end

Journal of Food Engineering 2 I( 1994) 395-409

Measurement Errors in Water Vapor Permeability of Highly Permeable, Hydrophilic Edible Films

Aristippos Germadios, Curtis L. Weller* & Charles H. Gooding b

Department of Agricultural and Biological Engineering, Clemson University, 235 McAdams Hall, Clemson, SC 29634-0357, USA, hDepartment of Chemical

Engineering, Clemson University, 211 Earle Hall, Clemson, SC 29634-0909, USA

(Received 28 May 1992; revised version received 2 October 1992; accepted 8 December 1992)

ABSTRACT

Water vapor transmission WVT of films is commonly measured using modifications of the ASTM E 96 Standard Method (cup method). A stagnant air layer exists between the underside of the film mounted on the cup and the sueace of the desiccant, saturated salt solution or distilled water contained in the cup. The method considers the air gap resistance to be negligible to water vapor transport. When high water vapor-transmit- ting hydrophilic edible films are measured with the cup method, the resistance of the stagnant air layer can be significant and, if neglected, can lead to underestimation of water vapor transmission rates. Equations were presented in this study to correct WVTdata for the air gap resistance. For both a methylcellulose and a corn zein film, water vapor permeabilities measured with air gaps of 1.0 and I.5 cm were statistically significantly (a = O-05) different. Values corrected to account for air gap resistance were not statistically significantly (a = 005) different. Literature data on water vapor permeability of other hydrophilic edible films were corrected to account for the air layer resistance. Underestimation of actual values ranged between 5 and 46X.

NOTATION

A Open mouth area of cup (m)

b Total molar concentration of air and water vapor (g mole/cm3) Diffusivity of water vapor in air (cm*/s)

*To whom correspondence should be addressed at: University of Nebraska-Lincoln, L.W. Chase Hall, East Campus, Lincoln, NE 68583-0726.

395 Journal of Food Engineering 0260-8774/94/$07.00 - 0 1994 Elsevier Science Limited, England. Printed in Great Britain

396

4, hi

L

NJ

P Pr Pwo

P WI P w2

PW3

w, T WVTR, WVTR,

APa

AP,

Aristippos Gennadios, Curtis L. Weller, Charles H. Gooding

Air gap between film and desiccant suspended over film (cm) Air gap between film and surface of distilled water, saturated salt solution or desiccant in the cup (cm) Fihn thickness (m) Measured value of water vapor transmission (WVT ) rate (g mol/cm2 s) Vapor pressure of water at the temperature of the test (Pa) Total atmospheric pressure (Pa) Partial pressure of water vapor in air at the surface of distilled water, saturated salt solution or desiccant in the cup (Pa) Partial pressure of water vapor at underside of fihn (Pa) Partial pressure of water vapor at the fihn surface outside the cup (Pa) Partial pressure of water vapor at the underside of desiccant suspended over film and cup (Pa) Corrected water vapor permeability (g/m s Pa) Measured water vapor permeability (g/m s Pa) Universal gas constant (8 306 600 Pa cm3/g mole K) Relative humidity over distilled water, saturated salt solution or desiccant inside the cup (Oh) Relative humidity outside the cup (Oh) Absolute temperature during testing (K) Corrected value of WVT rate ( g/m2 day) Measured value of WVT rate (g/m2 day) Apparent water vapor partial pressure difference across the film (Pa) Real water vapor partial pressure difference across the film (Pa)

INTRODUCTION

Edible films and coatings from proteins, polysaccharides and lipids have received increased interest in recent years as potential food protective materials. Research findings on production, properties and potential applications of edible films have been recently reviewed (Guilbert, 1986, 1988; Kester & Fermema, 1986; Krochta, 1992). Protein-based films from wheat, corn and soy proteins have been discussed by Gennadios and Weller (1990, 1991). WVT of these films is an important property, indicating their ability to control water vapor transport between a food system and its surroundings.

