Several cell disruption methods were tested on Nannochloropsis gaditana, to evaluate their efficiency in terms of cell disintegration, energy input and release of soluble proteins. High-pressure homogenization (HPH) and bead milling were the most efficient with >95% cell disintegration, ±50% (w/w) release of total proteins and low energy input (<0.5kWh.kg). Enzymatic treatment required low energy input (<0.34kWh.kg), but it only released ±35% protein (w/w). Pulsed Electric Field (PEF) was neither energy-efficient (10.44kWh.kg) nor successful for protein release (only 10% proteins w/w) and cell disintegration. The release of proteins after applying HPH and bead milling always required less intensive operating conditions for cell disruption. The energy cost per unit of released protein ranged from 0.15-0.25 €.kg in case of HPH, and up to 2-20 €.kg in case of PEF.

A mild biorefinery process was investigated on the microalga Nannochloropsis gaditana, to obtain an enriched fraction of water soluble proteins free from chlorophyll. After harvesting, a 100g.L solution of cells was first subjected to cell disruption by either high-pressure homogenization (HPH) or enzymatic treatment (ENZ). HPH resulted in a larger release of proteins (49%) in the aqueous phase compared to the Alcalase incubation (35%). In both cases, an ultrafiltration/diafiltration (UF/DF) was then performed on the supernatant obtained from cell disruption by testing different membrane cut-off (1000kDa, 500kDa and 300kDa). After optimising the process conditions, the combination of ENZ→UF/DF ended in a larger overall yield of water soluble proteins (24.8%) in the permeate compared to the combination of HPH→UF/DF (17.4%). A gel polarization model was implemented to assess the maximum achievable concentration factor during ultrafiltration and the mass transfer coefficient related to the theoretical permeation flux rate.

An isolation procedure for proteins from duckweed was optimized based on a previously developed method for protein isolation from sugar beet leaves. Optimization included the protocol for disrupting cells and protein recovery. With the optimized protocol, protein was isolated (protein yield 14.2%, RuBisCO yield 27%). The concentrate was off-white and contained 67.2% protein. The isolation procedure resulted in a large enrichment in RuBisCO (from 48% to 92%). Denaturation of duckweed protein concentrate was observed at 62 °C at pH 7, while heating at pH 4 did not show denaturation peaks. Solubility was good far from the iso-electric point and showed a minimum around pH 5. Gelling was better at pH 7 than at pH 4. At pH 7, duckweed gels were much stronger than soy and only slightly weaker compared to egg white protein, while at pH 4 duckweed gel strength was similar to soy and lower than egg white.

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