Dairy Systems - Practical Food Rheology - page 153

and the degree of compression. The tendency for stress at fracture to
increase with compression speed has been found for cheddar, Cheshire
and Leicester (Dickinson and Goulding, 1980; Ak and Gunasekaran,
1992; Brown et al ., 2003). At higher deformation rates, the viscoelastic
cheese has less time to relax during compression, giving higher val-
ues of the measured stress. Crosshead speed may be selected to mimic
chewing speeds, which will vary from person to person and depend
on type of foods, etc. Reported rates of jaw movement during chewing
range from 15 mm/s to 30 mm/s (Langley and Marshall, 1993). The
average bite velocities from chewing 15.9 mm 3 cheese sample ranged
from 19.8 to 35.1 mm/s (Meullenet et al ., 2002). More information on
sample preparation for uniaxial compression test can be found in van
Vliet and Peleg (1991).
With feta cheese, Wium et al . (1997) found the high correlation be-
tween instrumental results (stress at fracture, work at fracture as calcu-
lated by the area under the stress-strain curve until fracture and deforma-
bility modulus as calculated by the maximum slope of the stress-strain
curve) and hand and oral firmness. The main component in cheese that
builds the structure and gives the solid character is casein. Decreasing
firmness, as shown by a decrease in fracture stress during ripening, cor-
responds to an increase in proteolysis and a decrease in intact casein
(Rynne et al ., 2004). Increasing pasteurisation temperature significantly
reduced fracture stress, fracture strain and firmness of cheeses due to in-
creased moisture content and thereby reduction in protein concentration
(Rynne et al ., 2004). Another possibility is that the level of denatured
whey proteins (which complex with casein micelles during heating)
increases and thus reduces the connectivity among the casein network.
Besides casein, fat also plays an important role in the texture of
cheese. The structure and texture of cheeses are affected by the inter-
actions between the surface of milk fat globules and the casein matrix.
Native milk fat globules do not interact with the protein network in
cheeses and act mainly as inert fillers or structure breakers, depending
on their size and number (van Vliet, 1988; Michalski et al ., 2002). Fat
has been replaced with other ingredients in making low fat cheeses.
Mozzarella cheese containing soy protein produced strong gel networks
as indicated by highest compressive stress, showing that soy protein
functioned like an increase in casein rather than a decrease in fat (Hsieh
et al ., 1993). Though egg, whey and caseinate significantly influenced
the magnitude of G and G , they showed no effect on compressive
stress. Everett and Olson (2003) studied the effect of the surface coating
on fat globules on rheological properties during ripening of cheddar
cheese. Stress at fracture (firmness) decreased during ripening as ex-
pected from proteolysis. Cheeses containing α s2 -casein-coated globules
fractured at a lower stress and a smaller strain than those containing