Seaweed as a sustainable source of food protein: maximizing seaweed protein content, protein recovery, and nutritional quality
Doctoral thesis, 2023

This thesis focuses on utilizing seaweed, such as Saccharina latissima and Ulva fenestrata, as sustainable food protein sources to complement terrestrial protein currently limited by land and water supply. While seaweed holds promise, its protein content is lower than pulses and antinutrients reduce protein nutritional quality. Additionally, S. latissima often contains excessive iodine, necessitating post-harvest blanching.

We aimed to produce protein-rich seaweed using food-process waters as nutrient sources; assess how blanching parameters impact downstream pH-shift-based protein extraction; create an efficient extraction method targeting aqueous-soluble and lipophilic proteins; and evaluate the influence of extraction on protein nutritional quality after in vitro digestion.

When food-process waters, mostly herring-derived, were added separately to the cultivation media of tank-cultivated U. fenestrata, protein content increased 2.4-fold compared to seawater media, reaching 24% per dry weight (dw). Growth rates generally remained unaffected and S. latissima was incompatible with this new nutrient loop.

Blanching sea-cultivated S. latissima at 45 or 80 °C for 2 minutes was equally effective at reducing iodine. However, biomass blanched at 45 °C provided higher protein extraction yields (23% vs. 14%). Iodine was still the limiting element for the daily adult consumption of extracts from blanched biomasses (0.5 g dw), though higher than extracts from crude biomass (0.1 g).

Employing 0.1-0.5% aqueous Triton X-114 during protein extraction from U. fenestrata followed by alkaline extraction provided a 3.3-fold increase in extraction yields (23%) compared to two alkaline extraction cycles. In both protocols, proteins were concentrated via isoelectric precipitation. It was confirmed that Triton disintegrated membranes, targeting also lipophilic proteins.

Digestibility of pH-shift extracts from U. fenestrata increased from 28% for crude biomass to 36%. Extraction also raised amino acid accessibility from 57% to 73%. When using the Caco-2 cell model, amino acids from U. fenestrata and extracts thereof were as bioavailable as casein.

Altogether, we raised seaweed protein content by recycling nutrients currently lost during food processing, improved protein extraction yields, and proved that extracts have higher digestibility than crude seaweed. Based on theoretical estimations, seaweed can offer a modest contribution to sustainable food systems, though this relies on scaling up seaweed production volumes.

protein isolation

intestinal permeability

detergent

thermal water treatment

protein shift

wastewater

elemental compositional

macroalgae

gastrointestinal digestion

cultivation

KC-salen, Kemigården 4,
Opponent: Professor Jöel Fleurence, University of Nantes, France

Author

João Pedro Trigo

Chalmers, Life Sciences, Food and Nutrition Science

Trigo, J. P., Stedt, K., Steinhagen, S., Krona, A., Pavia, H., Abdollahi, M., Undeland, I. Harnessing the power of surfactants and alkaline aqueous solutions to efficiently solubilize and precipitate proteins from the seaweed Ulva fenestrata.

The rising global population and increasing demands for protein-rich diets pose a problem - we are running into limitations in our efforts to expand traditional protein sources, especially livestock-based. The issue arises from high carbon footprints connected to red meat production, finite amount of farmable land to produce more feed, and freshwater scarcity. For instance, 25 kg of feed is needed to produce 1 kg of beef, and around 68% of agricultural land is used for pasture. Increased use of marine protein sources can be part of a sustainable solution, not least seaweed which does not require feed, farmable land, and can grow in saltwater.

While some seaweeds have good amounts of protein, the contents are still lower than in legumes like soy. Also, seaweed contains many dietary fibers which makes it hard for our bodies to digest the proteins and take up its building blocks - the amino acids. Another problem is that certain brown seaweed species, like sugar kelp, can contain too much iodine. Despite being an important micronutrient, iodine can be harmful to us in high amounts. To reduce iodine amounts, the industry often soaks sugar kelp in warm water for a few minutes in a process called blanching.

This thesis explored the idea that we can increase the concentration of protein in seaweed using two strategies: adding nutrient-rich food process water residues to the seaweed cultivation media or producing seaweed protein ingredients. For the latter, we used both raw and blanched seaweed and tested two different extraction methods. Also, we examined the impact of these extraction methods on the digestibility of the seaweed and its protein ingredients using an "in vitro" test tube model.

By adding residual herring and shrimp process waters to tanks with seawater, we could more than double protein amounts in green seaweeds like sea lettuce, compared to cultivating them just with seawater. In the work with sugar kelp, we found that low blanching temperatures were equally good as high blanching temperatures at removing iodine. However, when using lower temperatures, we could extract more proteins from the seaweed. Furthermore, the protein ingredients we got from blanched sugar kelp - regardless of treatment temperature - could be consumed in 5 times higher amounts without reaching the tolerable upper intake of iodine, compared to protein extracts from unprocessed sugar kelp. An important learning was that seaweed protein extraction methods, such as the one we applied to sugar kelp, need to become more efficient and simplified to allow the commercialization of seaweed protein ingredients. Therefore, we invented an extraction method that combined solutions that can access proteins located in all parts of the seaweed. This new method was tested on sea lettuce and extracted more than 3 times the amount of protein compared to traditional methods. By simulating human digestion in vitro, we proved that an extracted seaweed protein ingredient was more digestible than the crude seaweed.

Altogether, this thesis opens possibilities for a more sustainable future, by transforming seaweed into a new food protein source. However, for this transition to happen, seaweed cultivation needs to be largely increased.

Seaweed as a vehicle for nutrients in a circular food chain - innovative steps to accomplish a protein shift (CirkAlg)

Formas (2018-01839), 2018-12-01 -- 2021-11-30.

Seaweed production systems with high value applications (Sweaweed)

Swedish Foundation for Strategic Research (SSF) (RBP14-0045), 2021-01-01 -- 2021-12-31.

Swedish Foundation for Strategic Research (SSF) (RBP14-0045), 2015-01-01 -- 2019-12-31.

Subject Categories

Food Science

ISBN

978-91-7905-886-9

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5352

Publisher

Chalmers

KC-salen, Kemigården 4,

Opponent: Professor Jöel Fleurence, University of Nantes, France

More information

Latest update

9/7/2023 6