Food ingredients from cultivated seaweeds-Improving storage stability and protein recovery

Doktorsavhandling, 2019

There is a global demand for new vegetarian protein sources, and seaweed have for multiple reasons been identified as a promising candidate. The overall aim of this thesis was to evaluate the potential of three different Swedish seaweed species as food protein sources, but also as sources of unsaturated fatty acids, vitamins and minerals. To accomplish this, the goals were to i) obtain a seaweed biomass high in protein and lipids, ii) find strategies to maintain the quality of the seaweed biomass during drying and storage and iii) develop an efficient method for recovering seaweed proteins.

The protein and lipid content in seaweed is related, e.g. to the nutrient access and physical parameters of the surrounding environment. By applying alterative cultivation, the protein and fatty acid content in U. lactuca were increased 3.4 respective 1.5 times by nitrate addition. Furthermore, the protein content was increased by cultivation at low temperature (12 °C) and light (50 μmol photons m-2 s-1), and the fatty acid content was increased by low light and high temperature (18°C).

The polyunsaturated fatty acids (PUFAs) in seaweed could make the dried biomass susceptible to lipid oxidation, with co-oxidation of pigments and vitamins. However, during long-term storage (≤520 days) of oven- and freeze-dried P. umbilicalis and U. lactuca, there was only a moderate development of the lipid oxidation-derived aldehydes, malondialdehyde, 4-hydroxy-trans-2-hexenal and 4-hydryoxy-trans-2-nonenal, while there was a great loss of unsaturated fatty acids and ascorbic acid. Light stimulated the fatty acid loss as well as bleaching of chlorophyll.

Several advantages are foreseen from concentrating the seaweed proteins. This calls for food grade and scalable fractionation methods. The pH-shift process, using alkaline protein solubilisation followed by isoelectric precipitation, was in this work adapted and improved for P. umbilicalis, U. lactuca and S. latissima, e.g. by including freeze-thawing-stimulated protein precipitation. When then comparing the new pH-shift process to two other fractionation methods, the pH-shift method resulted in extracts with the highest protein content: 71%, 51% and 41% per dry weight for P. umbilicalis, U. lactuca and S. latissima, respectively. The protein contained 37-41% essential amino acids. The highest achieved protein yields using the pH-shift method were 23%, 6% and 25%, respectively, for the listed species. For U. lactuca, the yield was further raised to 29% by incorporating a pre-incubation step at pH 8.5 prior to further protein solubilisation at pH 12. The pH-shift process was also successfully used as a first step in a sequential recovery of proteins, carrageenan, pectin and cellulose from P. umbilicalis, showing potential as a biorefinery tool. Throughout the work, the effect of different protein analysis methods on the achieved concentrations of protein in seaweed and seaweed protein extracts was evaluated. This revealed that the analytical choice has a profound impact on the results, especially for the extracts.

To summarise, it was possible to increase the level of proteins and lipids in seaweed through alterative cultivation protocols, and the seaweed proteins could then be further concentrated using the pH-shift process. These results strengthen the possibility that seaweed biomass can be a valuable complement to terrestrial vegetarian food protein sources. If considering the seaweed PUFAs as an added value to recover along with proteins, caution must, however, be taken when it comes to pre-processing storage of the seaweed biomass; these nutrients easily degrade during storage of dried seaweeds, especially in the presence of light.