Structural Characteristics,Properties, and In Vitro Digestibility of Rice

Using rice samples derived from normal rice cultivars and endosperm starch mutant, we investigated key factors contributing to the enzyme digestibility of steamed rice grains. The chemical composition of polished rice grains, structural features of endosperm starch, and enzyme digestibility of steamed rice grains were examined. The protein content of polished rice grains was 4.6–9.1%, amylose content was 4–27%, the DP_n of purified amylose was 900–1,600, the amylopectin short/long chain ratio was 1.2–5.9, and the enzyme digestibilities of steamed polished rice grains were 0.9–12.6 °Brix. Amylose content and RVA parameters (viscosity, breakdown, and setback) correlated significantly with enzyme digestibility of steamed rice grains. Multiple regression formulas were constructed to predict digestibility of steamed rice grain as a function of the molecular characteristics of the starch. When both amylose content and the short/long chain amylopectin ratio were used as predictor variables, they accounted for >80% of the observed variance in digestibility of steamed rice grains. Multiple regression revealed that the more digestible rice samples had starch with a lower amylose content and more short‐chain amylopectin. Reassociation of amylose‐lipid complex and recrystallization of amylopectin in the stored steamed rice grains was monitored by differential scanning calorimetry (DSC), and the observed retrogradation properties were related to the structural characteristics of starch and to the enzyme digestibility of steamed rice grains.     

Structure-retrogradation relationship of rice starch in purified starches and cooked rice grains: a statistical investigation

Amylose content and amylopectin chain length distribution, the two most commonly used structural parameters of starch, have significant effects on starch retrogradation. In the present work, starches were separated and purified from 18 rice cultivars. The amylopectin was purified from each starch. Amylopectin chain length distribution was analyzed by high-performance size-exclusion chromatography after debranched using isoamylase. The blue value was used to measure the amylose content before and after the defatting of starch. The amount of amylose associated with lipid was calculated. Pulsed nuclear magnetic resonance was used to follow the retrogradation of starch both in cooked rice grains and in the purified form. The Avrami equation was employed to describe the retrogradation kinetics of rice starch. To look into the relationship between the starch structure and retrogradation behavior, the structural parameters were correlated with retrogradation kinetics parameters using both Pearson and partial correlations. The results indicated the following: first, the retrogradation behavior of rice starch remains similar in both the purified form and cooked rice grains; second, the peak value of amylopectin short-chain length has a significant positive relationship with the amylopectin crystallization rate constant k; third, the amylose content after defatting has a significant positive relationship with the parameter k and a negative relationship with the Avrami exponent n; and fourth, the amount of amylose associated with lipid has a negative relationship with the parameter k.     

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.     

Characteristics of pregelatinized ae mutant rice flours prepared by boiling after preroasting

As ae mutant rice, such as EM10, lacks the starch branching enzyme IIb, its amylopectin contains more long-chain glucans than that of ordinary Indica and Japonica rice grains. Although boiled grains of ae rice cultivars are too hard and nonsticky for table rice, they are promising in terms of biofunctionality, such as prevention of diabetes. The present paper investigates the characterization of a novel group of four ae mutant rice cultivars (EM72, EM145, EM174, and EM189). They were subjected to the evaluation for their main chemical components, physical properties, and enzyme activities at different grain conditions (raw milled rice, roasted rice, boiled rice, and rice boiled after preroasting). These mutant rice grains are characterized by high apparent amylose, high protein and high glucose contents, high pasting temperature, high α-amylase activities, high resistant starch, and low degree of gelatinization. A novel method was developed to maintain the high resistant starch contents of gelatinized rice grains. Rice boild after preroasting showed a higher ratio of resistant starch and a lower amount of glucose than ordinary boiled rice. It became possible to produce high-quality and biofunctional pregelatinized rice flours by boiling with frozen fruits, such as tomatoes, after rice grains had been preroasted. These ae mutants were found to be suitable materials for rice/fruit or rice/vegetable products to serve as palatable, low-glucose, and high resistant starch rice products.     

