Characterisation of resistant starch from wheat and maize

Biochemical and compositional studies were made of resistant starch (RS) and other starch fractions derived from wheat, amylomaize and waxy maize starches, as a function of various pretreatments. Operationally, RS was isolated as the material that remained as a solid residue from digestions of starch fractions with alpha-amylase and pullulanase. These crude preparations of RS contained adventitious protein (alpha-amylase), and lipid. More extensively purified RS, obtained from wheat starch, contained essentially no protein and had a lipid content which was less than 35 % of that in the original starch. Purified RS was largely carbohydrate, of which the sugar composition was greater than 96 % glucose. Gel permeation chromatography of crude and purified wheat RS and crude amylomaize RS, solubilised with 90% dimethylsulphoxide, showed a distribution of low molecular weight glucans with a peak degree of polymerisation (DP) at about 60. Incubation of wheat RS with the debranehing enzyme, isoamylase, led to no change in apparent DP, indicating that RS was composed of linear α-glucans: spectrophotometric observations of iodine binding were consistent with this conclusion. RS could also be obtained from gelatinised waxy maize starch that had been debranched with pullulanase; in this case the subsequent autoclaving and amylolysis gave resistant linear α-glucans of lower DP (~40 at peak). By contrast, branched α-glucans from Lintnerised waxy maize starch gave only very low levels of RS, even after autoclaving.