Weak Gel Model Research Articles

Post-liquefaction normospermic human semen behaves as a weak-gel viscoelastic fluid.

The rheological behavior of human semen is overlooked and essentially unexplored in the scientific literature. Here, we provide the first quantitative experimental evidence that post-liquafaction normospermic human semen behaves as a viscoelastic fluid and the shear moduli can be scaled according to the weak-gel model.

A Chemometric Approach to Assess the Rheological Properties of Durum Wheat Dough by Indirect FTIR Measurements

Rheological measurements and FTIR spectroscopy were used to characterize different doughs, obtained by commercial and monovarietal durum wheat flours (Cappelli and Karalis). Rheological frequency sweep tests were carried out, and the Weak Gel model, whose parameters may be related to gluten network extension and strength, was applied. IR analysis mainly focused on the Amide III band, revealing significant variations in the gluten network. Compared to the other varieties, Karalis semolina showed a higher amount of α-helices and a lower amount of β-sheets and random structures. Spectroscopic and rheological data were then correlated using Partial Least Squares regression (PLS) coupled with the Variable Importance in Projection (VIP) technique. The combined use of the techniques provided useful insights into the interplay among protein structures, gluten network features, and rheological properties. In detail, β-sheets and α-helices protein conformations were shown to significantly affect the gluten network's mechanical strength.

Sugar and no sugar added fruit microalgae-enriched jams: a study about their physicochemical, rheological, and textural properties

Jams are preparations of fruits whose main preserving agent is sugar. Due to this, health concerns of consumers have resulted in a sugar reduction, and its replacement using alternative sweeteners and introducing new ingredients enhancing nutritional properties. In this study, four types of fruits jams (plum, strawberry, apple, and peach), with or without sugar, were prepared using two microalgae biomass, Arthrospira platensis (Spirulina) and Chlorella vulgaris, and Dunaliella salina extract as ingredients at different mix levels of concentrations, 0.10%–0.10%–0.05% respectively, for plum; 0.04%–0.00%–0.01 respectively, for strawberry; 0.06%–0.04%–0.00% respectively, for apple and 0.00%–0.01%–0.04% respectively, for peach. Physicochemical, rheological, and textural parameters were evaluated. Substitution of sugar/fructose syrup in the jam’s preparation caused changes in pH values, solid soluble content, and rheological and textural properties compared to sugar jams. Using sugar or sweeteners as isomalt, stevia and sucralose and microalgae biomass or extract showed significant changes in colour coordinates, however, these differences were not perceptible by the human eye. Jams containing microalgae biomass-extract showed higher G', G'', G*, and η* values than their corresponding control samples. All the jams presented weak-gel characteristics, distinguishing fruit jams. However, the results of weak-gel model analysis suggest that the influence of the different ingredients in the food system depends not only on their concentration but also on the interactions in the gel structure. Strawberry and apple jams showed no significant differences between microalgae biomass-extract samples and control samples, for both sugar and no sugar added jams being the best-obtained samples.

Starch replacement in gluten free bread by cellulose and fibrillated cellulose

This study investigated starch reduction and replacement by purified cellulose (FC0) and fibrillated cellulose (FC60) which included a comprehensive investigation on dough properties, proofing behaviours, cooking performance, and bread qualities. Replacing flour with FC0 and FC60 was found to strength the doughs without, however, altering the extension of the structural network according to the weak gel model. The relaxation time calculated by the generalised Maxwell model was found to be shorter than the deformation rate during proofing which suggests that the doughs behave like fluids during proofing. The relaxation time was less influenced by the additions of FC0 and FC60. Although the initial stage of proofing was less influenced, the later stage was significantly affected by the additions of FC0 and FC60 which increased dough rigidity and restrained the volume growth. The pasting properties were significantly influenced by the competition for water and volume between FC0/FC60 and flour. The bread qualities were characterised in terms of loaf volume and crumb properties. Loaves containing FC0 and FC60 had smaller specific volume and harder crumb. However, the addition of FC0 and FC60 is beneficial to the generation of a finer crumb structure. Fibrillation process is detrimental to maximise the addition of fibres. However, a small amount of fibrillated cellulose is beneficial to workability and crumb structure.

