Carbohydrate Polymers 115 (2015) 305–316 Contents lists available at ScienceDirect Carbohydrate Polymers j ourna l ho me page: www.elsevier.com/locate/carbpol Rice starch granule amylolysis – Differentiating effects of particle size, morphology, thermal properties and crystalline polymorph Sushil Dhital a , Vito M. Butardo Jr. b,1 , Stephen A. Jobling b , Michael J. Gidley a,∗ a Centre for Nutrition and Food Sciences, ARC Centre of Excellence in Plant Cell Walls, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, St Lucia, QLD 4072, Australia b Plant Industry and Food Futures Flagship, Commonwealth Scientific and Industrial Research Organisation (CSIRO), P.O. Box 1600, Canberra, ACT 2601, Australia a r t i c l e i n f o Article history: Received 21 July 2014 Received in revised form 12 August 2014 Accepted 13 August 2014 Available online 2 September 2014 Keywords: Amylopectin Amylose Amylose Extender Digestibility High amylose Starch structure a b s t r a c t The underlying mechanism of amylolysis of rice starch granules was investigated using isolated starch granules from wild-type, as well as SBEIIb mutant and down-regulated lines. Fused granule agglomerates isolated from mutant and transgenic lines were hydrolysed at similar rates by amylases, and had similar crystalline patterns and thermal properties as individual granules. Surface pores, a feature previously only reported for A-polymorphic starch granules, were also observed in B- and C-polymorphic rice starch granules. Although the microscopic patterns of hydrolysis among granules with different crystalline polymorphs were qualitatively similar, the extent and the rate of amylolysis were different, suggesting that B-type crystalline polymorphs are intrinsically more resistant to enzymatic hydrolysis than A-type in rice starch granules. It is proposed that the slightly longer branch lengths of amylopectin which leads to the formation of more stable B-type double helical structures compared to their A-type counterparts is the major parameter, with other factors such as granule size, surface pores and interior channels having secondary roles, in determining the rate of enzymatic hydrolysis of rice starch granules. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction The rate and extent of enzymatic hydrolysis of starch has attracted much attention not only due to its central role in convert- ing starch to glucose as the major source of energy in human and animal diets, but also as an important biological (e.g. germination of grains and sprouting of tubers) and industrial process (e.g. syrups and bio-ethanol production). The action of amylolytic enzymes on starch granules may be affected by various factors including granule morphology (e.g. shape and size, presence of compound granules), surface features (e.g. surface damage, pores leading to interior channels, and other as yet incompletely characterised zones of enzyme susceptibility), presence of non-starch components (e.g. proteins, lipids and cell wall remnants), and molecular compo- sition and conformation (e.g. size and amount of amylose and ∗ Corresponding author. Tel.: +61 7 33652145; fax: +61 7 33651177. E-mail addresses: s.dhital@uq.edu.au (S. Dhital), v.butardo@irri.org (V.M. Butardo Jr.), steve.jobling@csiro.au (S.A. Jobling), m.gidley@uq.edu.au, mike.gidley@uq.edu.au (M.J. Gidley). 1 Present address: International Rice Research Institute, Los Ba˜ nos 4031, Philippines. amylopectin, type and amount of crystallinity) as described else- where (Bird, Lopez-Rubio, Shrestha, & Gidley, 2009; Colonna, Leloup, & Buleon, 1992; Gallant, Bouchet, Buleon, & Perez, 1992). However, the relative importance of these factors has been difficult to determine as they are typically correlated with each other, and there is a lack of systems available for which individual factors vary whilst other factors are kept constant. Starch has a broad array of granule sizes ranging from ca. 1 m (e.g. rice, amaranth, and quinoa starch) to more than 100 m (e.g. potato and canna starch). Smaller granules, either from differ- ent botanical origins (Fukai, Takaki, & Kobayashi, 1994; Ring, Gee, Whittam, Orford, & Johnson, 1988) or fractionated from the same origin (Dhital, Shrestha, & Gidley, 2010; Franco & Ciacco, 1992; Franco, Ciacco, & Tavares, 1998; Noda et al., 2005; Tang, Ando, Watanabe, Takeda, & Mitsunaga, 2001), are found to be more sus- ceptible to amylolysis than their larger counterparts, consistent with the relatively higher surface area per unit mass available for enzyme adsorption (Warren, Royall, Gaisford, Butterworth, & Ellis, 2011). Cracks, holes or surface damage in granules can further increase the effective surface area enhancing the rate of enzy- matic adsorption and binding to macromolecules. Cereal starches such as maize and sorghum are known to have naturally occurring surface pores and interior channels (Fannon, Hauber, & BeMiller, http://dx.doi.org/10.1016/j.carbpol.2014.08.091 0144-8617/© 2014 Elsevier Ltd. All rights reserved.