Page 363 - Biosystems Engineering

340 Cha pte r Ele v e n

propagate a tear is measured as the tear strength of the material. The
Elmendorf test is one of the most common methods for measuring
the tear strength of paper and plastic films. The tear in packages is
initiated by sharp edges of products (frozen beans, French fries) or
any other physical defects (microholes, microbubbles, sudden thick-
ness changes, cracks, etc.) in a package.

Stiffness This is the resistance of the paper, film, or sheet to distor-
tion or bending. Young’s modulus, the cantilever test, and Handle-O-
Meter stiffness tester are used to quantify the stiffness of packaging.

Creep Resistance Packages and packaging materials may be sub-
jected to low but constant stress for very long periods of time, for
example, packages stacked on a shelf and pressurized containers.
Creep resistance is the deformation measured over time under con-
stant stress. Most packaging materials and packages intended for
long storage under constant stress are tested for creep analysis.
Crease Resistance Resistance to repeated flexure or creasing of a film
or a paper may result in increased permeability to gases, reduced ten-
sile properties, or change in optical properties. Crease resistance is
measured as the number of cycles to failure or significant changes of
a material’s properties.
Cushioning This is an ability of a material to absorb energy of dynamic
stress (vibration). Materials with higher energy-absorption ability
provide better cushioning for food. The cushioning property of pack-
aging is important in providing protection to packaged foods against
dynamic stresses during manufacturing, transport, and distribution.

Coefficient of Friction This is a measure of ease with which the sur-
face of one material will slide over another. This property is impor-
tant for all packaging materials used for packaging foods. Knowing
the coefficient of friction of different materials is valuable for stacking
packages over each other, rolling films/paper in printing and lami-
nating equipment, sliding, or moving packages over conveyor belts.

11.3.2 Thermal Properties
Knowledge of thermal properties is required to quantify heat trans-
port in packages during package manufacturing, in-package process-
ing, and storage. Heat transfer occurs in materials during package
fabrication and may also occur in packages during processing (e.g.,
in-container sterilization and pasteurization) and storage (under fluc-
tuating temperatures).

Specific Heat This is defined as the amount of heat required to raise
a temperature of a unit mass by one degree. A larger specific heat of a