Water relationships in Haematoccoccus pluvialis and their effect in high-pressure agglomeration for supercritical CO₂ extraction

Stickiness and caking of fine powders such as in dry disrupted microalgae should be avoided in supercritical (sc) CO₂ extraction, because they negatively impact extraction rate and yield. To establish limits in water content of H. pluvialis cysts and extraction temperature, this work studied water-state diagrams of the powder. The powder's squeeze flow behavior as a function of water content was useful to characterize the transition between glassy and rubbery states as water content increased. Water sorption (W versus a_w) at 20 °C was represented using the Guggenheim-Anderson-de Boer equation, with a monolayer water content of 3.67% (d.b.). The glass transition diagram (T_g versus W_w) was represented using the Gordon-Taylor equation, with T_gs = 88.3 °C (glass transition temperature of the anhydrous solids) and k = 3.49. The compression pressure necessary for squeeze flow behavior decreased 2.5-3 times at ambient temperature (ca. 23 °C) as a result of an increase in water content from 3.8% (d.b.) to 10-15% (d.b.) at which level glass-rubber transitions manifested, and then kept relatively constant when the water content increased even further. Alternatives to prevent caking of H. pluvialis during scCO₂ extract include reducing the initial water content of the powder, increasing particle size by high-pressure agglomeration, and/or reducing the extraction temperature so as to prevent the glass-rubber transition that is responsible for sample stickiness. Taking into account that the scCO₂ extractions are carried out above ambient temperature (40 ≤ T ≤ 60 °C), we recommend reducing the water content of H. pluvialis powder to W ≤ 5% (d.b.).