title: Molecular, mesoscopic and microscopic structure evolution during amylase digestion of maize starch granules.
abstract: Cereal starch granules with high (>50%) amylose content are a promising source of nutritionally desirable resistant starch, i.e. starch that escapes digestion in the small intestine, but the structural features responsible are not fully understood. We report the effects of partial enzyme digestion of maize starch granules on amylopectin branch length profiles, double and single helix contents, gelatinisation properties, crystallinity and lamellar periodicity. Comparing results for three maize starches (27, 57, and 84% amylose) that differ in both structural features and amylase-sensitivity allows conclusions to be drawn concerning the rate-determining features operating under the digestion conditions used. All starches are found to be digested by a side-by-side mechanism in which there is no major preference during enzyme attack for amylopectin branch lengths, helix form, crystallinity or lamellar organisation. We conclude that the major factor controlling enzyme susceptibility is granule architecture, with shorter length scales not playing a major role as inferred from the largely invariant nature of numerous structural measures during the digestion process (XRD, NMR, SAXS, DSC, FACE). Results are consistent with digestion rates being controlled by restricted diffusion of enzymes within densely packed granular structures, with an effective surface area for enzyme attack determined by external dimensions (57 or 84% amylose - relatively slow) or internal channels and pores (27% amylose - relatively fast). Although the process of granule digestion is to a first approximation non-discriminatory with respect to structure at molecular and mesoscopic length scales, secondary effects noted include (i) partial crystallisation of V-type helices during digestion of 27% amylose starch, (ii) preferential hydrolysis of long amylopectin branches during the early stage hydrolysis of 27% and 57% but not 84% amylose starches, linked with disruption of lamellar repeating structure and (iii) partial B-type recrystallisation after prolonged enzyme incubation for 57% and 84% amylose starches but not 27% amylose starch.
authors.affiliation: Centre for Nutrition and Food Sciences and ARC Centre of Excellence in Plant Cell Walls, The University of Queensland, Hartley Teakle Building, St. Lucia, Brisbane, Qld 4072, Australia; School of Science and Health, University of Western Sydney, Hawkesbury Campus, Richmond, NSW, Australia; Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DSC, NSW 2232, Australia.
authors.forename: Ashok K
authors.initials: AK
authors.lastname: Shrestha
authors.affiliation: Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DSC, NSW 2232, Australia; CSIRO, Food Futures National Research Flagship, Riverside Corporate Park, North Ryde, NSW, Australia.
authors.forename: Jaroslav
authors.initials: J
authors.lastname: Blazek
authors.affiliation: Centre for Nutrition and Food Sciences and ARC Centre of Excellence in Plant Cell Walls, The University of Queensland, Hartley Teakle Building, St. Lucia, Brisbane, Qld 4072, Australia.
authors.forename: Bernadine M
authors.initials: BM
authors.lastname: Flanagan
authors.affiliation: Centre for Nutrition and Food Sciences and ARC Centre of Excellence in Plant Cell Walls, The University of Queensland, Hartley Teakle Building, St. Lucia, Brisbane, Qld 4072, Australia.
authors.forename: Sushil
authors.initials: S
authors.lastname: Dhital
authors.affiliation: CSIRO, Food Futures National Research Flagship, Riverside Corporate Park, North Ryde, NSW, Australia; CSIRO Division of Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia.
authors.forename: Oscar
authors.initials: O
authors.lastname: Larroque
authors.affiliation: CSIRO, Food Futures National Research Flagship, Riverside Corporate Park, North Ryde, NSW, Australia; CSIRO Division of Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia.
authors.forename: Matthew K
authors.initials: MK
authors.lastname: Morell
authors.affiliation: Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DSC, NSW 2232, Australia.
authors.forename: Elliot P
authors.initials: EP
authors.lastname: Gilbert
authors.affiliation: Centre for Nutrition and Food Sciences and ARC Centre of Excellence in Plant Cell Walls, The University of Queensland, Hartley Teakle Building, St. Lucia, Brisbane, Qld 4072, Australia.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Gidley
mesh_terms: D000681:Amylases / Q000378:metabolism*; D000687:Amylopectin / Q000737:chemistry / Q000378:metabolism; D000688:Amylose / Q000737:chemistry / Q000378:metabolism; D004063:Digestion; D006801:Humans; D006868:Hydrolysis; D053838:Scattering, Small Angle; D013213:Starch / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure*; D014961:X-Ray Diffraction; D003313:Zea mays / Q000737:chemistry / Q000378:metabolism*
chemical_list: D013213:Starch; D000688:Amylose; D000687:Amylopectin; D000681:Amylases