Marvin A. Tung

Compression strength and deformation of gellan gels formed with mono- and divalent cations

Abstract The influence of divalent (Ca 2+ and Mg 2+ ) and monovalent (Na + and K + ) cations on the failure stresses and strains in gels formed with gellan polymer were investigated. Gellan gels containing 0.6-2.2% polymer and varying cation concentrations were tested using large compressive deformation until failure. Maximal true shear stresses and corresponding shear strains represented gel strength and extensibility. At small cation concentrations, the strength of gellan gels increased while the extensibility decreased with cation concentrations. The strongest gels were obtained at cation levels corresponding to 0.5 cations per carboxylate group in the repeat gellan unit for gels crosslinked with divalent cations and 10 to 30 cations per repeat gellan unit for gels with monovalent cations. At optimum cation levels, gellan gels with Ca 2+ were stronger than with Mg 2+ , and gellan gels with K + were stronger than with Na + . Above the optimum cation levels, gellan gels became brittle, and the strength of the gels decreased with increasing cation concentrations.

Sorption and transport of water vapor in nylon 6,6 film

The sorption and transport of water in nylon 6,6 films as functions of the relative humidity (RH) and temperature were studied. Moisture-sorption isotherms determined gravimetrically at 25, 35, and 45°C were described accurately by the GAB equation. Water-vapor transmission rates were enhanced above ≈ 60–70% RH, primarily due to the transition of the polymer from glassy to rubbery states. The glass transition temperatures (Tgs) of nylon 6,6 were measured at various moisture contents using differential scanning calorimetry. The results showed that the sorbed water acted as an effective plasticizer in depressing the Tg of the polyamide. Fourier transform infrared spectroscopy (FTIR) was utilized to characterize the interaction of water and the nylon. Evidence from FTIR suggested that the interaction of water with nylon 6,6 took place at the amide groups. Based on the frequency shift of the peak maxima, moisture sorption appeared to reduce the average hydrogen-bond strength of the NH groups. However, an increase was seen for the CO groups.

Water absorption in EVOH films and its influence on glass transition temperature

Moisture sorption kinetics of nonoriented ethylene vinyl alcohol copolymer (EVOH) film (EF-E15) were studied at 25, 35, and 45°C. Anomalous diffusion was observed for the polymeric film at high relative humidities (RH) and higher temperatures. Diffusion and solubility coefficients of water were found to be concentration dependent. The moisture sorption isotherms of three types of EVOH films (EF-E15, EF-F15, and EF-XL15) determined at 25, 35, and 45°C, were well described using the GAB equation. Glass transition temperatures (Tg) of the EVOH films, as influenced by RH, were measured using differential scanning calorimetry. Tg values decreased with increasing RH due to the plasticization effect of water, and were found to be dependent on ethylene content and orientation of the EVOH films.

Characterization of gellan gels using stress relaxation

Abstract The effect of testing parameters on stress relaxation characteristics and the relationship between those characteristics and intrinsic gel properties were investigated. Gellan gels were tested in stress relaxation using different crosshead speeds (3–300 mm/min) to apply strain (3–20%) to specimens of 21 and 31 mm diameter with aspect ratios of 0.5, 1.0 and 1.5. Relaxation appeared to be associated with a release of hydraulic pressure in the gel matrix. The relaxation rate was inversely related to specimen diameter, but positively related to the pore sizes in the gel matrix. Increasing the applied strain resulted in larger initial stresses but slightly lower equilibrium stresses. The equilibrium stresses were positively related to the strength of the gels and, thus, to the degree of crosslinking in the polymer network. As long as the polymer networks were not permanently damaged during relaxation tests, the gel structure could be restored upon soaking in water.

Gelling temperature, gel clarity and texture of gellan gels containing fructose or sucrose

The effects of fructose and sucrose on the gelling temperature, clarity, and texture properties of gellan gels crosslinked with calcium or sodium ions were studied by dynamic rheological, visible light absorption, and compression tests. The gelling temperatures of gellan solutions generally increased due to the addition of sucrose, except when both cation and sucrose concentrations were high. Addition of fructose up to 35%w/v had no effect on the gelling temperatures. Incorporation of fructose and sucrose resulted in a marked increase in the gel clarity. At low cation concentrations, sucrose strengthened the gels; but at high cation concentrations, sucrose weakened the gels. Gels formed by slow cooling in air were more turbid and generally stronger than the gels formed by rapid cooling in water. q 2001 Elsevier Science Ltd. All rights reserved.

Gellan polymer solution properties: dilute and concentrated regimes

Abstract Flow properties of dilute and concentrated gellan polysaccharide solutions were investigated under low and intermediate shear rates, respectively, to study the behavior of the polymer in water. An apparent stiffness parameter, B′-value, was calculated from the low shear intrinsic viscosity data obtained at various Na+ concentrations. B′-value data suggested that gellan polymer chains are partially flexible. Critical concentration, C ∗ , for gellan solutions was found to be 0.064% w/v, under the measurement conditions used. Flow curves as a function of increasing gellan concentration approached power-law flow. With decreasing temperature, gellan polymer solutions showed a transition to power-law behavior between 28 and 25°C. This transition was attributed to partial gelation of gellan polymer below the transition temperature range.