The Rheological Properties of Semolina Doughs: Influence of the Relative Amount of Ingredients

In the present study, the rheological properties of doughs, prepared by mixing semolina with water, yeast and salt, were investigated with different relative amounts of the ingredients. The rheological measurements were carried out by an Anton Paar MCR 102 rheometer, equipped with a plate-plate fixture. In more detail, frequency sweep tests were performed. Indeed, frequency sweep measurements showed to be informative about the force and extension of the gluten network that, in turn, demonstrated to depend on the relative amount of ingredients. The oscillatory experiments were analyzed by means of the “weak gel model”, whose parameters are directly related to such physical properties. It was found that doughs obtained with different amounts of ingredients showed significant differences in the rheological responses. The addition of yeast led to a significant viscosity and force network decrease, whilst the presence of salt produced an improve of the three-dimensional gluten network characteristics and, consequently, to the machinability of the dough. The study aims at contributing to a more efficient controllability in the industrial production of flour and semolina dough based foods, and in particular to that of “pani carasau”, a typical high quality and valuable Sardinian bread, since this process is high energy consuming (mainly in the baking step), in order to decrease the amount of off-specification products, and so making the processes more energy efficient and sustainable.

Attempt of the quantitative evaluation of flow properties of agar microgel suspensions using a weak-gel model

Attempt of the quantitative evaluation of flow properties of agar microgel suspensions using a weak-gel model

Dynamic rheological and structural characterization of fish gelatin – Gum arabic coacervate gels cross-linked by tannic acid

The propose of this study was to investigate the rheological and structural properties of fish gelatin (FG)–gum arabic (GA) complex coacervate gels treated with oxidized or non-oxidized tannic acid (OX-TA or NO-TA, respectively) at different concentrations ranged from 0.0 to 0.3% (v/v), using compositional, dynamic oscillatory rheological, and Fourier transform infrared (FTIR) analyses. The results revealed that the degree of NO-TA incorporation for cross-linking with FG via hydrogen bindings into the system was more pronounced than affinity of OX-TA for reaction through covalent bindings as evidenced by greater loss in volume fraction, moisture content, and biopolymer content values into the FG–GA coacervate phase. FTIR analysis showed addition of NO-TA led to more molecular disorder into the system, so that higher concentrations (0.2 and 0.3%) of the cross-linker disrupted electrostatic interactions through formation of stronger hydrogen bindings with proteins. Rheological results mentioned that addition and enhancement of (NO- or OX-) TA concentration improved gelling ability and mechanical properties of the FG–GA coacervate gels. However, frequency sweep test results implied that all the gels obtained can be classified as weak gels with shear-thinning behavior. Based on weak gel model, the gels obtained by treated FG–GA coacervates with NO-TA had more developed network structures and stronger intermolecular connectivities than those of obtained by the coacervates modified by OX-TA. The results of the current study provide basic knowledge necessary for the use of reinforced FG–GA complex coacervate gels in many useful applications, such as microencapsulation and hydrogel formation, in food and pharmaceutical industries.

Development and characterization of reconstituted hydrogel from Aloe vera (Aloe barbadensis Miller) powder

Structural and rheological characterization of reconstituted hydrogels developed from A. vera non-fibrous alcohol insoluble residue (NFAIR) powder using different methods [viz., shaking (S), heating-shaking (HS), and heating (H)] and concentrations (viz., 0.2–1.6 %, w/v) was carried out. Functional group distribution by FTIR spectroscopy and Congo red (CR) method revealed the presence of acetylated acemannan in A. vera powder. Dynamic oscillation studies of A. vera (NFAIR) fluids at all concentrations of 0.2–1.6 %, w/v, showed gel strength in the order of H > HS > S method. However, in H method, increase in concentration from 0.2 to 1.6 %, w/v showed the conformational transition from semi-diluted solution to weak gel nature. Rheological models described the effect of heating temperatures (HT); 30–90 °C, and times (Ht); 15–60 min on viscoelastic behavior in reconstituted A. vera fluids. The reconstituted A. vera hydrogel prepared with a concentration of 1.6 %, w/v using 50 °C (HT) and 30 min (Ht) condition showed a good agreement with the Power law (storage modulus, G′) and Weak gel model (complex modulus, G*) fitted data (R2 > 0.94) resulting higher viscoelastic moduli intercepts; G′0 (71.5 Pa s n′), G″0 (33.5 Pa s n″), lower slopes; n′ (0.22), n″ (0.06), higher network strength (A F , 121.3 Pa s1/z ) and number of network (z, 5.3) values. The obtained results suggested that heating at 50 °C/30 min can develop aqueous weak gel networks of A. vera with enhanced gel strength which may be utilized as a novel gelling agent for wide variety of targeted applications in food and pharmaceutical sectors.