Partial beta-amylolysis retards starch retrogradation in rice products

Starch retrogradation is the main cause of quality deterioration of starch-containing foods during storage. The current work investigated the effect of partial beta-amylolysis on the retrogradation of rice starch and the potential of beta-amylase in preparing rice products with extended shelf life. Isolated amylopectin, whole rice starch, and rice flour from a regular rice cultivar were partially hydrolyzed by either reagent-grade or food-grade beta-amylase. The degree of beta-amylolysis was expressed as average external chain length () for isolated amylopectin or the degree of hydrolysis (%) for other starch systems. Pulsed nuclear magnetic resonance was used to monitor starch retrogradation during storage at 4 degrees C. The results indicated that partial beta-amylolysis using reagent-grade beta-amylase retarded amylopectin retrogradation by shortening the of amylopectin. When was below DP 11.6, the amylopectin retrogradation was essentially inhibited. Partial beta-amylolysis had a similar effect on the amylopectin retrogradation in the whole starch system. The maltose produced in beta-amylolysis might slightly attenuate the retrogradation-retarding effect of partial beta-amylolysis. The effect of food-grade beta-amylase on starch retrogradation was also evident, although less effective than that of reagent-grade beta-amylase. The retrogradation-retarding effect of food-grade beta-amylase was also demonstrated in rice flour system, indicating a potential method for controlling the starch retrogradation of rice products.     

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.     

Comparison of Starch Physicochemical Properties of Waxy Rice Cultivars with Different Hardening Rates

The objective of this study was to investigate the starch characteristics of a novel waxy rice cultivar Hitachimochi 36 (H36) with an extremely slow hardening rate of waxy rice cake and the relationships between starch physicochemical properties and texture of waxy rice cake. Starch isolated from H36 showed significantly higher digestibility than that from other waxy rice cultivars, and the starch digestibility highly correlated with the starch crystallinity. The compressive force of the starch gel prepared from H36 was significantly lower than that from other cultivars when stored at 5°C for five days, which reflected the differences in endothermic enthalpies corresponding to retrograded amylopectin. Various textural parameters of cooked waxy rice cake prepared from H36 were also remarkably different from those of cooked waxy rice cake prepared from other waxy rice cultivars. The cooked waxy rice cake prepared from H36 exhibited the lowest compressive force and breaking force by tensile and rupture tests, respectively. The analysis of amylopectin chain length distribution indicated that amylopectin of H36 contained higher proportions of the short chains. The difference in amylopectin chain length distribution and lower crystallinity of starch contributed to the unique textural properties of waxy rice cake prepared from H36.     

Physicochemical,Textural, and Nutritional Characterization of Mexican Rice Cultivars

The physicochemical properties, textural properties, and starch digestibility of rice cultivars grown in Mexico were evaluated. Variations existed in grain dimensions, and the rice grains were classified as medium, long, and extra long. Huimanguillo had the highest amylose content (30.4%), and A06, A92, A98, and Champoton presented the lowest amylose content (24.3–25.2%). The protein content was 7.1–11.0% and the lipid level was 0.47–1.22% among these Mexican cultivars. Champoton showed the highest temperature and enthalpy of gelatinization, and this with A98, Culiacan, and Huimanguillo had the highest enthalpy of retrogradation. Cotaxtla had the highest pasting peak, setback, and final viscosity. The texture assessment in cooked rice showed that A06 had the highest hardness, and A96 and A98 had the highest stickiness. There was little difference in the rapidly digestible starch, slowly digestible starch, and resistant starch content of tested Mexican rice cultivars. The differences in the physicochemical properties could be used to determine the end use of these Mexican rice cultivars.     

Effects of Environmental Temperature on Structure and Gelatinization Properties of Wheat Starch

The effects of environmental temperature on gelatinization properties and amylopectin structures of wheat endosperm starch were examined by isolating starches from four wheat cultivars matured in growth chambers at daytime temperatures of 15, 20, 25, or 30°C. Kernel weight and starch content per kernel were reduced by high maturation temperature. Amylose content showed no significant change at high maturation temperature in some cultivars; in other cultivars, there was a slight increase. Principal component analysis of data on relative peak areas of debranched amylopectin showed that amylopectin from wheat grown at a lower temperature had a greater proportion of shorter chains. Amylopectin branch chains were classified into three groups based on the correlation coefficients between the data of branch chain length distribution and principal component scores, degree of polymerization (DP) of 6–12, DP 13–34, and DP ≥ 35. The gelatinization temperature of starches increased markedly at a higher maturation temperature, with increases exceeding 10°C at high maturation temperatures. Gelatinization properties correlated significantly with amylopectin chain length distribution.     

