2.9. Cryo scanning electron microsc

2.9. Cryo scanning electron microscopy (Cryo-SEM)
Gel samples (1.5 wt%) were placed into ultra-low temperature
freezer for 3 h. The vacuum freeze dryer (FD1, Boyikang Company,
Beijing, China) was used to remove water in the gel (about 36 h).
The drying samples were cut into small pieces and fixed onto metallic sample holders with conducting silver glue and then sputtered
with a layer of gold. The ready samples were examined by scanning
electron microscopy (JSM-840, Jeol, Japan).
3. Results and discussion
3.1. FT-IR spectroscopy
The FT-IR spectra of raw agar and agar acetate with DS = 0.365
are shown in Fig. 1. Compared with the FT-IR spectrum of raw agar
(curve a), the FT-IR spectrum of agar acetate (curve b) was almost
the same as the one of raw agar except for the appearance of new
absorption peaks at 1742 cm
−1
and at 1254 cm
−1
. The absorption
peak at 1742 cm
−1
indicated the presence of carbonyl group, corresponding to C O of acetic ester. The absorption peak at 1254 cm
−1
in the fingerprint region could be attributed to the C O C (acetic
ester group) bond stretching vibration.
3.2. Physicochemical properties
Physicochemical properties of the prepared agar acetates were
tested and shown in Table 1. It could be seen from Table 1 that
the gelling temperature, the gel melting temperature and the gel
strength of agar acetates were all decreased with the increase of DS.
Among which, the variation trend of the gelling temperatures with
the increase of DS was in agreement with the data reported in the
patent by Kenneth and Guiseley (1976). It was worthwhile to note
that the relationship between gel strength and DS values (in the
range from 0 to 0.365) was in good linear correlation (correlation
coefficient R = −0.99834) as it could be seen in Fig. 2.
3.3. Optical rotation analysis
Studies had shown that the mechanism of agar solution from
sol to gel was due to the double helix structure formed between
molecules (Arnott et al., 1974; Nordqvist & Vilgis, 2011). When agar
water solution cooled, agar molecules turned into double helix, further cooled, the double helix was gathered and generated the hard
gel. Temperature dependence of the optical rotation for solutions
of raw agar and agar acetate (DS = 0.365) on cooling was shown in
Fig. 3.