Scaling approach to water sorption isotherms of hydrogels and foods

Mechanisms affecting water sorption isotherms of foods, at high water activities, were evaluated by the processes of swelling and shrinking of model systems of uncharged and charged hydrogels. Their water activity was found to be determined by the combined effect of their osmotic pressure and the network pressure of the polymeric network. The scaling approach in polymer science indicates that in an uncharged network the osmotic pressure is affected by the mobility of the polymer segments and thus by the degree of cross-linking. However, in charged gels, which behave in a similar manner to many food systems, the counterions play the dominant role in determining the osmotic pressure. The network pressure may shift from a positive to a negative value during shrinking (dehydration), thus counteracting or co-operating with the osmotic pressure in decreasing the water activity. A scaling sorption isotherm, at high water activity, is formulated for a food system with physically significant parameters.     

From egg yolk/κ-Carrageenan dispersions to gel systems: Linear viscoelasticity and texture analysis

The effect of pH (3.5–6) and polysaccharide concentration (0–0.5 wt%) on the linear viscoelastic behaviour of egg yolk/κ-Carrageenan (EY/κC) mixtures in aqueous solution was studied by using Small Amplitude Oscillatory Shear (SAOS). Native egg yolk containing 45 wt% solids was used for all the samples. Thermally set EY/κC gels were also studied by SAOS and texture analysis. A variety of linear viscoelastic behaviours depending on κC concentration and pH were exhibited by EY/κC dispersions that may be explained in terms of the contributions of electrostatic attractive interactions and an exclusion volume effect between protein and polysaccharide macromolecules. This last effect seems to be dominant as pH shifts towards the isoelectric point (IEP) of egg yolk proteins, whereas, at pH far from the IEP a certain enhancement in the degree of compatibility, and even some κC autohydrolysis, seems to take place. The results obtained either from rheological or textural characterization of gels were consistent with that balance. In any case, the results obtained suggest that the microstructure of gels is governed by the protein ability to form gels, where hydrophobically driven interactions and subsequent cross linking among protein segments play a dominant role.     

Effect of high pressure processing on the gel properties of salt-soluble meat protein containing CaCl2 and κ-carrageenan

The effects of high pressure processing (HPP) on the water-binding capacity and texture profile (TPA) of salt-soluble meat protein (SSMP) containing 0.2% CaCl₂ and 0.6% κ-carrageenan (SSMP-CK) gels were investigated. The results showed that 300–400 MPa improved water-binding capacity and decreased TPA parameters of SSMP-CK gels (P < 0.05), while 100 MPa could increase hardness and chewiness of the gels. The thermal transition temperature peak for the myosin head (T_peak1) of SSMP disappeared on addition of CaCl₂ and κ-carrageenan. 300 MPa produced a new peak, and caused a shift of the NH-stretching left peak and amide I and the disappearance of NH-stretching right peak. The destruction of network structure and the weakening of molecular interaction within the pressurized gels could result in the decrease of TPA parameters. Thus gelling properties could be modified by HPP, κ-carrageenan and Ca^2 +. It is of interest to develop low-fat and sodium-reduced meat products.     

Protein–Protein Interactions in High Moisture-Extruded Meat Analogs and Heat-Induced Soy Protein Gels

Two commercial soy protein isolates were made into fibrous meat analogs by high moisture extrusion or into gels by heating and cooling, at varying concentrations and/or temperatures. Protein–protein interactions by extrusion or gelation were investigated through protein solubility studies of raw and finished products. All samples except for extrudates exhibited similar patterns of solubility in four selected extractants. Phosphate buffer (PB) extracted the least amount of protein. Addition of dithiothreitol (DTT) to PB improved protein solubility, indicating the presence of disulfide bonds. PB + Urea and PB + Urea + DTT gave the highest and almost equal amount of extractable proteins from all samples, except for the extrudates from which protein could not be extracted effectively by PB + Urea, implying that disulfide bonding was more pronounced during extrusion than gelation. The results support our hypothesis that soy protein gels and extrudates both have the same types of chemical bonds, namely covalent disulfide bonds and non-covalent interactions. It is the relative proportion of each type of bonds in their structures that differentiates the two with respect to reversibility and structure rigidity. In forming protein gels during heat-induced gelation, non-covalent bonds play a dominant role over disulfide bonds; whereas for forming the fibrous structure of protein extrudates, non-covalent bonds and covalent disulfide bonds are both important.     

