Modelling of physical ageing in starch using the TNM equation.

Gelatinised wheat starch, freeze dried and equilibrated at different RH, was aged at different temperatures and for different times. The Tool-Narayanaswamy-Moynihan (TNM) model was used to describe the ageing for all samples under all conditions. Three TNM parameters: x, Δh* and A were determined experimentally using, respectively, the peak shift method (x) and the dependency of T'(f) (the limiting value of T(f)) on the cooling rate (Δh* and A). The non-linearity parameter x and the non exponential parameter ß were also estimated by optimising a fit of the experimental normalised specific heat at different ageing times and temperatures to curves generated using the TNM model. The TNM model successfully described the normalised experimental data. It was found that the intermolecular forces were strong and the relaxation times depended more strongly on the glass structure than the glass temperature. The hydration level of the starch had a direct impact of the breadth of the relaxation time distribution. A dependency of the non-linearity parameter x on ageing temperature (peak shift method) was observed. This suggests that physical ageing is more complex than is described by TNM formalism.

Modeling of temperature-induced near-infrared and low-field time-domain nuclear magnetic resonance spectral variation: chemometric prediction of limonene and water content in spray-dried delivery systems.

The influence of temperature on near-infrared (NIR) and nuclear magnetic resonance (NMR) spectroscopy complicates the industrial applications of both spectroscopic methods. The focus of this study is to analyze and model the effect of temperature variation on NIR spectra and NMR relaxation data. Different multivariate methods were tested for constructing robust prediction models based on NIR and NMR data acquired at various temperatures. Data were acquired on model spray-dried limonene systems at five temperatures in the range from 20 degrees C to 60 degrees C and partial least squares (PLS) regression models were computed for limonene and water predictions. The predictive ability of the models computed on the NIR spectra (acquired at various temperatures) improved significantly when data were preprocessed using extended inverted signal correction (EISC). The average PLS regression prediction error was reduced to 0.2%, corresponding to 1.9% and 3.4% of the full range of limonene and water reference values, respectively. The removal of variation induced by temperature prior to calibration, by direct orthogonalization (DO), slightly enhanced the predictive ability of the models based on NMR data. Bilinear PLS models, with implicit inclusion of the temperature, enabled limonene and water predictions by NMR with an error of 0.3% (corresponding to 2.8% and 7.0% of the full range of limonene and water). For NMR, and in contrast to the NIR results, modeling the data using multi-way N-PLS improved the models' performance. N-PLS models, in which temperature was included as an extra variable, enabled more accurate prediction, especially for limonene (prediction error was reduced to 0.2%). Overall, this study proved that it is possible to develop models for limonene and water content prediction based on NIR and NMR data, independent of the measurement temperature.

Solvent-induced lysozyme gels: effects of system composition and temperature on structural and dynamic characteristics.

The gelation process of lysozyme in water/tetramethylurea in the presence of salt was investigated as a function of temperature and system composition by rheology, infrared spectroscopy, and microcalorimetry. Times and temperatures of gelation were determined from the variation of the storage (G') and loss (G'') moduli. It was found that gelation times follow exponential decays with both protein and tetramethylurea (TMU) concentrations and with temperature. The activation energy for the overall process shows a linear dependence on TMU mass fraction. A strongly increased beta-sheet content and reduced alpha-helix occur with the increase of TMU concentration in the binary solvent. Also, a linear decrease of lysozyme denaturation temperature and enthalpy on TMU concentration is found for the TMU mass fraction up to 0.5, above which no denaturation signal can be detected.

The effect of thermal history on the maillard reaction in a glassy matrix.

To test whether the extent of physical aging affected the reaction rate, Maillard reaction kinetics were studied in glassy model preservation systems subjected to two different thermal histories. The glass transition temperature and physical aging of the matrix were determined using differential scanning calorimetry, and the normalized heat capacities were modeled using the Tool-Narayanaswamy-Moynihan approach. Samples prepared using the different thermal histories initially had different degrees of aging, but these were practically indistinguishable after 10 h under the reaction conditions (65 degrees C); the samples underwent rapid structural relaxation at that temperature. The reaction of glucose and lysine in an amorphous trehalose/sucrose matrix was followed using spectrophotometric and chromatographic analysis. A difference in reaction rate could only be distinguished in the rate of consumption of glucose, which was approximately 20% faster in the minimally aged matrix; no significant differences were seen in any other indicator of reaction.

Physical aging of starch, maltodextrin, and maltose.

The physical aging of low water content, amorphous starch/water, maltodextrin/water, and maltose/water mixtures in the glassy state was examined using mechanical testing and calorimetry. Stress relaxation measurements showed that upon storage of the glassy materials there was a time-dependent increase in both flexural modulus and mechanical relaxation time. The mechanical relaxation time increased with depth of quench below the calorimetric glass transition temperature and with aging time at the quench temperature. Calorimetry of the aged materials showed an overshoot in heat capacity in the vicinity of the glass transition. The logarithm of the mechanical relaxation time showed a simple linear relationship with the size of the overshoot expressed as an enthalpy change. The calorimetric behavior could be modeled using the Tool-Narayanaswamy-Moynihan method.

