Isomalto/malto-polysaccharide structure in relation to the structural properties of starch substrates

•The amount of α-(1→6) linkages is consistently higher in IMMP LMW fractions.•GTFB-ΔN modification is limited by α-(1→4,6) linkages in amylopectin and amylose.•Amylose is essential for the incorporation of α-(1→6) linkages in amylopectin.•The GTFB-ΔN enzyme solely hydrolyses amylopectin in the absence of amylose. Isomalto/malto-polysaccharides (IMMPs) are soluble dietary fibres produced by the enzymatic modification of starch with 4,6-α-glucanotransferase (GTFB). The structure, size, and linkage distribution of these IMMPs has remained largely unknown, since most structural information has been based on indirect measurements such as total α-(1→6) content, iodine staining and GTFB hydrolytic activity. This study provides a deeper understanding of IMMP structure in relation to its respective starch substrate, by combining preparative fractionation with linkage compositition analysis. IMMPs were produced from a variety of amylose-rich and amylose-free starches. The extent of modification was investigated per IMMP molecular weight (Mw)-fraction, distinguishing between linear α-(1→6) linkages introduced by GTFB and starch’s native α-(1→4,6) branching points. It emerged that the amount of α-(1→6) linkages was consistently higher in IMMP low Mw-fractions and that GTFB activity was limited by native α-(1→4,6) linkages. The presence of amylose turned out to be a prerequisite for the incorporation of linear α-(1→6) linkages in amylopectin.