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This work describes a method to study glass transition on native starch powders, based on dynamical mechanical thermal analysis using compression tests, and was applied to wheat flour (13.5% water content). This method will allow the determination of Tg in native (unprocessed) starchy materials, with minimal disturbance of the natural structures. The influence of the test conditions (heating rate, frequency and strain) on the glass transition measurements was determined using factorial designs. The values of Tg determined as the maxima of the energy dissipation (peaks in E″) of native flour and of freeze-dried pre-gelatinized flour were not statistically different (around 64 °C). The heating rate did not affect the measurements in the range tested (0.25 to 1 °C min−1). An interactive effect of the strain amplitude and the frequency was detected. The significance of this interaction can be caused by differences in mechanical energy dissipation, which would indicate that not only temperature but also the total energy input may affect this transition. Slight effects of phase separation between gluten and starch were found on native flour.
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The protein adsorption capacity of different materials was evaluated to assess their potential efficacy as alternative adsorbents for the removal of proteins from wines, with the purpose of finding suitable low-swelling materials that could be used in a percolated bed. The adsorbents tested were thermally treated sodium bentonite, low-swelling adsorbing clays, ion exchange resins and other protein adsorbents (silica gel, hydroxyapatite and alumina). The materials were evaluated by analysing both the capacity to stabilize untreated white wines, according to a heat test, and by characterizing the adsorption isotherms of bovine serum albumin (BSA) in a model wine (water, ethanol and K-bitartrate). Breakthrough curves in a packed bed were determined experimentally for some materials, showing the influence of the adsorption isotherm shape. Some ion-exchange resins showed a favourable behaviour and have good potential as alternative adsorbents.
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The inactivation kinetics of horseradish peroxidase were determined at several values of pH (3, 4, 7, 9 and 10), temperature (40 to 70 °C) and pressure (7, 8 and 9 kbar). The data were obtained by taking samples from the moment that pressure inside the hydrostatic high pressure vessels reached the value specified, and therefore relate to isothermal and isobaric conditions. The inactivation kinetics were non‐log linear and could be individually well described by the conventional two‐fraction model, but the overall consistency of the kinetic parameters of this model was poor and the model was clearly not robust. Inversely, the kinetic model based on the Weibull probability distribution function yielded good individual fits and also a good overall consistency of the kinetic parameters. The whole set of 327 data points could be well described by an overall model considering that the shape parameter (ß) varied with pressure but not with temperature and that the rate parameter (1/α) varied with temperature according to an Arrhenius law, with a pressure‐independent activation energy, and varied exponentially with pressure. The activation energy and the pressure‐sensitivity parameter did not vary with pH up to pH 9. The results suggested a discontinuity of the kinetic behaviour at pH 10, with a different inactivation mechanism prevailing.
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The kinetics of thermal degradation of ascorbic acid was studied in a Maltodextrin matrix at different temperatures between 75 and 140 °C. Isothermal experiments were performed with samples earlier equilibrated in environments of known aw at 4 °C, enclosed in hermetically sealed vials, with water contents (w) between 0.17 and 1.68 g water/g solids. Sampling times were selected according to an optimal experimental design in terms of minimum confidence regions of the parameters estimated, using the Bigelow model and estimates from preliminary experiments. The results showed a quadratic dependence of the reference D-value and linear dependence of the z-value with moisture content: Dr(140 °C) = 150.07 − 290.73w + 269w2 (min); z = 12.14 + 22.99w (°C) and showed the applicability of D-optimal designs for determining kinetic parameters in complex situations, with limited experimental requirements. The dependence of the rate constants with water content could not be described by the WLF model in qualitative terms, although in absolute values this model could be used with constants similar to those expected from glass transition theory. The thermodynamic analysis of the results showed a good application of the compensation theory in the whole range of water contents.
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Between 2000 and 2009
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