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- Comparative characterization of whey protein concentrates from ovine, caprine and bovine breedsPublication . Pintado, Manuela E.; Silva, J.A. Lopes da; Malcata, F. XavierWhey protein concentrates (WPC) obtained from ovine, caprine and bovine wheys were studied in terms of chemical composition (concentration of lactose, protein, moisture, ash, calcium, sodium and potassium) and physical properties (onset denaturation temperature and viscoelastic behaviour). Ovine, caprine and bovine WPC exhibited decreasing protein content and increasing lactose content, in this order. The differences in ash content were relatively small, but ovine and caprine WPC were quite different from bovine WPC in terms of calcium and potassium. Important differences in protein content have also been observed; ovine WPC showed the highest concentration in the protein bgr -lactoglobulin, bovine WPC the highest concentration in the protein agr - lactalbumin and caprine the highest concentration in the protein immunoglobulin G. No significant differences existed in terms of denaturation onset temperature between the three types of WPC; however, differences were found within WPC types between those cases where NaCl was present or absent. The gel maturation at 85°C was characterized by storage moduli that were close to one another for ovine and caprine WPC, but these were far from those of bovine WPC. The decrease in temperature from 85 to 20°C had a strong effect on the viscoelastic moduli of WPC gels, but the initial structure could be recovered by reheating the gels from 20 to 85°C.
- Studies on a purification method for locust bean gum by precipitation with isopropanolPublication . Silva, J. A. Lopes da; Gonçalves, M. P.A commercial sample of locust bean gum was dissolved in hot water, centrifuged, and the supernatant and the sediment were recovered. Part of the supernatant was freeze-dried and the rest was poured into isopropanol to yield a precipitate of purified gum. This precipitate was recovered and a part of it was subjected to a second purification with isopropanol; an almost pure galactomannan sample was obtained. Another sample of the crude gum was fractionated on the basis of its solubility in water at different temperatures. Five fractions were obtained with mannose to galactose ratios (M/G) increasing with the temperature of fractionation. The purified samples exhibited higher M/G ratios and number average molecular weights (Mn) than the crude gum, whereas the opposite was observed with the freeze-dried sample. The polydispersity index decreased quite significantly in purified samples, meaning that they were more homogeneous than the original sample. Intrinsic viscosities were lower for the purified samplesv The flow behaviour of aqueous solutions of the gums was studied in the concentration range 0.6–1.2 g/dl: the solutions of the crude gum had higher viscosities than those of the purified and the freeze-dried gums at similar concentrations, throughout the shear rate range.
- Kinetics and thermal behaviour of the structure formation process in HMP/sucrose gelationPublication . Silva, J. A. Lopes da; Gonçalves, M. P.; Rao, M. A.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 non-equilibrium 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.
- Viscoelastic behaviour of mixtures of locust bean gum and pectin dispersionsPublication . Silva, J. A. Lopes da; Gonçalves, M. P.; Rao, M. A.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.
- Rheological study into the ageing process of high methoxyl pectin/sucrose aqueous gelsPublication . Silva, J. A. Lopes da; Gonçalves, M. P.The ageing process of high methoxyl pectin (HMP)/sucrose gels was followed at different ageing temperatures by small amplitude oscillatory experiments. Dynamic mechanical measurements allowed the characterisation of the point at which the system undergoes the sol/gel transition. The HMP/sucrose system is extremely sensitive to temperature variation during ageing, especially in the lower temperature range. The viscoelastic behaviour through the gel point changes with the ageing temperature, probably due to variations in mobility of the pectin chains, and consequently, in the lifetime of junction zones. Weaker pectin networks are formed under thermal conditions unfavourable to the development of hydrophobic interactions. Gel time and elastic modulus have a complex dependence on temperature, which could be attributed to the different thermal behaviour of the intermolecular interactions that stabilise the nonpermanent cross links of these physical networks.
- Microbiological and rheological studies on Portuguese kefir grainsPublication . Pintado, Manuela E.; Silva, J. A. Lopes da; Fernandes, Paulo B.; Malcata, F. Xavier; Hogg, TimThe native bacteria and yeasts present in Portuguese kefir grains stored under four distinct sets of environmental conditions have been isolated and identified on the basis of morphology and biochemical tests. The microbial population of the kefir grains as a whole has been characterized in terms of rates of biomass production and formation of lactic acid and ethanol. The rheological properties of the purified polysaccharide (kefiran) produced by the microflora of the grains and accumulated therein were studied in a low water activity solvent and as a component of a binary gel containing either k-carrageenan or xanthan gum.
