Dynamical considerations of network properties

Elasticity is discussed as an aspect of viscoelasticity, which is described by the tube model. The effects of both crosslinks and entanglements contribute to this model and a discussion of how these effects can be quantified is given. At high enough concentration, entanglements ensure the existence of elastic effects even without crosslinks, and a theory is presented on how this dynamical phase change comes about.     

不同因素对动态热机械分析仪(DMA)测试结果的影响

通过对高Tg板材的动态热机械分析(DMA),研究了升温速率、样品厚度、有无覆金属层对测试结果的影响。文章旨在通过分析这些因素的影响机理,深入了解板材的粘弹性能,为PCB板材热性能测试提供参考借鉴。     

猪鞋面革应力松弛时间谱的研究(Ⅰ)——猪鞋面革应力松弛行为的模拟和应力松弛谱的求解

研究了猪鞋面革的应力松弛模量,采用1～8个Maxwell单元组成的广义Maxwell模型,模拟了其应力松弛行为。结果表明:采用5个或5个以上Maxwell单元组成的广义Maxwell模型,可以较好地模拟猪鞋面革的应力松弛行为。利用模型模拟获得的应力松弛表达式和Schwarzl松弛时间谱二阶近似求解方法求解了松弛时间谱,得到了猪鞋面革的应力松弛谱图。     

Impulse viscoelastic characterization of wheat flour dough during fermentation

A new technique, named impulse viscoelasticity, was introduced and applied to investigate the rheological properties of dough. Specifically, the effects of mixing and fermentation times on the extensional properties of dough were characterized under small extensional strains of short duration. From the stress and strain responses measured, the tensile storage and loss moduli of dough were obtained as a function of mixing and fermentation times. There were significant changes in the tensile moduli of the dough samples over mixing times and in general high extensional properties were measured for the optimally-mixed dough. Fermentation caused the reduction of both tensile storage/loss moduli with a greater decrease in the storage moduli, clearly showing the loss of elasticity of the fermenting dough. The results showed that the short duration of the applied strain allows the impulse viscoelastic technique to be appropriately used in characterizing the rheology of foods which can vary during processing.     

Soybean Protein Dispersions at Acid pH. Thermal and Rheological Properties

The influence of pH, protein concentration, and ionic strength, on rheological properties of thermally treated acidic soy protein dispersions, was studied. Structural changes due to pH effect and thermal treatment were analized. DSC-thermograms at pH 3.5 showed a shoulder at 74.11±0.16°C that could be attributed to both β-conglycinin and the hexameric form of glycinin; and a peak at 81.88±0.29°C corresponding to 11S dodecameric form. At pH 2.75 one endotherm corresponded to denaturation of β-conglycinin. The acidic dispersions presented pseudoplastic behavior with_app values higher than those at pH 8.0. At pH 3.50 the ±_app was higher than at pH 2.75.The maximum viscoelasticiy was obtained with addition of 0.1 and 0.25M NaCl in the dispersions of pH 3.50 and 2.75, respectively. The increase in viscoelasticity was enhanced by the formation of 11S native fraction dimers.     

Viscoelastic Characterization of Surimi Gel: Effects of Setting and Starch

Rheological properties of Alaska pollock surimi gels were affected by setting and addition of corn starch. Setting increased (p < 0.05) both elastic and loss moduli of gel in the absence of starch. However, in the presence of corn starch, setting decreased (p < 0.05) the elastic and loss moduli of the gels. In stress relaxation, the residual forces increased from 37.1 Newton (N) (nonsetting) to 51.8N (setting) in the absence of starch. However, in the presence of starch, the residual force decreased from 49.1N (nonsetting) to 39.0N (setting) which indicated an antagonistic effect between corn starch and setting. A Maxwell model was useful to quantify relaxation behavior of surimi gel and to evaluate the effects of changes in processing conditions.     

