Key Proteins Causing Changes in Pasting Properties of Rice During Aging

Key components that cause changes in pasting properties of rice during storage aging were investigated in this work. The main nonstarch components in rice were sequentially removed from fresh and aged rice, the aging effect of the component was separated, and thus the aging contribution rate of the component (CACR) on rice aging could be deduced. The results showed that the largest contributor to rice aging was albumin with a CACR of 65%, followed by globulin and prolamin with the CACRs of 38 and 14%, respectively, and the CACR of glutelin was small (1%). In contrast, the CACRs of fat and crude starch were –7 and –11%, respectively. These findings suggest that albumin and globulin are predominantly responsible for changes in pasting properties of rice during storage aging. This conclusion directs future researchers to the changes occurring in albumin and globulin for disclosing the mechanisms of rice aging.     

Effect of Alkaline‐Soluble Proteins on Pasting Properties of Nonwaxy Rice Flour

The pasting properties of rice flours and reconstituted rice flours from mixing a common starch with proteins extracted from different rice cultivars at different total protein content levels were studied. Results showed that not only the total protein content but also the protein composition had an effect on the pasting properties of the rice flours. Among the different strands of rice proteins, globulin had the strongest influence on the pasting properties, followed by glutelin, whereas prolamin had the least influence. At the subunit level of the proteins, proteins with a molecular weight of 17,000, most likely from globulin, had the strongest effect on the peak viscosity of the rice flour, followed by those of 33,000. In comparison with that of the rice starch, the influence of proteins in rice was limited. The effect of interactions between the rice proteins and the starch, such as the role of starch‐granule‐associated proteins, was not isolated in this study, and further investigation is required to quantify this effect.     

Degree of Milling Effects on Rice Pasting Properties

The effects of degree of milling on pasting properties of medium‐grain (cv. Bengal and Orion) and long‐grain rice (cv. Cypress and Kaybonnet) were quantified using a Brabender ViscoAmylograph and a Rapid Visco Analyser. For all the cultivars tested, surface and total lipid contents decreased as the degree of milling increased. The peak viscosities for all rice increased with the degree of milling and the rates of increase were higher for medium‐grain than long‐grain cultivars. Degree of milling did not have a consistent effect on final viscosity for all the cultivars tested.     

Genotype and Environmental Influences on Pasting Properties of Rice Flour

The pasting behavior of flour from several Australian rice (Oryza sativa L.) cultivars, differing in amylose content and grown in three different locations and three seasons, were determined using the Rapid Visco Analyser. Genotype, growth season, and growth location all affected the pasting behavior of rice flour. The amylose content of the same cultivar was significantly higher in the coolest growing season, resulting in RVA traces with lower peak viscosity and higher setback than samples with lower amylose content. When the same cultivar of rice was grown in different locations in the same season, there were no significant differences in the total starch, protein, lipid, and amylose content of the flour, but there were significant differences in the pasting behavior. This indicates that environmental as well as genetic factors influence the pasting behavior of rice flour. Flour from parboiled and quick‐cooking rice did not paste and had low viscosities compared with unprocessed rice. Results from this study showed that the pasting behavior of rice flour was related to genotype and was influenced by environmental factors that brought about subtle changes in the grains that were not picked up by chemical analyses.     

Impact of Rice Flour Cold‐Water‐Soluble Fraction Removal on Gelatinization and Pasting Properties

The influence of the cold‐water‐soluble fraction on gelatinization and pasting properties of rice flour was investigated. The cold‐water‐soluble fraction was removed by water extraction under room temperature. The gelatinization properties of untreated and treated flour were analyzed with a differential scanning calorimeter, and pasting profiles were measured with a rapid viscosity analyzer. The removal of the cold‐water‐soluble fraction resulted in the formation of a loosened starch granule structure, a morphological alteration of protein bodies, a markedly lower gelatinization temperature, and a significantly higher pasting enthalpy. The impact on paste viscosity followed different trends. In some cultivars that had lower endogenous amylase activity, the paste viscosity was greatly reduced by the removal of the cold‐water‐soluble fraction. In others, the higher level of endogenous amylase activity led to more soluble saccharides being released through starch hydrolysis. Removing the soluble fraction caused a remarkable increase in peak viscosity. The overall effect on paste viscosity of removing the cold‐water‐soluble fraction was attributed to multiple factors, involving loosening of the starch granule structure, alteration of morphology of protein bodies, and the release of saccharides by endogenous amylase activity.     

