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The chain reorganization of cooked starch during storage plays an important role in the performance of starchy products such as rice foods. Here, different analytical techniques (such as small-angle X-ray scattering) were used to reveal how microwave cooking influences the chain assembly of cooked indica rice starch undergoing storage for 0, 24, or 48 h. While stored, more short-range double helices, long-range crystallites, and nanoscale orders emerged for the microwave-cooked starch than for its conventionally cooked counterpart. For instance, after storage for 24 h, the microwave-cooked starch contained 46.8% of double helices, while its conventionally cooked counterpart possessed 34.3% of double helices. This could be related to the fact that the microwave field caused high-frequency movements of polar groups such as hydroxyls, which strengthened the interactions between starch chains and water molecules and eventually their assembly into double helices, crystallites, and nanoscale orders. This work provides further insights into the chain reassembly of microwave-cooked starch undergoing storage, which is closely related to the quality attributes of starch-based products. [ABSTRACT FROM AUTHOR]

Cold-chain cooked rice is a widely-consumed instant food. While the quality of cooked rice as affected by processing has been widely studied, it remains largely unexplored how concurrent cold-chain conditions (e.g., refrigeration time with specific water contents) tailor the structure and starch digestibility of cooked rice. Here, as shown by combined techniques (e.g., scanning electron microscopy and small angle X-ray scattering), the cold storage (1 to 3 days) of cooked rice at 1.1:1 w /w water-to-rice ratio increased the uniformity of the rice matrix, strengthened the nonperiodic structure, and allowed more B-type starch crystallites and short-range orders. This induced an increase in the slowly digestible starch (SDS) content (from ca. 33.7% to 38.5%) as the refrigeration time rose. In contrast, for cooked rice with 1.5:1 w/w water-to-rice ratio, the cold storage (mainly 1 day) strengthened the matrix uniformity and the nonperiodic structure, and eventually increased the resistant starch (RS) content from ca. 10.3% to 17.7%. The present data could facilitate the design of cold-chain cooked rice with tailored starch digestibility. • Refrigeration increased slowly digestible starch of cooked rice at 1.1:1 w /w water content. • Refrigeration increased resistant starch of cooked rice at 1.5:1 w /w water content. • Refrigeration enhanced uniformity of rice matrices at varied water contents. • Refrigeration strengthened nonperiodic structure of cooked rice at 1.1:1 w/w water content. • Refrigeration led to more starch orders of cooked rice at 1.1:1 w/w water content. [ABSTRACT FROM AUTHOR]

This study explored how microwave reheating (to about 73 °C at different power levels) affects the microstructure and digestion characteristics of cooked rice with different water contents (1.1 and 1.5 times that of rice in weight). Irrespective of water content, mainly the V-type crystallites remained after microwaving reheating, with slight changes in other multi-scale structural features. Only at a relatively high water content (1.5) and with a power level high enough could short-range order be reduced. Such microwave reheating increased the digestion resistance of cooked rice. At a water content of 1.1 times, increasing the microwave power led to a decreased rapid digestible starch (RDS) content and an increased resistant starch (RS) content. With a higher water content (1.5), the enhancement of digestion resistance with higher microwave power was less significant but still, a reduced slowly digestible starch (SDS) content and a higher RS content were observed. [Display omitted] • Cooked rice with high water contents were microwaved at different power levels. • V-type crystallites remained after microwaving reheating. • Microwave reheating increased the digestion resistance of cooked rice. • Rice with 110% water content had increased RS content with reduced RDS content. • Rice with 150% water content had increased RS content with reduced SDS content. [ABSTRACT FROM AUTHOR]

