Characterization of phosphorylated cross-linked resistant starch by 31P nuclear magnetic resonance (31P NMR) spectroscopy

The structure of phosphate esters in resistant starch (RS4) is important to understand its digestibility by pancreatic α-amylase. Wheat starch was reacted at pH 11.5, 45 °C, and 3 h with 12% (starch basis, sb) of a 99/1 (w/w) mixture of sodium trimetaphosphate/sodium tripolyphosphate (STMP/STPP) to give modified starches with ∼0.4% phosphorus (P). Exhaustive digestion of the phosphorylated starch with α-amylase followed by amyloglucosidase gave the α,γ-dextrin whose 31P NMR spectrum indicated the presence of a ∼1/1 molar mixture of distarch monophosphate (DSMP) and monostarch monophosphate (MSMP). Approximately one-half of STMP/STPP was recovered unreacted after the typical reaction, and it was reacted with a second batch of starch after replenishing with 7% (sb) STMP/STPP. RS4 wheat starches prepared at pH 9.5–10.5 showed a decreasing ratio of DSMP/MSMP plus an increasing level of monostarch diphosphate (MSDP) and cyclic monostarch monophosphate as reaction pH decreased. Starch triphosphate ester was not detected in the RS4 products, all of which had been prepared at reaction times greater than three hours. A series of RS4 wheat starches were prepared where all contained ∼0.4% P but with varying levels of DSMP from 41% to 80% of organophosphates. The level of DSMP was positively correlated with the level of RS4 (r = 0.96) and with the level of total dietary fiber (r = 0.90).