Enthalpy relaxation of bovine serum albumin and implications for its storage in the glassy state.

Two endothermic peaks could be observed for five commercial samples of bovine serum albumin (BSA). The smaller peak observed by differential scanning calorimetry (DSC) corresponded to enthalpy relaxation. This peak was followed on storage of BSA, in its glassy state, after it had been heated above its denaturation temperature. Enthalpy and peak temperature increased with duration of storage. On storage for one week at 60 degrees C, a sample at 8.3% moisture showed a peak at 100 degrees C with an energy value of approximately 2 J per g protein. BSA samples were heated within the DSC sufficiently to eliminate the lower enthalpy peak but without altering the denaturation enthotherm. The amount of physical aging shown by these BSA samples was similar to that of the heat-denatured samples. It was concluded that the heating endotherms of dry BSA reflect both the storage and thermal history of the sample. Possible implications of the enthalpy relaxation of BSA on the behavior of this important protein are considered.

Comparative study of the retrogradation of intermediate water content waxy maize, wheat, and potato starches.

The retrogradation of extruded starches from three different botanical sources was studied in concentrated conditions (34 +/- 1% water) at 25 degrees C using differential scanning calorimetry (DSC) and isothermal calorimetry, Fourier transform infrared spectroscopy (FTIR), and wide-angle X-ray scattering. Potato starch showed the highest rate of retrogradation (approximately 0.17 h(-1)) followed by waxy maize (approximately 0.12 h(-1)), while the retrogradation of wheat starch was the slowest (approximately 0.05 h(-1)). In addition to the kinetics, the extent of molecular order in the retrograded samples was studied in detail in terms of "short-range" (helical) and "long-range" (crystalline) distance scales. The amylopectin crystallinity indices were essentially the same (approximately 47-51% amylopectin basis) for the three starches. However, significant differences were found in the enthalpy of melting measured by DSC after "full" retrogradation (potato, 11.6 +/- 0.7; waxy maize, 9.0 +/- 0.5; and wheat, 6.1 +/- 0.3 J/g of amylopectin). The degree of short-range molecular order in the retrograded state determined by FTIR was waxy maize > potato > wheat. The effect of amylopectin average chain length and the polymorphism of the crystalline phase were taken into account to explain the differences in the retrogradation enthalpies.

FTIR study of state and phase transitions of low moisture sucrose and lactose.

Mid-infrared spectra of freeze-dried sucrose and lactose systems were acquired over a range of temperatures (30-200 degrees C) and water contents (0-6.3%). Starting from the glassy state, the experimental conditions were selected to cover the main thermal transitions: the glass-rubber transition, the crystallisation and, for some samples, the subsequent melting. The FTIR spectra were very sensitive to the physical state. While subtle but systematic spectral differences between the glassy and rubbery states were detectable throughout the spectrum, a very pronounced increase in spectral resolution was observed as crystallisation occurred and was followed by the expected spectral broadening during melting. The temperatures at which these changes occurred were in satisfactory agreement with the transition temperatures measured by differential scanning calorimetry (DSC). The increase in molecular mobility as a result of increasing temperature or plasticisation by water led to a significant shift of the O-H stretching band to higher wavenumbers indicating a weakening of hydrogen bonding. This shift reached a maximum as the DSC measured crystallisation temperature range was approached. As expected, the crystallisation led to a highly effective hydrogen bonding network. This was more significant for lactose than for sucrose. No significant step change in hydrogen bonding was observed at Tg. As anticipated, the temperature at which these transitions occurred decreased with increasing water content but overlapped when observed in the context of the shifted temperature (T-Tg).

Monitoring chemical and physical changes during thermal flavor generation.

On-line techniques were developed to monitor chemical and physical changes occurring during the heating of skim milk powder (SMP). Atmospheric pressure chemical ionization mass spectrometry (APCIMS) followed the generation and release of volatile compounds from SMP in a packed-bed reactor. Operating conditions were optimized to avoid condensation of high boiling compounds such as maltol, and the system was highly reproducible (CV < 7%). Differential scanning calorimetry (DSC) of SMP identified a potential glass transition at an onset temperature of 67.9 degrees C and a series of exothermic events that were related to different stages of the Maillard reaction. No lactose crystallization was found after heating. Using a heated stage reflectance FTIR device, spectra were obtained at different temperatures. Analysis of the data showed a correlation between the intensity ratio at wavenumbers 1017 and 1064 cm(-1) and the glass transition measured by DSC. This FTIR system was not sensitive enough to detect Maillard intermediates. Combining data from the three techniques provides a fuller picture of the physical changes during the Maillard reaction and their effects on the chemical reactions.