Comparative study of the retrogradation of intermediate water content waxy maize, wheat, and potato starches

The retrogradation of extruded starches from three different botanical sources was studied in concentrated conditions (34 +/- 1% water) at 25 degrees C using differential scanning calorimetry (DSC) and isothermal calorimetry, Fourier transform infrared spectroscopy (FTIR), and wide-angle X-ray scattering. Potato starch showed the highest rate of retrogradation (approximately 0.17 h(-1)) followed by waxy maize (approximately 0.12 h(-1)), while the retrogradation of wheat starch was the slowest (approximately 0.05 h(-1)). In addition to the kinetics, the extent of molecular order in the retrograded samples was studied in detail in terms of "short-range" (helical) and "long-range" (crystalline) distance scales. The amylopectin crystallinity indices were essentially the same (approximately 47-51% amylopectin basis) for the three starches. However, significant differences were found in the enthalpy of melting measured by DSC after "full" retrogradation (potato, 11.6 +/- 0.7; waxy maize, 9.0 +/- 0.5; and wheat, 6.1 +/- 0.3 J/g of amylopectin). The degree of short-range molecular order in the retrograded state determined by FTIR was waxy maize > potato > wheat. The effect of amylopectin average chain length and the polymorphism of the crystalline phase were taken into account to explain the differences in the retrogradation enthalpies.     

Determination of the molecular and structural characteristics of okenia, mango, and banana starches

Starches were isolated from nonconventional sources (banana, mango, and okenia) and their characteristics were examined using polarized light microscopy, X-ray diffraction pattern, Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). Banana starch granules were of an ellipsoidal shape with size between approximately 8 and 20 microm; okenia had the smallest granule size, between approximately 2 and 5 microm. The three starches showed the Maltese cross, indicative of an intact granule structure. Okenia and mango starches had the A-type X-ray diffraction pattern, common to native cereal starches, whereas banana starch showed a mixture between A- and B-type pattern. Banana starch had the highest temperature (77.6 degrees C) and enthalpy (23.4 J/g) of gelatinization in excess water conditions; okenia had the lowest temperature (71.2 degrees C) and enthalpy (15 J/g), which may be related to the X-ray diffraction pattern and its small granule size. Both the okenia and mango starches had a higher molar mass and gyration radius than banana starch, which may be related to the differences determined in their crystalline structures.     

Solid-State 13C CP/MAS NMR studies on aging of starch in white bread

The effects of storage methods and glycerol on the aging of breadcrumbs were studied using solid-state (13)C CP/MAS NMR. After baking, a shift in C(1) peaks from triplet (A-type) to singlet (V-type) was observed. Addition of glycerol reduced the carbon peak intensities of fresh and aged breads, which correlated well with the DSC amylopectin "melting" enthalpy (r(2) = 0.91). Upon storage of bread with crust in hermetically sealed containers (when moisture migrated from the crumbs to the crust), the (13)C CP/MAS NMR peak intensity increased more rapidly during aging than when the bread was stored without crust. Although addition of glycerol retarded the starch retrogradation, as observed by (13)C CP/MAS NMR and DSC, it accelerated the firming rate. Therefore, bread firming in this case was controlled not only by starch retrogradation but also by other events (such as local dehydration of the matrix or gluten network stiffening).     

Effect of Storage Time on In Vitro Digestibility and Resistant Starch Content of Nixtamal,Masa, and Tortilla

Nixtamal, masa, and tortilla samples were stored for 24–96 hr and their chemical composition, retrogradation, and in vitro starch digestibility features were evaluated. Ash and fat contents in the three products were smaller than in the original corn sample, but protein levels were higher, all in accordance with previous studies. In general, a minor decrease in available starch (AS) content was observed with storage time. Masa showed the greatest AS values, followed by tortilla and nixtamal. Tortilla presented slightly higher retrograded resistant starch (RS3) values (1.1–1.8%, dmb) than masa (0.7–0.9%) and nixtamal (0.7–0.8%) and only minor increases were observed after 24 hr of storage, suggesting that retrogradation phenomenon in these samples takes place very rapidly and is more pronounced in the final product (tortilla). The development of RS3 explains the observed decrease in AS. Higher total resistant starch values were found in all samples at a range of 2.1–2.6% for nixtamal and masa, and a range of 3.1–3.9% in tortilla. This indicates that, apart from retrograded resistant starch, some ungelatinized fractions appear to contibute to the indigestible content of these products. The α‐amylolysis rate of the three materials decreased with storage. Tortilla showed the greatest hydrolysis indices. Differential scanning calorimetry (DSC) analysis showed that the nixtamal, masa, and tortilla did not show differences in amylopectin crystal melting temperature with storage time, but tortilla exhibited higher enthalpy values after 72 hr of storage, in accordance with the greater total RS contents recorded after prolonged storage.     

