas permea-

tion properties of the coatings after being placed on

fruits. Therefore, separate at lms need to be prepared

and tested. Two primary known methods of preparation

of at lm were described by Kamper and Fennema [22]

and Aydt et al. [23]. An OX-TRAN 1000TM (Mocon

Modern Control, Inc., Minneapolis, MN) is usuallyused to measure oxygen permeability, and WVP was

measured using a variation of the ASTM Standard

Method E 96 (ASTM, 1987), known as the ``cup

method''. CO2 permeability can be measured using a

modied permeability cell designed by Gilbert and

Pegaz [24] . Gas and water vapor permeabilities of the

coatings can be calculated as shown in Box 1.

Oxygen, carbon dioxide and water vapor perme-

abilities of edible coatings reported in the literature are

presented in Table 1 and compared with other conven-

tional plastic lms. The oxygen permeabilities of most

edible coatings was lower than the conventional plasticlms [2932]. Oxygen permeability (OP) of SPE coat-

ings is 13 times higher than those of polyethylene lm

and is 410 times higher than those of polypropylene

lm. OP of SPE coatings are similar to cellulose lm

values but are higher than those of protein edible coat-

Box 1. Gas permeability

The permeation can be described mathematically byFick's rst law. The ux (J) which is proportional to theconcentration gradient can be dened in one direction asfollows:

t hdgad I

Where, J is the ux, the net amount of solute that dif-fuses through unit area per unit time (g/m2.s or ml/m2.s),D is the diusivity constant (m2/s), C is the concentrationgradient of the diusing substance and X is the thicknessof the lm (m) [2528].

With the two assumptions, (1) the diusion is in steadystate and (2) there is a linear gradient through the lm,the ux (J) is given by

t hgP gIa ae P

Where, Q is the amount of gas diusing through the

lm (g or ml), A is area of the lm (m2

) and t is the time(s). After application of Henry's law, the driving force isexpressed in terms of partial pressure dierential of gasand a rearrangement of terms yields the following equa-tion in terms of permeability.

ae hpP pIa pa Q

Where, S is the Henry's law solubility coecient (mole/atm), p is partial pressure dierence of the gas across thelm (Pa) and P is the permeability ((ml or g) m/m 2.s.Pa).

Then, the permeabilities of O2, CO2 and H2O vaporcan be calculated from the following equation [2528];

aep R

H.J. Park / Trends in Food Science & Technology 10 (1999) 254260 255


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ings such as zein. CO2/O2 permeability ratios of edible

coating lms are higher than those of plastic lms. The

permeability ratios of protein lms are lower than those

of cellulose lms. SPE coatings present very high water

vapor barriers compared with other edible coatings [30].

Water vapor permeability (WVP) of SPE coatings is

lower than that of polyethylene lm and more than 100

times lower than the values for cellulose and protein

lms. These high oxygen and water vapor barrier prop-

erties will make SPE coatings desirable for fresh pro-duce as a replacement for wax. WVP of other edible

coating lms are much higher than those of plastic lms

[33]. WVP of wheat protein lm was 0.6030.630 ng.m/

m2.s.Pa, which was the highest among all edible lms

tested [34]. Plastic is the most widely used food wrap,

but water vapor commonly condenses on the inner sur-

face of plastic packaging materials thus leading to

potential microbial contamination in fresh produce [35].

Thus, a lm with greater WVP is desirable, although an

extremely high water vapor permeability of a lm is also

not desirable as it can result in excessive moisture loss of

fruits during storage.

Diusivity determination on fruit skin and eshKnowledge of diusivities of gases in bulky plant

organs is essential in understanding the physiological

changes, gas exchanges and internal gas composition.

The internal gas composition of fruits is determined by

the diusivities of skin, esh and stem [3638]. Burg and

Burg [36] designed a system to determine gas resistance

factors that can be used to estimate gas diusivities of

bulky plant organs as the ratio of internal concentration

to the ratio of the production of carbon dioxide and

ethylene at steady state. Diusivities of gases in bulky

plant tissue can be calculated as shown in Box 2.

Several reports exist on determining the diusivities of

the bulky plant organs [3641]. Burg and Burg [36]

dened a resistance factor (R) which could be estimated

for bulky plant organs, banana and tomato, as the ratio

of internal concentration to the ratio of production of

carbon dioxide and ethylene at steady state. They esti-

mated that more than 60% of gas exchange takes place

through the stem scar in tomatoes. But this resistance

factor is only an empirical value without conventional

dimensions, and is not constant with changes in thesurface to volume ratio. Cameron and Yang [37] mea-

sured the eux of a metabolic inert gas, ethane which, is

neither produced nor metabolized to a signicant degree

by the tissue. It was shown that over 97% of gas

exchange of tomato fruits occurs through the stem scar.

