Study On The Structure And Physicochemical Properties Of Rhizoma Gastrodiae Starch

Rhizoma Gastrodiae(also called “Tianma” in chinese) is a perennial orchidaceae, which has been used as traditional chinese medicine in the treatment of various diseases for thousands of years. It contains about 14.50% of starch, whose granule size is extremely small and has broad application prospects.In this paper, Rhizoma Gastrodiae starch was first isolated by water extraction, then purified via enzymatic method. The starch, protein and lipid content of the purified starch(without considering moisture) were 94.92%, 0.44% and undetected, respectively.The research results of micro-structure of Rhizoma Gastrodiae starch showed that the Rhizoma Gastrodiae starch granules were irregular polyhedron, with an average particle diameter of 1185 nm. Rhizoma Gastrodiae starch belong to B-type crystals, with crystallinity of 45.56%. The amylopectin content of Rhizoma Gastrodiae starch was 92.75%, which indicated that Rhizoma Gastrodiae starch belong to high amylopectin starch. The average chain length, external chain length, internal chain length, A: B and the degree of branching are 24, 15, 10, 2.72: 1 and 3.72, respectively, indicating a high degree of branching of Rhizoma Gastrodiae amylopectin. IR showed no big difference between Rhizoma Gastrodiae, maize and pea starch, except in 3200~3500 cm-1 and 1000~1200 cm-1, where the peaks of Rhizoma Gastrodiae starch were more wide and flat compared with maize and pea starch.The research results of physicochemical properties of Rhizoma Gastrodiae starch showed that the color parameters of L*、a* and b* were 97.00, 0.23 and 2.84, indicating that the whiteness of Rhizoma Gastrodiae starch was good and would leave a good impression on people. When the temperature increased from 50 ℃ to 90 ℃,the solubility and swelling of Rhizoma Gastrodiae starch increased from 3.45% and 2.74% to 20.75% and 25.24%, respectively, and were significantly higher than maize, pea and sweet potato starch. The transparency(10.6%) of Rhizoma Gastrodiae starch was the lowest among the four kinds of starchs. The pasting temperature of Rhizoma Gastrodiae starch with the content of 6% was 67.17℃, and was significantly lower than the other three. Meanwhile, it showed a minimum paste viscosity of 1911 cp among the four kinds of starch. Retrogradation was not found when Rhizoma Gastrodiae starch paste was stored at room temperature and 4℃, except at-18℃, when syneresis rate were 9.22%, 17.01% and 19.32% after three times freeze-thaw, respectively. Every parameter of gel texture of Rhizoma Gastrodiae starch was the lowest, indicating a weak gel. The RDS of Rhizoma Gastrodiae starch was the highest(90.98%), and the RS of it was the lowest(2.38%), suggesting that Rhizoma Gastrodiae starch was not healthy for diabetics.The rheological property parameters of Rhizoma Gastrodiae starch was measured by TA rheometer. Steady-shear tests showed shear-thinning behavior, indicating a pseudoplastic fluid. The viscosity of twice gelatinized Rhizoma Gastrodiae starch was significantly lower than once gelatinized Rhizoma Gastrodiae starch, suggesting the different structure between the two pastes. The apparent viscosity of gelatinized Rhizoma Gastrodiae starch paste was positively correlated with concentration and was negatively correlated with temperature. These data were well fitted to the power-law model and Arrhenius equation. Dynamic oscillation tests showed that the complex viscosity(η*) of Rhizoma Gastrodiae starch paste was negatively correlated with angular frequency(ω), While the storage modulus(G′) and loss modulus(G″) were both positively correlated with angular frequency(ω). G″: G′=0.36~0.55 showed that the elastic contribution is higher than the viscous contribution in Rhizoma Gastrodiae starch gels, which belong to elastic weak gels. These data were well fitted to the power-law model. But the data of apparent viscosity(η), complex viscosity(η*), shear rate(γ) and angular frequency(ω) were not well fitted to Cox-Merz rule, indicating the difference between anti-shearing and anti-oscillation abilities of Rhizoma Gastrodiae starch paste.