Agar gels due to the presence of the agarose fraction in the crude agar at typical concentrations between 0.5% and 2.0%. Unlike carrageenan agar does not require the presence of any particular ions to gel. One of the classic uses of agar is for the preparation of microbial plates where the combined properties of low syneresis, ion independent and a low set temperature make agar ideal.
Agar has a uniquely large hysteresis between its melting and setting temperature. Typically agar need to be heated above 90oC to form a good solution and depending on the seaweed source the setting temperate can be as low as 30oC and is typically between 30-45oC for a 1.5% solution. To overcome the very high dissolution temperature of agar several companies manufacture a form of agar that has been specially dried to allow the agar to dissolve at lower temperatures.
|Figure 1. Agar forming macroreticuated gels|
According to Rees agar forms antisymmetric double helices on cooling that hydrogen bond to form clumps of helices. These clumps can then form larger groupings that from a large porous gel structure. Agar is known to form a very porous gel and the pore size can be roughly measured by assessing the size of particulates that are excluded from the gel in a gel permeation experiment. It has been shown that agar gels can allow molecules up to 30M daltons in size to percolate through it structure. An agar gel as the unusual property of behaving like a sponge. An agar gel of a particular shape can be dried and upon rehydration it will swell to its original size and shape.
Agar synergy's are not as commercially important as they are for xanthan or carrageenan and tend to be rather small in magnitude. Gelidium agar is known to form a small synergistic interaction with locust bean gum that is not seen in products based on gracilaria.
Agar forms a synergistic interaction with sucrose and is used in some confectionery products. Tannic acid on the other hand may actually inhibit gelation. Agar is reasonably acid stable compared to other polysaccharides and does not show any protein reactivity. Agar can be used in acidic dairy products such as yoghurts where carrageenan would cause excessive flocculation due to the protein reactivity of the carrageenan. Recently a synergy has been reported between low gel strength agar and guar gum in patent by Rachid Lebbar of Setexam
Agars all have negative optical rotations whereas carrageenans are positive. This can be used to distinguish the two when identification is tricky. Sulphate level is often used and whereas a low sulphate level would indicate an agar you cannot definitively say that a high sulphate level is always a carrageenan.