- Characterization of requeijão and technological optimization of its manufacturing processPublication . Pintado, M. E.; Silva, J. A. Lopes da; Malcata, F. X.In attempts to characterize Portuguese whey cheese (Requeijão) and optimize the manufacture thereof I7 whey cheeses were produced according to a factorial design using heating time, heating temperature and fractional addition of ovinelcaprine milk as manipulated technological variables. Chemical analyses were carried out for the 17 cheeses, whereas sensorial and rheological analyses were carried out for eight selected whey cheeses and a reference (i.e. a whey cheese produced locally according to traditional procedures). A true local maximum exists for moisture content (at a temperature of about 93°C heating time of about 30 min and addition of about 17% ovine milk) which lies well within the range chosen for experimentation. Fat content of Requeijão was positively affected by heating temperature (especially via its quadratic effect) and, to a lesser extent, by heating time (especially via its linear effect); nitrogen content was especially affected by heating temperature (via its quadratic effect); and moisture content was affected especially by heating temperature (via its quadratic effect). The sensorial analyses showed that the eight whey cheeses produced were prefered with respect to the reference whey cheese. For rheological analyses the most significant observations pertain to the high strain dependence of the dynamic moduli, absence of a true equilibrium storage modulus, and relatively low difference between the loss and the storage modulus.
- Influence of temperature on the dynamic and steady-shear rheology of pectin dispersionsPublication . Silva, J. A. Lopes da; Gonçalves, M. P.; Rao, M. A.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.
- Effect of galactomannans on the viscoelastic behaviour of pectin/calcium networksPublication . Silva, J. A. Lopes da; Gonçalves, M. P.; Doublier, J. L.; Axelos, M. A. V.An investigation was carried out on the effect of the addition of galactomannans to pectin/calcium networks with different structural and rheological characteristics. For those pectin/calcium gels characterized by an elastic equilibrium modulus, the addition of the galactomannan increased both the storage and loss mod&i, especially at short time scales. This increase was greater than that which could be expected by simple additivity of the viscoelastic properties of each isolated system. The pectin/calcium network remained the continuous gel matrix controlling the viscoelastic behaviour of these systems at low frequency. For pectin/calcium systems close to the sol-gel transition or at low pH, the mixed systems evolved towards the behaviour of Viscoelastic liquids in the presence of increasing concentration of the non-gelling polymer. These overall results suggest that there is no specific interaction and that the changes in the rheological properties of the pectin gels are due to galactomannan microphase separation limited by the entrapment of these macromolecules in the pectin-calcium network.
- Rheological characterization under shear of a fraction of polymer produced via fermentation of whey-related media by Rahnella aquatilisPublication . Pintado, M. E.; Silva, J. A. Lopes da; Pintado, Ana I. E.; Malcata, F. XavierProduction of lactan, a polysaccharide composed of mannose, galactose and galacturonic acid (at the ratio of 5:3:2), starting from a semidefined medium containing lactose via fermentation with Rahnella aquatilis was described previously. In this communication, such polysaccharide was produced from five alternative fermentation media: (1) a synthetic (defined) medium, plain whey (under (2) aerobic and (3) anaerobic conditions), (4) whey permeate and (5) whey with 2% NaCl (w/v). The effect of the concentration of polysaccharide, pH and ionic strength at harvest on the rheological properties of the polysaccharide was studied using lactan-enriched fractions recovered from each medium and analysed in solution under steady shear flow. Lactan solutions showed a shear-thinning behaviour in all cases, and increases in viscosity were observed at increasing concentrations of polysaccharide, as expected. The polysaccharide fraction produced from whey with 2% (w/v) NaCl and plain whey under anaerobic conditions exhibited lower viscosity than that produced from the other media, an observation that is associated with the lower concentration of polysaccharide. Post-harvest addition of salts (KCl or CaCl2) and changes in pH (3–11) affected slightly the viscosity of the polysaccharide solutions. q1998