Physicochemical properties of the water and alkali extracted mucilages from the leaves of Asplenium australasicum（J. Sm.）Hook

• Main Authors: Hui-yuan Liang, 梁惠媛
• Other Authors: Lih-Shiuh Lai
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/53422055284289283002

碩士 === 國立中興大學 === 食品暨應用生物科技學系所 === 98 === Asplenium australasicum（J. Sm.）Hook, a kind of Pteridophyta, belongs to the family of Aspleniaceae. The tender leaves of Asplenium australasicum are getting lots of interest recently as one type of wild vegetables in Taiwan. During chopping of Asplenium australasicum plants, significant amount of mucilage were noticed. However, researches on the physicochemical properties of Asplenium australasicum mucilage are quite limited. Therefore, the objective of this study is to isolate and characterize the physicochemical properties of mucilage extracted from the tender leaves of Asplenium australasicum. Specifically, effects of extraction conditions, including solvent types (water and sodium bicarbonate) and extraction temperatures (25, 50, 70 and 90oC) would be evaluated. It was found that the yields of mucilage from Asplenium australasicum is significantly influenced by extraction solvents (NaHCO3 >> H2O), but not extraction temperature. Analysis of the molecular size by gel permeation chromatography (GPC) showed that, the weight average molecular size of Asplenium australasicum mucilages obtained from alkali and water extractions were about 6.30x107 and 2.63x107, respectively. However, the protein fractions overlapped with the low molecular weight mucilage fractions. Such results inferred that the mucilage from Asplenium australasicum was composed of mainly carbohydrates, in conjunction with some glycoproteins. The protein fractions and color matters could be mostly removed by active carbon, nevertheless accompanied with a significant decrease in mucilage yields. The conformation of Asplenium australasicum mucilage in 0.1 M NaCl was basically sphere, as evidenced by the value of Mark-Houwink exponent. HPLC-UV analysis revealed that the water-extracted mucilage contained about 85.67% neutral sugar (mainly galactose) and the ionic part was mainly glucuronic acid. In contrast, the alkali-extracted mucilage contained about 58.43% neutral sugar and the ionic part was mainly galactouronic acid. After being treated with active carbon, the amount of neutral sugar (mainly glucose and arabinose) also decreased, indicating active carbon could remove not only protein, but also the low molecular size mucialges. Rheological results of the Asplenium australasicum mucilage with different concentrations of mono-valent or divalent salts showed that in the dilute region (conc. ≤ 0.2%), the specific viscosity increased with increasing mucilage concentration, but decreased with increasing salt concentration (0-100 mM NaCl or KCl, 0-0.85 mM MgCl2 and 0-0.0085 mM CaCl2), possibly due to the electraostatic effect. The salt tolerance parameter of 70oC water extracted mucilage was significantly higher than that of 90oC alkali extracted mucilage, implying a stiffer backbone. However, alkali extracted mucilage formed ordered structure or solution gel under a CaCl2 concentration higher than 0.017 mM, as indicated by a significant higher viscosity as compared to the control sample. As analyzing the rheological properties in the semi-dilute and concentrated region, it was found that the steady shear viscosity as well as the dynamic shear storage (G’) and loss (G”) moduli of mucilage generally increased with increasing concentration, accompanied with moving the critical shear rate from Newtonian flow to shear thinning or cross-over frequency of G’ and G” toward smaller values, indicating an increase in molecular entanglement. However, the viscoelastic properties of the mucilage extracted by water was less affected by ionic strength, in contrast to the behavior of alkali extracted mucilage, which showed significant increase with increasing salt concentration. Furthermore, the mechanical spectra of alkali-extracted mucilage changed from a typical diluted solution to concentrated solution or even weak gel under appropriate salt concentration. The ionic effect was in the order of Ca2+>Mg2+ >>K+ and Na+, which was consistent with the rheological findings from dilute region.