M. A. Rao

Rheology and structure development during gelation of low-methoxyl pectin gels: the effect of sucrose

Abstract A non-isothermal kinetic model described G ′ values during gelation induced by cooling dispersions of low-methoxyl (LM) pectin+Ca 2+ , and 10 and 30% (wb) sucrose at pH 4 and {2(Ca 2+ )/(COO − )}=0.58. Two different gelation regimes, corresponding to two different temperature ranges, were observed. During the aging process, increasing the pectin concentration resulted in a significant increase in G ′ increasing the sucrose content also increased G ′. Using the plateau values of G ′ and the rubber elasticity theory, the number average molecular weight of chains between cross-links, M c , was estimated. At the same pectin concentration, M c of 0% sucrose gels was greater than those of 10 and 30% sucrose gels. These results reflect the important role of the hydroxyl groups of sucrose in gel formation and that sucrose may stabilize the structure of junction zones.

Heat transfer to model non-Newtonian liquid foods in cans during end-over-end rotation

Abstract Heat transfer coefficients were determined for aqueous guar gum solutions (0·3%, 0·4%, 0·5%, and 0·75%) during end-over-end rotation in copper cans. The variables investigated were size of can, speed of rotation and radius of rotation. The centre temperature within the can was found to oscillate. Over the range of variables studied, the heat transfer coefficient was found to be independent of the radius of rotation and the size of the can. The heat transfer data for non-Newtonian fluids were correlated in terms of Nusselt (Nu), generalised Reynolds (GRe), and generalised Prandtl (GPr) numbers: Nu = 1.41GRe 0.482 GPr 0.355 The correlation was found to be similar to that developed earlier for Newtonian liquids.

KINETICS AND THERMAL BEHAVIOUR OF THE STRUCTURE FORMATION PROCESS IN HMP/SUCROSE GELATION

The concentration and temperature dependence of the gelation kinetics of high-methoxyl pectin (HMP; 60% sucrose, pH 3) was investigated using measurements of small-amplitude oscillatory shear. The rate of gelation close to the gel point can be described as a second-order rate process using the kinetic model of Ross-Murphy (Carbohydr. Polym. 1991, 14, 281) and a critical exponent close to that predicted by the percolation approach. The modulus after a long ageing time showed a power concentration dependence with an exponent around 3.1, higher than the classical square of concentration dependence, which was probably either due to the nonequilibrium state of the HMP gels even after long ageing times, or due to the proximity of the concentration range studied to the critical gelling concentration. The gelation rate of HMP/sucrose systems is strongly dependent on the temperature. An Arrhenius relationship was applied to describe this dependence. Two different processes are proposed to explain the discontinuity observed, each one having rates with different temperature dependence. The applicable kinetics at longer times are quite different, with a lower dependence on polymer concentration and ageing temperature. A non-isothermal kinetic model was used to describe the gelation process of the HMP/sucrose system during cooling.

Influence of temperature on the dynamic and steady-shear rheology of pectin dispersions

Abstract The influence of temperature on the dynamic and steady-shear rheology of ionic polysaccharides, high-methoxyl and low-methoxyl pectins, has been studied and compared with the behaviour of locust bean gum, a virtually neutral biopolymer. Using the time-temperature superposition principle, the rheological parameters were reduced to an arbitrary reference temperature. Activation energies were calculated and their dependence on temperature and shear rate analysed. Concerning the viscosity dependence on temperature, two approaches have been considered: one associated with the theory of absolute reaction rates leading to an Arrhenius type equation, and the other associated with the free volume theory, expressed by the Williams-Landel-Ferry equation. The difficulties encountered in the superposition of the dynamic properties of the pectin dispersions, the high activation energies, yield values and elastic plateau at low oscillatory frequency, are consistent with a macromolecular organization of these polymers dominated by important aggregation phenomena, which could be attributed to important intermolecular interactions like hydrogen bonding and hydrophobic interactions, especially in conditions of low degree of ionization of the carboxylic groups.

Viscoelastic behaviour of mixtures of locust bean gum and pectin dispersions

Abstract Viscoelastic properties of single-component systems — locust bean gum (LBG), high-methoxyl (HMP) and low-methoxyl (LMP) pectins- and of HMP/LBG and LMP/LBG blends, were studied using dynamic oscillatory and steady-shear methods. The mechanical spectra of HMP/LBG blends showed characteristics intermediate between the individual components. A possible weak antagonistic interaction was found between LMP and LBG, especially in the mixture with similar amount of each single-component solution. Correlation between dynamic and steady-shear viscosities was compared with the empirical rule of Cox and Merz. Departures from the Cox-Merz rule were observed for the HMP dispersions, whereas for the LMP and the LBG dispersions the Cox-Merz rule prediction was satisfactory. The different behaviour of LMP relative to the HMP dispersions can be attributed to the lesser extent of intermolecular association and higher hydrodynamic volume, due to the higher charge density.