Morphology of starch in surimi gels

 The purpose of this work was to study the changes undergone by starch during heat-induced surimi gel preparation either with or without added egg white, and their effects on the structure of gels using light and scanning electron microscopy. Gels were made from SA-grade Alaska pollack (Theragra chalcogramma) surimi with: (1) salt (3%, w/w); (2) salt and waxy corn starch (3% and 5%, respectively w/w); or (3) salt, waxy corn starch and egg white (3%, 5% and 5%, respectively, w/w). Final moisture was adjusted to 73% or 83%. The gels were prepared by prior setting (40°C, 30 min, followed by 90°C, 30 min) or cooking (90°C, 30 min). The prepared gel was frozen and stored at –20°C (±1°C) until analysis. Samples were observed by light and scanning electron microscopy. The results show that the starch granules alter according to the processing conditions, with the predominance of crystalline or amorphous morphology depending upon the availability of heat and water. Large cavities formed in the protein gel matrix during setting can trap water; as a result, water availability is limited for starch to swell and gelatinize even in the high-moisture gel. Received: 13 March 1997     

Mucoadhesive and Physicochemical Characterization of Carbopol-Poloxamer Gels Containing Triamcinolone Acetonide

The viscosity and bioadhesive property of Carbopol-Poloxamer gels containing triamcinolone acetonide to mucosa were tested according to various concentrations of Carbopol gels of various pH. The increase in Carbopol concentration caused increased viscosity and bioadhesiveness. The neutralization of pH in various concentrations of Carbopol gels showed the increased viscosity, showing the highest viscosity and highest bioadhesiveness when neutralized to pH 6. A relationship between the viscosity and bioadhesive strength was shown from the neutralized Carbopol gels. The physicochemical interactions between triamcinolone acetonide and polymers were investigated by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectrophotometry. According to FTIR and XRD studies, the drug did not show any evidence of an interaction with the polymers used and was present in an unchanged state.     

A kinetic study of the reduction of chromium(VI) to chromium(III) by thiourea

The reduction of chromium(VI) to chromium(III) by thiourea is a key aspect of the chromium(VI)–thiourea–polyacrylamide gel polymer system used in oil recovery processes. A study was undertaken to develop a kinetic model for the reduction reaction. Reaction mixtures were prepared and chromium(VI) concentrations were determined from the mixtures's visible absorbance. The reaction rate was found to depend on the concentrations of chromium(VI), thiourea, and polyacrylamide and on solution pH. A kinetic model for the reactions was developed using the experimental data and previously proposed reaction mechanisms. The model accurately predicts the reaction rate in solutions that do not contain polyacrylamide and predicts a rate that is approximately correct for solutions containing polyacrylamide.     

VISCOMETRIC MEASUREMENT OF CHROMIUM(III)-POLYACRYLAMIDE GELS†

Gelled polymers are being used increasingly to modify the movement of injected fluids in secondary and enhaced oil recovery processes. A common gelation process involves the reduction of Cr(VI) to Cr(III) in the presence of polyacrylamide. The Cr(III) reacts or interacts with the polymer to form a gel network. Although correlations of gelation time with principal process variables have been obtained, viscometric data have not been reported during or after gelation. These data are needed for fluid flow calculations in surface equipment and estimation of flow behaviour in reservoir rocks.

A Weissenberg Rheogoniometer, with cone and plate geometry, was used to obtain viscometric data for the gelation of polyacrylamide and chromium (III). Solutions consisting of polyacrylamide polymer, sodium dichromate-dihydrate and sodium bisulfite were gelled under a steady shear field at constant temperature. The shear stress versus time profile for the galation process was interpreted to define a gelation time and to determine the apparent viscosity of the gelled fluid. The gelation time decreased as the applied shear rate increased up to about 14.25 sec^-1 and was affected by shear rate history. Viscometric properties of the gelled solutions were determined. Apparent viscosity of the gelled solutions decreased as the shear rate under which they were formed increased.

Post gelation studies indicated that gels exhibited a residual stress at zero shear rate and behaved as Bingham plastics under steady shear. Gels formed at low shear rates were more viscous than gels formed at high shear rates. However, the structure of these gels was susceptible to shear degradation.