This work concerns how starch-protein interplay affects the multi-scale structures (e.g., short- and long-range orders, nanoscale structure and morphology) of starch undergoing thermal processing (pasting) involving heating and cooling at high water content. An indica rice starch (IRS) and three proteins (whey protein isolate, WPI; soy protein isolate, SPI; casein, CS) were used. By inspecting rheological profiles of mixed systems before and after adding chemicals, IRS-WPI and IRS-CS showed mainly hydrophobic molecular interaction; and IRS-SPI exhibited hydrophobic, hydrogen bonding and electrostatic interactions. The RVA results revealed that, with starch and proteins as controls, starch-globular protein (WPI or SPI) interplay accelerated the swelling of starch granules (faster viscosity increase at initial pasting stage), and reduced the paste stability during heating (higher breakdown) and during cooling (higher setback); however, the starch-casein interactions resulted in opposed effects. Moreover, starch-protein interactions varied the multi-scale chain reassembly of starch into different structures during cooling. Observed could be fewer short- and long-range starch orders, and larger nonperiod structure (or colloidal clusters) on the nanoscale. On even larger scale to micron, IRS-globular protein molecules generated larger grids (with reduced number) in the gel network, and IRS-casein formed a more continuous gel network with less prominent tunnel-like features. • Starch-globular protein interaction accelerated starch's water uptake during heating. • Starch-casein interaction stabilized swollen granules and paste stability during heating. • Starch-protein interaction reduced the amount of ordered structures during cooling. • Starch-protein interaction allowed larger or more nanostructures during cooling. • Starch-casein formed a more continuous matrix in the gel network on micron scale. [ABSTRACT FROM AUTHOR]

Colored brown rice shows better health benefits than polished rice, and there is great interest in the design of demanded cooking manners for brown rice. This investigation concerns how the change of cooking mode can slow starch digestion of colored brown rice by inspecting mainly starch structural changes during cooking. Compared to cooking Mode 1, the soaking of Mode 2, at higher temperature, probably enhanced water diffusion into rice matrices, thus strengthening starch lamellae without apparently altering rice morphology and ordered structures such as starch polymorphs and helices. The followed heating of Mode 2, with reduced time, more sufficiently disrupted rice structures, causing less prominent nonperiodic molecular organizations. At the final cooking stage, the braising of Mode 2, having faster temperature reduction for prolonged time, leading to similar rice morphology, reduced molecular orders (e.g., helices) and probably denser amorphous regions of nonperiodic organizations. Such features tended to increase packing density of molecular chains (mainly starch) in rice matrices, thus slowing the enzyme diffusion in rice matrices and associated binding and catalysis events. In this way, a reduced starch digestion rate and increased resistant starch (RS) and slowly digestible starch (SDS) were seen for the rice following Mode 2 cooking. • Mode 2 cooking slowed starch digestion of cooked colored brown rice. • Mode 2 soaking stage (higher temperature) enhanced starch semicrystalline lamellae. • Mode 2 heating stage (shorter time) caused less prominent nonperiodic organizations. • Mode 2 braising stage probably allowed higher chain packing density of rice matrices. [ABSTRACT FROM AUTHOR]

This work concerns the multi-scale structures and digestion behaviors of indica rice starches with apparent amylose contents (AAC s) of ca. 14%–24%. A higher AAC not always allowed a lower digestion rate (k), different from previous findings that starches containing more amylose normally show slowed digestion. Here, the proportion of stable molecular orders (melting temperature above ca. 73 °C) and AAC synergistically governed starch digestion rate. With a similar AAC , a higher amount of stable orders (R h/l , ratio of enthalpy of stable orders to that of the rest) led to a lower k. Also, with a similar R h/l , a larger AAC caused a lower k. Consistently, JPZ starch had almost the highest AAC and R h/l , showing the lowest k. Regarding this, amylose and stable orders played roles in increasing the bulk density of starch structure matrices, and eventually slow the diffusion, absorption and catalysis events of digestion enzymes within the matrices. • Starches contained varied amounts of stable molecular orders melted above 73 °C. • Stable orders and amylose content synergistically affect starch digestion rate. • With similar amylose content, more stable orders reduced starch digestion rate. • With similar stable orders, more amylose reduced starch digestion rate. [ABSTRACT FROM AUTHOR]

