RHEOLOGY OF GELS

This review covers the rheology of gels formed by macromolecular food additives. In particular agar, alginate, carrageenan, gelatin, pectin and microbial polysaccharide gels are considered. Empirical and fundamental techniques that have been applied to gels are listed and the relationship between rheological parameters and sensory assessment of texture is briefly discussed. Certain general conclusions about the dependence of the rheological properties on network composition and structure are put forward.     

EFFECT OF EMULSION DROPLETS ON THE RHEOLOGY OF WHEY PROTEIN ISOLATE GELS

The effects of droplet size and emulsifier type on the rheology of whey protein isolate (WPI) gels containing emulsion droplets was studied. Gels were prepared by dispersing droplets of corn oil (20 wt%, d₃₂= 0.7 – 4 μm) in a 10 wt% WPI solution (pH 7.0, 50 mM NaCl), and heating at 90C for 15 min. Gel strength was determined by measuring the stress of gels at 20% compression using an Instron Universal Testing Machine. Droplets stabilized by WPI increased the gel strength, those stabilized by non-ionic surfactants (Tween 20 and Triton X-100) decreased it slightly, and those stabilized by SDS decreased it drastically. Gel strength increased as the droplet size decreased for droplets stabilized by WPI, but was relatively insensitive to the size of droplets stabilized by the small molecule surfactants. These observations may be explained in terms of the interactions between the emulsifiers and the protein network. Droplets coated with emulsifiers which can be incorporated into the protein network reinforce the structure and so increase gel strength, whereas droplets coated with emulsifiers which cannot be incorporated into the protein network disrupt the three dimensional structure of the gel and decrease its strength.     

THE RHEOLOGY OF GELS

The following aspects of the rheological behavior of polysaccharide and protein gels are discussed with particular emphasis on recent investigations: (1) Viscoelasticity of gels; (2) Validity of rubber elasticity theory; (3) Rupture strength of gels; and (4) Single-point measurements of gel strength.
It is concluded that in contrast to most food materials gels show linear viscoelastic behavior up to strains of the order of 0.1. Results obtained from creep and stress relaxation experiments would suggest that noncovalent crosslinks in gels move or break when the gel is stressed. Activation energies associated with crosslink breakage have been estimated from the temperature dependence of viscoelastic parameters. For gelatin and polysaccharide gels energies ranging from 5–65 Kcals/mole have been reported.
The stiff and extended nature of polysaccharide chains make it unlikely that polysaccharide gels obey rubber elasticity theory, though it is possible that this theory holds for gelatin gels and also for ovalbumin gels in 6 M urea.
It is emphasized that the rupture strength of a gel is not necessarily related to its elastic modulus and therefore 'single point' measurements of 'gel strength' based on rupture tests will not always rank a series of gels in the same order as tests which involve small deformations without rupture. One of the reasons for this is that the elastic modulus and rupture strength depend in different ways on the primary molecular weight of the polymer from which the gel is formed.     

RHEOLOGY OF SODIUM CASEINATE-CARRAGEENAN MIXTURES

The main effects and interactions of mixtures of sodium caseinate, k- or t- carrageenan, sodium chloride, calcium chloride and hydrogen ions on the rheological properties in aqueous systems cooled to 5 C were quantified. Response surface methodology was used to display the results for five different response variables: gelation temperature, consistency index (k*), dynamic power law factor (n*), critical strain (γ_c), and syneresis. The gelation profiles observed for both k- and t-carrageenan systems demonstrated that the sol-gel transition in sodium caseinate-carrageenan mixtures was due to electrostatic interactions of free carrageenan loops or tails with the intervention of cations. Caseinate molecules did not play a major role in this process. The electrostatic nature of the interactions was further demonstrated by the significant influence of salts and pH on the consistency and brittleness of the gels (represented by k*and γ_c, respectively). Consistency and brittleness were inversely related as well as consistency and syneresis. Syneresis was negligible in t-carrageenan gels and was reduced by sodium caseinate and sodium chloride in the k-carrageenan systems.     

