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The agar was originally thought to be made up of simple sulfated poly galatose structure wise. After the development of electron microscopy in 1937, it was shown that agar can be separated into two different fractions—agarose and agaropectin. The agarose works as gelling component of agar. The percentage of agarose and agaropectin may differ in different species. A higher ratio of agarose to agaropectin gives higher gel strength. By the late fifties more research was done on the structure of agar and agarose has been described as linear polymer, consisting of alternate D-galactose and 3,6 anhydro-Lgalactose. However its structure is a complex polysaccharide that varies from source to source. A study in 1991 showed that agar has at least 11 different agarobiose structure [12]. The variation in structure depends upon the gender of the species, the environment it grows in, the time of year, agar got extracted, etc. In summary agar is a complex of interchanging of β-(1–3)-D and α-(1–4)-L linked galactose residues [13]. Out of which α-(1–4)-L residues are converted by the 3,6 anhydro bridge as shown in Figure 5.1. Or we can say it is a composition of heterogeneous molecules that differ in their physico-chemical properties. However sulfates, pyruvates, urinate or methoxyl groups are also some of the modifications in the agarose complexes. Agarose is typically a molecule of higher molecular weight and low in sulfates. Agaropectin is just opposite to agarose, it has lower molecular weight compared to agarose and higher sulfates at about 5–8%. Some agars also have xylose in it.

Figure 5.1 Chemical structure of agarobiose (the fundamental repeating unit of agar). Source: Europian Food Cunsumption Database.

Modern alkaline treatment method helps in increasing the anhydrous bridges within molecule, thus increasing gel strength. Methoxy levels in agar determine the gel setting temperature. The level of methoxy content in gel is directly proportion to its strength. Thus higher level of methoxy in agar leads to the higher gel setting temperature and vice versa.