Box 1 Gas permeability

The permeation process can be described mathematically by Fick's first law. The flux (J) which is proportional to the concentration gradient can be defined in one direction as follows:

where J is the flux, the net amount of solute that diffuses through unit area per unit time (gm-2s-1 or mlm-2s-1), D is the diffusivity constant (m2 s-1), C is the concentration gradient of the diffusing substance and X is the thickness of the film (m) (Chang, 1981; Crank, 1975; Jost, 1960; Landrock and Proctor, 1952).

With two assumptions, (1) that the diffusion is in a steady state and (2) that there is a linear gradient through the film, the flux (J) is given by:

where Q is the amount of gas diffusing through the film (g or ml), A is the area of the film (m2) and t is the time (s). After application of Henry's law, the driving force is expressed in terms of the partial pressure differential of gas and a rearrangement of terms yield the following equation in terms of permeability:

where S is the Henry's law solubility coefficient (molatm-1), Ap is partial pressure difference of the gas across the film (Pa) and P is the permeability ((ml or g) mm-2s-1 Pa-1).

Then, the permeabilities of O2, CO2 and H2O vapor can be calculated from equation [16.4]:

coatings such as zein. The OP permeabilities of protein films were lower than those of polyethylene (low density), polyethylene and polyvinyl chloride, and were close to that of polyester film. The OP permeabilities of protein films, corn-zein and wheat were also lower than those of cellulose films, methyl cellulose MC(L) and hydroxypropyl cellulose HPC(L) both with low levels (L) of plasticizer. The addition of lipid (Myvacet 7-00TM) into HPC film decreased the OP permeability only slightly.

The CO2 permeabilities of protein films, corn-zein and wheat were lower than those of plastic films, polyethylene (low density), polyethylene and polyvinyl chloride, with the exception of polyester film which exhibits a greater barrier to CO2 permeation (Table 16.1). CO2 permeabilities of cellulose films, MC(L) and HPC(L), were higher than those of plastic films. The addition of lipid (Myvacet

Table 16.1 O2, CO2 and H2O vapor permeabilities of edible coatings

Film

Permeability

Table 16.1 O2, CO2 and H2O vapor permeabilities of edible coatings

Film

Permeability

bO2

bCO2

cH2O Vapor

SPE

2.10 ± 0.0001

-

0.00042 ± 0.04

Chitosan (15 cp)

0.0014

-

0.49

Zein

0.36 ± 0.16

2.67 ± 1.09

0.116 ± 0.019

Wheat gluten

0.20 ± 0.09

2.13 ± 1.43

0.616 ± 0.013

MC (L)

2.17 ± 0.45

69.0 ± 19.33

0.092 ± 0.003

HPC (L)

3.57 ± 0.03

143.9 ± 3.76

0.110 ± 0.004

HPC/lipid

3.44 ± 0.06

81.7 ± 4.58

0.082 ± 0.003

Cozeen

0.89

5.25 ± 26.10

0.407

PE

8.30

26.1

-

PP

0.55 ± 0.005

-

0.00065 ± 0.06

PVC

0.09-17.99

1.35-26.98

0.00071

PET

0.13-0.30

0.67-1.12

-

PE is polyethylene, PP is polypropylene, PVC is polyvinyl chloride, PET is polyester (Aydt et al., 1991; Kamper and Fennema, 1984; Park, 1999; Park and Chinnan, 1995a, 1995b; Park et al., 1993, 1994a,d 1998).

b Unit of permeability is in flmm-2s-1Pa-1; f is an abbreviation for femto (10-15). c Unit of permeability is ngmm-2s-1Pa-1; n is an abbreviation for nano (10-9).

PE is polyethylene, PP is polypropylene, PVC is polyvinyl chloride, PET is polyester (Aydt et al., 1991; Kamper and Fennema, 1984; Park, 1999; Park and Chinnan, 1995a, 1995b; Park et al., 1993, 1994a,d 1998).

b Unit of permeability is in flmm-2s-1Pa-1; f is an abbreviation for femto (10-15). c Unit of permeability is ngmm-2s-1Pa-1; n is an abbreviation for nano (10-9).

7-00tm) into HPC film decreased the CO2 permeability by 43.2%. CO2/O2 permeability ratios of edible films were higher than those of plastic films (Kader et al., 1989).

SPE coatings provide very high water vapor barriers compared with other edible coatings, as shown in Table 16.1. WVPs of SPE coatings were lower than that of polyethylene film and more than 100 times lower than the values for cellulose and protein films. These high oxygen and water vapor barrier properties will make SPE coatings desirable for fresh produce as a replacement for wax (Risse et al. 1987; Segall et al. 1974). The WVPs of other edible coating films were much higher than those of plastic films. The WVP of wheat protein film was 0.603-0.630ngmsPa-1, the highest of all edible films tested. Wheat protein film exhibited high permeability to water vapor probably because wheat protein was dispersed by addition of ammonium hydroxide (6N) as part of the formulation, and also contained a higher concentration of plasticizer, 40% (wt. plasti-cizer/wt. protein). The addition of lipid (Myvacet 7-00TM) into HPC film decreased the water vapor permeability by 24.7%. Plastic is the most widely used food wrap, but water vapor commonly condenses on the inner surface of plastic packaging materials thus creating a potential source of microbial contamination in fresh produce (Ben-Yehoshua, 1985). Thus, a film with greater water vapor permeability is desirable, although a film with extremely high water vapor permeability is also not desirable as it can result in excessive moisture loss from fruits during storage.

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