VISCOELASTIC PROPERTIES OF WHITE FRESH CHEESE FILLED WITH SODIUM CASEINATE

The yield, moisture loss and viscoelastic properties of white fresh cheeses containing sodium caseinate were determined and compared to those of a reference cheese without sodium caseinate. Added sodium caseinate resulted in higher cheese yields and lower moisture losses during aging. Creep compliance tests were performed on the cheeses using a parallel plate viscoelastometer. Multiple regression analysis of the experimental data provided first order models that explain the variation of most of the viscoelastic parameters of the white fresh cheeses in terms of sodium caseinate level, moisture content, aging and pH. The instantaneous elastic compliance and the Newtonian viscosity increased with moisture content and pH, respectively. The retarded elastic compliances increased and the first retardation time decreased during aging. The influence of sodium caseinate on most of the viscoelastic parameters was significant.     

VISCOELASTIC PROPERTIES OF HEAT-SET WHEY PROTEIN EMULSION GELS

The viscoelastic properties of heat-set whey protein gels and whey protein-stabilized emulsion gels have been studied using the dynamic oscillatory rheometry technique. The storage modulus was monitored and analysed for pure protein gels and emulsion gels over a wide range of protein concentrations. The dependence of storage modulus on protein concentration is different for gels of low and high modulus. At low protein concentrations, the increase of storage modulus is much more sensitive to the increase of protein concentration. The protein-coated oil droplets behave as active filler particles and dramatically enhance the gel strength. The effect of the oil volume fraction on the rheology has been investigated for emulsion gels containing 11 vol. %, 20 vol. % and 45 vol. % Trisun oil. The formula of van der Poel fails to describe the experimental results. This is attributed to the strongly flocculated state of the emulsion system.     

PHYSICOCHEMICAL AND STRUCTURAL CHARACTERISTICS OF SODIUM CASEINATE BIOPOLYMERS INDUCED BY MICROBIAL TRANSGLUTAMINASE

Some physicochemical properties and structural characteristics of microbial transglutaminase (MTGase)‐induced biopolymers of sodium caseinate (SC) were investigated. The sodium dodecyl sulfate–polyacrylamide gel electrophoresis and size‐exclusion–high‐performance liquid chromatography analyses showed that all components of SC were easily polymerized or transformed by MTGase to form high‐molecular weight biopolymers, and the susceptibility order of individual components was κ‐Casein (C) > α‐C > β‐C. The emulsifying properties of biopolymers depended on the incubation time with MTGase. The emulsifying activity index of biopolymers persistently increased with the MTGase (0–12 h) incubation time. The emulsion stability also increased with the incubation time (< 4 h), then declined a little with longer incubation (4–12 h). The differential scanning calorimetry analysis showed that the thermal properties of the biopolymers obtained after a 12‐h incubation were different from that of native SC or biopolymers obtained after a shorter incubation time (< 4 h), suggesting that the former has higher thermal stability. In addition, the ultraviolet (UV) spectra showed that the UV absorbance (at 275 nm) of MTGase‐induced biopolymers of SC decreased with an increasing incubation time with MTGase, and the maximal emission wavelength (λ _max ) slightly shifted to the “blue side.” The fluorescence spectra showed that the λ _max was related with incubation time with MTGase, slightly shifting to the “blue side” after 4 h with no further changes; its relative fluorescence intensity also increased. These results suggest a relationship between the functionalities and structural characteristics of the MTGase‐induced biopolymers of SC.     

RHEOLOGY OF PURE AND MIXED KAPPA-CARRAGEENAN GELS IN LACTIC ACID FERMENTATION CONDITIONS

Texture profile analysis and rupture tests, respectively, at a deformation of 5 and 80%, were carried out on commercial kappa-carrageenan gel cylinders (3% w/v concentration) and on mixed gel cylinders (kappa-carrageenan/iotacarrageenan or locust bean gum [LBG], in different proportions), to select the best gel composition as an immobilization matrix for continuous lactic acid fermentations of dairy media. Mixed gel cylinders were soaked in a model system for lactic fermentations consisting of 36 dairy media: milk, whey and whey permeate, with 4 lactate concentrations (5, 20, 35 and 50 g/L of medium), combined with 3 pH values (6.5, 6.0, 5.5). GENUGEL X-0909 n°5180850 was the most resistant of all commercial kappa-carrageenans available; mixing this kappacarrageenan with LBG in the respective proportions of 2.75%/0.25% gave the best rheological properties to the resulting mixed gel. Our study of the impact of fermentation parameters showed that pH has a significant effect on hardness, cohesion and percentage of deformation at rupture in milk (but not in the other 2 media). A pH drop of milk from 6.5 to 5.5 resulted in more brittle gels which may be explained by an increase in the amount of soluble calcium cations. Generally, soaking in milk produced the most brittle gels due to its calcium content, whey produced the hardest gels and whey permeate, the softest. An increase in the amomonium lactate concentmtion significantly improved hardness, rupture force and percentage of deformation at rupture, while reducing cohesion and resilience: NH₄+ cations would account for this effect. The differences in rheological behuviour of immobilization gel beads during continuous Eactic fermentations of milk and whey permeate would be due to the cation content and more specijically to the higher calcium concentmtion in milk.     