Effect of Stabilized Rice Bran Fractions on the Formation of Rice Flour Pasting Properties

Rice flour composition played a key role in determining the changes in pasting properties of rice flour. The influence of incorporating defatted rice bran (DFRB), rice bran fiber (RBF), rice bran protein (RBP), and stabilized rice bran (SRB) fractions on the mechanism of rice flour pasting viscosities was investigated. Pasting properties of long‐ and medium‐grain rice flour substituted with 5, 10, 15, 20, and 100% bran fractions resulted in a significant decrease (P < 0.05) in rice flour pasting property values. Flour substituted with RBP had the lowest pasting property measurements compared with other fractions, and the greater the percentage substituted, the lower the pasting property values. DFRB and RBF were least affected properties when used as a replacement. Results were attributed to the contribution of rice starch in the mechanism of rice paste formation, in which decreasing starch in a rice flour sample, as a result of substituting with fractions of SRB, may have resulted in faster swelling of starch granules to the maximum extent and increased their susceptibility to be disrupted by shear, resulting in low paste viscosities. Results also suggested that protein structural integrity and the nature of starch–protein bonding affected rice flour pasting mechanism formation.     

Effects of Cultivar and Processing Condition on Physicochemical Properties and Starch Fractions in Parboiled Rice

Starch can be classified into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) according to its resistance to amylolytic enzymes. This study investigated the effects of cultivar and feedstock under varying parboiling conditions on the physicochemical properties and starch fractions of parboiled rice. Rice (rough or brown) was soaked, steamed under pressure, dried immediately or stored at room temperature for 24 hr prior to drying, and then treated with or without a repeated steam cycle prior to milling. The storage treatment significantly increased the retrograded amylopectin enthalpy and amylose‐lipid complex melting temperature of parboiled rice. Parboiled rice samples prepared from brown rice feedstock had higher peak melting temperatures but lower enthalpy values of retrograded amylopectin than samples prepared from rough rice after the storage treatment. The pasting viscosity of parboiled rice was most affected by the repeated autoclaving treatment and cultivar. Starch fractions in parboiled rice were significantly affected by cultivar and storage and by the interactions of cultivar and parboiling conditions. The storage treatment significantly increased SDS and generally decreased RDS in parboiled rice. Parboiled rice with different SDS and RS contents can be produced by varying rice cultivar and parboiling conditions.     

Impact of Starch Gelatinization and Kernel Fissuring on the Milling Breakage Susceptibility of Parboiled Brown Rice

Parboiling, a hydrothermal treatment of paddy or brown rice, improves the texture and nutritional characteristics of cooked rice. We investigated milling breakage susceptibility of brown rice parboiled under different soaking and steaming conditions, resulting in samples with different degrees of starch gelatinization and levels of fissured grains and white bellies, that is, parboiled grains with translucent outer layers and an undesirable opaque center. The milling breakage susceptibility was 2.1% for raw rice and ranged from less than 1% up to 11.3% for parboiled rice. Parboiled samples with increased milling breakage susceptibility contained higher levels of white bellies and fissured grains. In white bellies, starch gelatinization is incomplete. Scanning electron microscopy revealed inhomogeneities in individual white bellies and fissured rice grains, indicating moisture gradients inside the grains during parboiling. Starch needs to be completely gelatinized to ensure the absence of white bellies and minimal fissured grain levels in the parboiled end product and, as a consequence, a decreased milling breakage.     

