The quantitative thermal analysis and interpretation on a molecular level have shown that starch has a surprising multistage glass transition which stretches over 100 degrees. Even dry starch has such a very broad partial transition which starts at about 250K, far below the decomposition temperature where the glass transition is not yet completed. Adding water, as in food applications, moves this transition to lower temperatures. This is the plasticizing effect of water which allows the study of more of the transition, and is, naturally, also the base of the application of starches in the food industry. Three steps were observed in starch-water systems (11-17 wt-% water). The first is indicated by a gradual deviation from the vibrational heat capacity of the glassy starch. The second is a jump in Cp, followed by the third, another deviation from linear Cp. The latter two are due to conformational and anharmonic motions. This new research resolves the old problem of gel-starch aging with moisture (11wt%) during food storage. Amorphous starch at room temperature absorbs 10-12 wt-% water. This starts at the gradual beginning of the transition and is far from the jump in Cp at about 372 K and ultimately leads to a semicrystalline starch. A similar, low-temperature beginning seems to exist in the glass-transition behavior of other biomaterials, such as proteins and poly(amino acid)s.
This work was supported by the Division of Materials Research, National Science Foundation, Polymers Program, Grant # DMR-0312233. Some use of equipment and laboratory space was provided by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC, for the U.S. Department of Energy, under contract number DOE-AC05-00OR22725.
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