Structure and Functionality of Barley Starches

Amylose contents of prime starches from nonwaxy and high-amylose barley, determined by colorimetric method, were 24.6 and 48.7%, respectively, whereas waxy starch contained only a trace (0.04%) of amylose. There was little difference in isoamylase-debranched amylopectin between nonwaxy and high-amylose barley, whereas amylopectin from waxy barley had a significantly higher percentage of fraction with degree of polymerization < 15 (45%). The X-ray diffraction pattern of waxy starch differed from nonwaxy and high-amylose starches. Waxy starch had sharper peaks at 0.58, 0.51, 0.49, and 0.38 nm than nonwaxy and high-amylose starches. The d-spacing at 0.44 nm, characterizing the amylose-lipids complex, was most evident for high-amylose starch and was not observed in waxy starch. Differential scanning calorimetry (DSC) thermograms of prime starch from nonwaxy and high-amylose barley exhibited two prominent transition peaks: the first was >60°C and corresponded to starch gelatinization; the second was >100°C and corresponded to the amylose-lipid complex. Starch from waxy barley had only one endothermic gelatinization peak of amylopectin with an enthalpy value of 16.0 J/g. The retrogradation of gelatinized starch of three types of barley stored at 4°C showed that amylopectin recrystallization rates of nonwaxy and high-amylose barley were comparable when recrystallization enthalpy was calculated based on the percentage of amylopectin. No amylopectin recrystallization peak was observed in waxy barley. Storage time had a strong influence on recrystallization of amylopectin. The enthalpy value for nonwaxy barley increased from 1.93 J/g after 24 hr of storage to 3.74 J/g after 120 hr. When gel was rescanned every 24 hr, a significant decrease in enthalpy was recorded. A highly statistically significant correlation (r = 0.991) between DSC values of retrograded starch of nonwaxy barley and gel hardness was obtained. The correlation between starch enthalpy value and gel hardness of starch concentrate indicates that gel texture is due mainly to its starch structure and functionality. The relationship between the properties of starch and starch concentrate may favor the application of barley starch concentrate without the necessity of using the wet fractionation process.     

Influence of storage conditions on the structure, thermal behavior, and formation of enzyme-resistant starch in extruded starches

Starch structures from an extrusion process were stored at different temperatures to allow for molecular rearrangement (retrogradation); their thermal characteristics (DSC) and resistance to amylase digestion were measured and compared. The structure of four native and processed starches containing different amylose/amylopectin compositions (3.5, 30.8, 32, and 80% amylose content, respectively) before and after digestion was studied with small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD). Rearrangement of the amylose molecules was observed for each storage condition as measured by the DSC endotherm at around 145 degrees C. The crystalline organization of the starches after processing and storage was qualitatively different to that of the native starches. However, there was no direct correlation between the initial crystallinity and the amount of enzyme-resistant starch (ERS) measured after in vitro digestion, and only in the case of high-amylose starch did the postprocess conditioning used lead to a small increase in the amount of starch remaining after the enzymatic treatment. From the results obtained, it can be concluded that retrograded amylose is not directly correlated with ERS and alternative mechanisms must be responsible for ERS formation.     

