Structure of Alginate
Alginate was originally thought to consist of a uniform polymer of mannuronic acid as shown in figure 1. However later studies showed the presence of guluronic acid residues and it is now understood that alginate is a linear co-polymer of β-D-Mannuronic acid and α±-L-Guluronic acid.
|Figure 1. Sodium polymannuronic acid|
Depending on the weed source and growing conditions the ratio of mannuronic and guluronic acid can vary. It is also known that the block structure within the alginate can vary significantly. The poly guluronic acid blocks bind significantly more effectively with calcium ions than the poly mannuronic acid blocks.
The weed types with the higher guluronic acid levels are normally the ones that show the strongest interaction with calcium and hence the strongest gel strength. However it is not quite that simple and only guluronic acid blocks over a certain size can be involved in calcium crosslinking and the larger the block the stronger the cross link. Hence to identify the alginate with the strongest calcium gel not only high guluronic acid levels are required but also significant block structure.
|Figure 2. Sodium polyguluronic acid|
Despite the stronger gel strength of the high guluronic acid containing weeds the major application for this product is in pet food. Most of the alginate sold into food and pharmaceutical applications today tends to be low in guluronic acid.