The potential of using pressurized hot water extraction to valorize the remaining crude agar in Gelidium sesquipedale waste stream after an initial industrial extraction was investigated. In this process, a four-factor Box-Behnken design was coupled with a response surface methodology. The impact of the operating temperature (°C), the internal pressure (bar), the extraction time (min), and the algae concentration (% w: v), as well as their quadratic effects and two-way interactions, on the physicochemical properties of the residual agar, was analyzed. The yield (%), gel strength (g/cm2), gelling temperature (°C), melting temperature (°C), 3,6-anhydrogalactose content (%), and the sulfate content (%) were all considered in the evaluation. A multiple regression statistical model was used to fit all the experimental responses to a second-order polynomial equation that confirmed the suitability of the approach. Temperature of 120 °C, low pressure of 3.28 bar, and an extended extraction time of 150 min along with a 3% (w: v) algae concentration were projected to be optimum conditions for a high extraction yield of 17.03%. The strength of the recovered agar hydrogel oscillated between a minimum of 25 g/cm2 and a maximum of 350 g/cm2. The key parameters impacting the fluctuation of the sulfate content in the recovered agar (2% ≤ sulfate content ≤10%, with R2 = 79.8%) appeared to be the temperature and the algae concentration, in addition to the quadratic effect of the solid concentration. By adjusting the parameters, the process can accommodate the physicochemical properties of agar for wider range of applications.
