Supercritical CO2 and Green Extraction methods for Added Value Products : Carotenoids, Chlorophylls and Phycocyanin from Spirulina Microalgae

Nowadays, the blue-green microalgae of the genus Arthrospira, commonly known as Spirulina, are commercially grown all around the world for their nutritional properties. The popularity of Spirulina as a food supplement is mainly due to its high protein content (up to about 70% by dry weight) and its richness in minerals, vitamins and provitamins, phytochemicals, essential amino acids, fibres and pigments 1. Among them, carotenoids, chlorophylls and phycocyanins are of high relevance as food and feed dyes. In particular, phycocyanin has been widely considered as a precious protein target because of its rare intense-blue colour, due to the presence of linear tetrapyrrole chromophores, covalently bound to cysteine residues via thioether bonds. Its protein-based structure is arranged in αβ protomers associated into trimers (αβ)3 and hexamers (αβ)6. Its absorption in the visible region (max=620 nm) and its natural fluorescence account for its application as marker in the medical field. The presence of the protein in the algae, carrying specific chromophores, is able to enhance the absorption range in the visible spectrum of light, facilitating the photosynthesis. Different strategies were developed for the isolation and purification of phycocyanin in the last decade, all of them however discarding the residual pigment fraction 2. This study suggests an integrated and “green” extraction chain that only leads to phycocyanin at the end. The body of the strategy involves two consecutive steps of extraction of carotenoids and chlorophylls through supercritical-CO2, a well-recognised “green” extraction method, before phycocyanin extraction 3. The biomass residue, exhausted in terms of carotenoids and chlorophylls, is finally extracted in water to yield phycocyanin. On the basis of recent and past literature on the topic, a strategy to yield the blue pigment with high purity was developed, keeping an eye on the scalability of the overall process in terms of cost and time consumption. Consecutive steps were carried out in order to enhance the phycocyanin purity, including electrocoagulation, dialysis and protein salting-out. These processes yielded 250 mg g−1 of phycocyanin (by dry Spirulina weight). A potentially scalable strategy to obtain the blue pigment with high purity (A620/A280 = 2.2) was set up. The practical application of the extracted blue phycocyanin pigment as a cotton-based tissue colorant was also experimented. References: [1] G. Chamorro-Cevallos, Int. J. Food Nutr. Sci., 2016, 3, 1-10. [2] R. Chaiklahan, N. Chirasuwan, V. Loha, S. Tia, B. Bunnag, Bioresour. Technol., 2011, 102, 7159–7164. [3] S. Marzorati, A. Schievano, A. Idà, L. Verotta, Green Chemistry, 2020, 22, 187-196.