Retrogradation of du wx and su2 wx Maize Starches After Different Gelatinization Heat Treatments

Retrogradation of du wx and su2 wx starches after different gelatinization heat treatments was studied by differential scanning calorimetry. Suspensions of 30% (w/w) starch were initially heated to final temperatures of 55–180°C. Gelatinized starch was cooled and stored at 4°C. Starch retrogradation in the storage period was influenced by initial heat treatments. Retrogradation of du wx starch was rapid: when initially heated to 80–105°C, retrogradation enthalpy was ≈10 J/g after one day at 4°C. The retrogradation enthalpy was ≈15 J/g after 22 days of storage, and reached a maximum of 16.2 J/g after 40 days of storage. For du wx starch, application of the Avrami equation to increases in retrogradation enthalpy suggests retrogradation kinetics vary with initial heating temperature. Furthermore, starch retrogradation may not fit simple Avrami theory for initial heating ≤140°C. Retrogradation of su2 wx starch was slow. After 30 days of storage at 4°C, the maximum retrogradation enthalpy for all initial heating temperatures tested was 7.0 J/g, for the initial heating to 80°C. This work indicates that gelatinization heat treatment in these starches is an important factor in amylopectin retrogradation, and that the effect of initial heat treatment varies according to the genotype.     

Influence of acyl chain lengths in mono- and diacyl-sn-glycerophosphatidylcholine on gelatinization and retrogradation of starch

The influence of starch with 1- or 2-monoacyl-sn-glycerophosphatidylcholine (GPC) having various chain lengths of fatty acids on gelatinization and retrogradation of starch was studied by the measurement of starch-GPC complex formation, complexing index, and differential scanning calorimetry. The addition of GPC to the starch sample slightly increased the blue value and lambda(max) with increasing chain length of GPC but decreased the phosphorus content and complexing index. The gelatinization onset and peak temperatures of starch complexes increased significantly with increasing chain length, but the enthalpies were statistically lower, except for the treatment with 1,2-distearoyl-sn-GPC when compared with that of the control. Among GPC (di and mono), 1- and 2-monomyristoyl-sn-GPC showed the highest complexing ability, whereas the complexing ability of the GPC decreased with the increasing chain length. According to the Avrami equation, the retrogradation rate (k, day(-1)) of starch was slower than that of the control, whereas the retrogradation rates of 1- and 2-monomyristoyl-sn-GPC were slowest among the GPCs. The positive linear relationship between k and the number of acyl groups of GPC suggests that a GPC with a shorter chain length could retard the retrogradation of starch during storage.     

Partial Characterization of Buckwheat (Fagopyrum esculentum) Starch

Laboratory-isolated buckwheat (Fagopyrum esculentum) starch was compared to commercial corn and wheat starches. Buckwheat starch granules (2.9–9.3 μm) were round and polygonal with some holes and pits on the surface. Buckwheat starch had higher amylose content, waterbinding capacity, and peak viscosity, and it had lower intrinsic viscosity when compared with corn and wheat starches. Buckwheat starch also showed restricted swelling power at 85–95°C and lower solubility in water at 55–95°C and was more susceptible to acid and enzymatic attack. Gelatinization temperatures, determined by differential scanning calorimetry, were 61.1–80.1°C for buckwheat starch compared to 64.7–79.2°C and 57.1–73.5°C for corn and wheat starches, respectively. A second endotherm observed at 84.5°C was an amylose-lipid complex attributed to the internal lipids in buckwheat starch, as evidenced by selective extraction. The retrogradation of buckwheat, corn, and wheat starch gels was examined after storage at 25, 4, and -12°C for 1–15 days. In general, buckwheat starch retrogradation was slower than that of corn and wheat starch, but it increased as storage time increased, as did that of the other starch pastes. When the values of the three storage temperatures were averaged for each storage period analyzed, buckwheat starch gels showed a lower percentage of retrogradation than did corn and wheat starch gels. Buckwheat starch also had a lower percentage of water syneresis when stored at 4°C for 3–10 days and had better stability to syneresis after three freeze-thaw cycles at -12 and 25°C.     

