M.P. Gonçalves

Solution properties of the galactomannans extracted from the seeds of Caesalpinia pulcherrima and Cassia javanica: comparison with locust bean gum

The galactomannans from the seeds of Caesalpinia pulcherrima and Cassia javanica were extracted from the milled seeds in water at room temperature. Both products, as well as a commercial sample of locust bean gum (LBG), were purified by precipitation in isopropyl alcohol. The intrinsic viscosity determined for LBG, [eta] = 15.2 dl/g, was slightly higher than those for the other two galactomannans. The dependence of the specific viscosity at zero shear rate on the coil overlap parameter, C[eta], revealed a similar behaviour for the three galactomannans. A master curve was obtained with a critical concentration, C*, at C*[eta] = 3.3. The slope of the curve in the concentrated regime is higher than the values in the range of 3.9-6.6, obtained for the generalized behaviour of several random coil polysaccharides. Dynamic experiments showed that, at the concentrations studied, the behaviour of the galactomannans was typical of systems with predominant entanglement networks in the region between the terminal and plateau zones of frequency response. The correlation between dynamic and steady shear properties (Cox Merz rule) was satisfactory for the three galactomannans.

Studies on a purification method for locust bean gum by precipitation with isopropanol

Abstract 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 (M n ) 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.

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.

Rheological study of the effect of Cassia javanica galactomannans on the heat-set gelation of a whey protein isolate at pH 7

Abstract The effect of Cassia gum galactomannan on the thermal gelation of a whey protein isolate (WPI) at 80 °C has been monitored by rheological dynamic measurements at fixed frequency and the mechanical spectra of the gels have been recorded at 80 °C and after quenching to 20 °C. The mechanical spectra were analysed quantitatively by fitting a Cole–Cole model to the storage and loss compliance versus frequency data. The concentration of WPI was 110 g/l and that of Cassia gum varied from 0 to 11 g/l. At low concentration, the presence of the galactomannan enhanced the aggregation rate and the strength of the protein gel but did not modify intrinsically the gelation process and the structure of the system. At higher galactomannan concentrations, the gels displayed substantially lower moduli values and the shape of the mechanical spectra was modified. This was tentatively interpreted as the consequence of a depletion-induced liquid–liquid phase separation occurring during the course of the aggregation process, responsible for the downturn observed on the loss modulus versus time curves monitored at 80 °C.

Influence of locust bean gum on the rheological properties of kappa-carrageenan systems in the vicinity of the gel point

Abstract Viscosity and oscillatory shear measurements have been performed on kappa-carrageenan alone and on kappa-carrageenan/galactomannan (guar gum or locust bean gum) blends in the vicinity of the sol-gel transition. From these measurements, a phase diagram showing the boundary limits for kappa-carrageenan/locust bean gum and kappa-carrageenan/guar gum was established. The viscoelastic behaviour at the transition showed that this process was sharp for the kappa-carrageenan/locust bean gum blend, while for the kappa-carrageenan/guar gum blend, it was more progressive. The kappa-carrageenan content at which the transition took place was estimated and was found to be constant, whatever the total polymer concentration of the mixture studied. This phase transition results in a system whose rheological properties are far removed from those of the original biopolymers. However, it is shown that kappa-carrageenan is primarily responsible for gel network formation.

Microstructure and flow behaviour of liquid water-gelatin-locust bean gum systems

Abstract Liquid water-gelatin-locust bean gum (LBG) systems, in the conditions of lowest compatibility (near the isoelectric pH of the protein), were explored using confocal laser scanning microscopy and viscometry. Confocal microscopy observation proved to be a more sensitive method to assess the phase state of the systems than the usual centrifugation or viscometric ones. It showed that in fact the system is already phase-separated well below the apparent binodal line determined by the other methods. When the continuous phase is enriched in LBG, the observed microstructure is of the expected type with spherical droplets of gelatin dispersed in the LBG phase. Bicontinuous systems were observed close to the inversion point, which is clearly detected by viscosity measurements; beyond this point, the gelatin enriched continuous phase contained large inclusions of LBG, apparently due to aggregation of the LBG disperse phase.For systems with a LBG enriched continuous phase, shear thinning behaviour, similar to that of a LBG solution, was observed; the low newtonian viscosity decreased as the volume fraction of the gelatin disperse phase increased, following approximately the usual logarithmic additivity law. In the reverse case, where the continuous phase is enriched in gelatin, flow curves with an apparent yield stress and a negative deviation from the logarithmic additivity law were observed. The variation of the viscosity with the volume fraction of the disperse phase obeyed approximately the equation of Palierne for systems of two viscoelastic phases when the interface is purely elastic.The microstructure of phase-separated mixtures depends on the phase viscosity ratio, which also determines the differences in flow behaviour.

Enzymatic Hydrolysis of Whey Protein Concentrates: Peptide HPLC Profiles

Abstract Hydrolysis of whey protein concentrates (WPCs) at different temperatures and pHs, using three enzymes: pepsin, trypsin, and Alcalase®, was monitored during more than 5 hr by reversed phase HPLC/UV, using a column containing a polystyrene‐divinylbenzene copolymer‐based packing, and an elution gradient from 8% to 80% acetonitrile containing 0.1% TFA. Peptides were separated according to their polarity and size, and degradation of α‐lactalbumin (α‐la) and β‐lactoglobulin (β‐lg) was evaluated. The three proteolytic enzymes (pepsin, trypsin, and Alcalase®) employed for hydrolysis of WPCs led to different kinetics of degradation of β‐lg. α‐la degradation after 15 min was almost complete for the three enzymes. The hydrolysis catalysed by each enzyme resulted in different peptide profiles by HPLC/UV. Hydrolysates produced by pepsin (HP) were resolved into three main fractions of high retention times, while tripsin hydrolysates (HT) were resolved into nine major peaks and Alcalase® hydrolysates (HA) were resolved into 12 major peaks, presenting a wide range of polarities and sizes. Although, with different β‐lg hydrolysis extension, chromatographic profiles of the degradation and formation of peptides can be used as a finger print of the type of enzyme used, because peptide profile is not affected either by temperature or pH.