Soybean Oil Volume Fraction Effects on the Rheology Characteristics and Gelation Behavior of Glucono‐δ‐Lactone and Calcium Sulfate‐Induced Tofu Gels

AbstractThe effects soybean oil volume fraction on the rheological characteristics and gelation behavior of acid and salt‐induced soft tofu‐type gels was investigated. High oil volume fraction resulted in lower gel point temperature and shorter gelation time (tgel) while G′ at the gel point increased as the oil volume fraction in the gels increased. A power law relationship tgel, gel point temperature and oil concentration was found and also the plot of G′ versus oil concentration fitted well in the model: G′∼CA. The exponent “A” was positive in salt‐induced gels but negative in acid‐induced gels. This gelation behavior trend was demonstrated by estimating the composite gel shear modulus, which showed that, in salt‐induced gels the stiffness of the oil droplets was higher than the one of the gel matrix while it was the opposite in acid‐induced gels. Furthermore, the weak gel model was used to analyze and describe the network extension and its strength.Practical ApplicationsIn this comparative study, soybean oil volume fraction effects on the rheological characteristics and gelation behavior of acid and salt‐induced soft tofu‐type gels was established. Different coagulation temperatures and coagulants were used. The study gives new insights on the protein‐to‐oil droplets interactions during gelation at different pHs in a tofu gel like product. The better understanding of the gelation mechanism, small and large scale deformations of soy protein gels with additional oil, is industrially relevant, as it will help in better process line design, monitoring and quality control which would potentially lead to the development of novel soy‐based and dairy free cheeses products. Additionally, the parameters and conditions used were closely related to the ones used during soft silken tofu making, therefore, ensuring the process implementation much easier.

Changes in minimally processed apple tissue with storage time and temperature: mechanical–acoustic analysis and rheological investigation

Texture and mesostructure of packaged fresh-cut ‘Fuji’ apple slices were studied during 30 days storage at 4, 10 and 20 °C. Instrumental mechanical–acoustic texture evaluations were carried out, and cell wall materials were isolated from the flesh tissue by means of an ethanol and methanol–chloroform solvents extraction procedure. The emphasis was on relating the macroscopic loss of fresh texture characteristics to the changes in the rheological properties occurring to the structuring elements of the cell wall materials (CWMs). The rheology of the reconstituted CWM water dispersion was evaluated by means of oscillatory dynamic testing, and viscoelastic data were interpreted according to the ‘weak-gel model’ theory. The dynamics of the flowing components at the mesoscale structural level was related to the texture softening, which yields decrease in firmness and rigidity parameters, and an attenuation of the acoustic emission (fewer acoustic peaks and lower sound pressure level), as it was measured in the combined acoustic–mechanical test. The kinetic reaction rate values are presented. Softening of apple flesh is accompanied by slight cleavage of molecular assemblies and a loss in cell adhesion, mediated by the increase in assemblies’ water uptake. The ability to handle the relationship between the sounds produced during flesh crushing and the structure of the cellular tissue would assist breeders and geneticists in the selection and control of the apple crispness, or would support technologists in preventing postharvest softening during storage and retail.

A novel laboratory scale method for studying heat treatment of cake flour

A lab-scale method for replicating the time–temperature history experienced by cake flours undergoing heat treatment was developed based on a packed bed configuration. The performance of heat-treated flours was compared with untreated and commercially heat-treated flour by test baking a high ratio cake formulation. Both cake volume and AACC shape measures were optimal after 15min treatment at 130°C, though their values varied between harvests. Separate oscillatory rheometry tests of cake batter at 80–100°C exhibited similar behaviour to the baking tests. The gel strength parameter in the weak gel model, measured at 100°C, was shown to correlate with flour quality and was identified as a possible alternative to test baking as a means of assessing flour quality after heat treatment.

The influence of thermal processing on emulsion properties of defatted hazelnut flour

In this study, the influences of roasting and the amount of hazelnut flour on the formation and stabilization of emulsions containing different amounts of oil were investigated. After hazelnuts were roasted in an oven at 140°C for 40min, the oil content was removed. The emulsions with defatted hazelnut flour containing corn oil at 3%, 10% and 50% were prepared. Roasting process significantly decreased the interfacial tension values of samples down to 1.9mN/m due to protein denaturation. There was no significant difference between the particle sizes of oil droplets in emulsions with roasted and raw hazelnut flour at the same concentration. However, diffusion coefficients of oil droplets increased for emulsions containing roasted defatted flour samples. The zeta (ζ) potential values of all emulsions increased when roasted hazelnut flour was used, indicating the stabilization of suspensions and the solution resistance against aggregation. Storage modulus (G′), loss modulus (G″) and complex viscosity (η∗) of emulsions were measured. G′ value was found to be greater than the G″ value, which fits into weak gel model. The roasting process resulted with lower transition temperatures but with increased transition enthalpies of the flour samples based on differential scanning (DSC) measurements. Lower transition temperatures may be attributed to the partial gelatinization of starch in the flour and partial denaturation of proteins. These results may help to tailor the properties of defatted hazelnut flour when it is used in food products containing emulsions such as sauces, dressings and creams for stabilizing purposes.

