RHEOLOGICAL PROPERTIES AND APPLICATIONS OF MESQUITE TREE (PROSOPIS JULIFLORA) GUM 2. RHEOLOGICAL PROPERTIES AND STABILITY OF O/W EMULSIONS CONTAINING MESQUITE GUM

Oil-in-Water emulsions stabilised by mesquite gum have been examined by their creep compliance-time response at a constant low shear stress and by shear stress-shear rate studies. The Theological data were influenced by pH, electrolyte (NaCl, CaCl₂) and aging time. Both pH and electrolyte altered the degree of compactness of the mesquite molecules adsorbed on the oil drops. Aging produced changes in the rheological properties, the general trend depending on the rate of drop coalescence and the ability of adsorbed mesquite layers on adjacent flocculated drops to interpenetrate and form new linkages. The drop coalescence rate was highest at pH 7.0, but this was offset by greater interpenetration so that the rheological parameters increased substantially during aging.     

EFFECT OF POLYSORBATE-60 ON THE STABILITY OF O/W EMULSIONS STABILIZED BY PROPYLENE GLYCOL ALGINATE AND XANTHAN GUM

The effect of polysorbate-60 in improving the stability of model oil-in-water emulsions formulated with xanthan gum and propylene glycol alginate was studied at two levels of the surfactant; 0.15% and 0.30%. Creep measurements and steady shear measurements as well as surface tension and oil droplet size measurements were conducted to relate rheological and surface properties to improved stability. Higher polysorbate-60 concentration resulted in lower surface tension values which shifted the oil droplet size distribution to a smaller size range; this distribution was a function of aging time at 0.15% weight concentration PS-60. The decay in rheological parameters, particularly Newtonian viscosity, was slower at 0.30% surfactant concentration.     

RHEOLOGICAL PROPERTIES OF CHEMICALLY MODIFIED RICE STARCH MODEL SOLUTIONS

Native rice starches have poor resistance to shear, and fair stability to retrogradation with soft texture, which can be altered through chemical modifications. Starch from broken pieces of rice of three rice cultivars (PUSA‐44, PR‐106 and PR‐114) was chemically modified by etherification and esterification reactions by different combinations of modification reagents to investigate the effect of modification on the rheological properties of rice starches. The modification resulted in shear stable gel with apparent pseudoplastic characteristics. The viscosity of starches increased upon acetylation and dual modification as a result of increase in solubility. However, cross‐linking had shown the reverse effect. The flow behavior index (n) and consistency coefficient (k) were significantly (P ≤ 0.05) improved upon modification in acetylated and dual‐modified starches. The effect of modification on the rheological properties was observed in similar pattern in all the three rice cultivars but varied significantly in variety PUSA‐44 may be because of its relatively higher amylose content.     

EFFECT OF SALT ON THE STABILITY OF PROPYLENE GLYCOL ALGINATE/XANTHAN GUM/POLYSORBATE-60 STABILIZED OIL-IN-WATER EMULSIONS

Stabilization mechanism of salt, as a dissolved electrolyte, in oil-in-water emulsions in the presence of stabilizers and/or emulsifiers, propylene glycol alginate, xanthan gum and/or polysorbate-60 has been studied using rheological techniques, particle size analysis and surface tension measurements. Salt affected the stability of the ternary system, propylene glycol alginate/xanthan gum/polysorbate-60, depending on the emulsifier/stabilizer ratio. Creep measurements indicated that low salt concentrations were particularly effective when PGA:X ratios were used in the presence of PS-60. In both PGA:X and PGA:X/PS-60 systems salt did not affect the particle size distribution during aging. Surface tension measurements were in agreement both with particle size distribution and high shear rate experiments.     

提高黄原胶热稳定性的发酵实践

黄原胶是一种具有广泛用途的微生物多糖。以野油菜黄单胞菌1.1781（Xanthomonas campestris）为生产菌株,进行了摇瓶条件下黄原胶发酵条件的优化研究,得到耐热性黄原胶形成的最佳条件为：蔗糖50.0g/L,蛋白胨5.0g/L,K2HPO4·3H2O4.0g/L,MgSO·47H2O0.2g/L,柠檬酸2.0g/L,碳酸钙3g/L,pH为7.0。所得黄原胶成品经121℃高温处理3h,测得的粘度为31.17mPa·s,比基本培养条件提高了24.13%。通过采用热重分析仪（TGA）对其耐热性进行研究,结果表明,黄原胶是一种热稳定性好的食品胶。     

变性淀粉及谷朊粉对面团特性的影响研究

研究淀粉及变性淀粉、谷朊粉对面团特性的影响。结果显示：在面粉中，按4％的比例添加马铃薯变性淀粉，对面团的稳定时间、弱化度及抗张强度等均有显著的影响（P〈0．01）；添加6％木薯变性淀粉，面团抗张力增加26、6％；添加3％的谷朊粉，面团形成时间及稳定时间均有较大增加。木薯原淀粉及其变性淀粉等对面团的特性影响不大。     

