This meant that the molecules of bo

This meant that the molecules of both raw agar and agar acetate
existed in the mixture form of single coil and double helix, and
were still evenly dispersed in solution when the solution temperature was in the range 50–70
◦
C. The change of the molecules of
both raw agar and agar acetate in solution from single coil to double
helix had no obviously effect on their solution apparent viscosity,
which indicated that the formed double helix had not begun to
gather to gel at the temperature higher than 50
◦
C. As the apparent
viscosity of the raw agar solution was mainly due to the intermolecular hydrogen bonding interaction between its molecules,
and the introduction of acetyl group could weak its intermolecular
hydrogen bonding interaction, this may be the reason why the solution apparent viscosity values of agar acetates were lower than the
one of raw agar under the same conditions. The solution apparent
viscosity of all samples increased sharply at the range of temperature 30–45
◦
C, which meant that the formed double helix began
to gather together, and all solutions of raw agar and its acetates
were gelling in this range of temperature. When the temperature
was lower than 30
◦
C, the hard gel was formed completely. The
temperatures of gel formation point (Tg) of agar and agar acetates
could be calculated from Fig. 4, which were 39
◦
C (raw agar), 37
◦
C
(agar acetate, DS = 0.149), 36
◦
C (agar acetate, DS = 0.210), 34
◦
C
(agar acetate, DS = 0.268), 33
◦
C (agar acetate, DS = 0.315), 32
◦
C
(agar acetate, DS = 0.365) respectively, which were in almost perfect agreement with corresponding values of gelling temperature
in Table 1, and further indicated that the gelling temperature of
agar acetate was decreased with the increase of DS.
3.5. Cryo-SEM
The gel skeleton structures of agar and agar acetates were characterized by Cyro-SEM, and the results are shown in Fig. 5.
It could be seen from Fig. 5 that the gel skeleton structures
of raw agar and agar acetate were similar and all of the porous
network microstructures. These photographs were similar to the
images captured by Sousa, Borges, Silva, and Gonc¸ alves (2013) and
Tuvikene et al. (2008) for agar and agarose, respectively. The pores
in the network microstructures came from the sublimation of ice
crystals in the frozen gels. The pores of gel skeleton structures of
agar acetate tended to be smaller and denser than the one of the
raw agar, and became even smaller and denser with the increase
of DS, i.e., the size of the water droplet trapped in gel skeleton
structures of agar acetate was smaller than that of raw agar, and
become even smaller with the DS increase of agar acetate. This
meant that the gel skeleton structure of agar acetate would have
much higher capillary force to hold the water in gels than that of raw
agar gel according to the capillary effect, i.e., the agar acetate would
have the higher gel water holding capacity than raw agar, and the
higher the DS of the agar acetate, the higher the gel water holding
capacity.
3.6. Texture profile analysis
The gel textural properties of agar and agar acetates were determined by the texture analyzer and shown in Fig. 6