Molecular mobility in sugar glasses

<p>Glasses are liquids that exhibit solid state behavior as a result of their extremely high viscosity. Regarding their application to foods, glasses play a role in the preservation of foods, due to their high viscosity and the concomitant low molecular mobility. This thesis focuses on sugar glasses. Sugar glasses are relevant as model systems for foods that contain sugars and have a low water content and/or that are frozen, since in both types the sugars can exist in the glassy state. Often, the stability of these types of foods can be attributed to the stability of the sugar glasses. Key factors controlling the stability are e.g. water content and temperature.</p><p>The work presented in this thesis aims at relating the stability of sugar glasses to molecular mobility, as a function of water content and temperature. More specifically, molecular mobility in sugar-water glasses was studied using two magnetic resonance techniques and subsequently related that to stability data obtained from the literature. Using the first technique, i.e. saturation transfer electron spin resonance (ST-ESR), the rotational mobility of a spin probe, added to the sugar-water mixture, was obtained. Using the second technique, a proton magnetic resonance ( <sup>1</SUP>H-NMR) technique, the relaxation behavior of the protons was determined. This is a measure of rotational proton mobility of both sugar and water molecules.</p><p>After the general introduction in chapter 1, a new data analysis of the ST-ESR spectra is proposed in chapter 2. This analysis has been developed to extend the range of rotational correlation times (<img src="/wda/abstracts/TAU.GIF" height="10" width="8" ALT="tau"/><sub>R</sub> ) up to values around 10 <sup>+4</SUP>s. This is necessary to obtain the<img src="/wda/abstracts/TAU.GIF" height="10" width="8" ALT="tau"/><sub>R</sub> values of very slow rotating spin probes, as present in a glassy material. The new data analysis of the saturation transfer ESR spectra is based on the diffusion as well as the recovery of saturation in competition with the field modulation. The previous data analysis was based on taking into account only the influence of diffusion on the spectra and, since the spectra are not sensitive for rotational diffusion with values of<img src="/wda/abstracts/TAU.GIF" height="10" width="8" ALT="tau"/><sub>R</sub> >10 <sup>-3</SUP>s, this was the limiting<img src="/wda/abstracts/TAU.GIF" height="10" width="8" ALT="tau"/><sub>R</sub> value. The new data analysis takes into account that the spectra are sensitive for the recovery process (T <sub>1</sub> ). This recovery process is linearly correlated with<img src="/wda/abstracts/TAU.GIF" height="10" width="8" ALT="tau"/><sub>R</sub> . In this way, the new analysis of saturation transfer ESR data provides values for very slow rotational mobilities up to<img src="/wda/abstracts/TAU.GIF" height="10" width="8" ALT="tau"/><sub>R</sub> values of around 10 <sup>+4</SUP>s.</p><p>Chapter 3 describes the use of the <sup>1</SUP>H-NMR technique to obtain molecular mobility of the water and sugar protons as a function of water content and temperature, in maltose-water glasses. In the <sup>1</SUP>H-NMR signal, slow decaying and fast decaying fractions of protons are distinguished, arising from mobile and immobile (<img src="/wda/abstracts/TAU.GIF" height="10" width="8" ALT="tau"/><sub>R</sub> > 3*10 <sup>-6</SUP>) protons, respectively. The assignment of mobile and immobile proton fractions to water and maltose protons is temperature dependent. Roughly however, water protons can be considered as mobile (except below T <sub>g</sub> ) and maltose protons as immobile (becoming mobile above T <sub>g</sub> ). By analyzing the relaxation behavior of the mobile protons, the mobility of the water molecules was determined. The method of second moments yielded information on the anisotropic mobility as well as proton density of the immobile protons. The mobility of both water and sugar protons were found to increase with temperature. The glass transition is found to be characterized by a continuous increase in mobility. Upon increasing water content, both sugar and water molecules are found to become more mobile.</p><p>In Chapter 4, both magnetic resonance techniques are applied to various sugar-water glasses with varying water content, as a function of temperature. In order to compare the mobility in different glasses, T <sub>g</sub> was taken as the reference point. By increasing the water content from 10 to 30 wt %, the spin probe mobility at T <sub>g</sub> was found to decrease, while the water mobility at T <sub>g</sub> was found to increase. This apparent paradox is explained on the basis of our experimental findings as follows. If water is added to a sugar glass, the average distance between the sugar molecules increases, leading to an increase in water mobility at T <sub>g</sub> . However the overall packing of water and sugar becomes denser at first, with increasing water content as deduced from Fourier transform infrared spectroscopy experiments. Thus the spin probe is hindered more in its rotation, leading to decreasing spin probe mobility at T <sub>g</sub> . The increasing amount of water yields on the other hand a larger mobility of the water molecules. We note that this effect of molecular packing is also exemplified by comparing the effects of carbohydrate molecules with increasing molecular weight (ranging from glucose up to maltoheptaose), while keeping the water content constant. Both water and spin probe mobility at T <sub>g</sub> increase upon increasing the molecular weight of the malto-oligomer. This can be explained by the fact that larger oligomers form less densely packed networks resulting in an increasing spin probe and water mobility.</p><p>Chapter 5 focuses on a second transition in mobility of the sugar protons. In concentrated glucose glasses this second transition occurs about 20 to 30 degrees above the glass transition temperature. Its dependence on water content is shown as a new line in the state diagram of glucose-water mixtures. In freeze concentrated glucose glasses a similar second transition is found. The second transition is interpreted as the so-called crossover temperature, where the dynamics changes from solid like to liquid like. In freeze concentrated glasses an increase in the amount of ice melting per degree is observed just above the temperature of the second transition. It is proposed that both in concentrated and freeze concentrated glucose glasses, this second transition relates to so-called collapse phenomena in sugar-water mixtures.</p><p>In Chapters 4 and 5, two different aspects of stability of sugar glasses have been related to molecular mobility. The long-term storage stability relates to molecular mobility <em>in</em> the glassy state (Chapter 4). It is shown how sugar, water and spin probe mobility in the glassy state depends on water content and type of sugar. The short-term stability, e.g. important during processing, relates to molecular mobility at the collapse temperature, i.e. ±20 degrees above T <sub>g</sub> . In Chapter 5, it is concluded that the origin of collapse phenomena signifies an abrupt increase in molecular mobility of the sugar protons at the collapse temperature. In conclusion, effects of water, temperature and type of sugar on molecular mobility have been established and related to aspects of stability of sugar glasses.</p&gt