Rheological properties of mixtures of kappa-carrageenan from Hypnea musciformis and galactomannan from Cassia javanica

Mixed gels of κ-carrageenan (κ-car) from Hypnea musciformis and galactomannans (Gal) from Cassia javanica (CJ) and locust bean gum (LBG) were compared using dynamic viscoelastic measurements and compression tests. Mixed gels at 5 g/l of total polymer concentration in 0.1 M KCl showed a synergistic maximum in viscoelastic measurements for κ-car/CJ and κ-car/LBG at 2:1 and 4:1 ratios, respectively. The synergistic maximum obtained from compression tests carried out for mixed gels at 10 g/l of total polymer concentration in 0.25 M KCl was the same for both κ-car/CJ and κ-car/LBG gels. An enhancement in the storage modulus (G′) and the loss modulus (G″) was observed in the mechanical spectra for the mixtures in relation to κ-car. The proportionally higher increase in G″ compared with G′, as indicated by the values of the loss tangent (tan δ), suggests that the Gal adhere non-specifically to the κ-car network.     

Thermal, Mechanical, and Molecular Relaxation Properties of Frozen Sucrose and Fructose Solutions Containing Hydrocolloids

Model frozen systems formulated with 20wt% sucrose or fructose and with the addition of 0.3 or 0.5wt% of xanthan gum (XG), guar gum (GG), locust bean gum (LBG), or a 50wt% mixture of XG and LBG were studied by differential scanning calorimetry, dynamic mechanical analysis, and ¹H-pulsed nuclear magnetic resonance. Melting onset of either the sucrose or fructose model systems was not affected by the addition of hydrocolloids. As expected, ice content was lower in fructose than in sucrose systems. Addition of hydrocolloids had no effect on ice content, except when the blend of XG and LBG was added to the fructose system, where ice content was significantly diminished. Hydrocolloids decreased molecular mobility for both frozen sucrose or fructose solutions, especially for the addition of XG/LBG blend. Relaxation times and storage modulus of the frozen systems with added hydrocolloids were significantly lower than the control frozen sugar solutions.     

Low Shear Rheology of Concentrated Tomato Products. Effect of Particle Size and Time

Time-dependent rheological properties of three tomato paste suspensions in the concentration range of 200–1,000 g paste/kg suspension have been investigated by using the vane geometry at shear rates . Creep tests were conducted to analyze the influence of the level of stress on the rheological behavior of the samples before and after homogenization. The experimental results indicate that the suspensions exhibit an elastic behavior at long times and relatively low stresses, which proves that this type of material can be characterized by a yield stress (σ _y). Applying stresses just beyond the yield stress, an initial rheopectic behavior appeared. This increase in viscosity at low deformations was markedly larger after homogenization, and this difference was attributed to changes in the aspect ratio, shape, and orientation of the particles induced by homogenization. These structural changes were also reflected in the transient viscosity when the samples were subjected to larger stresses (σ >> σ _y): before homogenization the suspensions exhibited a steady-state viscosity at large deformations, whereas after homogenization, the transient viscosity continuously decreased. That behavior was attributed to flocculation of the particles. This work was partially presented at the “4th International Symposium on Food Rheology and Structure” in February 19–23, 2006, Zürich, Switzerland.     

Synthesis and rheological properties of hydrogels based on amphiphilic alginate-amide derivatives

New amphiphilic derivatives of sodium alginate were prepared by covalent attachment of dodecylamine onto the polysaccharide via amide linkages at different substitution ratios, using 2-chloro-1-methylpyridinium iodide (CMPI) as coupling reagent. The aim was to limit the progressive loss of associative behaviour which occurs in the case of previously described dodecyl ester alginate derivatives due to hydrolysis of ester bonds. A series of hydrogels was obtained which differed by the amount of attached dodecyl tails. The stability and viscoelastic properties were evaluated and compared to those of hydrogels obtained with alginate esters. The observed differences were discussed in relation to the synthesis procedures. The advantages of amide links are underlined, especially with regard to long-term stability of hydrogels.     

Dissolved oxygen as principal parameter for conidia production of biocontrol fungi Trichoderma viride in non-Newtonian wastewater

Dissolved oxygen (DO) concentration was selected as a principal parameter for translating results of shake flask fermentation of Trichoderma viride (biocontrol fungi) to a fermenter scale. All fermentations were carried out in a 7.5 l automated fermenter with a working volume of 4 l. Fermentation performance parameters such as volumetric oxygen transfer coefficient (k _L a), oxygen uptake rate (OUR), rheology, conidia concentration, glucose consumption, soluble chemical oxygen demand, entomotoxicity and inhibition index were measured. The conidia concentration, entomotoxicity and inhibition index were either stable or improved at lower DO concentration (30%). Variation of OUR aided in assessing the oxygen supply capacity of the fermenter and biomass growth. Meanwhile, rheological profiles demonstrated the variability of wastewater during fermentation due to mycelial growth and conidiation. In order to estimate power consumption, the agitation and the aeration requirements were quantified in terms of area under the curves, agitation vs. time (rpm h), and aeration vs. time (lpm h). This simple and novel strategy of fermenter operation proved to be highly successful which can be adopted to other biocontrol fungi.     