But the measurement of ethane eux introduces several

uncertainties because they did not measure the diusiv-

ities of exocarp, pericarp and stem scar separately.

Solomos [40] , in a review of principles of gas exchange

in bulky plant organs, considered stationary states for

carbon dioxide diusion through spherical and cylind-

rical shaped plant organs and determined diusivities ofesh and skin of apple with the peeled and intact fruit.

But the eect of the stem for gas transfer was not con-

sidered in determining the apparent diusivities of

apple. The wax undoubtedly serves as a gas barrier to

oxygen, carbon dioxide and water vapor and other

metabolic gases and also provides protective functions

(for example, mechanical damage, fungal and insect

attack). Therefore, it can be assumed that the primary

factor that regulates the internal concentration of gases

is the skin in bulky plant organs. In apple, the resistance

of apple skin to gas diusion was 10 to 20 fold greater

than that of the esh, depending on the cultivar [40, 41].

Chinnan and Park [38] constructed a diusion cell from

Table 1. O2, CO2 and H2O vapor permeabilities of edible coatings

PermeabilityFilma

O2b CO2

b H2O Vaporc

SPE 2.100.0001 0.000420.04Chitosan 0.0014 0.49Zein 0.360.16 2.671.09 0.1160.019Wheat gluten 0.200.09 2.131.43 0.6160.013MC (L) 2.170.45 69.019.33 0.0920.003HPC (L) 3.570.03 143.93.76 0.1100.004HPC/Lipid 3.440.06 81.74.58 0.0820.003Cozeen 0.89 5.2526.10 0.407PE 8.30 26.1 PP 0.550.005 0.000650.06PVC 0.0917.99 1.3526.98 0.00071PET 0.130.30 0.671.12

a SPE is Sucrose Polyester; (L) refers to low level of plasticizer; MC is Methyl cellulose; HPC is Hydroxypropyl cellulose; PE is Polyethylene;PP is Polypropylene; PVC is Polyvinyl chloride; PET is Polyester [22,29,3133]b Unit of permeability is in .m/m2.s.Pa; f is the abbreveation for femto (1015)c Unit of permeability is ng.m/m2.s.Pa; n is the abbreviation for nano (109)

256 H.J. Park / Trends in Food Science & Technology 10 (1999) 254260


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PlexiglassTM and an apparatus for diusivity measure-

ment as shown in Figs. 1 and 2. Gas diusivities of

exocarp plus pericarp, pericarp and stem scar increased

as the tomatoes developed from green stage to red stage.

During the ripening process the progressive loss of

rmness is the result of a gradual transformation ofprotopectin into pectin which is degraded by enzyme,

polygalacturonase, in the cell wall [42]. This enzymatic

degradation of pectin is probably attributed to greater

diusion of gases in the bulky organs of fruits.

Measurement of internal gas compositionA cylindrical plug of tissue can be removed from

individual fruits (orange, apple, tomato, cantaloupe,water melon, and pineapple) using a rubber stopper

corer. A glass tube can be sealed around the hole in the

surface of the produce sample. In order to measure

internal gas composition, gas in the glass tube can be

allowed to equilibrate with internal gases [16,17,43].

Then a gas sample can be taken from the glass tube with

a syringe injected through the sealing stopper. By

immersing both the produce sample and the attached

glass tube in water, atmospheric contamination at the

point of syringe insertion can be prevented. Gas samples

can be analysed by gas chromatography. Required

equilibrium times (when gas composition of the insideof the glass tube is constant) need to be determined by

periodically monitoring gas changes inside the glass

tube. Equilibrium time can be expected to vary with

variety, ripeness, temperature and harvesting season for

various fruits but, two hours is usually enough time [44].

Prediction of internal gas compositionUsing gas permeation data of edible coatings, diu-

sivity data of skin and esh of the fruits, and the math-

ematical models, internal gas composition can be

predicted for selected fruits. Predictions of internal gas

compositions with and without coatings will enable

better matches to be made between individual fruits andindividual edible coatings. The mathematical model

could be veried by comparing predicted and measured

internal gas composition for various coating materials

and thickness on selected fruits.

