The development and application of nuclear magnetic cross relaxation spectroscopy to model and retrograding starch systems

An examination of the methods for nuclear magnetic cross relaxation spectroscopy (CRS) data collection and analysis was conducted using water and an aqueous waxy corn starch suspension to better perform and interpret the results obtained using CRS. The CRS data collection properties evaluated were the time to achieve steady state saturation, the direct saturation of liquid protons, generation of transverse magnetization, and the dependence of the offset frequency and RF field strength of longitudinal relaxation in the presence of radio frequency (RF) saturation. Effects were evaluated for variations of input values of RF saturation field strength, apparent cross relaxation rate, and solid longitudinal relaxation rate on the results for solid content and solid internal mobility from fitting NMR data to modified theoretical expressions. Conditions under which acceptable results can be obtained with limited data sets are discussed. The retrogradation of 5, 10, 15, and 25% dent corn starch was measured with DSC, rheology, and NMR spectroscopy techniques. From this analysis the retrogradation of normal dent starch can be divided into two components: (1) the amylose or fast (<24 hr) component and (2) the amylopectin or slow (>24 hr) component. During the initial stage of retrogradation, an increase in G′ corresponding to an increase in was observed for all four of the concentrations. During the latter stage of retrogradation, amylopectin recrystallization becomes the dominant process as measured by an increase in ΔH for the 25% starch gel, which corresponds to an increase in $MB0$. Also, an increase in G′ was observed for the first 2 days. A decrease in the molecular mobility of the liquid component was observed by decreases in 17O T₂, 1H D₀, and T_2A. Even though several of the other parameters did not measure changes in the starch gels over time, $MB0$ continued to decrease over time for all four starch concentrations. The value for T_2B did not change over time or concentration indicating that the mobility of the solid component does not change over time. A better understand the retrogradation of low solid systems was obtained by collectively relating the results from DSC, rheology, and NMR.