Effect of high-pressure treatment on oscillatory rheology, particle size distribution and microstructure of microalgae Chlorella vulgaris and Arthrospira platensis

Microalgae are rich in protein and bioactive compounds with a wide application in food, pharmaceutical, cosmetics and nutraceutical industries. However, the rigid cell wall remains the bottleneck for the extraction and release of cellular constituents in microalgae. High-pressure (HP) processing an athermal process has been explored to overcome the challenges. The objective of this study was to investigate the influence of high-pressure (HP) treatment (300–600 MPa/15 min) on the rheological, thermal, structural and particle size distribution of dry powdered form of two selected microalgae, namely Chlorella vulgaris (Chlorella) and Arthrospira platensis (Spirulina). HP-treatment of microalgae successfully transformed the liquid-like (G″ > G′) suspensional behavior to a solid-like (G′ > G″) gel by disrupting the cell wall of the biomass and denatured algal proteins. The gel rigidity (G′) increased with increasing the pressure from 300 to 600 MPa for both microalgae suspensions; however, the Spirulina exhibited the most significant improvement in the rigidity (6219 Pa) than the Chlorella (2340 Pa). HP-treatment at 300 MPa resulted in partial rupture of the Chlorella cell wall, which disintegrated completely at 600 MPa with exposure of the inner structure. The tubular shape of the Spirulina was not observed in the untreated sample; however, it appeared in the micrographs when the sample was treated above 300 MPa with the disintegration of the cellular mass. Chlorella had a bimodal particle size distribution (PSD) and the volumetric median particle size (Dv50) did not change significantly with the pressurization. Conversely, a unimodal PSD appeared in Spirulina and those particles were significantly affected by the pressure. The obtained information could be useful for food product development and tailoring the gel rigidity of the designed food products.