Physical Properties of Raw and Parboiled Paddy

The physical properties namely, size and shape, bulk density, true density, and angle of repose at moisture contents ranging from 7·19 to 28·28% d.b. for raw paddy (IR-36) and from 12·24 to 43·53% d.b. for parboiled paddy were determined using standard techniques. In the case of raw paddy, the thousand grain weight increased from 23·04 to 27·16 g with an increase in moisture content from 10·45 to 32·13% d.b. Bulk density and angle of repose increased from 522 to 566 kg/m³ and 42·35 to 49·30°, respectively, with an increase in moisture content from 7·19 to 27·86% d.b. True density and porosity decreased from 1405 to 1348 kg/m³ and from 62·84 to 58·01% respectively, with an increase in moisture content from 7·19 to 27·86% (d.b.). In the case of parboiled paddy, bulk density and angle of repose increased from 507 to 564·8 kg/m³ and 39·90 to 43·89°, respectively, with an increase in moisture content from 12·24 to 43·53% d.b. True density and porosity decreased from 1411 to 1342 kg/m³ and from 64·08 to 57·91% respectively, with an increase in moisture content from 12·24 to 43·53%. The physical properties were linearly dependent upon moisture content.     

A Comparative Study on Pasting and Hydration Properties of Native Rice Starches and Their Mixtures

Four rice starches were isolated from waxy and nonwaxy rice cultivars collected from different places in China. Individual rice starches were examined, along with their corresponding mixtures in different ratios, in terms of pasting and hydration properties. Analysis by micro‐viscoamylography (MVAG) showed that waxy rice starch and its blends had higher peak viscosity (PV), breakdown (BD), and setback (SB) than the remaining starches and mixtures. Apparent amylose content (AC) was 16.95–29.85% in nonwaxy individual rice starches and 13.69–25.07% in rice starch blends. Incorporating waxy rice starch (25%) significantly decreased the AC. AC correlated negatively with swelling power (SP) (r = ‐0.925, P < 0.01). SP exhibited nonlinear relationship (r² = 0.8204) with water solubility (WS) and both increased with temperature. The correlation showed that WS is also an index of starch characteristics and the granules rigidity affected the granule swelling potential. The results show that turbidity of gelatinized starch suspensions stored at 4 ± 0.5°C generally increased during storage up to five days.     

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.     

Influence of Early and Delayed Transplantation of Paddy on Physicochemical,Pasting, Cooking,Textural, and Protein Characteristics of Milled Rice

Milled rice of four indica cultivars (PR114, PR116, PR118, and PAU201) obtained from early (June 5 and 15) and delayed (June 25 and July 5) transplanted paddy was evaluated for physicochemical, pasting, cooking, and protein characteristics. During the grain‐filling and development stage, early transplanted paddy experienced average maximum and minimum temperatures of 32.8 and 22.3°C, respectively, against 30.8 and 15°C, respectively, for delayed transplanted paddy. Lightness (L*) value, protein content, and lipid content of milled rice were lower, whereas apparent amylose content, cooked grain hardness, final viscosity, and setback viscosity were higher in the delayed transplanted paddy. Whiteness of milled rice of PR114, PR116, and PR118 from delayed transplanted paddy was lower than milled rice from early transplanted paddy. PAU201 did not show any change in whiteness with delayed transplantation. Cooked rice of PAU201 showed exceptionally higher adhesiveness and lower hardness compared with other cultivars. Protein profiling showed a lower accumulation of prolamines in milled rice from PR114 and PAU201 upon delaying the transplantation from June 5 to July 5, whereas PR116 and PR118 showed a decrease in prolamine accumulation from transplantation on June 25. The changes in milled rice caused by delayed transplanting of paddy might be because of cool nighttime air temperature that affected the accumulation of starch, protein, and lipid.     

