The role of structural biopolymers in the functionalization of microalgae for food processing: Potential as structuring agent and implications for nutrient bioaccessibility

Microalgae are considered promising functional food ingredients due to their balanced composition, containing various (essential) nutrients as well as health-beneficial components. However, their functionality in food products might not be limited to health aspects, as microalgae could also play a structuring role in food. Microalgae are actually rich in structural biopolymers such as proteins, storage polysaccharides, and cell wall related polysaccharides, and their presence might possibly alter the rheological properties of the enriched food product. In that case, microalgae could be considered as multifunctional food ingredients, combining nutritional enrichment of the food products with structural benefits, possibly reducing the need for additional thickening or gelling agents. However, the functionality of microalgae and their structural biopolymers has only very limitedly been studied in this context. A first approach to benefit from these structural biopolymers consists of isolating the cell wall related polysaccharides, including cell wall bound polysaccharides and extracellular polysaccharides, for use as food hydrocolloids. While macroalgal polysaccharides have been successfully commercialized as thickening or gelling agents in food, polysaccharides of microalgae received surprisingly little attention in this context. Therefore, the first part of this doctoral thesis aimed to characterize the molecular composition of cell wall related polysaccharides of ten different microalgae species in terms of monosaccharides, uronic acids, and sulfate groups. As a result of the large evolutionary diversity of microalgae, a very diverse cell wall composition was observed for the different microalgae species, displaying little analogies with conventional food hydrocolloids. Based on the promising thickening properties reported for extracellular polysaccharides of Porphyridium cruentum, different cell wall fractions were isolated from this microalga and characterized in detail, in relation to their rheological properties. In contrast to the cell wall bound polysaccharides, extracellular polysaccharides were characterized by high molecular weight polymers, yielding higher intrinsic viscosities compared to several commercially used hydrocolloids. However, large amounts of co-extracted proteins and minerals limited their functionality in aqueous solutions, and attempts to induce gelation in presence of cations and/or heating-cooling cycles were unsuccessful. A second approach to introduce structural biopolymers is the incorporation of the whole microalgal biomass into food products. This would actually be a more effective and more sustainable approach, since other nutritional and health-beneficial components are also introduced in the food product and no waste streams are generated. However, due to the lack of literature data on the rheological properties of microalgal suspensions and the impact of processing thereon, there was an obvious need for research studies in order to evaluate the potential of this application. As expected from the variable biomass composition and physical properties of the different microalgae species, a large diversity in rheological properties was observed, with and without processing. In fact, whereas some microalgae such as Nannochloropsis sp. displayed Newtonian behavior, implying that their biomass could be used without disturbing the structural properties of the food matrix, other microalgae showed pseudoplastic flow properties and viscoelastic behavior. The importance of mechanical and thermal processing was clearly shown, not only in obtaining enhanced rheological properties, but also in tailoring microstructural properties. In fact, specific processing strategies were identified for suspensions of Chlorella vulgaris and Porphyridium cruentum to maximally exploit their structuring potential, resulting in different microstructural properties. The importance of these microstructural properties was shown in the last part of this doctoral thesis, focusing on the lipid digestibility and in vitro bioaccessibility of carotenoids and ω3-LC-PUFA in microalgal biomass. It was actually hypothesized that the microalgal cell wall would act as a physical barrier limiting the digestibility and bioaccessibility of intracellular components, assuming that the cell wall is not degraded by the digestive enzymes in the gastrointestinal tract. This hypothesis could be confirmed, as higher values for lipid digestibility and bioaccessibility of carotenoids and ω3-LC-PUFA were observed for disrupted Nannochloropsis sp. biomass than for untreated biomass. However, relatively low bioaccessibility values were still observed, which were attributed to the location of the carotenoids and/or the presence of other macromolecules in the biomass, since carotenoids and ω3-LC-PUFA were significantly more bioaccessible when supplied as extracted oil. This doctoral thesis provided the scientific knowledge base for evaluation of the potential of microalgae as structuring agents for food applications. It is obvious that the large diversity in physicochemical properties of microalgae requires a justified selection of appropriate microalgae species for specific applications. Incorporation of the whole microalgal biomass in food products is considered the most promising approach, since rheological properties can be tailored by using specific processing strategies, and nutritional and health-beneficial components are simultaneously introduced in the food product. However, additional research is required, for instance on the stability of these health-beneficial components upon processing and the impact on the sensory properties, for a complete evaluation of the design of high-quality food products enriched with microalgae.status: publishe