Unicellular protein: isolation, techno-functionality and digestibility

Samenvatting

Unicellular, photosynthetic microalgae and cyanobacteria are potential new protein sources for both food and feed. Variations between these sources are reflected in chemical variations (e.g. in the chemical composition) and in structural variation (e.g. in cell wall robustness). The aim of this thesis was to understand how chemical and structural variations between unicellular photosynthetic sources affect the application of unicellular protein as techno-functional ingredients and as a fish feed ingredient.

To study the applicability of unicellular protein as techno-functional ingredient in food, proteins were extracted and isolated from the cyanobacterium Arthrospira (spirulina) maxima and the microalgae Nannochloropsis gaditana, Tetraselmis impellucida and Scenedesmus dimorphus. Chemical variations observed between photosynthetic unicellular sources were reflected in variations in protein extractability. The isolates (62–77% w/w protein) varied in protein solubility as a function of pH and ionic strength, especially at pH < 4.0. Isolates from N. gaditana, T. impellucida and A. maxima were able to form emulsions (d3,2 0.2–0.3 µm) at pH 8.0. The amount of each isolate needed to form emulsions varied between the isolates (9–74 mg protein / mL oil), but was within the range of proteins from both similar (photosynthetic) sources (algae and sugar beet leaves) and other protein sources (dairy, legume and egg). Minor differences were observed in the pH dependence of flocculation amongst the isolate stabilized emulsions.

To study the applicability of microalgae and cyanobacteria as dietary protein sources for fish, Chlorella vulgaris, S. dimorphus, N gaditana, and A. maxima biomass was incorporated in fish feed (30% inclusion) and fed to Nile tilapia and African catfish. The cell walls of these unicellular sources used were quantified to vary in their robustness to mechanical degradation (structural variations). Although protein digestibility varied between the unicellular sources (ranging from 67–83%), the protein digestibility did not relate to the variations in unicellular cell wall robustness. There was no difference between both fish species regarding the nutrient digestibilities of the unicellular sources. Subjecting N. gaditana biomass to treatments that decrease its cell wall integrity increased in vitro accessibility of microalgae nutrients up to 4 times. The increased in vitro accessibility correlated with an increased in vivo digestibility of protein in Nile tilapia, confirming that nutrient accessibility plays an important role in the nutrient digestibility of microalgae in fish.

In conclusion, chemical variations observed between photosynthetic unicellular sources were reflected in variations in protein extractability. Structural variations between the sources were reflected in variations in in vivo protein accessibility and subsequent protein digestibility.