葛根淀粉糊流变学特性的研究

以葛根淀粉糊的动态及稳态流变特性为研究对象,以储存模量(G')、损耗模量(G\")和表现黏度(η)为主要试验指标,利用旋转流变仪,研究淀粉浓度、氯化钠、蔗糖以及麦芽糊精对淀粉糊流变学特性的影响,旨在探究淀粉糊的弹性、黏性、表观黏度的变化,为葛根食品的工业生产提供一定理论依据。研究结果表明:葛根淀粉糊是典型的剪切变稀的非牛顿流体,其G'、G\"及η均受这4种因素的影响。1)动态流变特性的研究结果显示,G'、G\"与淀粉浓度呈正相关;随特定添加范围内的氯化钠、蔗糖以及麦芽糊精的添加量增加,G'、G\"均有先升后降的趋势。2)稳态流变特性的研究结果显示,同一剪切速率下,η与淀粉浓度呈正相关,与麦芽糊精的添加量呈负相关;在特定添加范围内,氯化钠、蔗糖的添加量增加,η有先升后降的趋势。在工业生产中,葛根淀粉糊剪切稀化的行为有利于流动的淀粉糊充模成型,节省能耗。添加麦芽糊精会降低淀粉糊的弹性和黏性,而添加适量的氯化钠和蔗糖一定程度提高淀粉糊的弹性和黏性。     

Rheological behaviour of acorn starch dispersions: effects of concentration and temperature

Rheological properties of acorn starch dispersions at different concentrations (4%, 5%, 6% and 7%) were evaluated under steady and dynamic shear conditions. The flow behaviours of the acorn starch dispersions at different temperatures (25, 40, 55 and 70 °C) were determined from the rheological parameters provided by the power law model. The acorn starch dispersions at 25 °C exhibited high shear-thinning fluid characteristics ( n  = 0.23–0.36). Consistency index (K) and apparent viscosity (η_a,100) increased with an increase in starch concentration, and were also reduced with increasing temperature. Within the temperature range of 25–70 °C, the η_a,100 obeyed the Arrhenius temperature relationship with a high determination coefficient ( R ² = 0.97–0.99), with activation energies (E_a) ranging between 16.5 and 19.0 kJ mol⁻¹. Both the power law and exponential type models were employed in order to establish the relationship between concentration and apparent viscosity (η_a,100) in the temperature range of 25–70 °C. Magnitudes of storage ( G' ) and loss ( G ) moduli increased with an increase in the starch concentration and frequency (ω). The magnitudes of G ' were higher than those of G over most of the frequency range (0.63–62.8 rad s⁻¹). The dynamic (η*) and steady shear (η_a) viscosities of acorn starch dispersion at 7% concentration follow the Cox–Merz superposition rule.     

破碎温度对番茄浆流变性质与果胶分子结构的影响

对在不同破碎温度下所得番茄浆的动态流变性质以及果胶物质分子量的分布和大小进行了研究。结果表明：随着破碎温度的增加,番茄浆中果胶物质的平均分子量增加,但当破碎温度为90℃时,果胶物质的平均分子量却有所下降。在80℃和85℃破碎时,番茄浆具有相当大的贮能模量,而在75℃和90℃破碎时,贮能模量值较小。贮能模量值对振荡频率具有一定的依赖性,故番茄浆表现出弱凝胶特性。贮能模量G‘和损耗模量G‘‘交汇点的位置随着破碎温度的升高而向低频区移动。这说明破碎温度越高,大分子物质被保留得愈多,体系呈较强的弹性;破碎温度越低,体系中大分子物质被保留得越少,体系呈较强的粘性。     

Changes in the rheological properties of Iranian UF-Feta cheese during ripening

The frequency sweep test was used to evaluate storage modulus (G′), loss modulus (G′′) and loss tangent [tan (δ)] of Iranian UF-Feta cheese during ripening period (3, 20, 40 and 60 days). With development of ripening, storage and loss moduli increased at varying rates. The rate of increase in G′ was greater than that in G′′ resulting in a reduction in tan (δ). That is, storage modulus was dominant to loss modulus and as a result the elasticity nature was greater than the viscous nature of cheese samples. Due to the disruption of fat globules and proteolysis, protein matrix rearranged and formed a more compact texture containing aggregates of casein. Ripening did not influence the pH level and also the concentrations of dry matter, fat, salt, and total nitrogen in dry matter. However, water soluble nitrogen increased significantly (P < 0.05).     

Viscoelastic properties of concentrated aqueous ethanol suspensions of α-zein

Viscoelastic properties of α-zein dispersed in aqueous ethanol were studied using oscillatory strain rheometry. In 55-80% v/v aqueous ethanol, zein was only partially soluble, forming a gel at a sufficiently high zein concentration. The strain dependence of the storage modulus and the loss modulus of gelled systems exhibited features characteristic to closely-packed swollen particles. Close-packing was found to occur at a lower zein concentration with increasing ethanol concentration as the threshold for gelation decreased from a zein concentration of ca. 29 to 20% w/v with increasing ethanol concentration from 55 to 80% v/v. A contrasting trend in the effect of the solvent quality was revealed at a constant zein concentration of 30% w/v, at which the storage modulus decreased from ca. 12,000-700 Pa with increasing ethanol concentration from 55 to 80% v/v. The two major factors determining viscoelastic properties of the partially solvated zein systems were identified to be: (1) the degree of dissolution of zein into the continuous phase that was negatively correlated with the volume fraction of the dispersed phase and positively correlated with the osmotic pressure of the continuous phase; and (2) the degree of swelling of partially solvated zein particles that was positively and negatively correlated with the volume fraction and the storage modulus of the dispersed phase, respectively.     

Rheological and thermal effects of galactomannan addition to acorn starch paste

Effect of galactomannans (guar gum and locust bean gum) at different concentrations on rheological and thermal properties of acorn starch pastes was examined. Steady and dynamic shear rheological tests indicated that the magnitudes of consistency index (K), apparent viscosity (η_a,100), Casson yield stress (σ_oc), dynamic moduli (G′, G″), and complex viscosity (η∗) of acorn starch-galactomannan mixtures were much higher than those of the control (0 g/100 g gum concentration), and that these values also increased with an increase in gum concentration. At temperatures ranging from 25 to 70 °C, the effect of temperature on η_a,100 was well described by the Arrhenius equation. In addition, DSC studies showed that the presence of galactomannans resulted in an increase in the transition temperatures (T_o, T_p, and T_c) and a decrease of the gelatinization enthalpy (ΔH). In general, these results suggest that the presence of galactomannans in acorn starch modifies the rheological and thermal properties, but that these modifications are dependent on the gum type and gum concentration.