Influence of Bran Particle Size on Bread‐Baking Quality of Whole Grain Wheat Flour and Starch Retrogradation

The influence of bran particle size on bread‐baking quality of whole grain wheat flour (WWF) and starch retrogradation was studied. Higher water absorption of dough prepared from WWF with added gluten to attain 18% protein was observed for WWFs of fine bran than those of coarse bran, whereas no significant difference in dough mixing time was detected for WWFs of varying bran particle size. The effects of bran particle size on loaf volume of WWF bread and crumb firmness during storage were more evident in hard white wheat than in hard red wheat. A greater degree of starch retrogradation in bread crumb stored for seven days at 4°C was observed in WWFs of fine bran than those of coarse bran. The gels prepared from starch–fine bran blends were harder than those prepared from starch–unground bran blends when stored for one and seven days at 4°C. Furthermore, a greater degree of starch retrogradation was observed in gelatinized starch containing fine bran than that containing unground bran after storage for seven days at 4°C. It is probable that finely ground bran takes away more water from gelatinized starch than coarsely ground bran, increasing the extent of starch retrogradation in bread and gels during storage.     

Effect of Sucrose on Glass Transition,Gelatinization, and Retrogradation of Wheat Starch

Differential scanning calorimetry (DSC) was used to study the effect of sucrose on wheat starch glass transition, gelatinization, and retrogradation. As the ratio of sucrose to starch increased from 0.25:1 to 1:1, the glass transition temperature (T_g, T_g′) and ice melting enthalpy (ΔH_ice) of wheat starch‐sucrose mixtures (with total moistures of 40–60%) were decreased to a range of −7 to −20°C and increased to a range of 29.4 to 413.4 J/g of starch, respectively, in comparison with wheat starch with no sucrose. The T_g′ of the wheat starch‐sucrose mixtures was sensitive to the amount of added sucrose, and detection was possible only under conditions of excess total moisture of >40%. The peak temperature (T_m) and enthalpy value (ΔH_G) for gelatinization of starch‐sucrose systems within the total moisture range of 40–60% were increased with increasing sucrose and were greater at lower total moisture levels. The T_g′ of the starch‐sucrose system increased during storage. In particular, the significant shift in T_g′ ranged between 15 and 18°C for a 1:1 starch‐sucrose system (total moisture 50%) after one week of storage at various temperatures (4, 32, and 40°C). At 40% total moisture, samples with sucrose stored at 4, 32, and 40°C for four weeks had higher retrogradation enthalpy (ΔH) values than a sample with no sucrose. At 50 and 60% total moisture, there were small increases in ΔH values at storage temperature of 4°C, whereas recrystallization of samples with sucrose stored at 32 and 40°C decreased. The peak temperature (T_p), peak width (δT), and enthalpy (ΔH) for the retrogradation endotherm of wheat starch‐sucrose systems (1:0.25, 1:0.5, and 1:1) at the same total moisture and storage temperature showed notable differences with the ratio of added sucrose. In addition, T_p increased at the higher storage temperature, while δT increased at the lower storage temperature. This suggests that the recrystallization of the wheat starch‐sucrose system at various storage temperatures can be interpreted in terms of δT and T_p.     

Effects of Heat-Moisture Treatment and Lipids on Gelatinization and Retrogradation of Maize and Potato Starches

Effects of heat-moisture treatment (HMT) and lipids on the structure and gelatinization of maize and potato starches were studied, and the retrogradation process of 20% HMT starch gels was also investigated. Maize starch was physically modified by HMT or by defatting. Potato starch was physically modified by HMT or by adding monoglycerides. The X-ray pattern of the HMT maize starch was assigned to a combination of A and V patterns, which indicated that HMT formed crystallized amylose complexes and recrystallized amylose in maize starch granules. However, the X-ray pattern of defatted maize starch did not change for HMT, so the lipids originally existing in starch granules were important to the formation of new crystallites during this treatment. Differential scanning calorimetry (DSC) results suggested that weaker structures in amylopectin crystallites were more susceptible to degradation after HMT, while crystallized amylose complexes developed thermal stability after treatment. The amylose contents increased with increasing degree of HMT, which suggested that the newly created amylose arose from exterior linear chains of amylopectin degraded by the treatment. Investigation of retrogradation process showed that HMT significantly promoted retrogradation of starch gels, especially the initiation of recrystallization.     

