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

Abstract

The structure of phosphate esters in resistant starch (RS 4) 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. RS 4 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 RS 4 products, all of which had been prepared at reaction times greater than three hours. A series of RS 4 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 RS 4 ( r = 0.96) and with the level of total dietary fiber ( r = 0.90).