Abstract
Experimental protocols aiming at the characterisation of glass transition often suffer from ambiguity. The ambition of the present study is to describe the glass transition in a complex, micro heterogeneous system, the dry rice pasta, in a most unambiguous manner, minimising the influence of technique-specific bias. To this end, we apply an unprecedented combination of experimental techniques. Apart from the usually used NMR and DSC, we employ, in a concurrent manner, neutron transmission, diffraction, and Compton scattering. This enables us to investigate the glass transition over a range of spatio-temporal scales that stretches over seven orders of magnitude. The results obtained by neutron diffraction and DSC reveal that dry rice pasta is almost entirely amorphous. Moreover, the glass transition is evidenced by neutron transmission and diffraction data and manifested as a significant decrease of the average sample number density in the temperature range between 40 and 60 °C. At the microscopic level, our NMR, neutron transmission and Compton scattering results provide evidence of changes in the secondary structure of the starch within the dry rice pasta accompanying the glass transition, whereby the long-range order provided by the polymer structure within the starch present in the dry rice pasta is partially lost.
Highlights
The glass transition concept, adapted from the science of polymer materials, has established itself very well to test the quality and stability of concentrated foods, i.e., low moisture content foods
Several physio-chemical and thermo-mechanical properties of food can change across a glass transition, and they form a physical basis of the determination of Tg by techniques such as differential scanning calorimetry (DSC), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), rheological methods, or dielectric analysis
The results obtained by neutron diffraction and DSC revealed that dry rice pasta is almost entirely amorphous
Summary
The glass transition concept, adapted from the science of polymer materials, has established itself very well to test the quality and stability of concentrated foods, i.e., low moisture content foods. The high sensitivity of the relaxation times to glass transition has allowed for the introduction of an entirely new concept in NMR studies of food, the NMR phase diagram, which has been extensively used in the context of shelf-life stability of complex food systems [14]. The temperature of change in the second moment and Tg determined classically by the DSC method may differ by at least several degrees due to the fact that the glass transition temperature of a given system does not have a unique value but rather depends on the cooling rates used to observe Tg, sample composition and its processing [13,15,16,18]. In more complex food systems, a broader glass transition temperature range may be observed by different methods, or some methods may even fail to determine it
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