Page 365 - Biosystems Engineering

342 Cha pte r Ele v e n

their aesthetic characteristics and viewing the food through the pack-
age. Transmittance of ultraviolet light though the packaging material
may be important for food protection.

Transmittance This is the ratio of the light passing through to light
incident on the material, and reflectance is the ratio of the reflected
light to the incident light. Haze indicates a material’s “milkiness” or
“cloudiness.” For transparent material, transmittance is measured as
the percentage of transmitted light that deviates more than 2.5 percent
from the incident radiation. Surface imperfections and in homogene-
ities in a material’s structure affect the haze of material. Clarity is a
measure of degree of distortion of an object when viewed through the
material. The ability of a material’s surface to simulate a mirror (shiny
surface) is defined as (specular) gloss. The refractive index is the mea-
sure of refracted light when it passes through a medium of one den-
sity to a medium of another density. The refractive index depends on
density of the (packaging) material. The higher the index of refraction
of a transparent material, the clearer the vision appears.

11.3.4 Properties of Mass Transport
Metal and glass are impermeable to gases and vapors and cellulose
and polymer-based packaging are not. The transport of water vapor,
oxygen, carbon dioxide, and other gases can influence the quality,
and the shelf life, of packaged foods. A decrease in moisture in fresh
produce can lead to texture changes and economic loss, whereas an
increase in moisture in dehydrated products can cause microbial
growth, increase enzyme activity, and change texture. Oxygen trans-
fer in a package may lead to quality deterioration in dehydrated
products (e.g., lipid oxidation in fatty foods) and loss of color and
nutrients. However, in some cases modification of the atmosphere
within packaging gases may be desirable, which is achieved by trans-
fer of gases through the packaging material, for example, modified
atmosphere packaging of fresh produce and freshly roasted coffee. 2
The transport of water vapor and gas through a polymeric material
is described by two processes: permeation and pore effect. Permeation
involves three steps: adsorption of gas or water vapor onto the high-
concentration side of the polymeric surface, diffusion across the film
thickness, and desorption of the gas or water vapor from the low-
concentration side of the polymeric surface. In the case of pore effect,
water vapor and gas flow through microscopic pores, pinholes, and
cracks in the materials. Fick’s first law of diffusion gases is used to
characterize transport of water vapor or gas through a polymeric film
[Eq. (11.1)]:

J =− D dc (11.1)
dx