Rotating Tempering Reshapes Rice Starch Structure in Rice-Type-Dependent Ways

Rice kernels gently rotated in a temperature-controlled drum during tempering leave a measurable imprint on the starch later extracted from them, according to a study published in Current Research in Food Science. The treatment, called low-speed rotational thermostatic tempering (LSRTT), was applied to intact kernels of indica, japonica, and waxy rice at 24 degrees Celsius, 50 revolutions per minute, and 2 hours, after adjusting each type to a moisture content of 26 percent on a wet basis. The kernels were then milled, and starch was isolated from the resulting flour using an alkaline dispersion and centrifugation procedure. The central finding is that a single, uniformly applied tempering protocol produced a shared increase in structural order across all three starches, yet the downstream functional consequences diverged sharply depending on rice type.

The structural changes were consistent at two different scales of organization. X-ray diffraction showed that every sample retained its dominant A-type crystalline pattern, with the characteristic peaks near 15, 17, 18, and 23 degrees 2-theta still present after treatment. Relative crystallinity, however, rose substantially: from 16.99 to 23.87 percent in indica starch, from 15.24 to 21.67 percent in japonica starch, and from 18.59 to 27.99 percent in waxy starch, corresponding to gains of 6.88, 6.43, and 9.40 percentage points respectively. Fourier transform infrared spectroscopy told a complementary story. In the deconvoluted 800 to 1200 per centimeter region, the ratio of the 1047 to the 1022 per centimeter band, an index of short-range molecular order, increased in all three starches, and no new absorption bands appeared. Together, the two techniques indicate greater ordering at both crystalline and local molecular scales.

The molecular-level measurements revealed that the three rice types did not respond identically. Gel permeation chromatography coupled with refractive index and multi-angle laser light scattering showed that weight- and number-average molecular weights decreased in tempered indica and tempered waxy starches but increased in tempered japonica starch. The polydispersity index rose from 1.86 to 2.08 in indica and from 1.22 to 1.29 in waxy, while it declined slightly from 2.27 to 2.25 in japonica. Amylose content, measured with a commercial assay kit, increased slightly in all three starches after treatment. The authors caution that these measurements do not directly establish chain cleavage or the formation of amylose-like structures, since kernel tempering, milling, and starch isolation could each influence which molecular fractions are recovered.

The amylopectin chain-length distribution, determined by ion chromatography after debranching, also shifted in a rice-type-dependent manner. All samples showed a typical bimodal profile with a primary peak at degree of polymerization 13 to 14 and a secondary peak at 38 to 42. In japonica starch, the proportions of A chains and B1 chains decreased while B2 and B3-plus chains increased; waxy starch showed the opposite pattern, with A and B1 chains rising and longer chains falling. Indica starch changed only slightly. The authors note that changes in relative proportions alone do not demonstrate ligation of short branches, cleavage of long branches, or conversion between chain categories. Interestingly, the waxy starch’s largest crystallinity gain coincided with an increase in its A-chain fraction from 26.36 to 31.17 percent, yet indica and japonica starches gained crystallinity without any such increase, showing that enhanced ordering cannot be attributed simply to more short chains.

Hydration behavior shifted uniformly upward. Both the water solubility index and the swelling power, measured by heating starch suspensions at 95 degrees Celsius for 30 minutes and centrifuging, increased significantly in all three rice types after treatment. A higher solubility index means more of the sample was recovered as soluble material in the supernatant, while greater swelling power reflects more water retained by the sediment relative to its insoluble dry matter. Notably, the direction of this response depends on processing conditions: previous heat-moisture treatment of rice starch reduced both indices, whereas the gentler kernel-level tempering used here raised them. Among the untreated samples, native waxy starch had a higher solubility index but lower swelling power than the other two, showing that solubilization and swelling do not follow the same pattern across rice types.

