3.2.2. Trapped air influence on cooling process
Concerning with the cooling stage, the shaped hard candies are
cooled through a cooling tunnel, where the candies moisture does
not change. It is well known that the presence of air bubbles in
hard candies increases the resistance to heat transfer (internal
thermal resistance) due to the low value of air thermal conductivity
and consequently the cooling process is not efficient. Certainly,our previous results showed that the thermal conductivity of hard
candy is the most relevant thermal property for the heat transfer
process during the cooling stage of hard candies [23]. The behavior
of the candy heat transfer model depends critically on the value of
thermal conductivity, and has a weak dependence on the other
parameters such as product density and heat capacity.
Regarding with the presence of air in solid matrixes, Sakiyama
and collaborators [24] studied the air influence on hydrogels. The
results revealed that for air-impregnated gels with low water content,
the effective thermal diffusivities measured at 20 C were in
good agreement with the predicted values. For air-impregnated
gels with high water content, the effective thermal diffusivities
were well approximated by the predicted values up to 50 C. At
higher temperatures, the model tended to overestimate the effective
thermal diffusivity especially for the gels with high porosity.
On the other hand, Shariatt-Niassar and collaborators [25] concluded
that the entrapment of fine air bubbles could not be
avoided in practical food processes like extrusion.
Figs. 5 and 6 clearly show the presence of trapped air bubbles
[B] at the nearby zones to hard candy edge. As was mentioned
above, this trapped air has implications for the hard candies processing.
Air bubbles appear in the candy dough due to a slowly
cooling during the processing stage of dough tempering and
kneading after the cooking stage, where the dough temperature
goes from 140 C to 85 C.
The presence of air entrapped bubbles with higher magnifications
can be seen from Figs. 7 to 9. Precisely, Fig. 7 shows a selected
area of Fig. 6 while Fig. 8 refers to the same zone with higher magnification
than Fig. 7. Fig. 9 exhibits another sample with the presence
of air bubbles, in which the phenomenon is also observable in the nearby zone located between the crust and the center of the
sample.
Another complication caused by the presence of entrapped air
in food products is the determination of food thermal properties
due to the need of high parameter precision for the control of
the main processing parameters (temperature and velocity of cooling
air, residence time). Limited experimental techniques with
high accuracy are available to calculate thermal conductivity and
diffusivity of solid foods. For this reason, it is common the application
of different regression models for parameter estimations,
which are then used for process modeling. A detailed review on
several correlations can be found in Sweat [26] and Heldman
[27]. The correlations developed by Choi and Okos [28] are the
most widely used because they consider the dependence of the
thermal properties as function of the moisture and they can be applied
for a wide variety of food products. However, these correlations
do not consider the effect of microstructural arrangements,
which in many cases have significant influences [29]. Therefore,
the use of such correlations introduces uncertainty in the estimation
of thermal–physical properties.
In addition, large variations are expected due to the complex
structure of foods, being in several cases multi-phase systems.
Some structural models considering parallel, series, mixed or random
phases for the estimation of thermal conductivity are reviewed
by Aguilera and Stanley [29]. However, the effects of
undesirable structural aspects on thermal property estimation, like
the presence of trapped air bubbles, have not been widely
considered.
The use of micrographics reveals the presence of trapped air.
The subsequent step is to determine if the presence of this critical
phenomenon influences the thermal conductivity. The analysis
was done using the Choi and Okos’s correlations [28]. It is expected,
with the presence of air, that the real thermal conductivity
will be lower than the estimated.
Images corresponding to multiple sections of the selected areas
with bubbles were employed for characterization and quantification
of the air content in the hard candies. The result of air content
for all the samples examined expressed as mean value ± standard
deviation was 5.71 ± 1.72%. When this value was taking into account
to compute the thermal conductivity using Choi and Okos’s
correlation, the results showed that the actual thermal conductivity
(ka, contampling the air content) was lower than the one estimated
without considering the air presence (ke), however the
value is included within the correlation standard error interval
(±5%). The results are shown as follows:
ka = 0.2729 ± 0.0044 W/m C,
ke = 0.2821 ± 0.0141 W/m C.
Thus, the presence of trapped air in hard candy can be neglected
because its effect on the numerical value of the thermal conductivity
decrease the same order of magnitude as the standard error (%)
considered by the Choi and Okos’s correlation.