Water vapor permeability of edible films 397

Most of the data on WVT of edible films available in the literature have been obtained by using the ASTM Standard Test Method E 96430 (ASTM, 19891, kn own as the cup method, or variations of it. According to this method, a cup with an open mouth of known area is filled with distilled water or desiccant. In a modification of the method, saturated salt solutions are also used. A film specimen is sealed orrthe open mouth of the cup, the assembly is weighed, and placed under controlled relative humidity and temperature conditions. Partial water vapor pressure difference pwo - pw2 provides the driving force for water vapor flux through the film. The weight change of the cup is monitored by periodi- cal weighings. Weight gain or loss is plotted over time and when steady state is reached the plot is a straight line. The slope of this line divided by exposed film area (cup open mouth area) yields the (WVTR), of a tested film. According to this standard method, cups should be filled with water to a level of 2 + O-5 cm from the film specimen. When the cups are filled with desiccant an air space of O-6 cm between the under side of the film and the desiccant surface is recommended. This air gap is considered necessary to avoid film contact with the water or desiccant while hand- ling the cup.

The air gap (hi) between a film and the surface of the liquid or desiccant inside the cup is assumed to offer negligible resistance to water vapor transport. This method apparently works well for testing films of low WVTR. These were the films for which the test was developed. Edible films containing high amounts of lipid materials are indeed good water vapor barriers and their WVT can be tested with this method with no problems. The same does not hold true for protein- and polysaccharide-based films which are, in general, moisture sensitive and charac- terized by high WVT. In this case, resistance provided by the stagnant air in gap hi is significant. Neglecting the effect of this air resistance can lead to considerable underestimation of actual film WVTR. The apparent driving force for water vapor flux through film is pwO -pw2, whereas the actual driving force is pw , - pwz .

The effect of stagnant air space inside the cup has been apparently overlooked by researchers working with protein and polysaccharide edible films. WVT rates and permeabilities reported in the literature have not been corrected to account for the air layer resistance. Only recently, Krochta ( 1992) addressed this problem and provided an equation to calculate pwl .

Existence of a stagnant air layer above the cup could also result in significant resistance to water vapor transport. This problem is usually avoided by blowing air over the cup. For instance, most environmental chambers used to control relative humidity outside the cup provide

398 Aristippos Gennadios, Curtis L. Weller, Charles H. Goading

adequate air circulation. A variation of this method was reported where desiccant (RH, = 0%) was suspended with cheese cloth at a distance h, above the film outside the cup (Hagenmaier & Shaw, 1990). Air gap h, provides additional resistance to water vapor transport similar to the one of the gap inside the cup. As a result, apparent water vapor pressure difference across a film is pwO - pw,, whereas the actual one is pwl - pw2. Both pwL and pwz must be calculated in this case to correct the measured rate. Obviously this variation of the method is limited to suspension of desiccant over the film, since water or saturated salt solutions would drip on the film.

A point regarding use of the term permeability with edible fihns, especially hydrophilic ones, should be made. Water vapor permeability of a film is a constant that should be independent of the driving force for WVT. In other words, when a fihn is subjected to different water vapor pressure gradients (at the same temperature) the flux of water vapor through the film differs, but its calculated permeability should be the same. This does not happen with hydrophilic edible films where water molecules interact with polar groups in the film structure causing plasti- cization or swelling. Water vapor permeability was found to vary with the applied water pressure gradient for cellulosic films (Karel et al., 1959; Woodruff et al., 1972) and for amylose films (Rankin et al., 1958). Perm- eability of edible films based on lipid materials could also appear dependent on water vapor gradient due to clustering of water molecules into a nonpolar fihn structure.

Another assumption inherent to the calculation of permeability is its independence from film thickness. Studies have shown that this assumption also does not hold true for hydrophilic edible films. For pectinate films (Schultz et al., 1949), for amylose films (Rankin et al., 1958), and for a number of cellulosic films (Pate1 et al., 1964; Banker et al., 1966; Hagenmaier & Shaw, 1990) water vapor permeability has been found to increase with film thickness. A similar behavior has been observed with soy protein isolate and wheat gluten protein films in our laboratory.

