Physicochemical Properties of Starch in Extruded Rice Flours

The effects of extruding temperatures and subsequent drying conditions on X‐ray diffraction patterns (XRD) and differential scanning calorimetry (DSC) of long grain (LG) and short grain (SG) rice flours were investigated. The rice flours were extruded in a twin‐screw extruder at 70–120°C and 22% moisture, and either dried at room temperature, transferred to 4°C for 60 hr, or frozen and then dried at room temperature until the moisture was 10–11%. The dried materials were milled without the temperature increasing above 32°C. XRD studies were conducted on pellets made from extruded and milled flours with particle sizes of 149–248 μm; DSC studies were conducted from the same material. DSC studies showed that frozen materials retrograded more than the flours dried at room temperature. The LG and SG samples had two distinct XRD patterns. The LG gradually lost its A pattern at >100°C, while acquiring V patterns at higher temperatures. SG gradually lost its A pattern at 100°C but stayed amorphous at the higher extruding temperatures. DSC analysis showed that retrograded flours did not produce any new XRD 2θ peaks, although a difference in 2θ peak intensities between the LG and SG rice flours was observed. DSC analysis may be very sensitive in detecting changes due to drying conditions, but XRD data showed gradual changes due to processing conditions. The gradual changes in XRD pattern and DSC data suggest that physicochemical properties of the extruded rice flours can be related to functional properties.     

Effects of Rough Rice Storage Conditions on Gelatinization and Retrogradation Properties of Rice Flours

Changes in gelatinization and retrogradation properties of two rice cultivars, Bengal and Kaybonnet, during rough rice storage were studied using differential scanning calorimetry (DSC). The storage variables included two storage moisture contents (12 and 14%), three storage temperatures (4, 21, and 38°C), and four storage durations (0, 3, 9, and 16 weeks). Rough rice cultivar, storage temperature, moisture content, and duration affected (P < 0.05) the enthalpies and temperatures of gelatinization and retrogradation of rice flour. Bengal had a higher gelatinization enthalpy (P < 0.005) but lower gelatinization temperatures (P < 0.0001) than the long-grain Kaybonnet. Rice stored at 38°C exhibited higher gelatinization enthalpy and temperatures (P < 0.05) than those stored at 4 or 21°C. Storage duration affected the gelatinization and retrogradation properties through a higher order, rather than a linear, relationship.     

Properties of Flours and Starches as Affected by Rough Rice Drying Regime

Flours and starches from rough rice dried using different treatment combinations of air temperature (T) and relative humidity (RH) were studied to better understand the effect of drying regime on rice functionality. Rough rice from cultivars Bengal and Cypress were dried to a moisture content of ≈12% by three drying regimes: low temperature (T 20°C, RH 50%), medium temperature (T 40°C, RH 12%), and high temperature (T 60, RH 17%). Head rice grains were processed into flour and starch and evaluated for pasting characteristics with a Brabender Viscoamylograph, thermal properties with differential scanning calorimetry, starch molecular‐size distribution with high‐performance size‐exclusion chromatography (HPSEC), and amylopectin chain‐length distribution with high‐performance anion‐exchange chromatography with pulsed amperometric detection (HPAEC‐PAD). Lower head rice and starch yields were obtained from the batch dried at 60°C which were accompanied by an increase in total soluble solids and total carbohydrates in the pooled alkaline supernatant and wash water used in extracting the starch. Drying regime caused no apparent changes on starch molecular‐size distribution and amylopectin chain‐length distribution. Starch fine structure differences were due to cultivar. The pasting properties of flour were affected by the drying treatments while those of starch were not, suggesting that the grain components removed in the isolation of starch by alkaline‐steeping were important to the observed drying‐related changes in rice functionality.     

