LETTERS

Like Dr. Doar at Clemson in the article, “Predicting Permeability and Transmission Rate for Multilayer Materials” by Kay Cooksey, Kenneth S. Marsh, and Leroy H. Doar (September, p. 60), I was similarly confused after consulting various packaging textbooks and refereed literature while developing a packaging course for our department’s IFT accreditation application.

The situation is such that, in what are otherwise good textbooks, the authors’ confusion (or neglect) over terms and units for permeance and permeability renders calculations on shelf life useless for technologists and obfuscating for students. Although the article will help to dispel some of the confusion, two amendments to their article would improve matters further.

First, of the terms permeance and permeability, the permeance (or transmission rate) of the film is met first, because a manufacturer or a student cannot measure the film’s permeability experimentally, only its permeance. After measuring how much vapor passes through the film in a given time (its permeance), we derive the permeance capacity of a unit thickness of that film (its permeability) by multiplying the measured permeance by film thickness. Thus, these extensive and intensive properties of the film are analogous to the feeling and capacity of feeling exhibited by Jane Austen’s characters in Sense and Sensibility. 

Second, the absence of parentheses in Table 1 would mean that a strict adherence to their units would lead to erroneous values for permeability and transmission rate, since all their terms except for film area are expressed as numerators; parentheses after the divisor symbol are recommended to eliminate this error (see Dodd, I. S. 1997. The ACS Style Guide, 2nd ed. Am. Chem. Soc., Washington, D.C.). However, a more desirable method of expressing units is to adhere to a rationalized coherent system of units (SI) and express denominators with a negative exponent, although in packaging applications we prevent canceling of units so that the contributing physical mechanisms are expressed in the units. In this way, permeance has units of kg m^–2 s^–1 Pa^–1, while permeability has units of kg m m^–2 s^–1 Pa^–1 (since at a given temperature gases of interest have a certain density). Such strict formalism in expressing units may seem unwieldy, but correct usage will insulate our profession from the vilification expressed by some physical scientists (see Chatwin, P. and Katan, L. 1997. Migration from packaging into food. Math. Today, Oct., pp. 142-145) when we get our dimensions wrong.

—Martin Scanlon, Associate Professor, Dept. of Food Science, University of Manitoba, Winnepeg, Manitoba, Canada

CORRECTION
The Ingredients column, “Sipping into the Beverage Mainstream” (November, p. 78) incorrectly identified Organic Echinacea Elder, a new tea from Traditional Medicinals, as “a blend of elderberry organic chai, and echinacea.” It is actually a blend of Echinacea purpurea, elder, ginger, peppermint, and yarrow, all organically grown. It contains no chai tea, organic or otherwise.