As a result, experimentally determined water vapor permeability values of most edible films apply only to the specific water vapor gradients used during testing and for the specific thickness of tested specimens. Because of this, researchers have proposed use of the terms effective permeability (Biquet & Labuza, 1988) or apparent permeability (Kester & Fennema, 1989~).

The objectives of the present study were (1) to provide the necessary equations to correct WVT rates of hydrophilic edible films measured with the cup method; (2) to demonstrate the use of these equations for a protein-based and a polysaccharide-based film, and (3) to correct water

Water vapor pemzeabiliv of edible films 399

vapor permeability values of hydrophilic edible films reported in the literature.

MATERIALS AND METHODS

Corrective equations for WVT

Air gap resistance to WVT can be accounted for by applying the analysis of diffusion through a stagnant gas film presented by Bird et al. ( 1960). A schematic diagram indicating the locations of water vapor pressure values and air gap heights used in the following analyses is shown in Fig. 1.

1st Case: RH, > RH,, h, = 0

In this case pwo >P,, >pw2, pwO =pRH,/lOO, and pw2 =pRH,/lOO. The value of p can be found from the literature (Felder & Rousseau, 1978). Apparent and actual water vapor partial pressure differences across a film are:

AP, = Pwo - Pw2

AP, = Pwl - Pw2

The value of pwl can be calculated by

(1)

(2)

Pwi = PT- (PT- P,o) exp (NwhilcD )

cup- /////////. /////////. water,

+ - saturated salt solution ////f////.

or desiccant /////////.

/////////.

Fig. 1. Schematic diagram of water vapor permeability measurement cup indicating locations of water vapor pressure values and air gap heights.

400 Aristippos Gennadios, Curtis L. Weller, Charles H. Gooding

where

N,=(6.43 x lo-)WVTR, (4) Molar concentration c can be estimated from the ideal gas law:

c= pT/RT (5)

The diffusion coefficient (II) at 1.013 X lo5 Pa ( 1 atm) can be estimated by the following empirical equation (Bretsznajder, 197 1):

D= (O-26)( T/298)8 (6) The corrected value of water vapor transmission rate is given by

WVTR, = WVTR,KP,, - P~Z )/(P,, - P~Z 11 (7) 2nd Case: RH, < RH,, h, = 0

1nthiscasep,,p,,~p,,~p,,,p,,=pRH,/100,andp,,=pRH,/100.

APa = Pwo - Pw3 (12)

AP, = Pwl - Pw2 (13)

Both pw, and pw2 are unknowns. Equation (3) can be used to calculate pw , . For pw2 we have

pw3=pT-(pT-~w2) exp (Kh0lcD)

Solving eqn ( 14) with respect to pw2:

Pw2=PT-(PT-Pw3)exp(-N,h0/cD)

The value of WVTR, is given by

WVTR, = ~%,,[(P~o - pw3 )/(P,, - pw2 )I

(14)

(15)

(16)

Determination of water vapor permeability

Measured water vapor permeability can be calculated by

P,=(ll57 x 105)WVTR,L/Ap, (17)


Corrected water vapor permeability can be calculated from either of the following two equations:

P, = (1.157 x lO+)WVTR,L/Ap, (18)

P, = (1.157 x 10-5)WVTR,L/Ap, (19)

The coefficient l-157 X 10m5 in the above equations satisfies unit conversions.

Preparation of film-forming solutions

Corn zein (CZ) film-forming solutions were prepared by mixing 10 g of CZ (Freeman Industries, Tuckahoe, NY, USA), 2 g of glycerol (ACS grade, Baxter Diagnostics, McGaw Park, IL) and 65 ml of 95% ethanol. Methyl cellulose (MC) film-forming solutions were prepared by mixing 4.5 g of MC (viscosity 25 cP, Aldrich Chemical Co, Milwaukee, WI, USA), 1.5 g of propylene glycol (USP grade, Fisher Scientific, Pitts- burgh, PA, USA), 100 ml of 95% ethanol and 50 ml of distilled water. Both types of mixtures were heated for 10 min while being stirred on a magnetic stirrer/hot plate. The heating rate was adjusted such that the temperature of the solutions at the end of the preparation time was 75 to 77C. Upon removal from the hot plate, mixtures were kept at room conditions for 2 to 3 min until bubbling stopped and subsequently were cast.

