1. IntroductionAgar, the gel-formin

1. Introduction
Agar, the gel-forming polysaccharides extracted from Gelidiaceae and Gracilariaceae species (Freile-Pelegrín & Robledo, 1997),
is linear polymers based on a disaccharide repeat structure of
1,3-linked -d-galactopyranose and 1,4-linked 3,6 anhydro--lgalactopyranose units (Arnott et al., 1974; Labropoulos, Niesz,
Danforth, & Kevrekidis, 2002). Agarose and agaropectin are the
two components of agar, the former consisting of neutral polysaccharides with a high gelling ability and the latter consisting of
ionic polysaccharides with a low gelling ability (Arnott et al., 1974;
Yaphe, 1984). Agar is widely used for medical, pharmaceutical,
food and electronic industrial and laboratory experimental purposes due to its combination of renewability, biological activities,
biocompatibility, biodegradability and nontoxicity (Cayre, Chang, &
Velev, 2007; García-González, Alnaief, & Smirnova, 2011; Scholten
& Pierik, 1998; Wijesekara, Pangestuti, & Kim, 2011; Wu, Geng,
Chang, Yu, & Ma, 2009).
In recent years, the esterification of different polysaccharides
has gained great interest. As described by Sweedman, Tizzotti,
Schäfer, and Gilbert (2012), starches modified with octenyl succinic anhydride had been used in a range of industrial applications,
particularly as a food additive. Superabsorbent hydrogels were prepared from native celluloses by esterification crosslinking with
1,2,3,4-butanetetracarboxylic dianhydride (Kono & Fujita, 2012).
∗
Corresponding author. Tel.: +86 532 85955589; fax: +86 532 85950518.
E-mail address: zhangxdqd@hotmail.com (X. Zhang).
Poplar wood fibers were chemically modified by esterification
(acetate, propionate, benzoate), and the properties were shown to
be more thermally stable and hydrophobic than unmodified fibers
(Wei, McDonald, Freitag, & Morrell, 2013). However, the literature
and reports on agar esterification were very few. Acetylation of agar
was firstly patented by Kenneth and Guiseley (1976), but no more
properties of the agar acetates were described except the gelling
temperature. In this research, a series of agar acetates with different DS were prepared, and their properties were determined and
analyzed.
2. Experiment
2.1. Materials
Gracilaria agar (3,6-anhydro-l-galactose content: 35.5%; sulfate
content: 3.2%; gel strength: 1930 g/cm
2
) was purchased from Qingdao Agar Manufacturing Company, Shandong Province, China. All
reagent used in this research were purchased from Laiyang Chemical Reagent Plant, Shandong province, China.
2.2. The preparation of agar acetates with different DS
Agar acetates were prepared by the method of Kenneth and
Guiseley (1976) described in US3956273. Degree of substitution
(DS) was determined by titration according to the method of Tupa,
Maldonado, Vázquez, and Foresti (2013)