Effect of Processing Conditions on Color Change of Brown and Milled Parboiled Rice

The effects of the soaking and steaming steps in rice parboiling on color changes and the levels of reducing sugars in rice were studied. Brown rice was soaked to different moisture contents (MC, 15, 20, 25, and 30%). The L*, a*, b* color parameters of the Commission Internationale de L'Eclairage (CIE 1976) indicated that during soaking, red and yellow bran pigments diffused from the bran into the endosperm. The increase in brightness brought about by soaking rice was attributed to migration of rice compounds (e.g., lipids) from the inner to the outer bran layers (rice surface). The levels of reducing sugars in brown and milled soaked rice samples increased with increasing brown rice MC after soaking. The total color difference (ΔE) between parboiled and nonparboiled rice increased with increasing MC after soaking and depended on the intensity of the steaming conditions as reflected in the degree of starch gelatinization. Parboiling affected yellowness more than redness in mildly steamed brown rice and most in intermediately steamed brown rice. Severe steaming of brown rice affected redness more than yellowness. All three parboiling conditions equally affected the yellow color more than the red color in milled rice. Linear regression analyses indicated that parboiling had a larger effect on ΔE of milled parboiled rice than of brown parboiled rice. Furthermore, the linear relationship between the level of gelatinized starch and ΔE of the milled parboiled rice samples showed that both parameters are indicators for the degree of parboiling. Reducing sugars were formed and lost during steaming, suggesting Maillard reactions during steaming.     

Effects of Rough Rice Storage Conditions on the Amylograph and Cooking Properties of Medium-Grain Rice cv. Bengal

Rough rice (cv. Bengal) was stored at four moisture contents (8.8, 10.7, 12.9, and 13.6% MC) and three temperatures (3, 20, and 37°C) for up to six months. The amylograph overall paste viscosity of the milled rice increased during storage. This increase was most apparent in all samples stored at 37°C. For rice stored at 20 and 37°C at all MC levels, a 30–50% increase in peak viscosity (PV) was observed during the first three months of storage. PV subsequently leveled off for rice stored at 12.9 and 13.6% MC but declined for samples stored at 8.8 and 10.7% MC. The final viscosities also increased during storage. The water-absorption ratio of the samples during cooking in excess water increased by an average of 15% over six months of storage. The amylograph and cooking properties were significantly affected (P < 0.05) by rough rice storage duration, temperature, MC, and their respective interactions.     

Effect of Ultrasonic Treatment of Brown Rice at Different Temperatures on Cooking Properties and Quality

This research studied developing quick cooking brown rice by investigating the effect of ultrasonic treatment at different temperatures on cooking time and quality. The medium grain brown rice was ultrasonically treated in water at temperatures of 25, 40, and 55°C for 30 min and then dried by air at 25°C to its initial moisture content (11.0 ± 0.6%, wb) before cooking. The microstructure of rice kernel surface, chemical composition, and optimal cooking time of treated brown rice were determined. The pasting and thermal properties and chemical structure of flour and starch from treated brown rice were also examined. The results showed that the optimal cooking times were 37, 35, and 33 min after treatment at 25, 40, and 55°C, respectively, compared to the control of 39.6 min. The ultrasonic treatment resulted in a loss in natural morphology of rice bran, allowing water to be absorbed by a rice kernel easily, particularly at high‐temperature treatment. Even through rice flour still maintained an A‐pattern in the pasting properties, the crystallinity significantly increased after treatment at 55°C. Ultrasonic treatment increased the peak, hold, and final viscosities and decreased the onset temperature (T_o) and peak temperature (T_p), significantly. Thus, ultrasonic treatment could be used for reducing cooking time of brown rice.     

Structure and Pasting Properties of Oat Starch

Following a period of declining food use, oats are now increasing in importance because of perceived nutritional benefits. The pasting properties of oat starch were regarded as similar to those of other cereal starches until the development of instruments with a more rapid mixing system than the amylograph showed characteristic differences in oats. These differences in pasting properties offer opportunities for novel products in both food and industrial areas. The structure, composition, and pasting properties of oat starch are reviewed, with particular emphasis on methods of measurement. Future directions of research in this area are suggested.     