Film casting and drying

Film-forming solutions were cast on flat glass plates with a thin-layer chromatography spreader bar (Brinkman, New York, USA) set at 1.5 mm. Casting areas on the plates were framed with thick layers of mask- ing tape to prevent spreading. Plates with cast solutions were placed in an air-circulating oven (Isotemp@, model 338F, Fisher Scientific, Pitts- burgh, PA, USA) maintained at 35C. After 15 h the plates were removed from the oven, films were peeled off, and specimens 7 cm x 7 cm in size were cut. Prior to testing for WVT rates, all specimens were conditioned for 48 h in a desiccator over anhydrous calcium sulfate (Drierite@, indicating, 6 mesh, Baxter Diagnostics, McGraw Park, IL, USA).

Thickness

The thickness of film specimens, which is necessary for water vapor permeability calculations, was measured with a hand-held micrometer

402 Aristippos Gennadios, Curtis L. Weller, Charles H. Good&

(B.C. Ames, Waltham, MA, USA) to the nearest 254 pm. Five measurements were taken on each specimen, one at the center and four around the perimeter, and their mean was used as the specimen thickness.

Measurement of WVT rate

Cups used to determine WVTR,,, were manufactured at the Department of Agricultural and Biological Engineering at Clemson University by modifying an original design provided by Dr J. M. Krochta (Department of Food Science and Technology, University of California, Davis, CA, USA). Each consisted of a cylindrical bottom made of poly(methy1 methacrylate) (Piedmont Plastics, Greenville, SC, USA), a lid of the same material, and a rubber O-ring (Fig. 2). The bottom had a diameter of 8.7 cm. A well 4.6 cm in diameter and 2.1 cm in depth was milled into the bottom. An O-ring (internal diameter 5.6 cm) was placed into a groove milled around the well. Four screws, symmetrically placed around the cup perimeter, were tightened to hold film specimens securely between the lid and the bottom part. Good sealing of film specimens between lid and bottom was provided by the O-ring.

Machine Screws

Fig. 2. Cup assembly used to measure WVT rates of edible films.

Water vaporpenneability of edible films 403

WVTR, of CZ and MC films was determined for two different air gaps inside the cup. From each type of fihn, eight samples were tested with hi = 1.0 cm and eight samples with hi = 1.5 cm. Wells in the cups were filled with distilled water (RH, = 100%). After films were mounted, the whole assembly was weighed and placed in an environmental chamber (Model 317332, Hotpack, Philadelphia, PA, USA) set at 25C and RH, = 50%. Additional weighings with an accuracy of 0.001 g were taken at c. 30 min intervals. Weight loss was plotted versus time and a straight line (steady state) was obtained after 2 to 3 h. Linear regression was used to estimate the slope of this line in g/day. Regression coeffi- cients of determination (R *) greater than 0.97 were calculated for all samples. WVTR, was calculated by

WVTR, = Slope/A (20)

Equations (3) and (7) were used to calculate pwl and WVTR,, respectively. Values of P,,, and P, were also calculated from eqns ( 17) and ( 18), respectively.

RESULTS AND DISCUSSION

Validation of corrective equations

Mean measured and corrected permeability values of the MC and CZ films are presented in Table 1. A difference in height of the air gap inside the cups resulted in different measured permeability values for both films. The mean P, value was significantly greater (a = O-05) when the air gap was smaller. Differences between P, values measured with larger and smaller air gaps were c. 26 and 17% for MC and CZ films, respectively. When permeability values were corrected to account for air resistance, differences due to unequal air gaps was diminished to c. 2 and 4% for MC and CZ films, respectively. A two-tailed Students t-test of significance showed that corrected values were not statistically different at the a = O-05 significance level.

