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Mastering the art of agar gel cooking

Agar, which is a natural jelly-like substance extracted from a red algae is often found in the pharmaceutical industry for the production of gels used for growth culture media or electrophoresis, as well as in food industry for pastries, candies and delis, etc. This polysaccharide comes as a powder that is dissolved into water by bringing to a boil the later mixture for a few minutes, before cooling it down to induce the gelation. But how long should one boil the polysaccharide solution before cooling it down ? Beyond a few minutes, what is the impact of the cooking duration on the jellifying properties of agar and the corresponding gels ? A team from the CRPP in collaboration with the bioMérieux company has conducted a study published in November 2016 in Food Hydrocolloids on the heat-induced aging of agar sol and its impact on the corresponding agar gel. A solution of agar is maintained at 80°C over several days in the laboratory. During this incubation period the viscosity and the pH of the solution both decrease, which indicates that the polysaccharides slowly hydrolyze and oxidize leading to smaller molecules that progressively aggregate. Hot samples withdrew at different incubation times are cooled down to form gels. Gels prepared from a freshly prepared agar solution show a fibrous-like microstructure, elastic properties and display a brittle rupture scenario that involves the growth of macroscopic cracks. Increasing the incubation time of the agar sol leads to gels with a much coarser microstructure made of aggregated polysaccharides (see image below). The latter gels are all the more softer and ductile that the incubation time of the agar sol is larger. Finally, the research team has identified a critical incubation time, which is a function of the type of agar, beyond which the gel mechanical properties are drastically modified. These results shed new lights on the agar gels produced by the industry where the use of large tanks of hot agar solution often trigger long delay between the preparation of the solution and the cooling down, leading to gels of variable properties. The present study provides a good estimate of the effect of prolonged heating on the properties of agar gels and suggests that one can use the incubation time of agar sol to tune the structural and mechanical properties of agar gels to reach specific desired properties.

Contacts : Patrick Snabre, directeur de recherche CNRS au Centre de Recherche Paul Pascal

Thibaut Divoux, chargé de recherche CNRS au Centre de Recherche Paul Pascal,

caption of the figure : (left) Cryo-SEM images of gels prepared from samples drawn after different incubation times (1hour, 3 days and 5 days). Increasing incubation times at 80°C of the agar sol leads to gels of coarser microstructure. (right) Stress-strain curves determined by indentation of agar gels prepared after different incubation times ranging from 1h to 5 days. Beyond 2 days of incubation of the agar solution at 80°C, the gels formed later on display lower elastic modulus and yield stress, which value decrease for increasing incubation times.

article paru sur le site de l’Institut de Chimie