Rheology of starch–clay nanocomposites

The effects of incorporating various montmorillonite nanoclays into wheat, potato, corn, and waxy corn starch samples were examined by rheology and X-ray diffraction. The nanoclays included the hydrophilic Cloisite Na+ clay as well as the more hydrophobic Cloisite 30B, 10A, and 15A clays. Frequency sweep and creep results for wheat starch–nanoclay samples at room temperature indicated that the Cloisite Na+ samples formed more gel-like materials than the other nanoclay samples. X-ray diffraction results showed no intercalation of Cloisite Na+ clays at room temperature, suggesting that starch granules interacted only with the clay surface and not the interlayer. When the various wheat starch–nanoclay samples were heated to 95 °C, the Cloisite Na+ samples exhibited a large increase in modulus. In contrast, the more hydrophobic nanoclay samples had comparable modulus values to the neat starch sample. These results suggested that during gelatinization, the leached amylose interacted with the Cloisite Na+ interlayer, producing better reinforcement and higher modulus values. X-ray diffraction results supported this interpretation since the data showed greater intercalation of Cloisite Na+ clay in the gelatinized samples. The samples containing wheat and corn starch showed comparable elastic modulus values during gelatinization. However, the potato and waxy corn samples had modulus values that rapidly decreased at higher temperatures.     

Colored Food Emulsions—Implications of Pigment Addition on the Rheological Behavior and Microstructure

Coloring foods with natural pigments enables the development of attractive products with nutritional advantages. The overall objective of this work was to study the consequences of adding the pigments lutein (lipophylic) and phycocyanin (hydrophilic) on the rheological behavior of oil-in-water food emulsions stabilized by pea protein isolate. The emulsions were characterized in terms of their linear viscoelasticity, and of their steady and transient flow behaviors. The rheological tests were monitored by using a microscope (optical analysis system) coupled to a controlled stress rheometer. Upon lutein incorporation, the emulsions became less stable, presenting lower rheological function values than the control emulsion (without pigment addition). On the other hand, phycocianin addition resulted in a significant reinforcement of emulsion structure, a higher resistance to structural breakdown becoming evident. An emulsion containing both pigments, in the same proportion, presented an intermediate rheological behavior resulting from a combination of the effects observed for the emulsions containing each of them.     

A computational study of leukocyte adhesion and its effect on flow pattern in microvessels

Three-dimensional computational modeling and simulation are presented on the adhesive rolling of deformable leukocytes over a P-selectin coated surface in parabolic shear flow in microchannels. The computational model is based on the immersed boundary method for cell deformation and Monte Carlo simulation for receptor/ligand interaction. The simulations are continued for at least 1 s of leukocyte rolling during which the instantaneous quantities such as cell deformation index, cell/substrate contact area, and fluid drag remain statistically stationary. The characteristic ‘stop-and-go’ motion of rolling leukocytes, and the ‘tear-drop’ shape of adherent leukocytes as observed in experiments are reproduced by the simulations. We first consider the role of cell deformation and cell concentration on rolling characteristics. We observe that compliant cells roll slower and more stably than rigid cells. Our simulations agree with previous in vivo observation that the hydrodynamic interactions between nearby leukocytes affect cell rolling, and that the rolling velocity decreases inversely with the separation distance, irrespective of cell deformability. We also find that cell deformation decreases, and the cells roll more stably with reduced velocity fluctuation, as the cell concentration is increased. However, the effect of nearby cells on the rolling characteristics is found to be more significant for rigid cells than compliant cells. We then address the effect of cell deformability and rolling velocity on the flow resistance due to, and the fluid drag on, adherent leukocytes. While several earlier computational works have addressed this problem, two key features of leukocyte adhesion, such as cell deformation and rolling, were often neglected. Our results suggest that neglecting cell deformability and rolling velocity may significantly overpredict the flow resistance and drag force. Increasing the cell concentration is shown to increase the flow resistance and reduce the fluid drag. The reduced drag then results in slower and more stable rolling of the leukocytes with longer pause time and shorter step distance. But the increase/decrease in the flow resistance/fluid drag due to the increase in the cell concentration is observed to be more significant in case of rigid cells than compliant cells.