Effect of concentration on rheological properties of acid-hydrolyzed amylopectin solutions

Rheological properties are sensitive to variations in molecular structure and they are useful in developing structure–function relationships for systems of polysaccharides in solution. Intrinsic viscosities of acid-hydrolyzed Amioca starches in 90% DMSO/10% water were determined using a modified approach for polyelectrolytes. Double logarithmic plots of specific viscosity, ηsp, against coil overlap parameter, c[η], were made and trends in both the dilute and concentrated regimes were found to be similar to those seen with other polysaccharides. Dynamic frequency sweeps indicated that all acid-hydrolyzed starches in 90% DMSO/10% water exhibited dilute solution or liquid-like behavior with G″ higher than G′. A contradiction in classification of 6–12% starch dispersions was found with shear tests (concentrated) and small strain oscillatory tests (dilute).

Thermal behavior of gum arabic in comparison with cashew gum

Abstract Thermal behavior of gum arabic and cashew gum, the exudate polysaccharides from Acacia and Anacardium occidentale L., containing different gum/water concentrations: 100/0, 80/20, 60/40, 50/50, 40/60, 20/80 and 5/95% w/w, were studied by differential scanning calorimetry (DSC) and thermogravimetry (TG). DSC thermal profiles for gum arabic with low water content (0–40%) showed an endothermic event at about 90°C (Tonset) and with increasing water content (50–80%) multiple melting endotherms and associated enthalpies were observed. Similar behavior was found in the DSC curves of cashew gum with Tonset of about 97°C. TG curves show two decomposition stages, the major decomposition occurred at 252°C (Tonset) to arabic and cashew gums pure.

Residence time distribution in a holding tube

Abstract The residence time distribution (RTD) of two Newtonian (water and 12% sucrose solution) and two non-Newtonian liquids (0·2 and 0·4% guar gum solutions), at a minimum of five different flow rates each, was determined in a holding tube consisting of 10 sections with a total length of 33 m, at 25±0·5°C. The response to a pulse of NaCl solution was detected by a conductivity transmitter and recorded. The relative variances of the experimental RTD curves were used to evaluate the dispersion and the tanks-in-series models. In general, vessel dispersion numbers were higher in laminar flow. For water in laminar flow; the number of tanks-in-series ranged between 14 and 20, whereas in turbulent flow the number was between 61 and 103. For the sugar solution the number of tanks-in-series was higher. For the 0·2% gum solution data the number of tanks was estimated to be between 18 and 34, while for the 0·4% gum solution data it was estimated to be between 7 and 12. Neither model can be said to be better than the other.

Convective Heat Transfer to Fluid Foods in Cans

Publisher Summary The chapter explains about the convection heating of foods in cans. Canning is the effective means to preserve large part of food supply. Heating characteristics of food products based on conduction and convection are illustrated. Conduction heated products exhibit slower heating rates and longer lag times and convection-heated products exhibit faster heating rates and shorter lag times. Methodologies to study convective heat transfer to foods are explained. The heat transfer rates to liquid foods are influenced by flow conditions such as viscosity, geometry, velocity of flow, and physical and thermal properties of liquid. Flow properties of Non-Newtonian fluids and viscoelastic fluids are also discussed. Heat transfer by natural convection takes place by conduction through a stagnant film of liquid on the inside surface of the can. Various factors affecting heat transfer by forced convection are explained in details. Methods of heat transfer in orbitor retorts, spin cooker, flame sterilization etc. are clearly illustrated. The process of heat transfer by convection in the presence of particulate matter is explained in detail. Heat is first transferred to the fluid and then to the particle. The presence of particulate matter will alter the flow patterns and contribute to the mixing of the cans contents due to their motion. The chapter also emphases the need for experimental studies that are applicable to non-Newtonian foods. It also illustrates the importance of dimensionless correlation in the field of heat transfer.

The isolation and characterization of a water extract of konjac flour gum

Abstract An aqueous extract from konjac flour was dried, milled and redispersed in water, prior to its characterization by viscometry and rheometry. The rate of change of relative viscosity with reciprocal absolute temperature was of a magnitude less than that of some other polar polysaccharides by a factor approximating 2. The volume-concentration relationship was non-linear in water, but linear in 0·04 m tartaric acid. The hydrophilicity of this extract in 0·04 m tartaric acid was only 0·68 g g −1 solute/100 g dispersion, much less than that of many of the commonly used polysaccharides. The intrinsic viscosity was 1320 ml g −1 , among the highest of the polysaccharides. Considering the relatively low water affinity, the high viscosity of the konjac extract was attributed mostly to solute-solute interaction, rather than to hydration, at functional use-levels. The dried, redispersed extract was characterized in both steady and dynamic shear by the Carreau and Cross equations, and by the Cox-Merz rule. Deviation from the Cox-Merz rule was attributed to molecular associations of time scales longer than non-specific physical entanglements.