Understanding carrageenan extraction principles to build insight in red seaweed cell wall structural organization

Carrageenan is a type of sulfated galactan found in the cell wall of red seaweeds called the carrageenophytes. In 2015, more than 60 thousand tonnes of dry carrageenophytes were processed globally to meet the demand of industrial carrageenan. These industries use carrageenan as a gelling or thickening agent in many of their product applications. Carrageenan gelling properties depend strongly on the carrageenan types and cations used to make the gels. In general, there are three major carrageenan types. The first two, κ- (kappa) and ι- (iota) carrageenan are used to form a strong and weak gel, respectively. These two carrageenan types are found at varying ratios in the gametophyte life phase of the carrageenophyte. The third type, λ-(lambda) carrageenan, is a non-gelling (precursor type) and acts as a thickening agent. λ-carrageenan exists by itself in the tetrasporophyte life phase of the carrageenophyte. The conventional carrageenan extraction process relies on high temperatures (80-90oC) and relatively long extraction times (4-8 h) to optimally solubilize carrageenan from the carrageenophyte cell wall, before the carrageenan is recovered through isopropanol (IPA) precipitation. For seaweed species that have a high ratio of gelling carrageenan (κ-,ι-carrageenan), alkali is usually added during the extraction process to transform the precursor carrageenan into gelling carrageenan, maximizing the gelling carrageenan concentration, thus improving its gelling properties. The combination of these high extraction temperatures, long extraction time, as well as the use of alkali effectively degrades the seaweed biomass, allowing the recovery of optimal carrageenan yields. However, as a consequence, many other temperature-sensitive seaweed components that are still valuable such as phycobiliproteins (light harvesting protein complex unique to red seaweeds and cyanobacteria) will be denatured under these intense conditions and will have lost their functionality (e.g. color). Furthermore, the use of the isopropanol filtration aid to separate the residual biomass from the water-soluble carrageenan makes it difficult to recover the biomass, as the biomass adheres to the filtration aid. This is unfortunate, since the biomass is still relatively rich in other compounds (e.g. proteins, pigments, fibres etc.). Due to the large volume of carrageenophytes being processed, valorization of even a fraction of the side stream components will add value to the carrageenan industr