• Microwave cooking-storage induced more orders than did conventional cooking-storage. • Microwave cooking-storage induced more amorphous-crystalline structures. • Microwave cooking-storage increased the content of slowly digestible starch. • Microwave cooking-storage lowered the digestion rate of starch. • The evolutions in starch digestion features were enhanced by the increased amylose. By inspecting starch hierarchical structural evolutions, this work explores how microwave cooking with storage tailors the slowly digestible features of indica (IRS) and waxy (WRS) rice starches. Particularly, relative to conventional cooking-storage, the microwave treatment especially at high powers (8 and 10 W/g) increased the molecular orders (crystallites) and periodic amorphous-crystalline structures. Such changes facilitated the formation of domains with intermediately-densely packed starch chains, being modestly accessible to the diffusion, absorption and catalysis of enzymes and dominantly showing slowly digestible features. Consistently, the microwave processed starches showed a higher SDS (slowly digestible starch) level and a lower digestion rate. This microwave-enhanced SDS generation became more prominent for IRS, and the treated IRS at 10 W/g showed the highest SDS content (ca. 54%). This is related to the enhanced reassembly of glucan chains into ordered/semicrystalline structures and the formation of slowly digestible domains as induced by the increase of amylose molecules. [ABSTRACT FROM AUTHOR]

From a supramolecular structure viewpoint, we concern how concurrent microwave and heat moisture treatment (M/HMT) tailors the pasting and digestion features of indica rice starch (IRS) and waxy rice starch (WRS), using combined analytical methods such as scanning electron microscopy and small-angle X-ray scattering. The results confirm that, for the starches (especially IRS), M/HMT led to a concave granule surface, weakened alignment of helices in lamellae, loosened packing of chains in amorphous regions, and somewhat disrupted crystallites. Consequently, the susceptibility of starch structures to hydrothermal effects was insignificantly altered, as reflected by unchanged pasting temperature; the enzyme absorption/hydrolysis could be properly accelerated, as affirmed by a modestly increased digestion rate and slightly reduced enzymatic resistance. Also, compared to WRS, IRS had more amylose with relatively long branches, which probably contributed to starch chain interactions during M/HMT. This tended to reduce the paste viscosity related to the swelling degree of starch granules, and increase the paste stabilities during heating and cooling related to the robustness of swelled granules and the reorganization of glucan chains. • Concurrent microwave-heat moisture treatment (M/HMT) reduced starch paste viscosity. • M/HMT strengthened starch paste stabilities during heating and cooling. • M/HMT properly increased the digestion rate and slowly digestible fraction of starch. • M/HMT moderately disrupt glucan chain assembly in starch supramolecular structure. [ABSTRACT FROM AUTHOR]

To date, how the pH conditions of thermal processing tailor the structure and digestibility of resulting starch-based complexes remains largely unclear. Here, indica rice starch (IRS), stearic acid (SA), and a whey protein isolate (WPI) were used as materials. Increasing the pH value from 4 to 8 during thermal processing (pasting) mainly suppressed the starch digestion of starch-WPI-SA complexes rather than starch-SA counterparts. The starch-SA complexes showed moderate structural changes as the pH value rose, and there was less rapidly digestible starch (RDS) only at pH 8. For the starch-WPI-SA complexes, an increased pH value allowed larger nonperiodic structures and more V-type starch crystallites, with almost unchanged short-range orders but apparently collapsed networks at pH 8. Such ternary complexes displayed more resistant starch (RS) as the pH value rose. The ternary sample at pH 8 contained ca. 29.87% of the RS fractions. [Display omitted] [ABSTRACT FROM AUTHOR]

The rice starch following microwave cooking with storage showed more slowly digestible starch and a lower digestion rate than did the conventionally treated counterpart. The underlying mechanism was interpreted by inspecting starch multi-level structural evolutions during digestion. Accompanying digestion, not only were starch matrices digested, leading to porous substrate and probably less polymorphs and nanoscale orders, but also starch chain reassembly occurred, causing A to B crystalline transform for untreated starch and formation of new molecular organization (repeat length: 5 nm) for treated starches. Hence, the digestion for native and treated starches was governed by concurrent matrix hydrolysis and molecular reassembly during digestion. The ultimate digestion of a specific structure was affected by the state of structural system. Unlike common views, the polymorphs in processed starches without native architecture were preferentially digested. Also, compared to conventionally treated counterpart, the microwave treated starch exhibited enhanced molecular reassembly during digestion, eventually displaying stronger slowly digestible features. Image 1 • Starch digestion was governed by concurrent matrix hydrolysis and chain reassembly. • Polymorphs in processed starch could be preferentially digested. • Microwave treated starch showed enhanced molecular reassembly during digestion. [ABSTRACT FROM AUTHOR]