MECHANICAL PROPERTIES OF FISH AND BEEF GELS PREPARED WITH AND WITHOUT WASHING AND SODIUM CHLORIDE

Excellent gels, as measured by fold test, were prepared from three species of fish without first washing the minced tissue. Dependent on species (red hake, cod, or winter flounder), excellent to adequate gels, as determined by the fold test, could be prepared from fish muscle without NaCl provided that the minced tissue was washed prior to gel formation. Postrigor beef formed inadequate gels while excellent to good gels could be produced from prerigor beef although salt was a requirement. The concentration of contractile proteins in fish gels was estimated and they did not appear to be a major factor determining gel quality. Soluble muscle protein did not appear to interfere with gel formation either. Percent expressible moisture, percent recovery, and modulus of deformability did not relate well to fold test scores.     

RHEOLOGY OF CHOCOLATE

Abstract . This review covers mainly the rheology of chocolate above its melting point. The validity of the Casson relationship is discussed and two alternative equations are given. Different factors affecting viscosity are considered: fat content, lecithin or emulsifiers content, water content, conching time, particle size, temperature, degree of temper and vibration. Instrumental methods of viscosity measurement as well as the problem of thixotropy are also covered. Some information is given on the rheology of chocolate in the solid state.     

RHEOLOGY OF STIRRED YOGURTS

Rheological characteristics of 2 commercial brands of stirred yogurt were evaluated using a Haake Rotovisco Model RV 20 with an M-5-Osc measuring head and MV-1 rotor assembly from measured shear stress values under a programmed 3-cycle up- and down-shear rate at 100 s⁻¹/min from 0–500 s⁻¹ (5 min). The upward shear-rate flow behavior of the yogurt samples could be described by a Herschel-Bulkley model while the downward shear-rate curves were essentially linear. Both upward and downward shear rate curves demonstrated progressive structural degradation with repeated shearing. The dependency of rheological parameters on temperature in the range 10–25°C followed both Arrhenius and Turian models.     

RHEOLOGY OF MULBERRY PEKMEZ

The rheological behavior of mulberry pekmez (concentrated mulberry juice, °Brix = 72.0) with different solid contents (62.8, 55.8 and 46.0) was studied over the temperature range of 10, 20, 30, 40, 50 and 60C using a Controlled Stress Rheometer. Mulberry pekmez was found to exhibit Newtonian behavior. The effect of temperature can be described by means of Arrhenius equation. The activation energies for flow of samples vary from 53.11 to 12.73 kJ/mol depending on soluble solid contents. The effect of soluble solids on viscosity can be described by an exponential equation. The best model to describe the combined effects of temperature and soluble solids content on viscosity is η= 3.40 X 10^-10 exp (0.12C + 3723/T).     

RHEOLOGICAL PROPERTIES OF PECTATE GELS

Creep compliance measurements were made on calcium pectate gels using a paralled plate viscoelasto-meter. Linear viscoelastic behaviour was observed and the response was consistent with a model containing a Maxwell element in series with three Voigt elements. When gels were prepared with the calcium level necessary for maximum gel strength a linear relationship was found between the reciprocal of the creep compliance and the square of the polysaccharide concentration.
The rheological behaviour of pectate gels had many similarities to gels prepared from alginates containing a high proportion of guluronic acid residues. However, pectate gels were more sensitive to calcium ions than alginate gels and at high calcium levels the Newtonian viscosity of the pectate gel was much higher than that found for the alginates. These differences have been interpreted in terms of the structure and molecular weight of the two polymers.     

RHEOLOGY OF EGG YOLK

The rheological behaviour of egg yolk over the temperature range 5° to 60°C was studied and found to fit the power law. For egg yolk of different moisture content it was found that the data gave a better fit to the power law than to an exponential equation.