Dairy Systems - Practical Food Rheology - page 154

globules coated with α s1 -casein, β-casein, κ-casein, α-lactalbumin or
β-lactoglobulin. Images from confocal microscopy revealed that larger
and more irregular fat globules existed in cheese where globules were
coated with α s2 -casein or natural milk fat control. It was concluded
that coating fat globules with highly charged α s2 -casein prevents the
development of a strong protein network.

7 . 3 . 2 . 2 Uniaxial tension

Uniaxial tension can be performed on the same machine capable of do-
ing uniaxial compression except that the stress is applied in the opposite
direction. In general, tension testing is not suitable for routine mea-
surements due to complications associated with gripping and lengthy
sample preparation. Another important consideration is the geometric
coupling of fracture stress and fracture strain. When the gel fracture cri-
teria is a stress level, fracture strain is a function of elongation needed
to reduce the specimen cross section to an area producing the critical
fracture stress (Hamann et al ., 2006). This results in similar increase
or decrease of fracture stress and fracture strain with varying variables
such as concentration. It has been found that fracture strain from ten-
sion is lower than compression and torsion (Tang et al ., 1994; Hamann
et al ., 2006). Stress increases more rapidly in tension, causing the ma-
terial to fail at small strain. Another reason is that defects are more
exaggerated in tension due to the volume-increasing nature of the test.
The advantage of the tension test is the absence of the friction problem
as that which occurs in a compression test. Sample slipping and grip-
ping problem can be eliminated with special grip design (Gunasekaran
and Ak, 2003). A test piece that is thin in the middle and gluing the
specimen to the grip can help avoiding the problem (Rosenthal, 1999).
Lelievre et al . (1992) and Stading and Hermansson (1991) used molds
to form specimen and were able to work with fragile gels.
Tensile testing is of practical importance for mozzarella cheese, which
is one of the major ingredients in pizza. Consumer acceptability of
mozzarella cheese is mostly determined by its functional and textu-
ral properties such as shreddability, stretchability, meltability, hardness
and springiness. Fracture strain in tensile testing has been related to
stretchability (Ak et al ., 1993). Mozzarella processing involves knead-
ing and stretching the curd in hot water, resulting in materials with
oriented structure which exhibits anisotropy. This anisotropic property
is one factor to consider in tensile testing. Studying the effect of sam-
pling direction (parallel or perpendicular to protein fiber orientation) on
fracture properties of mozzarella cheese, Ak and Gunasekaran (1997)
found that fracture stress and strain were greater when cheeses were
pulled in the parallel direction than in the perpendicular direction. They