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In this process, the gelling property of colloids is used, by which the integrated water is simply removed by applying high pressure. In Japan it was used mainly in extraction of semi-refined agar from Gracilaria. Stone blocks were used to press the gel packs of agar, removing water. Afterwards, it is pressed with miniature hydraulic presses to remove the remaining water. In 1964, Prona from France, developed modern syneresis technique of concentrating the extracted agar gel from all type of agarophytes. Later this company expanded its industry to Mexico, Chile, South Africa, and also modernised the plants in Spain, Morocco and Portugal. This improved company’s global headship in agar production. Prona only joint their business with INA of Japan and Algas Marinas of Chile.

In 1971 Okazaki [11] wrote that syneresis was possibly only for the Gracilaria. Later when semi-automated process was developed and got improved with time. It leads to the spread of syneresis technique over the globe due to its low production cost and less dependency on weather/cold temperature. In old freezing-drying methods, 80 to 100 times more ice was required to produce 1 ton agar.

In syneresis method, the purity of agar is way better than freezing method. It is 20% higher than the freezing–drying method which was 11%. Which means agar, produced by freezing method, has more impurities like water and other soluble impurities, than the syneresis method.

Treatment: alkaline solution is used to extract polysaccharide from the cell wall of seaweed. For example: for Gracilaria any stronger base is required to extract agar from its cell wall. Strong base helps in alkaline hydrolysis of the sulfates resulting stronger agar gel strength.

Extraction: Extracted agar is later dissolved into boiling water above atmospheric pressure and on a high pH to obtain a high yield.

Filtration: Most standard filtration techniques can be used in this step.