Investigation of polysaccharide extraction efficiencies from diatom biomass using green extraction techniques Green extraction of diatom polysaccharides considering a biorefinery concept and preservation of biogenic silica.

Sammanfattning

Diatoms, the abundant unicellular microalgae encapsulated by a silica cell wall calledfrustules, have received interest as a feedstock for biomass valorization. Despite itspromising content, challenges remain regarding the down stream processing of diatomswhich suffers from long processing time and reliance on toxic solvents. Thisstudy has explored two green extraction techniques, Ultrasound Assisted Extraction(UAE) and Microwave Assisted Extraction (MAE), to investigate extraction yieldsof polysaccharides. While polysaccharides were the extraction target, a biorefineryapproach has been considered, meaning the extraction yields of soluble proteins,lipids and pigments, and the impact on the frustules have been tested. High pressurehomogenization (HPH) treated biomass was subjected to extractions in water,which were performed in a 22-factorial design of experiments for both extractionmethods. The extract supernatant was used for quantification of extraction yield,monosaccharide composition and soluble protein content. The separated pellet wasused for lipid and pigment analysis, and observed under a scanning electron microscope(SEM) to assess the structural integrity of the cell walls. The green extractionmethods were compared with a conventional control of hot water extraction (HWE).Overall, MAE presented a gentle physical treatment, with an increased extracellularmonosaccharide content compared to HWE. The overall extraction yields of MAEand HWE were similar, displaying values in the range of 20-25%, where the MAEextracts contained the highest amount of monosaccharides (12%). However, therewas no dramatic increase compared to the 9-10% content measured in UAE andHWE, or between the different conditions. SEM imaging confirmed a marginalimpact on the frustule integrity by MAE. Moreover, pigment yield was enhancedcompared to initial biomass, whereas soluble protein yield decreased, likely due todenaturation in elevated extraction temperatures. UAE conditions resulted in thehighest extraction yield at approximately 45%, twofold that of MAE and HWE.Furthermore, UAE increased soluble protein yield by 85% compared with untreatedbiomass and showed a content twofold that of MAE, but induced the most damageto the frustules by fragmenting them. The disruption of cells increased the fractionof extracellular polysaccharides, but reduced the levels of glucose, possibly due todegradation of the polysaccharides into oligo- and monosaccharides, which should bestudied further. The performed extraction screening presents promising results, withpossibilities of method alterations and optimization for the future use of diatoms asa biorefinery feedstock.