Rheology of Biological Soft Matter by Isamu Kaneda

Author:Isamu Kaneda
Language: eng
Format: epub
Publisher: Springer Japan, Tokyo

7.9 Validation of Conclusion and Hypothesis

Under standard operation condition for instrumental test, results from the gellan study are comparable with those from the agar study when S50 is used as artificial tongue, and the gel fracture can relate to the oral strategy for the first size reduction when the initial fracture strain of gellan gels is smaller than 60 %. This is the case of less deformable gels from series A and B. Results are different from the agar study, on the other hand, when the initial fracture strain of gellan gels is larger than 70 %. This is the case of highly deformable gels from series D. These validate conclusions from the agar study but show a limitation at the same time for evaluation of highly deformable gels such as from series D.

Regarding the boundary strain and stress to change the oral strategy for size reduction from tongue–palate compression to mastication, hypothesis drawn by the agar study is in accordance with previous videofluorography inspection [1], both elucidating the critical strain at around 10–12 %. The hypothesis can be true when the initial fracture strain of gellan gum gels is smaller than 60 % with a shift of the critical strain to a larger one, ca. 20–30 %, while cannot be true when the initial fracture strain is larger than 70 %. From sensory evaluation, all subjects use tongue–palate compression for the first size reduction when the initial fracture force of gellan gum gels is equal to or lower than 15 N, whereas some subjects use mastication when the initial fracture force is equal to or higher than 20 N. This is also consistent with the hypothesis drawn by the agar study that oral strategy for size reduction should change from tongue–palate compression to mastication at a boundary fracture force of 10–35 N. A new finding from the gellan study is that a threshold force to change the oral strategy should decrease with increasing the initial fracture strain.

In the case of agar, gels always fracture through instrumental compression on artificial tongue when apparent Young’s modulus of the gel is smaller than that of artificial tongue, while they do not when apparent Young’s modulus of the gel is larger than that of artificial tongue. That is, the difference in apparent Young’s modulus between the gel and artificial tongue determines the gel fracture on the instrumental evaluation system. However, this is not necessarily true for gellan gum gels, and the fracture profile of the gels cannot be always explained by the difference in apparent Young’s modulus between both entities. Results should be discussed in relation to the rheological characters representing a large deformation behavior rather than a small deformation behavior such as Young’s modulus. From this perspective, it may be worth comparing the strain-hardening index between gellan gum gels and artificial tongue, and the comparison was thus tried. However, the strain-hardening index was difficult to determine experimentally due to poor data reproducibility and did not work well for explaining the fracture profile of the gels (data not shown). This should be considered in the future work.

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