Sterols in Microalgae : Euglena gracilis and Selenastrum sp.

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Title: Sterols in Microalgae : Euglena gracilis and Selenastrum sp.
Author: Zhang, Yangyang
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Food and Environmental Sciences
Publisher: Helsingin yliopisto
Date: 2017
Language: eng
Thesis level: master's thesis
Discipline: Elintarvikekemia
Food Chemistry
Abstract: The literature review introduced the chemistry of sterols and presented the sterols found in microalgae, and placed emphasis on the analytical methods used for studying sterols in microalgae. A brief discussion about application of microalgae-derived sterols was also included. The aim of this work was to learn about the sterol compositions in microalgae: Euglena gracilis and Selenastrum sp.. The common analytical methods of sterols are not suitable when applied to microalgae. Traditional alkaline hydrolysis may lead to an underestimation of total sterol content, because it cannot break acetal bond in steryl glycoside (SG). Additional acid hydrolysis for determining SG may lead to isomerization or decomposition of Δ7-sterols, which are the main sterols in green algae. A combination of alkaline hydrolysis and enzymatic hydrolysis was performed in this study. Firstly, sterol contents were determined using two methods: direct saponification and accelerated solvent extraction followed by saponification. Secondly, sterol classes: free sterol (FS), steryl ester (SE), and SG were determined by fractionation using solid phase extraction, followed by alkaline hydrolysis (FS and SE) and enzymatic hydrolysis (SG). Sterols were quantified using an internal standard and determined by GC-FID as their trimethylsilyl ether derivatives and identified by GC-MS. Euglena gracilis contained three major sterols: ergosterol and corbisterol, and Selenastrum sp. contained Δ7-ergosterol, chondrillasterol, and Δ7-chondrillasterol. Sterol contents ranged from 0.68-3.24 mg/g dry matter in Euglena gracilis, of which ergosterol constituted 68-93%. Sterol content in Selenastrum sp. was > 9 mg/g dry matter, with 36% Δ7-ergosterol, 12% chondrillasterol, and 52% Δ7-chondrillasterol. Comparison between the two extraction methods showed that ASE had a lower sterol yield than direct saponification. In E. gracilis, SE compromised 20-24%, FS 60-65%, and SG 11-12%. In Selenastrum, SE compromised only 1%, FS 74%, and SG 25%. The findings suggested that data on sterol composition ought to be viewed with caution. Underestimation of total sterol content may result from missing remarkable amounts of SG in certain microalgae species.
Subject: plant sterol

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