Pulsed Nuclear Magnetic Resonance (PNMR) Study of Rice Starch Retrogradation

Although pulsed NMR (PNMR) has been used for qualitative study of starch retrogradation in selected systems, validation is necessary for its application to new systems. PNMR was used to analyze the retrogradation of rice starches in purified form, in rice flour, and in cooked rice grains. The standard curves between the relative solid content (S′, %) by PNMR and the percentage of gelatinized starch (GS, %) were determined for common rice flour, common rice starch, and waxy rice starch at different moisture contents. The coefficients of linear regression for these curves (R²) were all >0.997. Starches with different amylose contents were tested for S′ values at the stages of freshly gelatinized, retrograded (4°C, 18 days), and reheated (90°C, 20 min). The S′ of reheated starch (S′_reheat) was similar to the S′ of freshly gelatinized starch (S′₀), so we concluded that the increase in S′ during storage corresponded to amylopectin retrogradation. The effect of moisture content on retrogradation of rice starch, rice flour, and cooked rice grains was studied by PNMR, and the data were interpreted using the Avami equation. Decreasing the moisture content increased the rate of retrogradation and led to a higher parameter k and a lower parameter n. For moisture content in the range studied, PNMR can be used to follow amylopectin retrogradation of different rice starch systems.     

Investigation of Digestibility In Vitro and Physicochemical Properties of A‐ and B‐Type Starch from Soft and Hard Wheat Flour

In this study, the functional properties of A‐ and B‐type wheat starch granules from two commercial wheat flours were investigated for digestibility in vitro, chemical composition (e.g., amylose, protein, and ash content), gelatinization, retrogradation, and pasting properties. The branch chain length and chain length distribution of these A‐ and B‐type wheat starch granules were also determined using high‐performance anion exchange chromatography (HPAEC). Wheat starches with different granular sizes not only had different degrees of enzymatic hydrolysis and thermal and pasting properties, but also different molecular characteristics. Different amylose content, protein content, and branch chain length of amylopectin in A‐ and B‐type wheat starch granules could also be the major factors besides granular size for different digestibility and other functional properties of starch. The data indicate that different wheat cultivars with different proportion of A‐ and B‐type granular starch could result in different digestibility in wheat products.     

Fine Structures of Starches from Long‐Grain Rice Cultivars with Different Functionality

The structural features of rice starch that may contribute to differences in the functionality of three long‐grain rice cultivars were studied. Dried rough rice samples of cultivars Cypress, Drew, and Wells were analyzed for milling quality, grain physical attributes, and starch structures and physicochemical properties. Drew was lower in head rice yield and translucency and higher in percentage of chalky grains compared with Cypress and Wells. Apparent amylose content (21.3–23.1%), crude protein (8.3–8.6%), and crude fat (0.48–0.64%) of milled rice flours were comparable, but pasting properties of rice flours as measured by viscoamylography, as well as starch iodine affinity and thermal properties determined by differential scanning calorimetry were different for the three cultivars. Drew had higher peak, hot paste, and breakdown viscosities, and gelatinization temperature and enthalpy. Molecular size distribution of starch fractions determined by high‐performance size‐exclusion chromatography showed that the three samples were similar in amylose content (AM) (20.0–21.8%) but differed in amylopectin (AP) (64.7–68.3%) and intermediate material (IM) (10.9–13.5%). Drew had highest AP and lowest IM contents, whereas Cypress had the lowest AP and highest IM contents. High‐performance anion‐exchange chromatography of isoamylase‐debranched starch indicated that the AP of Drew was lower in A and B1 chains but higher in B2, B3, and longer chains.     

Rice amylopectin fine structure variability affects starch digestion properties

The possibility to identify or develop new rice cultivars with low glycemic response was investigated. Twelve rice cultivars with a narrow range of amylose contents were selected based on their wide variation in rapid viscoanalyzer (RVA) pasting breakdown to study the relationship between starch digestibility and amylopectin fine structure and pasting properties. Rice flour samples were cooked for in vitro digestibility analysis using the standard Englyst assay. RVA was performed for pasting properties of starches. Results showed that rapidly digestible starch (RDS) was highly and negatively correlated (r = -0.86, p < 0.01; r = -0.81, p < 0.01) with FrI long and FrII intermediate/short debranched amylopectin linear chains, respectively, and positively correlated (r = 0.79; p < 0.01) with FrIII very short linear chains. Slowly digestible (SDS) starch was positively correlated (r = 0.80, p < 0.01; 0.76, p < 0.01) with FrI and FrII, respectively, and negatively correlated (r = -0.76, p < 0.01) with FrIII. RVA breakdown viscosity was positively correlated (r = 0.88, p < 0.01) with RDS and negatively correlated (r = -0.89, p < 0.01) with SDS. Thus, the RVA method potentially could be used as a screening tool for starch digestion properties. This study reveals a molecular basis in amylopectin fine structure variability for starch digestion properties in rice cultivars and could have value in identifying slowly digesting cultivars as well as developing a breeding strategy to produce low glycemic rice cultivars. Keywords: Rice; starch; RVA; amylopectin; digestibility.     

