3. Results and discussion
3.1. Glass transition
The glass transition takes place over a range of temperature.
Currently, there is no agreement on the definition of Tg point on
a DSC curve among the various points that may be chosen (onset,
midpoint, end-point), since none of them has a clear physical
meaning. However, it is widely accepted that the glass transition
should be reported with at least two parameters indicating its onset
or midpoint and the width of the transition.
Fig. 3 shows the temperature profile obtained from the DSC
experiments. The heat flow curve begins at the top (endothermic
down), and the sigmoidal change is construed as evidence of vitrification
phenomena. The midpoint of this thermal event is readily
detectable and is considered as the glass transition obtained from
DSC thermogram.
As is shown in Fig. 3, the experimental values of the glass transition
temperatures (mean value ± standard deviation) graphically
calculated as it was described in the work of Shamblin and Zografi
[16] as well as Liu and collaborators [17] are:
Tg onset = 35.36 ± 1.48 C,
Tg midpoint = 35.85 ± 1.51 C,
Tg endpoint = 36.37 ± 1.63 C.
The experimental values of hard candy Tg are above room temperature,
just as was described by Liu and collaborators [17]. Also,in agreement with McFetridge and collaborators [18], Tg were in
the temperature range between the Tg values of the individual sugar
components.
The knowledge of the hard candy glass transition is important
not only to ensure the quality during the storage [19] but also to
control the temperature range during the cooling stage in order
to reach the glassy structure.
In organic glasses, the increase of moisture content and storage
temperature plays an important role on the rate of deteriorative
reactions. The most important change affecting the behavior of
amorphous carbohydrates is the plasticization which occurs at a
quite narrow temperature range above Tg. This phenomenon leads
to a dramatic decrease in viscosity, and therefore an increase in
molecular mobility [20] which cause different time-dependent
structural transformations during storage (stickiness, cold flow
and crystallization) [21].
In the case that the storage temperature of an amorphous product
is lower than its corresponding Tg, the product exists in a highly
viscous glassy state and the diffusion-limited processes, like
crystallization, become extremely slow [13]. Nevertheless, the
moisture sorption in amorphous products during storage can
dramatically lower the Tg. When this temperature is lower than
the storage temperature, the amorphous state becomes less
viscous and consequently the crystallization may exist [13]. The
addition of glucose syrup and fructose enhances the physical
stability interfering with crystallization [20].
With these results, it is easy to conclude that the lower the storage
temperature that below its glass transition temperature, the
bigger the prevention effect of undesired changes.
On the other hand, the glass temperature (Tg) is also important
to determine the cooling operating conditions in order to obtain
adequate temperature leaving the cooling tunnel. The maximum
admissible final temperature is 34 C to reach the glassy structure.
In addition, because of hard candy composition, its thermal conductivity
is very low (approximately 0.28 W/m C), therefore a radial
temperature transient is expected within a hard candy item.
Thus, temperatures lower than 34 C must be reached at the center
of each hard candy article. Taking into account these considerations,
it is then useful to develop a mathematical model to determine
the optimal operating conditions (air cooling temperature
and velocity as well as residence time). Moreover, the model will
allow contemplating explicit constraints associated with quality
aspects, for example, to impose the minimum value of the radial
temperature difference.
It is generally known that Tg of food products depends on its
composition, especially with the water content. For example, some
studies were conducted to demonstrate that Tg is directly related to
the water entrapped in the matrix by Cardoso and Abreu [10] as
well as Johari and collaborators [11] for sugar-related glasses. So,
it is expected that Tg values for different candy samples with similar
sugar formulation but different moisture contents will take a
similar behavior that the last cited work [11]. From our experimental
results (not shown), it can be concluded that Tg decreases as the
water content increases due to the low glass transition temperature
of water (135 C) as is mentioned above.
According to this, the determination of Tg for each candy formulation
is required in order to assure a high product quality during
the cooling stage and storage.