As shown in Table 1, neglecting the stagnant air layer resistance resulted in a large error and a significant underestimation of actual permeability values of hydrophilic MC and CZ films. The higher the WVT rate through a film the more important is air gap resistance and the larger the error produced from overlooking this effect. In general, when corrected transmission rates and permeability values are within 5% of measured values, air layer resistance can be assumed negligible. Under the experimental conditions of the present study (T= 25C, RH, = 100%

404 Aristippos Gennadios, Curtis L. Weller, Charles H. Gooding

TABLE 1 Measured Water Vapor Permeability Values of Methyl Cellulose and Corn Zein Edible

Films and Corrected Values to Account for Stagnant Air Layer Resistance~b~c

Airgap (cm)

1.0 15

1.0 l-5

P, x IO (g/m s Pa)

26 f 0.3 2.1 f 0.1

4.0 + 0.2 3.4 _+ 0.2

P, x 1o0 (g/m s Pa)

Methyl cellulose 6.2 f 05 6.1 + 0.4 Corn zein 5.1+ 0.3 49k@4

Errold (%)

58 66

22 31

Measured and corrected permeability values are the mean of eight samples plus/minus one standard deviation. hTesting conditions were 25C and 50% (lOO%-50%) relative humidity gradient across the films. cMean thickness of the methyl cellulose and the corn zein films were 23 f. 1 pm and 89 f 10 pm, respectively. Error was calculated as [(PC- P,,,)/P,]lOO.

and RH, = 50%) corrected values will be within 5% of the measured values when WVTR, < 130 g/m2 day for hi = 1.5 cm.

Correction of Iiterature data

g/m* day for hi = 1.0 cm and WVTR, < 85

Water vapor permeability data of several hydrophilic edible films reported in the literature were corrected using the equations presented in this study. Reported and corrected values are shown in Table 2. Errors due to neglecting air gap resistance ranged between 5 and 46%. In the articles referenced in Table 2, information on air gap depth was available making correction of these data possible. In several other sources, water vapor permeability values of hydrophilic edible fihns are given without specifying an air gap depth. Such cases include films from wheat gluten (Gontard et al., 1992), hydroxypropyl cellulose and methylhydroxy- propyl cellulose/ethyl cellulose (Banker et aZ., 1966) and cellulose esters (Pate1 et al., 1964; Munden et al., 1964; Lachman & Drubulis, 1964).

As mentioned earlier, WVT rates of edible films containing high amounts of lipids are fairly low, unless the films are very thin. As such, neglecting resistance of the air layer inside the cup is usually justified as demonstrated by the following. The calculated WVTR, value of a bilayer film made of hydroxypropyl methylcellulose plasticized with polyethy-


lene glycol and beeswax (Kamper & Fermema, 1984) was within 0.02% of the reported WVTR, value. For a 14.7 pm thick hydroxypropyl methycellulose film containing 42% stearic acid (Hagenmaier & Shaw, 1990) the calculated corrected value was within 0.2% of the measured value.

The effect on WVT of a stagnant air layer in the cup was noticed by Schultz et al. (1949) in their water vapor transmission rate study of pectinate films. The transmission rate of a calcium sodium pectinate film was reduced by 5% when the dead air space was increased by 50%. From this observation these researchers concluded that the error intro- duced by the whole dead air space did not exceed 11%. Their conclusion assumed a linear relationship between air gap depth and error intro- duced from it, which is erroneous as illustrated by the equations presented here. Some permeability values from Schultz et al. (1949) were corrected to account for air space, and an error of 30 to 41% was estimated (Table 2).

In a few recently conducted studies, Permatran-W measuring systems (Macon, Minneapolis, MN) were used to determine water vapor permeability of edible films (Greener & Fennema, 1989a, b; Kester & Fennema, 1989b; Germadios et al., 1990, 1993). These systems operate according to the ASTM Standard Method F 1249-89 (ASTM, 1989). Film specimens are mounted between the upper and lower halves of temperature-controlled diffusion cells. A couple of absorbent pads moistened with distilled water or saturated salt solution to achieve a desired RH, are placed in the lower half of the cells. Water vapor trans- mitted through the films is carried to an infrared detector by dry nitrogen (RH, = 0%) flowing over the upper side of films. As with the cup method, an air gap of O-2 to 0.3 cm exists between the saturated pads and the underside of the film. When edible barriers that are high water vapor transmitters are tested, the effect of this gap could be important. The underestimation error would be greater than 5% when WVTR, values approach or exceed 1000 g/m* day. Gennadios et al. ( 1990,199 1) used a Permatran-W600 instrument to measure WVT rates of wheat gluten-based films at 23C and RH, = 11%. Due to the small relative humidity gradient applied across the films, errors induced by neglecting air gap resistance were smaller than 2%.

