High-pressure treatment of the green and orange Dunaliella salina biomass: effect on particle size distribution, small amplitude oscillatory shear rheology, and microstructure.

Dunaliella salina, a halophilic microalga, is well known for its ability to produce β-carotene and has significant commercial applications. The actively growing green culture turns to orange color due to photosensitization, during which there is a significant reduction in chlorophyll content (chlorophyll A and B: 16.04 and 2.80-1.70, 0.21 mg/g dry basis, respectively) with an increase in carotenoids (α- and β-carotenes: 1.60 and 4.81 mg/g dry basis). This change has been accompanied by a considerable variation in protein content (green: 34.27% and orange: 18.57%) and ash content (green: 38.37% and orange: 58.11%). To avoid extreme heat sensitivity, high-pressure (HP) processing, a nonthermal technology, has been applied to pigment-rich Dunaliella. This research aimed to examine the effects of HP treatment (300-600 MPa/15 min) on the rheological, structural, and particle size distribution of Dunaliella in two consecutive cell growth stages (e.g., green and orange). Oscillatory rheology data displayed a distinct protein denaturation at 57.87 °C for untreated green cells, whereas orange cells did not. Conversely, several denaturation peaks appeared in the HP-treated orange cell suspensions, and those peaks remained unaffected by pressure treatment. Isothermal heating exhibited liquid-like behavior for green cells, whereas the solid-like behavior was evident for orange cells. PSD displayed a shift of unimodal to bimodal distributions of Dunaliella cells after the HP treatment. Orange cells exhibited PSD parameters of Dv: 8.60 μm, Dv: 71.6 μm, and Dv: 255 μm. XRD patterns of both green and orange cells are almost identical, exhibiting several peaks that were attributed to metal ions absorbed by the cells from the growth media. Overall, a significant difference in compositional and functional properties was observed between the green and orange Dunaliella biomass.