Stress-strain relationships for gellan gels in tension, compression and torsion

Abstract Formulated foods often contain small amounts of polymeric ingredients that interact to form three-dimensional networks which stabilize structure and provide desirable textural quality. One goal of food engineers is to understand how gels are formed and to be able to predict their mechanical properties. We have studied calcium-crosslinked gels of gellan, a polysaccharide recently introduced to the food industry, in order to understand their stress-strain behavior in tension, compression and torsion. This information will be applied when designing stability and texture for a variety of food systems. Gels were prepared by dispersing gellan polymer in 90 °C water, adding calcium chloride and cooling in molds to form the test specimens. Four levels of polymer, ranging from 0.6 to 1.8% w/v, were used with seven calcium concentrations between 1.5 and 60 mM. Molds of differing construction provided specimen shapes that were cylindrical with enlarged ends for tensile tests, cylindrical for compression tests and capstan-shaped for torsion tests. Specimens for tension and torsion testing were modified with plastic adapters to facilitate attachment in mechanical testing machines; whereas compression was carried out between lubricated parallel Teflon plates. Gel properties were influenced strongly by the content and combination of polysaccharide and ion. Shear stress relationships in small deformations were almost identical in the three testing modes for a given gellan and calcium concentration. In large deformations, gels were more rigid in tension than in compression and torsion, but they failed at the same maximum shear stress regardless of testing mode. Stress-strain responses were analyzed using constitutive equations based on energy functions. Mooney-Rivlin constitutive equations were applicable for stress-strain relationships for small strains; however, equations based on three-term energy functions fitted the data more accurately at larger strains.

Thermorheological studies of food polymer dispersions

Abstract Reversible thermal gelation of methylcellulose (MC) was characterized by measuring the dynamic shear storage modulus (G′) as temperature was increased from 20 to 67° C at scanning rates of 1·0, 0·50 and 0·25 C°/min, and then decreased in a similar manner. The thermally induced gelation of 15·0% dispersions of soy or canola protein and meat emulsion systems with and without canola protein were also studied by measuring G′ as temperature was raised from 20 to 90°C and then lowered at a rate of 2·0C°/min. The storage modulus for solutions of MC decreased from 20 to 40°C. From 40–67°C, log G′ decreased in three stages, first slowly with temperature, then suddenly increasing three orders of magnitude between 54 and 62°C, with only small increases above 62°C. When cooled, G′ for the sample decreased in three similar stages, giving almost the mirror image of the gelation behavior. When heated from room temperature, soy and canola protein dispersions exhibited different viscoelastic behavior. G′ for the canola dispersion decreased slightly more than one order of magnitude when heated to 60°C, while the soy dispersion showed only a slight decrease in G′. At 60–90°C, structure formation in the canola dispersion was detected as G′ increased two orders of magnitude. Structure development for the soy dispersion commenced at about 78°C with G′ increasing only 10-fold. Both proteins exhibited similar increased rigidity throughout cooling. The meat emulsion containing canola experienced structural changes on heating and cooling intermediate to that of isolated canola dispersions and all-meat emulsions. The all-meat protein emulsion showed greater rigidity overall and achieved maximum structure development at a lower temperature than the canola-substituted emulsion. Thermorheological scanning provided a convenient method to explore the mechanisms involved in structure formation in these polysaccharide and protein gelling food systems.

Failure Testing of Gellan Gels

Abstract Gels, formed using a 1% gellan gum solution containing 7 m m calcium ions, were tested to failure in compressive, tensile and torsional modes. The gels were brittle and fractured at relatively small strains. In the compression experiments samples failed in shear at low deformation rates, and in a combination of compression and shear at higher strain rates. Tensile fracture was always evident in uniaxial tension measurements, and in the torsion experiments at low strain rates. At high deformation rates the torsional samples underwent a combination of shear and tensile fracture. The three tests were compared on the basis of the shear stress and strain at failure. The shear stress agreed in all cases. The strain only agreed in the compressive and torsional modes and was an order of magnitude lower in tension.

Limonene transport and mechanical properties of EVOH and nylon 6,6 films as influenced by RH

The transport properties of d-limonene through ethylene vinyl alcohol copolymers (EVOH) and nylon 6,6 films as functions of relative humidity (RH) and temperature were studied. Permeation properties of these polymers were strongly influenced by temperature and RH. Compared to the EVOH films, the nylon 6,6 film had much greater limonene permeability. Mechanical property studies indicated that both the tensile modulus and yield strength of the EVOH films decreased with an increase in RH. The polymer changed from being stiff and brittle at low RH to being soft and ductile at high RH. In addition, ethylene content and orientation were found to affect the transport and mechanical properties of limonene through EVOH polymers.