Advances in the biorefinery of Sargassum muticum: Valorisation of the alginate fractions
Introduction
Seaweeds are a potential sustainable source of biopolymeric compounds, which are not found in terrestrial plants. Depending on the pigment (green, brown or red), the seaweed properties can notably vary, being fucoxanthin the specific pigment in brown seaweeds. The main gelling and bioactive components in these last seaweeds are alginate, laminarin, fucoidan, phlorotannins, minerals, lipids, among others. Their content and properties depends on the geographical location, season, age, environmental factors and the extraction technology. These factors confer to the seaweed unique characteristics in terms of chemical characteristics and biological activities. Alginate is a linear biopolymer with two types of anionic monomers, β-d-mannuronic (M) and α-l-guluronic acid (G); depending on how these compounds are linked the properties change (Stiger-Pouvreau et al., 2016). It is the main constituent of the cellular walls of brown algae, representing 17–45% of the dry weight in the brown seaweed (Vera et al., 2011). The algal origin and their seasonal variations are responsible about the proportion of alginate types M and G and their proportions. When the alginates are rich in MG/GM blocks usually these will be less viscous, more acid soluble and, have the possibility of forming more flexible chains. On the other hand, when the GG blocks number is larger, the gelling properties have shown a great stability. Based on these properties, alginate is widely used as thickener, stabiliser or gelling agent in food and non-food industries (Usov and Bilan, 2009; Abdulbari et al., 2014; Rioux and Turgeon, 2015). Another polysaccharide of interest is fucoidan, mainly composed of fucose, other monosaccharides and sulfate groups. It is only present in brown seaweeds and has interesting biological properties for human health, such as antiviral, antitumor, antioxidant, anti-inflammatory, immunomodulatory and antiangiogenic (Li et al., 2008; Santoyo et al., 2011; Lee and Mooney, 2012; Dore et al., 2013; Hamid et al., 2015; Cong et al., 2016; Palanisamy et al., 2017; Wang et al., 2017).
It should be pointed out that commonly used extraction technologies show many drawbacks that need to be overcome. Long extraction time, low selectivity and relatively high solvent consumption are some disadvantages of conventional technologies to extract different fractions from seaweed or other raw material. For this reason, green alternative extraction technologies (such as autohydrolysis or subcritical water extraction) could be an attractive possibility to achieve functional extracts with a high extraction yield, improving solubility and saving extraction time using high temperature and high pressure. In addition, extraction technologies based on pressurized conditions, using water as a solvent, are considered as environmentally friendly techniques, avoiding the use of organic solvents (Flórez-Fernández et al., 2018).
It should be highlighted that autohydrolysis, based on an autocatalysed reaction in mild acidic media, could cause depolymerisation and solubilisation of polysaccharides. In addition, the physicochemical properties of water: polarity, surface tension, diffusivity, dissociation coefficient or viscosity, change compared to those found at atmospheric conditions. The solvation properties can be manipulated changing temperature and extraction time (Conde et al., 2010; Flórez-Fernández et al., 2018). In this context, this work is part of a project motivated on producing compounds of industrial interest from the brown seaweed Sargassum muticum using autohydrolysis as an innovative environmental friendly technology, following the biorefinery concept to fractionate the raw material. Specifically, this work has been focused on the recovery and characterisation of the alginate fractions to complement the results obtained in previous (González-López et al., 2012a, 2012b; Balboa et al., 2013) and in a parallel work (Flórez-Fernández et al., 2017b), where the bioactive fractions from autohydrolysis liquor were studied. For this purpose, the physicochemical, rheological, bioactive and biological properties of the precipitated alginate from brown seaweed Sargassum muticum were studied at the conditions previously selected in terms of bioactive potential of the liquid fractions obtained by autohydrolysis in the later work.
Section snippets
Raw material
Sargassum muticum was collected in Praia da Mourisca (Pontevedra, Spain), in July 2014. After cleaning, washing and grinding algae were stored wet in hermetic plastic bags in darkness at −18 °C until further use.
Autohydrolysis
The algal samples were mixed with distilled water in a stainless stirred reactor using a liquid:solid ratio (LSR) 30:1 w/w. The suspension was heated up to 150 or 170 °C in a pressurized reactor (Parr Instruments 4848, USA). Once the reactor achieved the temperature selected, then it
Physicochemical properties
The proximal composition (in dry basis) of the brown seaweed Sargassum muticum was summarised on Table 1. The results were similar to others works using this seaweed as raw material. Namely, the composition was in well harmony with the data reported by Balboa et al. (2013), with seaweed collected in the same place (June 2010). The slight compositional differences found between both samples can be justified by environmental factors variations.
A simplified scheme of this environmental friendly
Conclusions
The proposed environmental friendly processing of S. muticum could be an attractive alternative for the integral use of this invasive seaweed. The results obtained in this work jointly with those previously reported indicated that autohydrolysis could be a promising technology to simultaneously extract alginate fractions, fucoidans and phlorotannins. Careful monitoring of the processing conditions is required for the subsequent revalorization of the different fractions of the marine sources.
Acknowledgements
Financial support from the Ministry of Science and Innovation of Spain (CTM2015-68503R), Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016-2019) and the European Union (European Regional Development Fund - ERDF), is gratefully acknowledged. M.D.T. thanks Spanish Ministry of Economy and Competitiveness for her postdoctoral grant (IJCI-2016-27535), and N.F.F. thanks Xunta de Galicia for her postdoctoral grant (ED481B 2018/071).
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