Starch Damage and Pasting Properties of Rice Flours Produced by Dry Jet Grinding

Milling method and particle size affect some properties of rice flour. To prepare ultra‐fine rice flour of <30 μm, hammer and dry jet grinding methods were examined and the effect of particle size on starch damage and pasting properties of the flour were elucidated. A jet mill could make finer flour (<10 μm mean size) with a narrower particle size distribution than a hammer mill could. Starch damage increased dramatically at a mean size of <10 μm. Particles of a similar size (<60 μm) had different levels of starch damage between mills. Not only the particle size, but also the milling method affected the level of damaged starch. Flour samples of ≥45 μm mean size had similar viscosity curves, but samples of <20 μm had different curves. Peak viscosity and final viscosity decreased sharply at <10 μm. Setback viscosity for particles of 3 μm from both brown rice and white rice were higher than the peak viscosity. Stability to heat and shearing stress were decreased for <20 μm flours as the breakdown viscosities decreased. Starch damage and pasting properties of flour ground from the nonwaxy japonica cultivar Koshihikari changed dramatically at a mean size of <10 μm.     

Effect of Gamma Irradiation on the Functional and Nutritive Properties of Rice Flours from Different Cultivars

The purpose of this work was to study the effect of minimal doses of γ‐irradiation to reduce microbial loads (Co⁶⁰, doses of 0, 1.5, and 3 kGy) on flour from three rice cultivars with different amylose contents (AC) grown in Argentina: RP2 (AC 25%), Yeruá (Y) (AC 19%), and Higokumochi (H) (AC 5%). Studies on some starch characteristics such as digestibility (rate of starch hydrolysis in vitro), gel viscosity and syneresis, and scanning electron microscopy (SEM) were performed. Results showed that γ‐irradiation affects rice functional and nutritive characteristics proportionally with the dosage used. With 3 kGy, original digestibility values increased 37.5, 60, and 76%; gel viscosity decreased 52, 77, and 90%; and syneresis increased 0, 193, and 546% on RP2, Y, and H, respectively. Amylopectin was the starch fraction most affected as shown by SEM. Although some of these changes in cultivar attributes might be considered negative for traditional food formulations, they could help in the design of new foods or for fermented beverages.     

Physicochemical Properties of Rice Starch Modified by Hydrothermal Treatments

Rice starches of long grain and waxy cultivars were annealed (ANN) in excess water at 50°C for 4 hr. They were also modified under heat-moisture treatment (HMT) conditions at 110°C with various moisture contents (20, 30, and 40%) for 8 hr. The modified products were analyzed by rapid-viscosity analysis (RVA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Generally, these hydrothermal treatments altered the pasting and gelling properties of rice starch, resulting in lower viscosity peak heights, lower setbacks, and greater swelling consistency. The modified starch showed increased gelatinization temperatures and narrower gelatinization temperature ranges on ANN or broader ones on HMT. The effects were more pronounced for HMT than for ANN. Also, the typical A-type XRD pattern for rice starch remained unchanged after ANN or HMT at low moisture contents, and the amorphous content increased after HMT at 40% moisture content.     

Effect of Physicochemical Properties of High‐Amylose Thai Rice Flours on Vermicelli Quality

Certain physicochemical properties, including chemical composition, pasting, thermal parameters, and the gel texture of flour prepared from six cultivars of Thai rice with similar high‐amylose content were determined. These properties were correlated with the quality of vermicelli prepared from these flours for cooking and textural properties. Flour prepared from a Thai rice cultivar currently used for commercial production of vermicelli served as reference for these comparisons. Many similarities, but some significant differences, in the physicochemical properties were observed between the test rice flours and to the reference cultivar. Vermicelli prepared from all of the test rice flours were within an acceptable range for cooking quality but showed greater variation in textural quality. Among the physicochemical properties, gel hardness was well correlated with cooking and textural quality of vermicelli, and was useful for predicting overall vermicelli quality.     

Milled Rice Breakage Due to Environmental Conditions

Milled, long-grain rice was exposed to air at temperatures (T) of 20, 30, and 40°C, and relative humidities (RH) ranging from 25 to 85%. The kernels then were subjected to a breakage test to determine the extent of damage that occurred during each exposure condition. Increasing air T levels produced higher amounts of broken kernels across the RH range. Milled rice at higher moisture content (MC) levels sustained more extensive stress crack damage at low RH conditions and less stress crack damage at high RH conditions relative to milled rice at lower MC levels. Varietal differences were also present, but were overshadowed by MC effects.     

