Effect of charged polysaccharides on the techno-functional properties of fractions obtained from algae soluble protein isolate
Graphical abstract
Introduction
Microalgae have long been considered a promising alternative protein source for foods. Initially, the interest in microalgae protein was based purely on its high nutritional quality (Becker, 2007). Recently, the combined cost-efficient production of high-value protein isolates and biofuels from microalgae biomass has triggered renewed interest in the isolation of proteins from microalgae as ingredients in food (Wijffels et al., 2010, Williams and Laurens, 2010). Despite the increased interest, the structural and techno-functional properties of microalgae proteins are poorly documented. To enable the study of techno-functional properties of microalgae protein a process has recently been developed for the mild isolation of the soluble protein fraction from the green microalgae Tetraselmis sp. (Schwenzfeier, Wierenga, & Gruppen, 2011). The final algae soluble protein isolate (ASPI) was free from green colour and contained 64% (w/w) protein, 24% (w/w) carbohydrates, composed for one fourth of uronic acids, and presumably 12% (w/w) remaining material containing mainly ash and other minor components. Part of the carbohydrates in ASPI is present in form of glycoproteins. The uronic acids present are expected to mainly represent building blocks of charged polysaccharides present in ASPI. These charged polysaccharides could form soluble, electrostatically stabilized complexes with a surface-active protein. These complexes promote the co-adsorption of the polysaccharides to the interface (Ganzevles, Cohen Stuart, van Vliet, & de Jongh, 2006), thereby influencing the foam and emulsion properties of ASPI.
For ASPI stabilized emulsions, the presence of charged carbohydrates on the emulsion droplet surfaces has been indicated by measuring their ζ-potentials (Schwenzfeier, Helbig, Wierenga, & Gruppen, 2013), which remained negative over the whole pH range investigated (pH 3–7). The co-adsorption of charged polysaccharides and proteins resulted in increased stability against droplet aggregation around pH 5, the pH at which emulsions stabilized with currently used food protein isolates (e.g. whey protein isolate) often do aggregate. An increase in ionic strength (≥100 mM NaCl) caused the ASPI stabilized emulsion to flocculate at pH 5, presumably due to the dissociation of protein-polysaccharide complexes. The foam stability of ASPI stabilized foams increased with increasing ionic strength at all pH values tested, but the foam properties were not found to be affected by the presence of polysaccharides (Schwenzfeier, Lech, Wierenga, Eppink, & Gruppen, 2013). Similar dilatational moduli at varying pH and ionic strength suggested the sole adsorption of protein to the air–water interface. Still, the stability of ASPI stabilized foams compared superior to the stabilities of foams stabilized with whey protein isolate or egg white albumin in the pH range 5–7 at similar isolate concentrations, despite the lower protein content of ASPI.
In the present study the influence of the polysaccharides present in ASPI on the techno-functional properties of the isolate was investigated. For this purpose the isolate was further fractionated. Subsequently, the fractions obtained were characterized with regard to their chemical composition and their foaming and emulsion properties were studied as function of pH.
Section snippets
Materials
Algae soluble protein isolate (ASPI) (53% [w/w] protein) was isolated from the commercially available microalgae Tetraselmis sp. (Reed Mariculture, Campbell, CA, USA) as described previously (Schwenzfeier et al., 2011). All chemicals used were of analytical grade and purchased from Sigma (St. Louis, MO, USA) or Merck (Darmstadt, Germany) if not stated otherwise. Proteinase K (order no. 92905, 825 U/mg) from Fluka (Ronkonkoma, NY, USA) was used for hydrolysis experiments.
Fractionation of ASPI
Fractionation of ASPI
Chemical composition
The ASPI batch used in this study contained 53% (standard deviation (SD) < 1) (w/w) proteins, 16% (w/w) (SD 1) carbohydrates, composed for approximately one third of uronic acids, and 17% (SD not determined) (w/w) chloroform/methanol (CHCl3/MeOH) extractable material (Table 1). The remaining 14% (w/w) unidentified material are expected to be polysaccharide and protein building blocks not included in the standard compositional analysis of those molecules. Possible reasons for the incomplete
Conclusions
The centrifugal filtration of ASPI applied resulted simultaneously in a concentration-induced fractionation and a size-dependent fractionation of the isolate. Compared to ASPI, increased amounts of charged polysaccharide moieties in ASPI-UA ensured electrostatic repulsion between the individual emulsion droplets and, therefore, emulsion stability over a broad pH range (4–7). It is suggested that at least part of these charged polysaccharide moieties co-adsorb in form of glycoproteins. The foam
Acknowledgements
This work was performed within the research theme ‘Algae’ of the cooperation framework of Wetsus, Center Of Excellence For Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs.
The authors are thankful to Frederik Lech for carrying out the air–water interfacial measurements described in this study.
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