Microalgae are rich in several nutritional and health-beneficial components, showing great potential as functional food ingredients. To this extent, knowledge of the biomass composition is essential in the selection of suitable microalgae species for specific food applications. Surprisingly, although cell wall polysaccharides are generally reported to play a role in functionality, limited attention has been given to the cell wall related polysaccharides of microalgae so far. Therefore, this study aimed to characterize dry biomasses of ten microalgae species with potential as functional food ingredients, with a particular focus on the composition of cell wall related polysaccharides. The investigated species were Arthrospira platensis, Chlorella vulgaris, Diacronema lutheri, Tisochrysis lutea, Nannochloropsis sp., Odontella aurita, Phaeodactylum tricornutum, Porphyridium cruentum, Schizochytrium sp. and Tetraselmis chuii. Lipids, proteins and ash made up a large fraction of the biomasses, except for the freshwater algae C. vulgaris and A. platensis which were mainly composed of proteins and polysaccharides. Generally, low amounts of storage polysaccharides (2-8%) were observed in the investigated microalgae species, while extracellular polymeric substances were only present in P. cruentum, O. aurita, C. vulgaris and A. platensis. Cell wall polysaccharides contributed approximately 10% of the biomass and were composed of heteropolysaccharides, showing at least five different monosaccharides. Moreover, the presence of uronic acids and sulfate groups provides anionic characteristics to the cell wall related polysaccharides of several microalgae. As a result, these polysaccharides show potential to display interesting functionalities as bioactive or technological substances.

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Microalgae show great potential for use as novel ingredients in food products, as they are rich in several nutritional and health-beneficial components. However, addition of total microalgal biomass might alter the structural properties of the food system. Therefore, information is required about their rheological characteristics towards selection of microalgae species for specific food products. This study comprises the rheological characterization of seven commercially available microalgae species in aqueous suspensions, before and after mechanical and thermal processing. Substantial differences in rheological properties were observed between the investigated microalgal suspensions. Among the untreated suspensions, Porphyridium cruentum, Chlorella vulgaris and Odontella aurita showed the largest structural properties and could be described as weak gels. All suspensions showed shear-thinning flow behavior at the examined concentration of 8% w/w, except for Nannochloropsis species. Shear-thinning behavior was also observed for the separated serum phase of P. cruentum, which might be attributed to the presence of sulfated exopolysaccharides. During processing, rheological properties were significantly altered. High pressure homogenization was used as a mechanical treatment, followed by a pasteurization or sterilization process. Whereas suspensions of Arthrospira platensis and C. vulgaris showed an increased storage modulus and viscosity after processing, the opposite was observed for P. cruentum and O. aurita. No clear effect of processing was observed for suspensions of Nannochloropsis sp., Schizochytrium sp. and Phaeodactylum tricornutum. Investigation of the microstructure revealed differences in degree of cell disruption by high pressure homogenization, with Nannochloropsis sp. being the most resistant. Subsequent thermal processing resulted in aggregation of released cell material and/or intact cells. In conclusion, the obtained results provide the scientific knowledge base for the selection of microalgae species towards food applications. Whereas some microalgae species hardly affect the structural properties of the food product, other microalgae species show large potential for use as a structuring agent in food applications.

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In the current study, the texturizing properties of partially pectin-depleted cell wall material (CWM) of apple, carrot, onion and pumpkin, and the potential of functionalization by high-pressure homogenization (HPH) were addressed. This partially pectin-depleted CWM was obtained as the unextractable fraction after acid pectin extraction (AcUF) on the alcohol-insoluble residue. Chemical analysis was performed to gain insight into the polysaccharide composition of the AcUF. The microstructural and functional properties of the AcUF in suspension were studied before HPH and after HPH at 20 and 80 MPa. Before HPH, even after the pectin extraction, the particles showed a cell-like morphology and occurred separately in the apple, onion and pumpkin AcUF and in a clustered manner in the carrot AcUF. The extent of disruption by the HPH treatments at 20 and 80 MPa was dependent on the botanical origin. Only for the onion and pumpkin AcUF, the water binding capacity was increased by HPH. Before HPH, the texturizing potential of the AcUFs was greatly varying between the different matrices. Whereas HPH improved the texturizing potential of the pumpkin AcUF, no effect and even a decrease was observed for the onion AcUF and the apple and carrot AcUF, respectively.

Cell wall related polysaccharides of the red microalga Porphyridium sp. were shown to be a promising source of new sustainable thickening agents. Isolated extracellular polysaccharides (EPS) consisted of high molecular weight polymers, showing a higher intrinsic viscosity compared to several commercially used hydrocolloids. Aqueous solutions of EPS (2% w/w) were characterized by substantial viscosities and weak gel behavior. Even though the extracted water soluble cell wall polysaccharides exhibited the same monosaccharide profile as EPS (composed of galactose, glucose, xylose and glucuronic acid), a lower molecular weight and intrinsic viscosity was observed for this fraction, resulting in poor rheological properties. Therefore, it was hypothesized that the physicochemical properties were related with a different molecular structural organization of these monosaccharides and sulfate groups. The main challenge for commercialization of extracellular polysaccharides of Porphyridium sp. remains the purification of these fractions to obtain polysaccharide extracts with low protein and salt contents.

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