Capsule formation by nonropy starter cultures affects the viscoelastic properties of yogurt during structure formation

The aim of this work was to study the structure formation of yogurt made with cultures containing ropy Lactobacillus delbrueckii ssp. bulgaricus (R), capsule-forming nonropy Streptococcus thermophilus (CNR), and noncapsule-forming nonropy cultures (NCNR). Similar gelation profiles were shown for milk fermented by ropy and nonropy lactic cultures. The gelation point occurred at lower pH values in milk fermented with R or NCNR compared to that of milk fermented with CNR culture. Differences between capsule forming ropy and nonropy cultures were observed in the aggregation behavior of the caseins. Gels made with R culture had the highest maximum in loss tangent (tan delta). However, this maximum occurred at the highest pH value when CNR culture was used. The earlier gelation of milk fermented by the encapsulated nonropy strain of Strep. thermophilus resulted in increased structure rearrangement as the pH dropped, interfering with the formation of a more compact structure.     

Characterisation of chickpea flour‐based gluten‐free batters and muffins with added biopolymers: rheological,physical and sensory properties

Chickpea flour (CF)‐based muffin formulations were made with CF alone and with added biopolymers [whey protein (WP), xanthan gum (XG) and inulin (INL)] to evaluate their suitability to be a wheat flour (WF) substitute in muffins. Structural characteristics of the batters and muffins were studied by means of rheometry, microscopy, physicochemical properties, and texture and sensory analysis. Partial replacement of CF with XG, alone (at 0.5 and 1%) or blended with either WP or INL, significantly increased the batter viscoelasticity as denser matrices developed; moreover, the muffins with XG added at 1% had similar hardness to wheat gluten muffins. The replacement of WF decreased the springiness, cohesiveness, chewiness, resilience and specific volume of the muffin. However, baked muffins with added XG also showed higher sensory sponginess and moisture associated with numerous air bubbles in the batter and were perceived to be easier to swallow and to have better general appearance.     

Temperature and pH as factors influencing droplet size distribution and linear viscoelasticity of O/W emulsions stabilised by soy and gluten proteins

The influence of pH and two post-emulsification treatments (pH modification and thermal cycles) over linear dynamic viscoelasticity and droplet size distribution, DSD, of O/W emulsions (75% oil) stabilized either by soy protein isolate, SPI, or wheat gluten, WG were studied in the present work. Rheological properties and droplet size of fresh emulsions showed an important dependence on pH as a consequence of the role of electrostatic interactions, not being possible to obtain a stable emulsion for pH values close to the protein isoelectric point, pI, (4–5 for SPI and 6 for WG). In order to overcome this inconvenient, an alternative emulsification procedure, basically consisting in a modification of pH after emulsification (indirect emulsification), was successfully developed. Emulsions obtained after this post-emulsification treatment, showed higher elastic (G′) and loss (G″) moduli and also larger oil droplets than fresh emulsions prepared at the same pH. Moreover, the application of upward/downward temperature cycles from 20 to 70 °C to emulsions directly prepared at a pH yielded to significantly higher values of the rheological functions when compared to those found for fresh emulsions. Accordingly, both post-emulsification treatments lead to apparent enhancements in emulsion rheology and microstructure, which is indicative of a good potential to improve long-term emulsion stability.     

Thermal, microstructural and textural characterisation of gelatinised corn, cassava and yam starch blends

The functional properties of mixtures of maize, cassava and yam starches and their relationships with microstructural characteristics were investigated. Experiments were performed following the simplex‐centroid design with internal points and restrictions (upper limit) for yam starch proportion. The statistical model used (Scheffé canonical equation) was a powerful tool to predict the pastes behaviour within the limits of the experimental area. Polynomials with second level interactions were applied to obtain the surface response. Viscoelastic attributes of mixtures differed from those of individual starches. As shown by differential scanning calorimetry and microscopical observations, physical properties of the mixtures depended on the type of networks obtained. These networks varied upon yam, corn and cassava starch proportions as each one has a particular characteristic: gelatinisation temperature, granule size, swelling capacity and amylose/amylopectin ratio among others.