Physical Properties of the Cakes
Cake Volume and Density. Figure 1 shows the cross
section of the cakes containing different levels of SPI
showing that the samples were different in terms of height
and porosity. Therefore, cake density, volume and height
were calculated for each sample (Table 1). The cake density
decreased from 0.396 to 0.366 (g/cm3), the volume
increased from 542.18 to 598.91 cm3 and the height
increased from 4.72 to 6.00 cm after addition of 30% SPI.
Ronda et al. (2011) indicated that addition of 10 and 20%
SPI in the gluten-free cakes resulted in the highest cake
volume. Sahin (2007) indicated that the protein content
significantly affected the density, volume and height of the
cakes, possibly by retaining more air produced during
mixing or the CO2 produced from the baking powder. Sufficient
batter consistency is necessary to retain enough
gasses in the batter during baking resulting in the suitable
cake volume (Stauffer 1990). Based on the results in
Table 1, increasing the SPI level resulted in an adequate
batter consistency leading to lighter cakes of higher
volume. In addition, SPI may stabilize the films form normally
by egg proteins and oil around the air bubbles to
prevent their collapse or early escape during baking. The
effects of SPI on starch and proteins during baking are
important because these two components have great influences
on the final cake volume. During baking, the batter
is converted to a porous semisolid mainly because of the
starch gelatinization and protein coagulation. Both these
transitions are obviously dependent on the starch and
protein origin. When an increase in starch gelatinization
and protein coagulation temperature takes place, the
change of batter from a fluid to a porous solid happens
later, allowing the cake to increase in volume for a longer
time (Stauffer 1990). It has been shown that soy proteins
can increase starch gelatinization temperature and
enthalpy of gelatinization (Li et al. 2007) that can result in
more expansion of the batter during baking. Moreover, it
has been signified that soy proteins unfold exposing reactive
cysteine residues at above 70C during baking and consequently
on cooling, a network strengthened by hydrogen
bonds develops. Therefore, covalent interactions between
gluten and soy proteins and the development of a gel
network would occur during baking (Roccia et al. 2009;
Bainy et al. 2010). The gel network can strengthen the cake
structure and hinder its collapse after baking.