RHEOLOGICAL PROPERTIES AND APPLICATIONS OF MESQUITE TREE (PROSOPIS JULIFLORA) GUM. 1. RHEOLOGICAL PROPERTIES OF AQUEOUS MESQUITE GUM SOLUTIONS

The viscosities of aqueous mesquite gum solutions were measured at gum concentrations extending to 16.7% (w/v). Very dilute solutions exhibited typical polyelectrolyte behavior with respect to the influence of gum concentration on the reduced viscosity in the absence and presence of electrolyte. Shear thickening was exhibited at concentrations of 1–16.67% (w/v) when the shear rate exceeded ∼100 s⁻¹, whereas shear thinning was exhibited at lower shear rates. Shear thickening was attributed to configurational changes in the gum molecules at high shear rates. Electrolytes (0.1N NaCl and 0.1N CaCl₂) reduced the viscosity of concentrated solutions at high shear rates to a larger extent than increasing the pH. Charge screening by electrolyte permits the gum molecules to assume a more compact configuration.     

EFFECT OF THERMAL PROCESSING ON STRUCTURE AND RHEOLOGICAL PROPERTIES OF CARRAGEENAN/CAROB GUM GELS

Optical rotatory dispersion and rheological studies have been used to investigate changes in carrageenan and carob gels on thermal processing. A synergistic effect was observed on the gel strength of both unprocessed and heat processed carrageenan carob gels when compared to gels without carob, although a lower gel strength was measured after processing. The reduction in strength of gels on heat processing appears to be due mainly to the depolymerisation of the network chains while the crosslinks of the gel are largely unaffected.     

EFFECTS OF INTERMOLECULAR INTERACTIONS AND MOLECULAR ORIENTATION ON THE FLUX BEHAVIOR OF XANTHAN GUM SOLUTIONS DURING ULTRAFILTRATION

Conformational changes of xanthan gum as a function of concentration were investigated to elucidate its unusual flux behavior during ultrafiltration (UF). The contribution of hydrogen bonding on structure formation and the molecular orientation of xanthan gum on the flow characteristics were studied rheomechanically and rheo-optically. Known to affect xanthan structure at low concentrations, hydrogen bonding unexpectedly did not show significant influence on xanthan rheological properties up to 2 wt %. The ordered layers formed on the membrane surface proved responsible for the enhanced water removal during UF. This unique behavior could be attributed to the formation of aligned molecular orientation in addition to the viscosity increases during the biphasic region. Significant differences were observed between xanthan fermentation broth and solutions made of commercial xanthan, suggesting the need to control the salt concentration in fermentation broth in order to make UF an effective recovery process for xanthan gum after fermentation.

PRACTICAL APPLICATIONS

The knowledge gained from the present study strengthens fundamental understandings on the conformational changes of xanthan biopolymers during the recovery of xanthan gum from fermentation broth using ultrafiltration (UF). By taking advantage of the aligned molecular orientation, along with controlling the salt concentration in the fermentation broth, UF could be operated at elevated flux to speed up the removal of water from the viscous fermentation broth. The technology should find broad applications in the fermentation industry, especially where viscosity is of concern during the separation and purification of the product.     

STUDIES ON ALBIZIA ZYGIA GUM 4.1 COMPARATIVE ASSESSMENT OF RHEOLOGICAL PROPERTIES OF ALBIZIA AND OTHER MUCILAGES

Rheological properties of albizia, tragacanth, methylcellulose and sodium carboxymethylcellulose were compared. All gums exhibited non-Newtonian flow with thixotropy which was affected by temperature, pH, concentration of the gum and presence of electrolytes. It is concluded that 1% lbizia mucilage should have the same suspending power as 0.4% tragacanth.     

RHEOLOGICAL STUDY OF O/W EMULSIONS CONTAINING DRIED WHOLE EGG AND LOCUST BEAN GUM

The influence of sucrose addition and type of salt (NaCl; Morton Lite salt: 50% NaCl and 50% KCl, and Mineral salt: 65% NaCl, 25% KCl and 10% MgSO₄) on the rheological properties of O/W emulsions containing locust bean gum was analyzed. Flow curves over the range 0.1 to 100 s⁻¹ and transient flow curves at 0.2 s⁻¹, at 20 and 30C were obtained for 12 different emulsions, containing 30% (w/w) of sunflower oil. The K and n power law parameters and the hysteresis loop area (HL) were evaluated from the flow curve. So were the Hahn parameters from time-dependent stress decay fitting. Rheological behaviour of emulsions was affected by the type of salt and the presence of sucrose. Changes in solvent properties of the continuous phase in the emulsions on the macromolecules present could be responsible for the different behaviour. The ageing effect on flow behaviour parameters and on the hysteresis loop area was also studied. Evolution of rheological parameters during emulsion storage suggests that increase of ionic strength and polar solutes concentration decrease the stabilizing properties of locust bean gum.     