Structures of Four Waxy Rice Starches in Relation to Thermal,Pasting, and Textural Properties

Waxy rice starches from three japonica cultivars (Taichung Waxy 1 [TCW1], Taichung Waxy 70 [TCW70], Tachimemochi) and one indica cultivar (Tainung Sen Waxy 2 [TNSW2]) were characterized for chemical and physicochemical properties. The amylopectin structures were different for the four waxy rice starches in terms of degree of polymerization (DP), average chain length (CL), exterior chain lengths (ECL), and distribution of chains, indicating the existence of varietal differences. The order of swelling power was TCW1 > TCW70 > TNSW2 > Tachimemochi; the order of water solubility index was TCW70 > TNSW2 > Tachimemochi > TCW1. The low water solubility index of TCW1 might be ascribed to a high DP. All starches shared similar gelatinization temperatures and enthalpies but showed distinct retrogradation patterns. TNSW2 showed the highest retrogradation rate, followed by TCW2, Tachimemochi, and TCW70. TCW70 exhibited the highest overall pasting viscosity, followed by TNSW2, TCW1, and Tachimemochi. The hardness of waxy rice starch pastes from a Brabender amyloviscograph increased rapidly after storage at 5°C for one day and remained the same or slightly increased after seven days of storage, whereas the opposite trend was observed for adhesiveness. The lower degree of retrogradation of TCW70 was probably a result of a larger amount of A chain and a shorter ECL. The changes in hardness correlated with the amount of A and B1 chains. The texture attributes of waxy rice starch pastes were significantly affected by amylopectin retrogradation during storage.     

Chalky and Translucent Rice Grains Differ in Starch Composition and Structure and Cooking Properties

Chalk is an important quality characteristic in rice and occurs most commonly when high temperatures are experienced during grain development. The aims of this report are to determine whether chalk affects cooking quality and to attempt to explain the effects on the basis of starch and protein in chalky and translucent grains. Three cultivars of rice were grown in the glasshouse at either 38/21°C or 26/15°C (day/night temperatures). Rice grown at the higher temperature contained more chalky grains. Grains in the inferior position were more susceptible to forming chalk than were those in the superior position. The presence or absence of chalk affected cooking quality but neither amylose content, amylopectin structure nor protein composition explained the differences in cooking quality. However, the shape, size, and packing of amyloplasts and cells in chalky grains differed from those in translucent grains and might offer an explanation for the differences in cooking quality. It seems likely that the processes involved in the initiation or packing of amyloplasts are susceptible to high temperatures.     

Rheological and Pasting Properties of Buckwheat (Fagopyrum esculentum Möench) Flours With and Without Jet‐Cooking

Pasting, rheological, and water‐holding properties of buckwheat (Fagopyrum esculentum) flour obtained from whole achenes separated into three particle sizes, and three commercial flours (Fancy, Supreme, and Farinetta) were measured with or without jet‐cooking. Fancy had instantaneous paste viscosity (measured using RVA) after jet‐cooking that was not observed for Supreme or Farinetta, and paste viscosity was lower for the latter two flours. Supreme jet‐cooked flour exhibited higher peak viscosity than flour without jet‐cooking, and paste exhibited high shear‐thinning. Fancy exhibited strongest viscoelastic properties (measured using a rheometer). Jet‐cooking damaged buckwheat flour structure, thereby reducing viscoelasticity. Buckwheat flour pastes experienced shear‐thinning over a wide range of shear rates. Jet‐cooking greatly enhanced water‐holding capacity. Buckwheat flour particle size did not greatly influence paste viscosity. Study showed buckwheat flours have unique pasting and rheological characteristics that have different food applications, which could especially be useful for people with celiac disease as buckwheat is gluten‐free.