EFFECTS OF BUTTER OIL AND WHEY PROTEINS ADDITION ON VISCOELASTIC PROPERTIES OF CONFECTIONARY DULCE DE LECHE

The effect of butter oil and whey proteins addition on viscoelastic properties of confectionary Dulce de Leche was evaluated. A two factors experimental design (0-8 % p/v of butter oil and 0-1 % p/v of whey proteins) was analysed. Frequency sweep tests were carried out and for all formulations studied a weak gel like behavior was observed. Therefore, results were interpreted using a weak gel model for foods. It was observed that the rheological behavior of the sample with addition of whey proteins and butter oil is significantly different than the others because rheological units interact with greater strength. Even more, the sample without butter oil and with whey proteins addition is composed by less rheological units than the others. It is concluded that the whole addition of butter oil and whey proteins generates a stronger gel matrix, more appropriate for use in confectionary.

Dynamic Viscoelastic Behavior of Vegetable‐Based Infant Purees

AbstractViscoelastic properties of four vegetable‐based infant purees were evaluated in temperature range of 5–80C. Samples behaved like weak gels, with the exception of rice and chicken puree at 35, 50 and 65C, which behaved like a macromolecular solution. At 5–65C, storage modulus (G′) and storage loss (G″) could be well described by a power function of the frequency (R2 > 0.92), and the dependency of their parameters with temperature was modeled by Arrhenius's model and quadratic functions. Approach named “weak gel model” was also applied to the baby foods, and both G′ and G″ values decreased with an increase in temperature between 5 and 50C. Master curves were obtained applying time‐temperature superposition principle, and horizontal shift factor was sensitive to choice of viscoelastic property being selected for shifting procedure. At 5–50C, infant purees behaved like thermoreologically complex materials. Rheological measurements were well supported by particle size distributions.Practical ApplicationsComposition of baby foods is being continually modified and adapted with the aim of producing healthier foods. However, there are no recent rheological studies of vegetable and fruit puree‐based baby foods. A comprehensive study on viscoelastic behavior of four current commercial types of Spanish vegetable‐based infant purees was carried out. The effect of a wide range of temperatures, which are encountered during processing and storage of baby foods, on rheological properties was documented. Based on frequency sweep data, the dynamic rheological behavior of these products ranged from that of a concentrated biopolymer solution to a “weak gel.” Controlling temperature, moisture content and particle size distribution may be employed to optimize applicability, stability and sensory of baby foods. Results provide a useful fingerprint of rheological behavior of these products. Generated data and findings can be extrapolated to other infant systems for developing new products for convenience to newborn consumers and manufacturers.

Temperature dependency of linear viscoelastic properties of a commercial low-fat soft cheese after frozen storage

The temperature dependency of linear viscoelastic properties of a commercial low-fat soft cheese containing microparticulated whey protein as fat replacer was studied considering the effect of freezing. After thawing, cheeses were held at 6°C during 48days for ripening. Refrigerated cheeses (stored at 6°C for 48days) were used as control samples. Frequency sweeps (0.01–10Hz) in the linear viscoelastic region at 10, 20, 30, 40, and 50°C were performed. Activation energies for complex viscosity at 1Hz were obtained from an Arrhenius-type equation. Also, the time–temperature superposition method, the modified Cole–Cole analysis and the weak gel model for foods were used to compare the behavior of frozen and refrigerated cheeses. The results obtained in this work indicated that the viscoelastic properties of the studied cheeses obtained at different temperatures were influenced by freezing.

Rheological characterisation of cake batters generated by planetary mixing: Comparison between untreated and heat-treated wheat flours

The rheological behaviour of high ratio cake batters prepared with untreated and heat-treated wheat flours was analysed at different stages of the manufacturing process, namely slurry, (on aeration) foams and (with fat addition) aerated emulsions, featuring air volume fractions up to 0.50. Both steady shear and viscoelastic behaviours were studied. All materials exhibited shear-thinning behaviour at 20 °C over the shear rate range studied (0.07–10 s −1). The generalised Cox–Merz rule could be applied to all samples. Materials prepared with heat-treated flours exhibited greater stability, as indicated by slurry thixotropy and cohesive energy, and the change in apparent viscosity and air content of foams and aerated emulsions on extended mixing. Foams and aerated emulsions showed significant elastic behaviour with G′∼ G″. The temperature dependency of aerated emulsions was studied by oscillatory shear testing from 20 to 100 °C and indicated three regimes in temperature dependence: below 40 °C G′and G″ were insensitive to temperature; between 40 and 70 °C the complex viscosity exhibited Arrhenius-type behaviour, while above 70 °C G′ and G″ increased as expected for gelatinisation and foam setting. The weak gel model for foods was used to analyse the latter data sets and confirmed that the gel network generated in aerated emulsions prepared with heat-treated flours was significantly stronger than those made with unheated flours. The differences between flour types were also observed in tests on un- and heat-treated flours obtained from a second and third harvest. The impact of these quantifiable differences in rheology on performance during baking is discussed.