Correlations Between the Fine Structure,Physicochemical Properties,and Retrogradation of Amylopectins from Taiwan Rice Varieties

Fourteen varieties of rice from Taiwan, including five Indica, five Japonica, and four waxy cultivars, were used in this study for the examination of fine structure and physicochemical properties of amylopectin. The results indicated that the amylopectin of Indica rice had lower molecular weight, lower average degree of polymerization (DP), and lower average chain number when compared to Japonica and waxy varieties. The shortest average DP was 6 glucose units for all 14 rice varieties. The average chain lengths (CL) of amylopectin were 18–22, 15–18, and 17–20 for Indica, Japonica, and waxy rice, respectively. Indica varieties with high amylose content had amylopectin that comprised a few extra long chains (DP >100). The CL distribution profiles of amylopectins for these 14 varieties could be divided into two factions: CL 10–15 and CL 40–44. Amylopectin of the Indica rice had a relatively high blue value and λ_max, implying that a high proportion existed as long branches. The amylopectin of three Indica varieties with lower DP exhibited higher intrinsic viscosity, which might be attributed to the more elongated rod conformation of the few extra long chain amylopectins. The proportion of short chains with DP 6–9 glucose units seemed to influence the rate of the retrogradation of amylopectins.     

Impact of Elevated Nighttime Air Temperatures During Kernel Development on Starch Properties of Field‐Grown Rice

The structural features of starch were examined to better understand the causes of variability in rice quality resulting from nighttime air temperature (NTAT) incidence during kernel development. Starch samples were isolated from head rice of four cultivars (Bengal, Cypress, LaGrue, and XL723) field‐grown in four Arkansas locations (Keiser, Pine Tree, Rohwer, and Stuttgart) in 2009 and 2010. Average NTATs recorded during the grain‐filling stages of rice reproductive growth in the four locations were 3.0–8.4°C greater in 2010 than 2009. Elevated NTATs altered the deposition of starch in the rice endosperm. Means pooled across cultivars and locations showed that amylose content was 3.1% (percentage points) less for the 2010 sample set. The elevated NTATs in 2010 resulted in a decrease in the percentage of amylopectin short chains (DP ≤ 18) and a corresponding increase in the percentage of long chains (DP ≥ 19) by an average of 1.3% (percentage points). The greater NTATs in 2010 also produced greater starch paste peak, final, and breakdown viscosities, whereas setback and total setback viscosities decreased. Changes in paste viscosity were highly correlated with the changes in the proportion of amylose and amylopectin. Onset gelatinization temperature was greater by 3.5°C, gelatinization enthalpy by 1.3 J/g, and relative crystallinity by 1.5% (percentage points) for the 2010 sample set. Changes in gelatinization parameters and granule relative crystallinity were highly correlated with the changes in amylopectin chain‐length distribution. Year × cultivar × location interaction effects were statistically insignificant, indicating that the four cultivars evaluated all showed some degree of susceptibility to the effects of temperature incidence during kernel development, regardless of the growing location.     

Properties of Starches from Several Low‐Amylose Rice Cultivars

The starch properties of five low‐amylose rice cultivars, Yawarakomachi, Soft 158, Hanabusa, Aya, and Snow Pearl, were compared with those of two normal amylose rice cultivars, Nipponbare and Hinohikari. There were no large differences in the distributions of the amylopectin chain length determined by high‐performance anion‐exchange chromatography, and the starch gelatinization properties determined by differential scanning calorimetry, between normal and low‐amylose rice cultivars. Results obtained using rapid viscosity analysis indicated that low‐amylose rice starches had lower peak viscosity, breakdown, and setback values than normal amylose rice starches. Starch granules from low‐amylose rice cultivars had a higher susceptibility to glucoamylase than those from normal amylose rice cultivars. The results of this study showed some differences between normal and low‐amylose rice starches in pasting properties and enzymatic digestibility.     

