Hydrothermal treatments – A quick and efficient alternative for agar extraction from Gelidium sesquipedale

The red seaweed Gelidium sesquipedale is recognized for its high-quality agar, a valued hydrocolloid with gelling capacity, an industrially desirable property strongly dependent on its structure and composition. Despite its importance, there is a lack of information regarding the effects of processing conditions, including alternative extraction technologies, on these properties. To shorten that gap, the present study aimed to evaluate the use of autohydrolysis for obtaining agar from this marine macroalgae. This was performed by assessing the effect of hydrothermal treatments' severity and time and temperature binomials (at the same severity factor) on the extraction yield, texturizing and rheological behaviour, structure, composition, and molecular weight of the polysaccharide. The results obtained indicated that the optimal processing severity for agar recovery is 2.1 ± 0.1. Not only does this condition preserve the extraction yield (ranging from 12.3 ± 3.4% to 23.4 ± 1.6%, depending on the time and temperature binomial chosen), but it also results in a gelling strength of 758 ± 44 g/cm2, all while allowing a sharp processing time reduction (e.g. 1 min at 150 °C and 7.5 min at 140 °C). Molecular weight, rheological behaviour, and texturizing capacity proved to be directly linked to treatment's severity, with no differences observed in time and temperature binomials (at the same severity factor). On the other hand, 3,6-anhydro-α-l-galactose content and overall structure appeared to be intrinsic to the biomass, and not affected by the extraction conditions. Overall, this work demonstrated that autohydrolysis can be used to quickly and efficiently obtain high-grade food-safe agars when properly tuned.