INFLUENCE OF CARRAGEENAN AND TOTAL SOLIDS CONTENT ON THE RHEOLOGICAL PROPERTIES OF LACTIC BEVERAGE MADE WITH YOGURT AND WHEY

The purpose of this work was to study the influence of carrageenan level (X₁) and total solids content of yogurt/whey mixture (X₂) on starter culture fermentation time, physicochemical properties (pH, fat and total solids contents) and rheological characteristics of lactic beverages using response surface methodology. The physicochemical characteristics of the lactic beverages were similar to those of commercial products. All beverages showed non-Newtonian fluid behavior with thixotropy and yield stress. The total solids content of the lactic beverages had a greater influence on the rheological behavior of these products than the carrageenan level. Wide ranges of consistency measurements were observed by varying the stabilizer level and total solids content.     

EFFECTS OF EMULSIFIERS ON DOUGH RHEOLOGICAL PROPERTIES AND THE TEXTURE OF EXTRUDED NOODLES

The effects of six emulsifiers on the rheological properties of wheat flour doughs were evaluated using capillary and oscillatory rheometers. Sodium stearoyl 2-lactylate (SSL) and sucrose fatty acid ester 160 (SFAE 160) significantly increased the apparent viscosities (η), storage moduli (G") and loss modulus (G") of the doughs. The η, G' and G" increased with the concentrations of SSL from 0.5 to 1.5%. Lecithin, SFAE 110 and 20W (low HLB values) had the opposite effects. Extruded noodle with SSL had the highest tensile strength and cutting force among all treatments. Therefore, SSL could be used to produce the extruded noodles with higher elasticity. Lecithin, MG, SPAE 110 and 20W decreased the strength of the extruded noodles. There was a positive correlation between viscosity and shear modulus of the doughs and the strength of extruded noodles. Values of HLB emulsifiers used in this study were positively correlated with G' and η*.     

GRANULE SIZE DISTRIBUTION AND RHEOLOGICAL BEHAVIOR OF HEATED MODIFIED WAXY AND UNMODIFIED MAIZE STARCH DISPERSIONS

The effect of granule size distribution, supported by microscopic analysis, on the steady shear and dynamic shear rheological behavior of 5% gelatinized modified waxy maize starch (MWMS) dispersions heated at 80C for time periods from 0.5 to 30 min was examined and data obtained with 5% unmodified maize starch (UMS) dispersions was used to provide additional insight. The MWMS dispersions showed a faster and more extensive swelling than the UMS dispersions at 80C. They also exhibited rheopectic (anti-thixotropic) time-dependent and shear thinning with yield stress behavior. UMS dispersions showed shear thickening characteristics after short heating times that gradually turned to shear thinning as the heating time was increased. Dynamic tests on MWMS dispersions showed G' was about ten times higher than G". G' showed a small dependence on frequency over 0.6–63 rad/s while G", after an increase at low frequencies, became less sensitive. The gelatinized MWMS dispersions can therefore be classified as 'weak gels.' The flow behavior index of UMS dispersions was strongly     

APPLICATION OF AN IN-LINE RHEOLOGICAL CHARACTERIZATION METHOD TO CHEMICALLY MODIFIED AND NATIVE CORN STARCH

The application of an in‐line ultrasonics‐based rheological characterization method for measuring the rheological properties of 6% (w/v) acid‐thinned and native corn starch suspensions and gels was studied. The measurements were performed in steady, laminar pipe flow using a 5 MHz frequency transducer to determine the radial shear rate distribution. Two pressure transducers were used to measure the pressure drop allowing the shear stress distribution to be calculated through the conservation of linear momentum. It was possible to obtain shear viscosity over a range of shear rates, 1–50 1/s, from a single velocity profile. A comparison of the shear viscosity function at different flow rates showed that this method could be a valid method of process rheometry during the manufacturing of starch‐based products. Both ultrasonics and rotational rheometry results showed that acid‐thinned and native corn starch suspensions exhibited Newtonian behavior before heat treatment and non‐Newtonian behavior after heating. Power law consistency index and flow behavior index of the acid‐thinned corn starch gel at 39°C were 0.60 Pa.s ^0.68 and 0.68, respectively; whereas those of the native corn starch were 5.90 Pa.s ^0.37 and 0.37 at 38C. Their consistency index increased and flow behavior index decreased with cooling and a thermoreversible change in the flow behavior index was observed in the native corn starch after storage.