1H NMR studies of molecular mobility in potato systems in relation to non-enzymatic browning

NURIA ACEVEDO; CAROLINA SCHEBOR; MARÍA DEL PILAR BUERA

Water Properties in Food, Health, Pharmaceutical and Biological Systems

Blackwell

Año: 2010; p. 437 - 443

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES; mso-fareast-language:ES;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Pulsed 1H NMR was used to determine molecular mobility of water and polymer in freeze-dried potato starch samples in relation to the sorption properties of the system. Native starch was obtained from fresh white potatoes. Potatoes were grinded and starch was extracted from the juice after rinsing with distilled water. An aqueous suspension was freeze-dried to obtain the dried starch powder. Water sorption data were determined at 25°C in a range of 43-98% R.H.. Thermal transitions were determined by differential scanning calorimetry. The states of water in freeze-dried potato starch were analyzed by 1H NMR CPMG measurements. The analysis of rapidly relaxing components following a single 90º pulse experiments (FID) were used to assess polymer mobility. The CPMG analysis gave a short component of the order of 0.05 to 0.1 milliseconds attributed to the `bound´ water (“multilayer” water) and a long component in the range of 0.5 to 3.5 milliseconds, whose value linearly increases with the water content up to aw= 0.84. These samples correspond to the second region of the sorption isotherm. This increase was independent of temperature in a range between 25 and 55ºC and of the physical state of the sample. The proportion of the long component increased with the increase in relative humidity, suggesting that it is directly related to the mobility of water. Above aw= 0.84 the long component T2 remains “constant”, and this samples correspond to the third region of the sorption isotherm. Up to aw= 0.94 there was no presence of freezable water as observed by DSC analysis. At higher aw values (above 0.97) the long component T2 increases markedly (above 7.7 ms) and freezable water was detected by DSC. The FID analysis showed a single T2 component which was associated to the mobility of the polymer chains. There was an increase in T2 with the increase in RH and temperature; however a clear change in the T2 values could not be identified around Tg.1H NMR could be employed to determine the water states of freeze-dried potato starch samples and relate them to the different regions of the sorption isotherm.