28 wt% of water) in aqueous solutions of a sugar alcohol, e.g. sugar,
isomalt, xylitol with various contents of 0, 5, 10, 15 and 20 g/100 g
water were investigated. The effects of sugar on the physical properties
(including mechanical stability and dispersal behavior) of
the produced tablets were presented in a previous study (Nguyen
and Ulrich, 2014). Here, these results were presented in comparison
to other sugar alcohol binders.
Fig. 2 shows the effect of binders on the tensile strength of the
produced tablets. It can clearly be seen that all three sugar alcohol
binders dramatically improved the mechanical stability of produced
tablets. The two-side freezing mode significantly enhances
the hardness of produced tablets. Using isomalt as a binder with
a content of 20 g/100 g water could result in more than 3,5 times
an increase of the applied crushing force of tablets produced by
the freeze casting process with the two-side freezing mode and a
twofold increase in case of the one side freezing mode can be found
(see Fig. 2(b)). The maximum crushing force value of 44.8 N (i.e.
0.18 N/mm2 diametral tensile strength, calculated by Eq. (1)
reported by Fell and Newton (1970)) could be reached in case of
isomalt, which is higher than the maximum crushing force value
of 34.5 N of the produced tablets using sugar as a binder (see
Fig. 2(a) and Nguyen and Ulrich (2014)). In case of xylitol
(Fig. 2(c)), there is a different behavior. The difference in the effect
of the freezing modes in the increase of the applied crushing forces
was not as pronounced as sugar and isomalt. The mechanical
strength of the produced tablets slightly increases when the xylitol
content increases from 0 to 15 g/100 g water, thereafter the tensile
strength decreases as the xylitol content reaches 20 g/100 g water.
This behavior is explainable by the high porous behavior of the
produced tablet which is shown in Fig. 3(d).
It is obviously to be seen that the looks and the porous structures
of the produced tablet bodies are different when the binders
were added (see Fig. 3). At the same binder content of 20 g/100 g
water, the looks of produced tablet body in case of isomalt looks
denser than the produced tablet in case of sugar and xylitol (see
Fig. 3(b–d)). To see more details of the morphology of the pores
in solid tablet bodies, the microscopic images of the vertical-cut
surface of produced tablets by the one side freezing mode were
presented in Fig. 4. The needle-like structure channels of the pores
(see Fig. 3(a) and the first left image of Fig. 4(a)) was gradually
substituted by a porous and size increasing sphere pore structure
since the binder contents were increased (see Fig. 4). It is interesting
that the sphere-like pattern of pores is well met with the
morphologies of ice crystals in ice cream when the sweeteners
were added, which was reported by Hagiwara and Hartel (1996).
However, there is a possibility that these big sphere pores were