Microalgae proteins: From bulk to techno-functional proteins with benefits for nutrition and sustainability

Abstract

Alternative food value chains have to be developed and implemented to nourish future humankind sustainably within the boundaries of global resources. In the course of this thesis, proteins from the emerging source microalgae have been assessed for potential applications from the bulk down to specific techno-functionalities of isolated protein fractions. Microalgae gain increasing importance owing to protein concentrations of up to 70 % while possessing a number of beneficial nutrients including vitamins, antioxidants, minerals and essential fatty acids. Coupled with a higher productivity per area and year than common crops and cultivation on non-arable land, microalgae are foreseen to impact on major challenges of the sustainable development goals. In an attempt to utilize the entire microalgae and its bulk protein, high moisture meat analogues (HMMA) have been produced with a fibrous texture building on a combination of soy protein concentrate and heterotrophic cultivated microalgae, namely Auxenochlorella protothecoides. Structured alternatives to animal proteins with similar texture, appearance and taste are demanded by an increasing group of consumers owed to increased awareness of sustainability, animal welfare and nutritional believes. High moisture extrusion cooking offers a viable structuring technique in this context. The incorporation of spray-dried microalgae biomass in up to 50 % per dry matter affected the formation of fibrous structures. It could be balanced by reducing moisture levels. The elevated fat content of microalgae biomass led to lubrication effects, while probably undisrupted microalgae cells acted as passive fillers and limited the access of intracellular proteins. Both effects may have reduced texturing but increased tenderness in comparison to pure soy based extrudates. By using heterotrophic cultivated A. protothecoides with a light-yellow coloration, a consumer-adverse visual appearance could be omitted. Microalgae integration improved the extrudate’s nutritional profile by incorporating vitamin Bs and E, whose retention was over 95 % in the final product. In a next step, the emulsification potential of protein extracts from Arthrospira platensis has been assessed. Functionality gives value to a biomass fraction and it allows to exploit multiple products from one biomass in a biorefinery approach maximizing its overall value generation. This is needed for the emerging protein alternative microalgae as technical readiness and missing economy of scale limit its economic viability. Arthrospira platensis, commonly known as Spirulina, is next to Chlorella vulgaris one of few microalgae strains that has been granted the generally recognized as safe (GRAS) status owed to their historic and safe usage. The emulsifying potential of A. platensis extracts could be demonstrated and a mechanistic change in the emulsion stabilization upon progressing purification is hypothesized. A microalgae suspension of A. platensis powder in phosphate buffer solution (pH 7, 0.1 M) was homogenized and separated by centrifugation. Proteins were precipitated at the identified isoelectric point at pH 3.5 and diafiltrated. Emulsions with 20 vol% MCT oil could be formed with all extracts of different degrees of purification. Normalized by protein concentration, smaller droplets could be stabilized with an increasing degree of purification. In interfacial shear rheology measurements, the build-up of an interfacial viscoelastic network was faster and final network strength increased with the degree of purification. It is suggested that isolated A. platensis proteins rapidly form an interconnected protein layer while coextracted surfactants impede protein adsorption for crude and soluble extracts. For potential applications in food, pharma and cosmetic product categories, the enhanced functionality has to be balanced against the loss in biomass while purifying microalgae proteins. Arthrospira platensis is also the source of phycocyanin. The only natural derived blue colorant for the food industry that serves as fluorescent marker in biomedical research and therapeutic agent in multiple diseases. The high-value component is a mix of two protein-pigment complexes, allophycocyanin (APC) and c-phycocyanin (CPC), which are part of special light harvesting complexes in cyanobacteria, the phycobilisomes. They allow cyanobacteria to expand their range in spectroscopic wavelength used for photosynthesis. Stability of colorants is concerning for food coloring matrices, particularly for the Spirulina-based phycocyanin as it comprises heat sensitive protein-pigment complexes. Although frequently encountered in food processing, the impact of short time heat treatments has not been studied systematically. An industry relevant phosphate buffered phycocyanin solution was heated in batch and continuous processing systems with especially the continuous modular micro reaction system being scalable by micro process engineering principles. Both systems are characterized by a high surface-to-volume ratio allowing isothermal conditions with residence times down to 5 s. UV-Vis absorbance scans revealed biphasic degradation of phycocyanin color activity to about 30 % within 30 s at T ≥ 70 °C. Kinetic modelling of the color decay via an nth order approach contradicts previously assumed linear first order kinetics with a best fitting empirical reaction order of n = 6. It shows that decay in phycocyanin color activity is not a single process but encompasses CPC and APC aggregate disintegration and denaturation. The structure-function relationship was further investigated in continuous high temperature short time (HTST) treatments of a purified phycocyanin solution. It could be shown by differential scanning calorimetry that a mixture of APC and CPC disassembled and denatured between 50 and 70 °C. Combined with UV-Vis absorbance and fluorescence spectra of samples treated in continuous HTST processing, three characteristic transition temperatures were allocated to specific quaternary aggregates. In contrast to sequential chemical denaturation, changes in phycocyanin’s chromophores coincided with alterations in the secondary protein structures in continuous HTST. The insights gained help to define processing windows and enable targeted process control for preserving maximum color shades, fluorescence properties and biological functionalities of phycocyanin for multiple applications in research and industry. In conclusion, the functionalities of microalgae proteins could be showcased by extruding whole microalgae biomass, stabilizing fluid-fluid interfaces with microalgae protein extracts and characterizing the coloring functionality of phycocyanin in thermal processes. It underlines the potential of microalgae proteins in various categories and scales relevant for food, pharmaceutical and cosmetic industries ranging from bulk to specific techno-functional fractions.