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

• Water sorption of H. pluvialis fits the GAB equation with Wm = 3.67% (d.b.).
• Tg of hydrated H. pluvialis fits the GT equation with Tgs = 88.3 °C and k = 3.49.
• Squeeze flow measurements can simply characterize hygroscopic powders for compaction.
• To avoid caking during scCO2 extraction, H. pluvialis should be dried to W ⩽ 5% (d.b.) to prevent a glass–rubber transition.
• High-pressure agglomeration of H. pluvialis powder prevents caking during scCO2 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 aw) 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 (Tgversus Ww) was represented using the Gordon–Taylor equation, with Tgs = 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.).