Effects of Heat Treatments on the Milling,Physicochemical, and Cooking Properties of Two Long‐Grain Rice Cultivars During Storage

During storage, the milling, physicochemical properties, and eating quality of rice change, which is generally termed “aging.” Aged rice is preferred by processors because of better processing characteristics, and therefore there are attempts to develop accelerated aging processes. In this study, the effects of various heat treatments and their influences on the milling, physicochemical, and cooking properties of two long‐grain rice cultivars during storage were investigated with a randomized complete block design with an 8 × 5 × 2 full‐factorial treatment design. Two long‐grain rice cultivars, Wells and XP723, were treated with eight different heat treatments, including two levels of UV irradiation, two levels of autoclaving, three levels of convection oven heating, and one control, and then stored for 180 days at room temperature. The heat treatments significantly influenced all properties, including head rice yield (HRY), surface lipid content, peak gelatinization temperature, pasting properties, and cooked rice texture. All properties except HRY exhibited a significant two‐way interaction of cultivar and heat treatment. The severe autoclaving treatment resulted in rice of significantly different protein compositions when compared with the control. Storage impacted all properties except HRY and peak gelatinization temperature. Autoclaving (particularly severe autoclaving) produced samples with more distinct characteristics for most properties. Cooked rice hardness and stickiness exhibited not only significant main effects but also significant two‐ and three‐factor interactions.     

Modeling Starch Gelatinization Kinetics of Milled Rice Flour

Milled long‐grain rice samples were evaluated by differential scanning calorimetry (DSC) to determine the kinetics of starch gelatinization. The experiments were conducted with milled rice flour with a 10.6% degree of milling. DSC thermograms were obtained from 35 to 110°C using heating rates between 1°C/min and 15°C/min. The rate constants were evaluated, and two activation energies were found for different temperature ranges. At <70.1°C gelatinization was not completed. It was assumed that at <70.1°C gelatinization would only affect the amorphous regions. During the subsequent phase the crystalline regions destabilized by the amorphous component begin to gelatinize. For moisture content of 70%, wet basis, and a heating rate of 12°C/min, the enthalpy of gelatinization reaches a constant value of 7.3 J/g.     

Properties and Structures of Flours and Starches from Whole,Broken, and Yellowed Rice Kernels in a Model Study

The objective of this study was to compare the structure and properties of flours and starches from whole, broken, and yellowed rice kernels that were broken or discolored in the laboratory. Physicochemical properties including pasting, gelling, thermal properties, and X‐ray diffraction patterns were determined. Structure was elucidated using high‐performance size‐exclusion chromatography (HPSEC) and high‐performance anion‐exchange chromatography with pulsed amperometric detection (HPAEC‐PAD). The yellowed rice kernels contained a slightly higher protein content and produced a significantly lower starch yield than did the whole or broken rice kernels. Flour from the yellowed rice kernels had a significantly higher pasting temperature, higher Brabender viscosities, increased damaged starch content, reduced amylose content, and increased gelatinization temperature and enthalpy compared with flours from the whole or the broken rice kernels. However, all starches showed similar pasting, gelling, thermal properties, and X‐ray diffraction patterns, and no structural differences could be detected among different starches by HPSEC and HPAEC‐PAD. α‐Amylase may be responsible for the decreased amylopectin fraction, decreased apparent amylose content, and increased amounts of low molecular weight saccharides in the yellowed rice flour. The increased amount of reducing sugars from starch hydrolysis promoted the interaction between starch and protein. The alkaline‐soluble fraction during starch isolation is presumed to contribute to the difference in pasting, gelling, and thermal properties among whole, broken, and yellowed rice flours.     

Factors Governing Pasting Properties of Waxy Wheat Flours

Waxy wheat (Triticum aestivum L.) contains endosperm starch lacking in amylose. To realize the full potential of waxy wheat, the pasting properties of hard waxy wheat flours as well as factors governing the pasting properties were investigated and compared with normal and partial waxy wheat flours. Starches isolated from six hard waxy wheat flours had similar pasting properties, yet their corresponding flours had very different pasting properties. The differences in pasting properties were narrowed after endogenous α‐amylase activity in waxy wheat flours was inhibited by silver nitrate. Upon treatment with protease, the extent of protein digestibility influenced the viscosity profile in waxy wheat flours. Waxy wheat starch granules swelled extensively when heated in water and exhibited a high peak viscosity, but they fragmented at high temperatures, resulting in more rapid breakdown in viscosity. The extensively swelled and fragmented waxy wheat starch granules were more susceptible to α‐amylase degradation than normal wheat starch. A combination of endogenous α‐amylase activity and protein matrix contributed to a large variation in pasting properties of waxy wheat flours.     