CONCLUSIONS

Water vapor permeability values of highly permeable, hydrophilic edible films, as determined with the cup method, are underestimated when air gap resistance in the cup is neglected. Furthermore, since the magnitude

TA

BL

E

2 W

ate

r Vapor

Perm

eabili

ty V

alu

es

of

Hyd

rophili

c Edib

le F

ilms

as

Report

ed in

the L

itera

ture

and C

orr

ect

ed fo

r Sta

gnant A

ir

Layer

Resi

stance

Ref

eren

ce

Film

T

hick

ness

P

,, x

IO

P

, x

100

E

rror

L1

Con

ditio

&

(pm

) M

m

s P

a)

(glm

s P

a)

(W

Ayd

t et

a/.

(19

91

) A

ydt eta

f.(1

99

1)

Hagenm

aie

r & S

haw

( 19

90

) H

agenm

aie

r & S

haw

(19

90

) H

agenm

aie

r & S

haw

(19

90

) H

agenm

aie

r & S

haw

(19

90

)

Hagenm

aie

r & S

haw

( 19

90

)

Kam

per &

Fennem

a ( 1

98

4)

Wheat g

lute

n/g

lyce

rin

Corn

zein

/gly

ceri

n

(Hyd

roxy

pro

pyl

)meth

ylce

llulo

se

(Hyd

roxy

pro

pyl

)meth

ylce

llulo

se

(Hyd

roxy

pro

pyl

)meth

ylce

llulo

se

(Hyd

roxy

pro

pyl

)meth

ylce

llulo

se

+ 2

% s

teak

aci

d

(Hyd

roxy

pro

pyl

)meth

ylce

llulo

se

+ 4

2%

ste

ari

c aci

d

(Hyd

roxy

pro

pyl

)meth

ylce

llulo

se

/Poly

eth

ylene gly

col

/Hydro

genate

d co

ttonse

ed and

soyb

ean oils

140

12

.5

23.1

46

89

4

.1

5.3

23

38

1.3

1.

7 24

28

0

.7

0.9

22

8 O

-5

0.8

38

19

0.7

@

9 22

4

125

O-0

8

1.8

O-0

9 1

1

27C

, O

/85%

RH

l-9

5

26C

, 5

0/1

00

% R

H

26c

, 5

0/1

00

% R

H

27C

. O

/85%

RH

27

C,

O/8

5% R

H

27C

, O

/85%

FW

27

C,

O/8

5% R

H

25C

85

/O%

RH

Kanig

& G

oodm

an ( 1

96

2)

Park

& C

hin

nan (1

99

0)

Park

& C

hin

nan (1

99

0)

Park

& C

hin

nan ( 1

99

0)

Ranki

n e

f al.

( 19

58

) R

anki

n e

t al.

(19

58

) Sch

ult

z et

al.

( 19

49

) Sch

ult

z et

al.

(1949)

Sch

ult

z et

al.

