Study On Physicochemical,rheological And Emulsifying Properties Of Polysaccharides Extracted From Stems Of Dendrobium Officinale

Dendrobium officinale,mainly distributed in southern China,is a perennial herb with high medicinal value for its active ingredients including alkaloids,flavonoids,polyphenols,amino acids,polysaccharides,etc.The current researches on Dendrobium officinale polysaccharides(DOP)mainly focus on the biological activities such as the ability to treat enteritis,immunomodulatory effects,antioxidant activities,etc..However,the rheological properties and emulsification properties of DOP are rarely studied.In this study,fresh Dendrobium officinale stems were employed as raw material to prepare DOP samples.The polysaccharide samples HEPDOP and EPDOP were prepared from D.officinale stems respectively with or without pretreating by hot water blanching before water extraction and ethanol precipitation.To observe the effects by the pretreatment of hot water blanching,the physicochemical and rheological properties of EPDOP and HEPDOP were investigated.On the basis of polysaccharides properties,the emulsifying ablility of DOP were studied.The coconut oil emulsions were prepared to discuss stability of the emulsions using polysaccharide complex as emulsifiers.The main achievements as followed:1.The juices crushed from Dendrobium officinale stems with pretreatments of hot water blanching revealed higher and more stable apparent viscosity than that without pretreatments.It was assumed the hot water blanching pretreatment significantly reduced the mannanase activity in D.officinale stems and the mannanase actions of degrading polysaccharides caused to the reduction of the viscosity of Dendrobium juice.2.The physicochemical properties of EPDOP and HEPDOP were investigated.The results showed that:(1)Through the infrared spectra scanning it was found there was no significant difference between EPDOP and HEPDOP,which suggested that the primary structure of polysaccharides could not be changed by the hot water blanching pretreatment;furthermore,by UV spectrum scanning for EPDOP and HEPDOP,the typical protein absorption peaks at 280 nm was observed,and due to more protein content,the peak intensity of EPDOP was higher than that of HEPDOP;(2)In the GPC spectrum,the molecular weight contribution curve of HEPDOP demonstrated a unimodal distribution(Mw=483.93 KDa),while that of EPDOP contained a main peak(Mw=411.41 KDa)and a heteropeak with a Mw of 98.88 KDa which potentially related to the protein in the sample;(3)By analysis of interface properties of polysaccharides,the?-potential or the interfacial tension of HEPDOP reached higher than that of EPDOP at the same concentration(0.5wt%),which may be due to the different content of protein in polysaccharide samples;(4)The TGA results further showed that the decomposition temperature of HEPDOP was 311°C,which was higher than that of EPDOP(306°C);(5)The results of the molecular fluorescence spectra of two polysaccharides(pyrene was the probe)showed that at the same 0.3%(w/w)concentration,the E₁/E₃ ratio of EPDOP was 1.218,which was lower than HEPDOP(1.344),indicating that EPDOP presented a larger hydrophobicity compared to HEPDOP.3.The rheological properties of EPDOP and HEPDOP were studied.The results showed that:(1)Both EPDOP and HEPDOP indicated shear-thinning behavior in solutions;(2)At the same concentration(2%(w/w)),HEPDOP had higher viscosity than EPDOP,and acidity,alkalinity,and the addition of salt ions was found to reduce their viscosity;(3)The dynamic oscillation test showed that the intersection of G'and G''values of HEPDOP occurred at a lower concentration compared to EPDOP,this phenomenon may be related to the length and number of molecular chains,which was caused by the difference in the degree of entanglement between EPDOP and HEPDOP molecular chains;(4)We observed that increasing temperature caused the decrease of viscosity and modulus of both DOPs,and in the experimental test temperature range,both EPDOP and HEPDOP showed thermal hysteresis,however,the gel transition temperatures were observed only in HEPDOP solution.4.The emulsification properties of Dendrobium officinale polysaccharide(DOP)were studied.Coconut oil oil-in-water emulsions were prepared by using Dendrobium officinale polysaccharide(DOP),propylene glycol alginate(PGA),gum arabic(GA)and their polysaccharide complexes as emulsifiers.The results showed that:(1)DOP mainly affected the apparent viscosity of complex polysaccharides,among the complex polysaccharide samples,PGA:DOP=2:8 and GA:DOP=2:8 had the highest viscosity,which were(380.8±1.39)m Pa·s and(205.6±0.35)m Pa·s respectively;(2)The interfacial activity and zeta potential of complex polysaccharides were mainly influenced by PGA or GA,the interfacial tension of the two polysaccharides were small,which were(7.69±0.015)m N/m for PGA:DOP=8:2 and(12.96±0.103)m N/m for GA:DOP=8:2;(3)Emulsions prepared with compound-polysaccharides exhibited smaller average particle sizes,the average particle size of the emulsion prepared by the compound-polysaccharide(PGA:DOP=2:8)was(8.356±0.061)?m,and the average particle size of the emulsion prepared by the compound-polysaccharide(GA:DOP=2:8)was(11.768±0.015)?m;(4)Fluorescence microscope observations showed that compared with single polysaccharides as emulsifiers,emulsions prepared by compound-polysaccharides had smaller droplets and less aggregation of droplets;(5)Emulsion rheology results showed that all emulsions exhibit typical non-Newtonian shear thinning behavior in the shear rate range of 1s^-1to 1000 s^-1,at the same shear rate,the viscosity of the emulsion prepared by compound-polysaccharide DOP/PGA(PGA:DOP=2:8,total concentration was 1.5%)and the viscosity of the emulsion prepared by compound-polysaccharide DOP/GA(1.5%DOP,2.0%GA)were relatively large,and the G'and G''of these two emulsions increased with increasing frequency,in the frequency range of 0.1 Hz-10 Hz,G''always dominated,indicating that both emulsions exhibited viscosity-dominated fluid-like behavior;(6)The stability analysis of emulsions by a dispersion analyzer LUMi Sizer showed that the emulsion prepared by compound-polysaccharide DOP/PGA(PGA:DOP=2:8,total concentration was 1.5%)and the emulsion prepared by compound-polysaccharide DOP/GA(1.5%DOP,2.0%GA)had better stability.