Functionality of Rice and Sorghum Flours in Baked Tortilla and Corn Chips

The effects of raw and gelatinized sorghum and rice flours on the structure and texture of baked corn and tortilla chips were evaluated. Dry masa flour was hydrated into masa, sheeted, and cut. Corn chips were baked in an air-impingement oven, and tortilla chips were baked first in a three-tier oven and then in an air-impingement oven. Baked tortilla chips required significantly greater force to break and were less susceptible to breakage during handling than baked corn chips. Raw and gelatinized, normal and waxy rice and sorghum flours significantly changed the texture and structure of baked chips. Waxy rice and sorghum flours reduced peak force and work, increased chip thickness, and improved overall acceptability (as assessed by a taste panel), but waxy rice and sorghum chips were more fragile and had a greater number of large central air cells. Waxy rice was more beneficial than waxy sorghum flour. Gelatinization of waxy flours increased thickness of baked chips, whereas gelatinization of nonwaxy flours had no improvement over waxy flours alone. Gelatinization of sorghum flour significantly decreased the peak force and work values for baked tortilla chips when compared with the control chips. Gelatinized rice flour tortilla chips were not significantly different than the control chips but were significantly harder than the other baked tortilla chips. The complex interactions that occur in baked corn and baked tortilla chips suggest that each ingredient acts differently in the two products. Thus, each ingredient must be evaluated for specific products and processes.     

Quality Characteristics of Long-Grain Rice Milled in Two Commercial Systems

Long-grain rice variety Kaybonnet was milled to three degree of milling (DOM) levels in two commercial milling systems (a single-break, friction milling system and a multibreak, abrasion and friction milling system) and separated into five thickness fractions. For both milling systems, the surface lipid content (SLC) and protein content of the milled rice varied significantly across kernel thickness fractions. SLC was influenced by DOM level more than by thickness, while the protein content was influenced by thickness more than by DOM level. Particularly at the low DOM levels, the thinnest kernel fraction (<1.49 mm) had higher SLC than the other kernel fractions. Protein content decreased with increasing kernel thickness to 1.69 mm, after which it remained constant. In both milling systems, thinner kernels were milled at a greater bran removal rate as indicated by SLC differences between the low and high DOM levels. For rice milled to a given DOM level, the multibreak system produced fewer brokens than did the single-break system.     

Observation and Measurement of Residual Bran on Milled Rice Using Hyperspectral Imaging

Residual bran on milled rice is directly related to its quality. This study proposes a method to measure the residual bran patterns on a single rice grain by using hyperspectral imaging (HSI). HSI is a sensing technique that combines both spatial and spectral information and may be used for chemical compound identification and quantification. In this study, HSI was applied to assess rice bran residue nondestructively. In the experiment, rice samples were milled and scanned with an HSI system. Afterward, the rice samples were dyed to enable the residual bran to be identified with optical microscopy and image processing algorithms. Classifiers were then developed to predict the rice bran residue by using the HSI measurements as inputs. The predicted images were compared with the micrograph images for classifier performance evaluation. The proposed approach can estimate the residual bran distribution on milled rice surface with an accuracy of 93.5%.     

Water Absorption Characteristics and Volume Changes of Milled and Brown Rice During Soaking

The water absorption characteristics and volume changes of rice with various degrees of milling during soaking were measured at five temperatures (5–40°C). The measured data were fitted to the exact solution for the infinite plane sheet diffusion model, which is an exact solution for the diffusion equation. The measured results agreed well with the model. The effects of temperature and the degree of milling on the water absorption rate constant were investigated. Volume changes of samples were determined by measuring particle density and bulk density. An empirical equation relating the moisture content of the sample during soaking and at temperatures of 10–50°C to specific volume was derived. In addition, bulk density was related to the quadratic function of the moisture content of the sample during soaking.