Fruit phase transition temperature

In the case of homologous series of ohgo- and polysaccharides, Tg and Tg increase with the molecular weight up to a certain hmit (Fig. 0.6). Table 0.8 hsts the phase transition temperatures Tg of some fruits and vegetables. 

Table 0.8. Phase transition temperature Tg of some fruits and vegetables...

Figure 15 Dependence of the structural order parameter of DPH in microsomal membranes derived from chiUing-resistant tomato fruit (a) and chilling-sensitive tomato fruit (b). The phase transition temperature of these biological membranes is an indication of the susceptibility of the fruit to cold temperature damage. (Data reanalyzed from Ref. 22.)...

Poly-N-isopropylacrylamide (NIPAM) solution in water precipitates above 40 °C, so NIP AM gels have provided the most fruitful example of a thermally driven phase change. A swollen cross-linked NIPAM gel deswells at the same temperature, 40 °C. Hompolymer gels show a discontinuous phase transition while copolymer gels with ionizable groups such as acrylic acid can show a continuous transition or critical point behavior. Having resolved the thermodynamics of the system, the kinetics of the phase change become important. 

Thermal analysis of polymeric materials can be particularly fruitful for the analyst. Melting points, phase transitions, pyrolysis, and curing conditions can all be gleaned not only from the temperature positions in DTA (DSC), but also from the width of endothermic and exothermic peaks. In addition, it is often possible to analyze gases that are liberated from the sample by gas chromatographs and mass spectrometers. 

The results of DSC analyses of freeze-dried plum (skin and pulp at the natural proportion) presented different behaviors for each domain. At Uy, 0.75, two glass transitions (Tg) were visible (Figure 58.1a) as a deviation in base line and shifted toward lower temperatures with increasing moisture content and caused by the plasticizing effect of water (Slade and Levine, 1991). The first one, clearly visible at lower temperatures, was attributed to the glass transition of a matrix formed by sugars and water. The second one, less visible and less plasticized by water, was probably caused by macromolecules of the fruit pulp. Two Tg are normally visible in systems formed by blends of polymers (Verghoogt et al., 1994) and in edible films (Sobral et al., 2002) caused by phase separation between polymers and between proteins and plasticizers, respectively. However, Sobral et al. (2001) and Telis and Sobral (2002) also observed two Tg for persimmon and tomato, respectively, at low domain. 

A number of kinetic isotope studies have been reported for gas-phase eliminations. Isopropyl bromide-dg decomposes more slowly than isopropyl bromide and the intramolecular isotope effects (A ,c2D4hx-c2D4)/ (C2D4hx-c2D.iH)) have been recorded for pyrolyses of ethyl acetate (2.0), chloride (2.20), and bromide (2.10) at 500°C. At the elevated reaction temperatures, these values correspond to the maximum predicted for complete loss of the C-H stretching vibration and they have been interpreted as indicating considerable weakening of the C-H bond in the transition state. Whether this is a homolytic or hetero-lytic bond fission, it is remarkably insensitive to beta substituent effects on rate. The intramolecular isotope effects in these cases could reflect predominantly a secondary isotope effect rather than the intended primary effect and dissection into an intermolecular and secondary isotope effect would prove more fruitful. (Section 2.2.1.)... 

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