Agriculture Reference
In-Depth Information
regression is interaction effect illustrating the connection of the drying temperature and the
temperature of rehydration. Similar conclusions of the regression coefficients significance
analysis of the polynomial model for the equilibrium moisture content for untreated sugar
beet pulp can be drawn. In this case, the most important factor is the rehydration temperature,
but it can be seen that its effect was reduced in comparison to that of the initial speed
rehydration. In fact, looking at the t-values of regression coefficients it can be concluded that
the effect of drying temperature is more pronounced at equilibrium moisture content. The
most important coefficient for the model of the equilibrium moisture content is intercept
which indicates that the calculated value of the equilibrium moisture content does not diverge
much from the calculated values of b
0
(Djuric et al. 2004). The main linear effect on the
rehydration initial rate of treated sugar beet pulp has a rehydration temperature, while its
quadratic influence is even more significant compared with the linear. The most important
coefficient in the equilibrium moisture content model is intercept, same as untreated sugar
beet pulp.
The effects of rehydration and convective drying temperature on rehydration initial rate
of untreated sugar beet pulp are given in Fig. 1. The rehydration initial rate of untreated sugar
beet pulp sharply rises with increasing rehydration temperature. Explanation of this
phenomenon lies in the fact that with the increase of water temperature in which rehydration
occurs, water viscosity decreases so that the resistance to water diffusion into the structure of
dietary fiber is smaller. At the beginning of the rehydration water penetration is limited to the
outer layer of the pulp, and it first fills larger channels and cavities, so that the dominant
resistance to mass transfer is external and in that connected to the viscosity of water. The
effect of drying temperature is less pronounced at lower rehydration temperatures, while at
higher temperatures the effect is significantly more pronounce. In fact, at lower rehydration
temperatures due to higher viscosity, as already mentioned the dominant resistance to mass
transfer is external. With increasing temperature, these resistances are reduced so that internal
resistances are notable. The increase of the resistance with increasing drying temperature is
associated with changes in the structure of the pulp that may occur during drying process. At
higher drying temperatures, shrinkage of the sugar beet pulp occurs; as result of that
collapsing of the channel network in the structure of plant tissue is present. Thus, an increase
in mass transfer resistance inside the plant tissue the rehydration initial rate decreases. For the
same reason, the effect of rehydration temperature increase is less pronounced for dietary
fibers dried at higher temperatures.
As said before, the most significant regression coefficients of the polynomial in the case
of equilibrium moisture content b
0
, which means that the values obtained by the model,
although statistically speaking different, not much differ from the calculated values of b
0
(Fig.
2.). The effect of drying temperature has the same trend at higher and lower rehydration
temperatures. With increase in rehydration temperatures the equilibrium moisture content has
gradually an increase that is much smaller in comparison with the increase in rehydration
initial rate. The reason for this difference may be found in a longer duration of rehydration
process during which is dominated by internal resistance to mass transfer, associated with the
structure of sugar beet pulp. Higher drying temperatures are the cause of changes in the fibers
tissue and thus increase the internal mass transfer resistance which results in the reduction of
the equilibrium moisture content.
