Water and starch chain mobility in cassava starch as monitored by NMR: Effects of heat-moisture treatments, growth conditions and harvest time
This study focuses on applying Nuclear Magnetic Resonance (NMR) to differentiate heterogeneous domains of starch Mobility and distribution of water and starch chains in cassava starch from crops grown in drought and rainy reasons, harvested at 6 months and 12 months, were investigated by deuterium (2H) solid state NMR and proton (1H) spin-spin relaxation time (T 2) distributions. Changes in water and starch chain mobility with heating, freezing, and acid hydrolysis were also studied.
In a very low moisture range (<5% dry basis), all water present was immobile (rigid) in both drought and rainy cassava (A-type crystals), and potato starches (B-type crystal). Thus, the amount of bound water was irrespective of degrees of crystallinity and crystal types. The “Monolayer” value estimated from water sorption isotherms (7–9% dry basis) over-estimated the level of bound water in starch (when compared with NMR data). With increasing water content, T, decreased to a minimum at a moisture range of 14–17% (dry basis) marking its onset of molecular plasticization of the starch chains. Packed beds of starch granules (54% water, dry basis) were subjected to heating and freezing treatments. NMR T2 distributions of water and starch protons were characterized with a continuous model. Intragranular water (T2 ∼ 3 ms, disappeared below 0°C) and extragranular water (T2 ∼ 20 ms) were observed. An extra long T2 (∼100 ms) population (postulated as water associated with leached amylose) was developed upon gelatinization.
The swelling and melting of starch from the drought crop were shifted to a higher temperature than those from the rainy crop resulting from a possible closer packing of rigid amylopectin and/or amylose chains. Upon gelatinization, rigid amylopectin (T2 ∼ 10 μs) disappeared, while the mobile amylopectin (in the amorphous region of semicrystalline lamellae, T2 ∼ 1 ms) increased. An additional fraction with very long T 2 (∼50 ms) emerged at 65°C and above indicating an increase in mobile/soluble starch.
This study demonstrated a possible use of NMR to monitor changes in starch chains and related water mobility in intact starch granules, which is a unique feature not offered by other methods.