Structure
The original structure of agar was believed to be a simple sulphated poly galactose, but it is actually a very complex polysaccharide that varies considerably in structure, depending on the source. Early work in 1930s showed that agar consisted of at least two separate polymers that could be fractionated – these being agarose and agaropectin. Later work has shown that the structure varies substantially across different raw materials, with species, environmental conditions and time of the year.
Agarose is the major component of agar and is the gelling fraction. It comprises repeating units of agarobiose which itself comprises alternating units of ⍺-(1-4)-L-galactose and β-(1-3)-D-galactopyranose with varying amounts of sulphate, pyruvate, uronate and methoxyl groups present. The α-(1-4) residues can also be modified by the presence of a 3,6 anhydro bridges. The presence of anhydro bridges has a significant impact on the gelling functionality and varies across raw material sources. Modern alkali treatment methods can be used to increase the level of anhydro bridging in the molecule and subsequently improve the gel strength.
Agarose is typically a high molecular weight polymer (>100kDa) and is low in sulphate whereas agaropectin is typically low molecular weight (<20kDa), high in sulphate (ca. 5-8%) and is non-gelling.
A note on Danish agar – Danish agar is the traditional name for a gelling polysaccharide that is extracted from Furcellaria seaweeds known as furcellaran. Furcellaran is not an agar, it is a kappa-beta type carrageenan that has had historical use in the Baltic regions of Europe. These days there is a small extraction industry in Estonia.
Further information on agar structure and properties can be accessed using the arrows in the Further Reading box below.