Ageing of Starch Based Systems as Observed with FT-IR and Solid State NMR Spectroscopy
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0038-9056 |
| Online Access: | http://www.viks.sk/chk/star_11_12_98_478_483.doc |
| Abstract: | Summaries. The retrogradation and physical ageing of model starch systems with respect to their glass transition temperatures Tg have been investigated by Fourier transform infrared spectroscopy and solid state NMR spectroscopy. Diffuse reflectance Fourier transform infrared (DRIFT) spectra demonstrate the commencing retrogradation of starch materials stored above their Tg by changes in peak lineshapes and intensities in the characteristic area between 995cm-1 and 1020cm-1. Solid state NMR proton relaxation times in the rotating frame (proton Tl) show a characteristic course in relation to the storage conditions (time, humidity), for which a distinction is made between physical ageing which occurs below the Tg and recrystallisation (retrogradation) which takes place above Tg. The proton Tl's of materials stored below Tg increase asymptotically in time due to physical ageing, whereas the proton Tl's of materials stored above Tg increase until a moisture content is reached that rises them above Tg, decrease due to further water absorption and then increase due to recrystallisation (retrogradation). Conclusions. FT-IR and solid state NMR spectroscopy are good techniques for observing physical ageing and retrogradation by means of spectral changes in lineshapes and linewidths and by the determination of relaxation times. In FT-IR spectroscopy changes in the area of 1050cm-1 and 995cm-1 imply-the retrogradation of starch based materials stored above Tg. Proton Tl relaxation times are very sensitive to changes in the degree of crystallinity. domain size, molecular arrangement and the moisture content during retrogradation. The influence of the storage humidity in relation to the Tg can be monitored via the relaxation times. Below the Tg the decrease in free volume results in an asymptotic increase in relaxation times. During water absorption the relaxation times are suggested to increase until the Tg is at about room temperature. Above the Tg further absorption of water increases the mobility of starch until the equilibrium moisture content has been reached. During and after this period retrogradation takes place and the relaxation times increase because of the development of crystallinity. |
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| ISSN: | ISSN 0038-9056 |