Gas diusion models will be determined according to

physical shape and composition of individual fruits. For

example, if one dimensional steady state diusion with a

constant diusion coecient is assumed, the gas diu-

sion model for a hollow sphere can be used to predict

internal oxygen composition of some fruits such as

apples and cantaloupes as follow: In one dimensional

diusion with a constant diusion coecient, the rate ofgas transfer in the sphere is [2527,40]:

dgad hdPgadrP Pardgad I

On substituting u=Cr in the equation (1), we have:

du/dt=D(d2C/dr2). At steady state, the dierential

equation of this case is:

drPdgadradr 0 P

In a hollow sphere where, r , if gas con-centrations are kept constant at the surfaces such that they

are C1 at r , and C2 at r , then g g1 r

Box 2. Diusivity calculation

Gas exchange in bulky plant tissue can be approxi-mated by Fick's rst law. The ux of a gas of Fick's law isdependent on gradient of concentration and diusivitiesof plant organs. But to determine the gradient of gases,Fick's second law can be employed [2528,40,41]. If dif-fusion is one dimensional and the diusion coecient is

constant, the rate of transfer through unit area becomes:

dgad hdgad I

At non-steady state, all the solutions can be obtainedeither by the method of separation of variables andFourier series or by the Laplace transformation.

If surface concentrations are constant, the followingboundary and initial conditions may apply:

g g1Y x 0Y ! 0

g 0Y x vY ! 0

g 0Y 0 ` x ` vY 0

The solution in the form of a trigonometrical series is:

gxY g11 xav

2apI

n1

g1an sinnxavxphn2p

2av2 P

As t approaches innity the terms involving theexpoential vanish and we simply have the linear con-centration distribution. The rate at which the gas emergesfrom unit area of the face x=L of the test sample is givenby hdgadxv, which is easily deduced from equa-tion (2). By integrating them with respect to t, we obtainthe total amount of diusing substance Qt which haspassed through the membrane in time t as follows:

avg1 hav2 1a6 2p

I

n1

1xphn2p2av2 Q

As t approaches innity, equation (3) approaches theline:

hgIav vPaTh R

This has a intercept L on the t-axis given by:

v vPaTh S

The intercept Lt is referred to as the `time lag'. Thus,the measured values of concentration of the diusionconstant from the linear portion of the plot [38,39].



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g2r ar . By integrating with respect to timet over the surface area, the total amount of diusing gas

Qt passing through the wall can be determined by:

RphppgP gIa Q

Where: Dapp is apparent diusivity of the hollow

sphere and a and b are constants for individual fruits

[2527,40].

However, at steady state the ux of oxygen passing

through the spherical fruit wall should equal the rate of

gas consumption, thus:

RphppgP gIa yP R

where: R(O2) is respiration rate of oxygen per fruit and

W is weight of the fruit.

Internal oxygen composition, C1, can be predicted

using equation (4). Correlation factors will be calculatedfrom actual measurement of internal gas composition.

Also, predicted internal gas composition of edible lm

coated fruits will be veried by measuring internal gas

composition [40,41,45]. Also optimum edible coating

thickness can be calculated for each produce-coating

combination as shown in Box 3.

Fig. 2. Apparatus for diusivity measurement: (1) diusion cell, (2) water bath, (3) ask, (4) mineral oil, (5) test gas inlet, (6) nitrogen inlet, (7)three-way valve, (8) three-way connector, (9) two-way valve, (10) sampling chamber, (11) silicone septum, (12) gas owmeter, (13) brass tub-ing. Diusivity measurement can be done by the following procedures in [38]. Each of the cored and sliced sample prepared for the studycan be placed in the diusion cell, and a pre-mixed gas (9.9% O 2, 10.1% CO2, 80.0%, N2) can be introduced to the supplying chamber. Theamount of CO2 and O2 diused through the sample in time t into the sampling chamber can be measured by gas chromatography. Thesampling interval is 5 min, and the total sampling period is 2 hr. The diusion cell is immersed in a water bath maintained at 21 C. All

equipment for determining gas diusivities are placed in a heat-insulated chamber, and the temperatures at several places inside thechamber monitored.

Fig. 1. Diusion cell: (1) sample holder, (2) gas chamber, (3) sample, (4) sample retainers, (5) threaded bush, (6) sealing O-ring, (7) tubingadapters, (8) thumb nuts, (9) thread rods. The diusion cell was constructed from PlexiglassTM for determining diusivities [38,39]. The cell iscomposed of three main parts: the sample holder, supplying chamber and sampling chamber. The face of each part be tooled for an O-ring

which provides a tight connection.



