Browsing by Author "Shrestha, AK"

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    Effects of processing high amylose maize starches under controlled conditions on structural organisation and amylase digestibility
    (Elsevier, 2009-01-22) Htoon, AK; Shrestha, AK; Flanagan, BM; Lopez-Rubio, A; Bird, AR; Gilbert, EP; Gidley, MJ
    The amylase digestibility of high-amylose maize starches has been compared before and after thermo-mechanical processing. Starches were analysed for enzyme-resistant starch yield, apparent amylose content, crystallinity (X-ray diffraction), and molecular order (NMR and FTIR), both before and after treatment with (x-amylase. All samples had significant (>10%) enzyme-resistant starch levels irrespective of the type and extent of thermal or enzymic processing. Molecular or crystalline order was not a pre-requisite for enzyme resistance. Near-amorphous forms of high amylose maize starches are likely to undergo recrystallisation during the enzyme-digestion process. The mechanism of enzyme resistance of granular high-amylose starches is found to be qualitatively different to that for processed high-amylose starches. For all samples, measured levels of enzyme resistance are due to the interruption of a slow digestion process, rather than the presence of completely indigestible material. © 2008, Elsevier Ltd.
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    Enzyme resistance and structural organization in extruded high amylose maize starch
    (Elsevier, 2010-05-05) Shrestha, AK; Ng, CS; Lopez-Rubio, A; Blazek, J; Gilbert, EP; Gidley, MJ
    Gelose 80, a high amylose maize starch, was extruded in a twin screw extruder at different feed moistures, cooled, stored for 12 days at 4°C, and cryo-milled. The raw and extruded starches were analysed for in vitro enzyme-resistant starch content (ERS), morphology, in vitro digestibility, long range (X-ray diffraction) and short range (FTIR) molecular order. Extrusion markedly increased the rate of starch digestion and reduced the ERS content, irrespective of whether B-type or B- and V-type polymorphs were present. Increasing feed moisture and storage resulted in only slight increases in ERS content, with X-ray diffraction and FTIR also showing small changes in long and short range molecular order, respectively. Analysis of residues from in vitro digestion showed the mechanism of enzyme resistance of granular and extruded high amylose starch to be markedly different, both qualitatively and quantitatively. Enzyme digestion of granular high amylose maize starch showed initial disorganization in structure followed by slow reorganization at later stages of digestion. In contrast, molecular reorganization took place throughout the enzyme digestion of extruded high amylose maize starch. Higher levels of crystallinity were found in digested extrudates compared with digested granules, showing that there is no direct correlation between starch crystallinity and enzyme digestion rates. © 2009, Elsevier Ltd.
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    Molecular rearrangement of starch during in vitro digestion: toward a better understanding of enzyme resistant starch formation in processed starches
    (American Chemical Society, 2008-07) Lopez-Rubio, A; Flanagan, BM; Shrestha, AK; Gidley, MJ; Gilbert, EP
    Resistant starch (RS) is defined as the fraction of starch that escapes digestion in the small intestine, serving as a fermentation substrate for beneficial colonic bacteria. Several studies have been focused on the description of the RS fractions from different starch varieties, but little attention has been paid to the digestion process itself that, from the present work, seems to play a key role in the generation of enzyme-RS (ERS), as determined in vitro. High-amylose starch samples, extruded at two different processing conditions, have been characterized at different stages of in vitro digestion using scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), infrared spectroscopy (FF-IR), solid state C-13 NMR spectroscopy, and X-ray diffraction (XRD). Control samples kept for 18 h in the digestion solution without starch hydrolyzing enzymes (alpha-amylase and amyloglucosidase) were used for comparison purposes. An increase in molecular order was favored by the hydrolytic action of the enzymes, reflected in an increase in double helical order observed by NMR, higher crystallinity measured by XRD, and corresponding changes in FT-IR spectra. An increase in the intensity of the scattering objects was also observed by SAXS as a function of digestion. SAXS from the dry ERS fractions reveals the 001 reflection of crystallites formed during the digestion process, corresponding to a characteristic dimension of the resistant crystalline fraction of similar to 5 nm. The changes found suggest that enzyme resistant starch does not refer to a specific structure present in predigested starches, but may in fact be formed during the digestion process through the rearrangement of amylose chains into enzyme-resistant structures of higher crystallinity. Therefore, the resistance to enzyme digestion of a specific processed starch is the result of a competition between the kinetics of enzyme hydrolysis and the kinetics of amylose retrogradation. © 2008, American Chemical Society
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    Molecular, mesoscopic and microscopic structure evolution during amylase digestion of maize starch granules
    (Elsevier Science Ltd, 2012-09-01) Shrestha, AK; Blazek, J; Flanagan, BM; Dhital, S; Larroque, O; Morell, MK; Gilbert, EP; Gidley, MJ
    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. © 2012, Elsevier Ltd.