Retrogradation of Maize Starch After Thermal Treatment Within and Above the Gelatinization Temperature Range

Studies of starch retrogradation have not considered the initial thermal treatment. In this article, we explore the effect of heating to temperatures within and above the gelatinization range on maize starch retrogradation. In the first experiment, 30% suspensions of waxy (wx) starch were initially heated to final temperatures ranging from 54 to 72°C and held for 20 min. On reheating in the differential scanning calorimeter immediately after cooling, the residual gelatinization endotherm peak temperature increased, the endotherm narrowed, and enthalpy decreased. Samples stored for seven days at 4°C showed additional amylopectin retrogradation endotherms. Retrogradation increased dramatically as initial holding temperature increased from 60 to 72°C. In a second experiment, wx starch was initially heated to final temperatures from 54 to 180°C and rapidly cooled, followed by immediate reheating or storage at 4°C. Maximum amylopectin retrogradation enthalpy after storage was observed for initial heating to 82°C. Above 82°C, retrogradation enthalpy decreased as initial heating temperature increased. A similar effect for ae wx starch was observed, except that retrogradation occurred more rapidly than for wx starch. These experiments show that heating to various temperatures above the range of gelatinization may profoundly affect amylopectin retrogradation, perhaps due to varying extents of residual molecular order in starch materials that are commonly presumed to be fully gelatinized. This article shows that studies of starch retrogradation should take into account the thermal history of the samples even for temperatures above the gelatinization temperature range.     

Modification of rice starch by selective degradation of amylose using alkalophilic Bacillus cyclomaltodextrinase

A cyclomaltodextrinase (CDase) isolated from alkalophilic Bacillus sp. I-5 (CDase I-5) exists in a dodecameric form, an assembly of six dimers, each catalytic site of which is located in a narrow groove at the interface of the dimeric unit. Because of the unique geometric shape of the catalytic site, the enzyme has the ability to discriminate the molecular size of substrates. An analysis of the hydrolysis reaction of the enzyme revealed that its kcat/Km value on amylose was 14.6 s(-1) (mg/mL)(-1), whereas that for amylopectin was 0.92 s(-1) (mg/mL)(-1), showing an exceptionally high preference toward amylose. CDase I-5 was applied to modify the starch structure to produce low-amylose starch products by incubating rice starch with this enzyme. We found that the amylose content of rice starch decreased from 28.5 to 9%, while the amylopectin content remained almost constant with no significant change in the side chain length distribution. When the CDase I-5-treated rice starch was stored at 4 degrees C for 7 days, the retrogradation rate was significantly retarded as compared to that in the control sample.     

Structures and physicochemical properties of six wild rice starches

Starches from six wild rice cultivars were studied for their chemical structures and physicochemical properties and compared with a long-grain rice starch. The six wild rice starches were similar in morphological appearance, X-ray diffraction patterns, swelling power, and water solubility index but different in amylose content, beta-amylolysis limit, branch chain length distribution, thermal properties, and pasting properties. The structure of the wild rice amylopectins was close to that of waxy rice amylopectin with more branching and a larger proportion of short branch chains of degree of polymerization 6-12 as compared with that of amylopectin from rice starch with a similar amylose content. The differences in branch chain length distribution of amylopectin and amylose content were assumed to contribute to the differences in physicochemical properties among the six wild rice starches as well as to the differences between the wild rice starches and the rice starch.     

Effect of Rice Starch-Lipid Complexes on In Vitro Digestibility,Complexing Index,and Viscosity

Effects of nonwaxy (21% amylose, 79% amylopectin) and waxy (100% amylopectin) rice starch-lipid complexes on the rate of in vitro digestibility were determined. Long-chain (≥C:18) saturated emulsifiers reduced digestibility more than short-chain (<C:18) saturated and unsaturated emulsifiers when complexed with nonwaxy and waxy rice starch. The largest decrease in digestibility (33%) was achieved with Polyaldo 10-1-2 (100% C18:0 with decaglyceryl monostearate modification) for nonwaxy rice. Waxy rice starch did not complex with most of the emulsifiers, in contrast to nonwaxy rice starch. Most of the emulsifiers that reduced digestibility by 10% or less were composed of unsaturated monoglycerides, including some acetylated and succinylated monoglycerides. The fluid behavior of nonwaxy rice starch-emulsifier solutions was more pseudoplastic than waxy rice starch-emulsifier solutions. The consistency index varied with emulsifiers. The nonwaxy rice starchemulsifier solutions and some of those prepared using waxy rice starch would be suitable for semisolid food applications. The waxy rice starchemulsifier solutions with low consistency (0.4–0.7) and high-flow behavior (0.7–0.8) indices would be suitable for beverage applications.