Influence of Alginate Concentration and Molecular Weight on Functional Properties of Mayonnaise

The rheological properties of mayonnaise containing alginates from Macrocystis pyrifera as stabilizers were studied using a controlled stress rheometer and a texture analyser penetrometer. Dynamic oscillatory tests of the emulsions were carried out in strain sweep and frequency sweep modes. The rheological parameters used for this study were the storage ( G′), loss ( G″) and complex ( G*) moduli and the strain, which determined the limit of the linear viscoelastic region (γ c). The effect of alginate concentration (0.2, 0.5 and 0.8 g/100 g) and molecular weight (MW: 76, 120 and 154 kDa) on physical properties was examined and the best combination useful to minimize differences between the alginate emulsion and mayonnaise with modified corn starch as commercial reference was identified (0.35 g/100 g and 108 kDa). The storage modulus increased with alginate content and it became independent of the molecular size above a critical value between 76 and 120 kDa. A positive effect of MW on γ c was observed at 0.2 g/100 g alginate and the critical strain decreased with increasing polymer concentration up to 0.5 g/100 g. Results under small-amplitude oscillatory shear tests (linear viscoelastic properties) were interpreted in the context of the weak gel model. Two material parameters were determined to describe the rheological structure of the emulsion: the coordination number ( z) and the parameter A ( G* at 1 Hz). Mayonnaises with alginate concentrations of 0.5 and 0.8 g/100 g and at all the MW had coordination numbers which were not significantly different from those of the reference sample. The emulsion strength of alginate samples with polymer concentration of 0.5 and 0.8 g/100 g was significantly higher than the control.

Filled-snacks production by co-extrusion-cooking. Part 3. A rheological-based method to compare filler processing properties

Filled snacks are new industrial co-extruded products made by a dough shell and an internal filler constituted by salty or sweet cream. Due to different rheological characteristics of fillers and dough, it is difficult to choose operational variables capable of guaranteeing a complete filling. This work aims to determine the optimal rheological properties of both filler and dough to make an optimal product without undesired voids. A typical dough and several fillers have been considered and rheologically characterised by means of dynamic oscillation, steady shear flow and creep tests. A dynamic temperature ramp test at 1 Hz was performed to determine temperature dependency of the tested material. A time–temperature shifting technique based on a modification of the Cox–Merz rule and on weak gel model was applied to extend the flow curve range and to overcome temperature experimental difficulties. The obtained data, expressed by a power law correlation, were inserted in a numerical algorithm previously developed, and the internal swelling of the dough followed by a recovery was computed during mixed flow inside the extrusion head. It was suggested that a greater recovery implies a better filling capability because the filler may follow the dough during swelling. Simulations indicated that a complete filling was obtained only for the materials exhibiting a quite stable network during the flow. The experimental results obtained by preparing different industrial filled snacks confirmed the simulative predictions. Quantification of the network shear stability was determined by comparing the loss modulus ( G″) and shear viscosity ( η) values. In particular G″ versus frequency and η versus shear rate were fitted by a power law model and the best fillers must present a ratio between their exponents close to 1.

A weak gel model for foods

From a general point of view food can be seen as either a continuous aqueous or lipidic phase in which other components are dispersed (for example long fibres, droplets). While these systems seem to be very different one from the other, they generally show similar behaviour due to the presence of a supramolecular structure, comprised of ordered or interacting droplets, particles etc. that determine these characteristics. Many food systems can be treated as gels, characterised by a three-dimensional network where weak interactions (like Wan der Waals interactions or hydrogen bonding) ensure the stability of the structure. The gel is assumed to coincide with the critical gel of Winter's theory, but a finite relaxation time is assumed to account for the flowing character of foods, that is interpreted in terms of theory of cooperative flow. Thus, apart from the usual two continuum material function, i.e. a truncated relaxation spectrum and an apparent elastic modulus, other two material parameters were determined and proposed to describe the network extension and its strength. This kind of approach, named weak gel model, was applied to different foods such as jams, dough and yoghurt and it proved to be quite adherent to food reality.