Thermal properties measured by differential scanning calorimetry diverged in a way that highlights how different structural indices need not move in parallel. Gelatinization enthalpy increased significantly in indica starch, from 13.96 to 17.60 joules per gram, alongside a significant rise in conclusion temperature, while peak temperature remained statistically unchanged. In contrast, enthalpy fell significantly in japonica starch, from 12.37 to 11.20 joules per gram, and in waxy starch, from 12.54 to 11.97 joules per gram; tempered waxy starch also showed significantly lower peak and conclusion temperatures than its untreated counterpart. Because chain-length distributions were nearly unchanged in indica, the enthalpy increase there may reflect altered molecular packing rather than redistributed chain lengths, and the enthalpy decrease in japonica occurred despite increased long-chain fractions, indicating that chain architecture alone cannot explain the thermal responses.

Pasting behavior, assessed with a Rapid Visco Analyzer, followed similarly type-specific trajectories. Indica starch showed significant decreases in peak viscosity, trough viscosity, final viscosity, and pasting temperature, with peak viscosity falling from 4691.50 to 3920.00 centipoise and final viscosity from 3675.50 to 2981.50 centipoise. Japonica starch’s peak viscosity rose significantly from 3837.50 to 4168.00 centipoise, with other parameters unchanged. Waxy starch showed significant decreases in peak viscosity, breakdown, and pasting temperature, while final viscosity and setback remained statistically unchanged. The directions of peak-viscosity change mirrored those of molecular weight, falling in indica and waxy and rising in japonica, which the authors suggest may point to a contribution of molecular size.

Rheological and textural tests on the gels added further nuance. All samples behaved predominantly elastically, with the storage modulus exceeding the loss modulus across the 0.1 to 10 hertz frequency range. Tempered indica starch showed lower moduli than its untreated counterpart, yet its gel hardness after 24 hours of refrigerated storage at 4 degrees Celsius increased significantly, illustrating that oscillatory measurements at 25 degrees Celsius and penetration tests after cold storage probe different physical states. Tempered japonica starch showed higher moduli together with significantly greater hardness and work of compression, suggesting greater resistance both to oscillatory deformation and to penetration after storage. For waxy starch, the storage-modulus curves of treated and untreated samples largely overlapped, and hardness and work of compression did not differ significantly, so the treatment’s structural changes produced no detectable change in waxy gel texture.

An exploratory Pearson correlation analysis across the six sample groups helped tie these threads together, though the authors emphasize that pooling rice types and treatments means the observed associations may reflect both type differences and treatment responses, and should be read as exploratory rather than causal. Molecular weight was positively associated with relative crystallinity and short-range order but negatively associated with peak viscosity, final viscosity, and gel hardness, while the B3-plus chain fraction correlated positively with gelatinization peak temperature and pasting temperature. The overall conclusion is that a common increase in structural order can accompany distinct processing outcomes: indica starch gained enthalpy but lost viscosity, japonica starch gained peak viscosity and gel firmness, and waxy starch gained a lower pasting temperature and reduced breakdown without texture change. The authors point to possible applications, such as instant glutinous-rice foods using treated waxy starch and firmer rice-based gels using treated japonica starch, but stress that these directions require validation in actual food formulations.

Subject of Research: Effects of low-speed rotational thermostatic tempering of rice kernels on the multiscale structure, hydration, thermal, pasting, and gel properties of starch isolated from indica, japonica, and waxy rice.

Article Title: Effects of low-speed rotational thermostatic tempering of rice kernels on the structure, hydration and gel properties of isolated starch

Article References: Dong, R., Zheng, Y., Fu, Z., Xia, J., Yang, Z., & Wang, Z. (2026). Effects of low-speed rotational thermostatic tempering of rice kernels on the structure, hydration and gel properties of isolated starch. Current Research in Food Science, Article 101613. https://doi.org/10.1016/j.crfs.2026.101613

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101613

Keywords: rice starch, low-speed rotational tempering, relative crystallinity, amylopectin chain-length distribution, gelatinization enthalpy, pasting properties, swelling power, gel texture, indica rice, japonica rice, waxy rice, food processing