( 19

49

) Sch

ult

z et a

l. ( 1

94

9)

Meth

ylce

llulo

se

Wheat g

lute

n/g

lyce

rin

Meth

ylce

llulo

se/p

oly

eth

ylene

gly

col

(Hyd

roxy

pro

pyl

)meth

ylce

llulo

se

/Poly

eth

ylene gly

col

Am

ylose

A

myl

ose

Sodiu

m h

ydro

gen pect

inate

C

alc

ium

sodiu

m p

ect

inate

C

alc

ium

sodiu

m p

ect

inate

C

alc

ium

sodiu

m

Pect

inate

/gly

ceri

n

5

0.1

45

0

.15

2

5

37

.8C

, 91

/O%

RH

4

00

6

.2

7.0

1

1

21C

85

/O%

RH

5

5

0.9

1

.1

18

21C

85

/O%

RH

50

1.1

1

.3

15

21C

85

/O%

RH

29

3

.3

3.6

8

2

9

2.2

3

.8

42

2

7

6-l

10

.0

39

3

5

6.1

8

.8

31

3

5

7.7

1

3.1

4

1

46

8

.7

12

.9

33

25

C, 1

00/l%

RH

25C

8 l/

29%

RH

25C

81

/31

%R

H

25C

81

/31

%R

H

$

3.5

C, 7

8/4

0%

RH

25C

81

/31

%R

H

i 2 w

((Err

or w

as c

alc

ula

ted as [

(PC

- P

,)/

PC

] 100

. 3

RH

s are

outs

ide and in

side th

e cu

p, r

esp

ect

ively

. B

A

ir gap d

epth

is n

ot dir

ect

ly s

peci

fied in

text.

A v

alu

e o

f 0

.6 cm

was u

sed fo

r th

e c

alc

ula

tions a

s in

dic

ate

d in

the A

STM

E 9

6 d

esi

ccant

g

meth

od fo

llow

ed b

y th

e re

searc

hers

. Q

Air

gap depth

is n

ot s

peci

fied in

the te

xt.

A v

alu

e o

f 1

.5 cm

was u

sed a

fter p

ers

onal c

om

munic

ation w

ith a

uth

or H

. J.

Park

. * &

g

408 Arisiippos Gennadios, Curtis L. Weller, Charles El. Gooding

of the air gap varies between different research studies, accounting for stagnant air layer resistance is an essential requirement prior to compar- ing water vapor permeabilities of hydrophilic edible Nms and coatings developed and tested at different laboratories.

ACKNOWLEDGEMENTS

This research was supported by a South Carolina Agricultural Experi- ment Station Enhancement in Packaging Research Competitive Grant. Technical Contribution No. 3301 of the South Carolina Agricultural Experiment Station, Clemson University, Clemson, SC, USA.

REFERENCES

ASTM (1989). Standard test methods for water vapor transmission of materials. E 96-80. In Annual Book ofASTM Standards, Vol. 1.5. American Society for Testing and Materials, Philadelphia, PA, USA, pp. 745-54.

Aydt, T. P., Weller, C. L. & Testin, R. F. ( 1991). Mechanical and barrier properties of edible corn and wheat protein films. Trans. ASAE, 34,207-l 1.

Banker, G. S., Gore, A. Y. & Swarbrick, J. ( 1966). Water vapor transmission properties of free polymer films. J. Pharm. Pharmac., l&457-66.

Biquet, B. & Labuza, T. P. (1988). Evaluation of the moisture permeability characteristics of chocolate films as an edible moisture barrier. J. Food Sci., 53,989-98.

Bird, R. B., Stewart, W. E. & Lightfoot, E. N. (1960). Transport Phenomena. John Wiley & Sons, New York, USA, pp. 522-6.

Bretsznajder, S. ( 197 1). Prediction of Transport and Other Physical Properties of Fluids. Pergamon Press, Oxford, UK.

Felder, R. M. & Rousseau, R. W. ( 1978). Elementary Principles of Chemical Processes. John Wiley & Sons, New York, USA, pp. 538-40.

Gennadios. A. & Weller, C. L. (1990). Edible films and coatings from wheat and corn proteins. Food Technol., 44 (lo), 63-9.

Gennadios, A. & Weller, C. L. ( 1991). Edible films and coatings from soymilk and soy protein. Cereal Foods World, 36, 1004-9.

Gennadios, A., Weller, C. L. & Testin, R. F. ( 1990). Modification of properties of edible wheat gluten films. ASAE Paper No. 90-6504. American Society of Agricultural Engineers, St Joseph, MI, USA.

