Freeze-Drying Changes the Structure and Digestibility of B‑Polymorphic Starches Bin Zhang, Kai Wang, Jovin Hasjim, Enpeng Li, Bernadine M. Flanagan, Michael J. Gidley, and Sushil Dhital* Centre for Nutrition and Food Sciences, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Brisbane, Queensland 4072, Australia ABSTRACT: Starch granules both isolated from plants and used in foods or other products have typically been dried. Common food laboratory and industry practices include oven (heat), freeze, and ethanol (solvent-exchange) drying. Starch granules isolated from maize (A-type polymorph) and potato (B-type polymorph) were used to understand the effects of different dehydration methods on starch structure and in vitro digestion kinetics. Oven and ethanol drying do not significantly affect the digestion properties of starches compared with their counterparts that have never been dried. However, freeze-drying results in a significant increase in the digestion rate of potato starch but not maize starch. The structural and conformational changes of starch granules after drying were investigated at various length scales using scanning electron microscopy, confocal laser scanning microscopy, X-ray diffraction, FTIR spectroscopy, and NMR spectroscopy. Freeze-drying not only disrupts the surface morphology of potato starch granules (B-type polymorph), but also degrades both short- and long-range molecular order of the amylopectin, each of which can cause an increase in the digestion rate. In contrast to A-polymorphic starches, B-polymorphic starches are more disrupted by freeze-drying, with reductions of both short- and long-range molecular order. We propose that the low temperatures involved in freeze-drying compared with oven drying result in greater chain rigidity and lead to structural disorganization during water removal at both nanometer and micrometer length scales in B-type polymorphic starch granules, because of the different distribution of water within crystallites and the lack of pores and channels compared with A-type polymorphic starch granules. KEYWORDS: starch granules, in vitro digestion, drying, crystalline structure, granule morphology ■ INTRODUCTION Starch is the main energy reserve in many plants, comprising mainly two types of glucose polymers, namely, amylose and amylopectin, in the form of semicrystalline granules. Amylose is a primarily linear polymer consisting of α-1,4-linked D-glucose units with few long branches, whereas amylopectin is a highly branched polymer made up of mainly α-1,4 linkages and ∼5% α-1,6 linkages forming a large number of short branches. In starch granules, the branches of amylopectin are often in a double-helical conformation and contribute to the crystallinity of granules (normally between 15% and 45%), whereas amylose is considered to be largely in an amorphous conformation with some single-helical V-type crystalinity. 1 Starch granules isolated from various botanical origins show different shapes (spherical, oval, disk-shaped, etc.) and sizes (submicrometer to larger than 100 μm), as well as surface morphologies, such as the presence of pinholes that connect the hilum to the surface through interior channels. Furthermore, the amylose content, the branch chain length, and the molecular weight of starch molecules vary among starches from various botanical origins. These structural differences can influence the digestibility and other functional properties of starch. Starch granules are commercially available in dry form for extended shelf life and for potential savings on transport and storage costs, and thus a drying process is essential after starch granules have been isolated from plants, such as from grains, legumes, and tubers. Common food laboratory and industry practices for drying isolated starch granules include oven (heat) drying, freeze-drying, and ethanol (solvent-exchange) drying. Drying conditions have been reported to damage the surface and alter the interior structure of starch granules, eventually affecting their properties, such as chemical reactivity, gelatinization, retrogradation, and pasting properties. 2−6 In a previous study, Apinan et al. 7 found that freeze-dried potato starch granules displayed higher enzymatic susceptibility than heat-dried potato starch granules, which was explained to be caused by the alteration of the surface structure during the drying process. However, the effects of freeze-drying on molecular structure and packing are less understood and have not yet been reported. Because of the inherent structural and morphological di fferences between the A- and B-type polymorphic starches, conclusions drawn from B-type poly- morphic starches, such as potato starch, might not represent the mechanism for A-type polymorphic starches, such as maize and wheat starches. Thus, the objective of the present study was to understand the changes in molecular, crystalline, and granular structure (including surface morphology) of starch granules caused by drying processes (oven, freeze, and ethanol drying) and how they affect the digestibility of starch granules. Starch granules were isolated from mature and immature maize kernels and mature potato tubers before being subjected to Received: November 17, 2013 Revised: January 20, 2014 Accepted: January 28, 2014 Published: January 28, 2014 Article pubs.acs.org/JAFC © 2014 American Chemical Society 1482 dx.doi.org/10.1021/jf405196m | J. Agric. Food Chem. 2014, 62, 1482−1491