Food Science: Courses: FS 532

15. Water Vapor Permeability

Procedure
  1. Measure the inside diameter of a petri dish with a ruler.
  2. Measure the length and width, as well as weigh to four decimal points, a strip of packaging film.
  3. Lubricate the groove of the metal template with petroleum jelly.
  4. Center the strip of packaging film on the template.
  5. Weigh out to one decimal point ca. 10g drierite in a small cup labeled on the bottom.
  6. Place the cup on the packaging film.
  7. Invert the petri dish and center it on the template.
  8. Pour molten wax into the grooves of the template that surround the inverted petri dish and place a weight on the dish.
  9. After the wax cools and hardens (few minutes), gently remove the petri dish from the template. Turn over the petri dish so that the drierite rests on the bottom.
  10. Cut away excess packaging film at the edge of the wax seal.
  11. Weigh the petri dish to four decimal points. This is the zero time value for the permeability experiment.
  12. Repeat steps 1-11 for a second petri dish and packaging film.
  13. Also, prepare a control sample by placing ca. 10g drierite in a petri dish and weighing the dish (with drierite) to four decimal points.
  14. Place all the three dishes in a dessicator, which has ca. 750ml water at the bottom. After the cover is placed on the dessicatant (drierite) is inside the petri dishes, 0% RH will be maintained there.
  15. At periodic intervals for ca. 24h, weigh each petri dish.
  16. Record each weight as a function of time.
Lab Report
  1. Graph weight change of each petri dish as a function of time.
  2. For the dishes with packaging films, also calculate the slopes of the plots using linear regression analysis.
  3. With the aid of the Appendix, calculate the WVTR and permeance of each film.
  4. Calculate the thickness of each film with the aid of the Appendix.
  5. Calculate the permeability (B) of each film from the permeance and film thickness with the aid of the Appendix.
Appendix

Gas permeability is based on Fick's first law of diffusion, and is often descrbied with the following equation:

GTR = P (p1 - p2) ...(1)

GTR = Gas Transmission Rate = Q/At

Q = quantity of gas transmitted (mass, volume or concentration)

A = film area

t = time

P = permeance

p1 - p2 = partial pressure difference of the gas in question across the film

For transmittance of water vapor, GTR is referred to as WVTR (Water Vapor Transmission Rate), and permeance is referred to as WVP (Water Vapor Permeance). Eq (1) then becomes

WVTR = WVP (p1 - p2) ...(2)

Partial pressure difference is usually expressed in terms of relative humidity (RH):

RH = p/pw ...(3)

p = partial pressure of water vapor

pw = water vapor pressure

Water vapor pressure is the same as the absolute pressure found on a steam table. Insertion of (3) into (2) yields

WVTR = WVP ( RH1 - RH2 ) pw ...(4)

Since WVTR is the same as Q/At in Eq. (1), Eq. (4) can be expressed

Q = WVP (RH1 - RH2)Apwt ...(5)

From (5), the slope of a plot of Q vs t equals

WVP (RH1 - RH2) Apw ...(6)

In the present experiment A, pw, and RH are known, so WVP can be calculated from the slope. The more fundamental parameter is permeability (B), however. Whereas permeance is dependent on film thickness, permeability, atleast in principle, is not. Permeability is given by

B = WVPz ...(7)

z = film thickness

Film thickness can be estimated from the density and dimensions of the film. For a rectangular piece of film, volume (V) is given by

V = xyz ...(8)

x = film length

y = film width

Density (d) is mass (m) per unit volume (V), so

d = m/V = m/xyz ...(9)

Film thickness can be calculated with the aid of Eq. (9), provided film density is known.

University of Wisconsin College of Agricultural & Life Sciences
Copyright © 2003, Department of Food Science,
University of Wisconsin-Madison. 		Updated February 20, 2003.
foodsci@facstaff.wisc.edu