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Measurement of quality and shelf life changeQuality criteria for edible lm coated fruits must be

determined carefully, and the quality parameters must

be monitored throughout the storage period. For

example, color change and rmness are very important

quality parameters in some fruits. Color change, rm-

ness loss, ethanol fermentation, decay ratio and weight

loss of edible lm coated fruits need to be monitored

[21]. Color change can be monitored by the change in

hue angle. An Instron universal test machine can be

used to measure rmness with a non-destructive method

[45]. Sensory evaluation and consumer acceptabilitytests need to be examined during storage.

References

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Box 3. Optimal thickness for edible coating

The hollow sphere model can be used also to determinethe edible coating optimal thickness in some fruits, suchas apple and cantaloupe. In the edible lm coated appleand cantaloupe, the ux of oxygen passing through thespherical fruit wall from the center to the interface of thelm coating and the fruit surface should equal the ux ofoxygen passing throguh the edible coating from theinterface of the lm coating and the fruit surface to theatmosphere, and should equal the rate of oxygen con-sumption of the edible lm coated apple and cantaloupeat steady state [40,41,45].

4phppg2 g1a

4phzg2 gxa2 y2 I

Where: Rc(O2) is the oxygen consumption rate ofcoated fruits, D

czis the diusivity of edible coatings and

X is the thickness of the edible coating. Cx is oxygenconcentration at the surface between the edible coatingand the surface fruits.

Optimal coating thickness which will create a desirablerange of internal oxygen concentrations (C1) in apples,(i.e., 23%), and cantaloupe, (35%) will be calculatedfrom the following equation:

RphzgP gxPayP P

Where b+X becomes b when X is very small. Cx isdetermined from equation (1) with C2=Cx.



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33 Park, H.J., Jung, S.T., Song, J.J., Kang, S.G., Vergano, P.J. andTestin, R.F. (1998) `Mechanical and Barrier Properties of Chit-

osan-based Biopolymer Film' in Chitin and Chitosan Research5, 1629

34 Park, H.J., Bunn, J.M., Vergano, P.J. and Testin, R.F. (1994)`Water Vapor Permeability and Mechanical Properties of GrainProtein-based Films as Aected by Mixtures of PolyethyleneGlycol and Glycerin Plasticizers' in Transaction of ASAE 37,12811285

35 Ben-Yehoshua, S. (1985) Ìndividual Seal-packaging of Fruit andVegetables in Plastic FilmA New Postharvest Technique' inHort. Science 20, 3237

36 Burg, S.P. and Burg, E.A. (1965) `Gas Exchange in Fruits' in Phy-siological Plantarum 18, 870884

37 Cameron, A.C. and Yang, S.F. (1982) À Simple Method for theDetermination of Resistant to Gas Diusion in Plant Organs' inPlant Physiol. 70, 2123

38 Chinnan, M.S. and Park, H.J. (1995) `Determining Oxygen and

Carbon Dioxide Diusivities of Exocarp Plus Pericarp, Pericarp,and Stem Scar of Tomatoes' in Applied Engineering in Agri-culture 11, 393396

39 Floros, J.D. and Chinnan, M.S. (1989) `Determining the Diu-sivities of Sodium Hydroxide Through Tomato and CapsicumSkins' in J. Food Engineering 9, 129141

40 Solomos, T. (1987) `Principles of Gas Exchange in Bulky PlantTissues' in Hort. Science 22, 766771

41 Solomos, T. (1989) `À Simple Method for Determining the Dif-fusivites of Ethylene in ``McIntosh'' apples'' in Sci Hort 39, 311318

42 Hobson, G.E. and Davies, J.N. (1971) ``The Tomato'' in The Bio-chemistry of Fruits and Their Products, (Hulme, A.C., ed), pp.437482, Academic Press, London and New York

43 Banks, N.H. and Kays, S.J. (1988) `Measuring Internal Gases and

Lenticel Resistance to Gas Diusion in Potato Tubers' in J.Amer. Hort. Sci. 113, 577580

44 Park, H.J. (1999) `Determining and Predict Internal Gas Com-position of Korean ``Fuji'' Apples and Tangerines' Paper # P07/26, Presented at the 10th World Congress of Food Science andTechnology, October 38, Sydney, Australia

45 Park, H.J. (1991) Èdible Coatings for Fruits and Vegetables:Determination of Gas Diusivities, Prediction of Internal GasComposition and Eects of the Coating on Shelf Life' Doctoraldissertation, Univ. of Georgia, Athens, GA


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Edible in Fruit