Gennadios, A., Weller, C. L. & Testin, R. E (1993). Modification of physical and barrier properties of edible wheat gluten-based films. Cereal Chem., 70, 426-9.

Gontard, N., Guilbert, S. & Cuq, J.-L. (1992). Edible wheat gluten films: Influ- ence of the main process variables on film properties using response surface methodology. J. Food Sci., 57,190-5, 199.

Greener, I. K. & Fermema, 0. (1989~). Barrier properties and surface characteristics of edible, bilayer films. J. Food Sci., 54, 1393-9.


Greener, I. K. & Fennema, 0. (1989b). Evaluation of edible, bilayer films for use as moisture barriers for foods. 1. Food Sci., 54,1400-6.

Guilbert, S. (1986). Technology and application of edible protective films. In Food Packaging and Preservation. Theory and Practice, ed. M. Mathlouthi. Elsevier Applied Science, London, UK, pp. 371-94.

Guilbert, S. (1988). Use of superficial edible layer to protect intermediate moisture foods: Application to the protection of tropical fruits dehydrated by osmosis. In Food Preservation by Moisture Control, ed. C. C. Seow. Elsevier Applied Science, London, UK, pp. 199-2 19.

Hagenmaier, R. D. & Shaw, P. E. ( 1990). Moisture permeability of edible films made with fatty acid and (hydroxypropyl)methylcellulose. J. Agric. Food Chem., 38,1799-803.

Kamper, S. L. & Femtema, 0. (1984). Water vapor permeability of edible bilayer films. J. FoodSci., 49, 1478-81, 1485.

Kanig, J. L. & Goodman, H. (1962). Evaluative procedures for film-forming materials used in pharmaceutical applications. J. Pharm. Sci., 5 1,77-83.

Karel, M., Proctor, B. E. & Wiseman, G. (1959). Factors affecting water vapor transfer through food packaging films. Food Technol., 13 (l), 69-74.

Kester, J. J. & Fennema, 0. R. (1986). Edible films and coatings: A review. Food Technol., 40 ( 12), 47-59.

Kester, J. J. & Fennema, 0. (1989a). An edible film of lipids and cellulose ethers: Barrier properties to moisture vapor transmission and structural evaluation. .I. Food Sci., 54, 1383-9.

Kester, J. J. & Fennema, 0. (1989b). Resistance of lipid films to water vapor transmission. JAWS, 66, 1139-46.

Krochta, J. M. (1992). Control of mass transfer in foods with edible coatings and films. In Advances in Food Engineering, eds R. P. Singh & M. A. Wirakartaka- sumah. CRC Press, Boca Raton, FL, USA, 5 17-38.

La&man, L. & Drubulis, A. (1964). Factors influencing the properties of films used for tablet coating. I: Effects of plasticizers on the water vapor transmission of cellulose acetate phthalate films. J. Pharm. Sci., 53,639-43.

Munden, B. J., Dekay, G. & Banker, G. S. (1964). Evaluation of polymeric materials. I: Screening of selected polymers as film coating agents. J. Pharm. Sci., 53,395-401.

Park, H. J. & Chinnan, M. S. (1990). Properties of edible coatings for fruits and vegetables. ASAE Paper No. 90-65 10. American Society of Agricultural Engineers, Chicago, IL, USA.

Patel, M., Patel, J. M. & Lemberger, A. P. ( 1964). Water vapor permeation of selected cellulose ester films. J. Pharm. Sci., 53,286-90.

Rankin, J. C., Wolff, I. A., Davis, H. A. & Rist, C. E. (1958). Permeability of amylose film to moisture vapor, selected organic vapors, and the common gases. Ind. Erg. Chem., 3, 120-3.

Schultz, T. H., Miers, J. C., Owens, H. S. & Maclay, W. D. ( 1949). Permeability of pectinate films to water vapor. J. Phys. Colloid Chem., 53, 1320-30.

Woodruff, C. W., Peck, G. E. & Banker, G. S. (1972). Effect of environmental conditions and polymer ratio on water vapor transmission through free plasticized cellulose films. J. Pharm. Sci., 61, 1956-9.

Documents

end