Valorization of red seaweed biomasses towards future sustainability (VALSEA), Multi-extraction of bioactive compounds from macroalgae

Due to the increase in the global population, the demand for food will increase dramatically during the coming 30 years and worldwide food production is facing substantial challenges. It is predicted that protein as a primary nutrient may be in short supply in the future. Hence, it is widely accepted that alternative sources and production methods for protein are needed.

Seaweeds are already an essential resource in the food industry where they are used for production of for example, stabilizing agents. Moreover, seaweeds are rich in both minerals and essential trace elements. In addition, seaweed is a viable alternative source of protein. Some species of seaweed are known to contain protein levels up to 47%.

The use of seaweed for protein production has several benefits compared with traditional high-protein crop use. For example, seaweeds have a higher protein yield per unit area and do not require freshwater to grow. Currently, only carrageenan and furcellaran are extracted from seaweed in large scale while all other compounds proteins, antioxidants, and pigments are wasted.

Thus, it would be advantageous to have a method of preparing carrageenan/furcelleran where other bioactive compounds from seaweed can be extracted as value-added products without decreasing the carrageenan/furcelleran yield or introducing adverse effects on their functional quality. . The global carrageenan and furcellaran production in 2014 amounted to 60,000 tonnes with a value of US$ 626 million.

From this, it can be estimated that the total dried seaweed consumption for this production was at least 300,000 tonnes per year. The protein content of these types of seaweed is typically in the range 4–28%. If just half of the total amount of protein could be extracted, more than 20,000 tonnes per year of a new, high-value protein product would be obtained.

The main hypotheses of this PhD study are that enzyme-assisted extraction (EAE) is the best method to extract proteins from Eucheuma denticulatum, Palmaria palmata and Furcellaria lumbricalis. EAE should be followed by alkaline extraction to increase the efficiency of protein extraction, and the protein extraction has no adverse effect on the carrageenan and furcellaran quality.

Therefore, the main focus of the present PhD study has been to design and develop a method to extract protein from Palmaria palmata that can be implemented in large scale and also to extract both protein and carrageenan/furcellaran from Eucheuma denticulatum and Furcellaria lumbricalis. In order to avoid detrimental effects on carrageenan and furcellaran, there was a pre-defined framework that all the experiments were performed at pH 7 at room temperature.

Moreover, the isolated carrageenan and furcellaran after protein extraction were evaluated with regard to yield, gel quality, and viscosity. The highest protein extraction efficiency (92 %) was obtained for Palmaria palmata. In contrast to Furcellaria lumbricalis and Eucheuma denticulatum, there were no pre-defined limitations with respect to the pH and temperature ranges that could be applied in the extraction procedure.

The obtained results for Eucheuma denticulatum indicated that Alcalase® at 0.2% w/w and pH 7 (59 % protein extraction efficiency) or Viscozymes® at 0.2% w/w and pH 7 (48 % protein extraction efficiency) were the optimal treatments for extracting protein from this seaweed species. The yield of carrageenan extraction for the blank sample (with no enzymatic treatment and no NAC-assisted alkaline extraction) was 17.8%, while it increased to 23.8% when only N-acetyl-L-cysteine (NAC) assisted alkaline extraction with no enzymatic treatment was tested for protein extraction.

Moreover, although the carrageenan yield when using the combination of Celluclast® and Shearzymes® with 35.5% was higher than when the treatments were performed with Viscozymes® (27.6%) or Alcalase® (27.7%), the gel quality and, in particular, the maximum gel strength (breaking strength), were lower for the combination of Celluclast® and Shearzymes® compared with Viscozymes® or Alcalase®.

Celluclast® and Shearzymes® resulted in a negative effect on breaking strength compared with the blank sample. In addition, the effect of protein extraction on the amino acid profile of extracted protein was evaluated. Compared with the untreated sample, the lowest increases of 44% and 30% in total content of amino acids and essential amino acids were observed for the sample that was treated with Alcalase®, while higher increases were obtained for the combination of Celluclast® and Shearzymes® with 146% and 159% respectively.

For Furcellaria lumbricalis, the lowest protein extraction efficiency was obtained by using different enzymatic treatments followed by NAC-assisted alkaline extraction. The maximum extraction efficiency was 23% when the combination of Fungamyl® 800 L, Viscozyme® L, and Alcalase® 2.4 L FG was used, followed by NAC-assisted alkaline extraction.

The effect of the enzymatic treatmentat higher temperatures than room temperature, namely 35, 45, 50, and 55˚C were also evaluated. There was no significant difference (p<0.05) between the maximum protein extraction efficiency at higher temperatures and at room temperature. Consequently, the protein extraction was conducted by chemical extraction using NaOH or KOH.

The results indicated that NaOH is more efficient than KOH for extracting protein. However, it also releases furcellaran to the extracted solution. Therefore, KOH was selected for further experiments. Different temperatures and extraction duration were examined to evaluate the possibility of protein extraction from Furcellaria lumbricalis.

The results showed a significant increase in extraction efficiency, which was up to 90% for the sample treated by KOH 8% for 2 or 6 hours at 80˚C. However, there was a detrimental effect on furcellaran quality for the treatments at 40˚C and 80˚C with KOH 4% and KOH 8%. The protein extraction with KOH 0.5% for 2 or 7 hours at 40˚C, and KOH 0.5% for 18 hours at room temperature had a positive effect on furcellaran quality.