Role of Phosphorus in Viscosity,Gelatinization, and Retrogradation of Starch

The effect of starch crystallinity and phosphorus on starch gelatinization and retrogradation were studied using wide-angle X-ray powder diffraction, cross polarization/magic angle spinning (CP/MAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy, ³¹P NMR spectroscopy, Rapid Visco Analyzer (RVA) and differential scanning calorimetry (DSC). Two starches differing significantly in peak viscosity (cv. Stephens, 283 BU; cv. Crew, 560 BU) were comparable in amylose content and starch crystallinity, while differing significantly in phospholipids content. Starch of lower peak viscosity had a higher phospholipids content and showed a slower rate of retrogradation. Starch from Stephens (0.098% phosphorus) had an enthalpy value of retrograded starch of 2.2 J/g after 14 days of storage, while starch from Crew (0.062% phosphorus) had an enthalpy value as high as 4.4 J/g. Defatting with a hot n-propanol and water (3:1) mixture caused substantial changes in peak viscosity. Peak viscosity for starch from Crew decreased by 75 RVU due to defatting, while starch from Stephens decreased by as much as 125 RVU. After defatting with the hot n-propanol water mixture, the rate and extent of starch retrogradation were comparable between the prime starches, which differed significantly in peak viscosity.     

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.     

Analysis of the retrogradation of low starch concentration gels using differential scanning calorimetry,rheology, and nuclear magnetic resonance spectroscopy

The retrogradation of 5, 10, 15, and 25% corn starch gels was measured using differential scanning calorimetry (DSC), rheology, and an array of NMR spectroscopy techniques. During the initial (<24 h) stage of retrogradation, an increase in G' corresponding to an increase in the number of solid protons participating in cross-relaxation (M(B)(0) was observed for all four concentrations studied. During the latter (>24 h) stage of retrogradation, amylopectin recrystallization becomes the dominant process as measured by an increase in deltaH(r) for the 25% starch gel, which corresponded to a further increase in. A decrease in the molecular mobility of the liquid component was observed by decreases in (17)O T(2), (1)H D(0), and T(2A). The value for T(2B) (the solid transverse relaxation time) did not change with concentration or time indicating that the mobility of the solid component does not change over time despite the conversion of the highly mobile starch fraction to the less mobile solid state during retrogradation.     

Effect of Holding Temperature on the Structures of Mung Bean Starch Gels and Noodles

The effect of storage temperatures (‐10, +1, and +10°C) on the structural organization of mung bean starch gels and noodles was studied by acid hydrolysis, X‐ray diffractometry, and gel‐permeation chromatography. The gels showed higher susceptibility to acid compared with the noodles as shown by the rate constants of the first stage of hydrolysis (k = 5.37–12.17 × 10^‐2/day and k = 4.19–4.61 × 10^‐2/day for gels and noodles, respectively). Acid hydrolysis showed no difference in the amount of resistant residues of both gels (42–46%) and noodles (44–45%), except for gels (38%) stored at ‐10°C. The acid‐resistant residues of both the gels and noodles had a B‐type X‐ray diffraction pattern (major reflections at 2θ = 19, 24, and 25°). The acid‐resistant residues of the unstored sample and those stored at ‐10°C for both gels and noodles contained chains with DP 46–54 and after debranching yielded two peaks with DP 29–39 and DP 15–19. The acid‐resistant residues of gels and noodles stored at +1 and +10°C contained chains with DP 35–37 and after debranching showed two chain populations with DP 31–33 and DP 14–19. These results indicate the greater participation of amylopectin in the retrogradation process occurring during storage at +1 and +10°C.     

Slowly digestible state of starch: mechanism of slow digestion property of gelatinized maize starch

The mechanism underlying the previously reported parabolic relationship between amylopectin fine structure, represented by the weight ratio of linear short chains [degree of polymerization (DP < 13) to long chains (DP >/= 13], and slowly digestible starch (SDS) content was investigated from the viewpoint of starch retrogradation and substrate susceptibility to enzyme hydrolysis. A maize mutant sample, termed "highest long-chain starch" (HLCS) representing group I samples with a higher proportion of long chains, showed a bell-shaped SDS pattern with retrogradation time, whereas insignificant changes in SDS were found for the sample termed "highest short-chain starch" (HSCS) representing group II samples with a higher proportion of short chains. This corresponded to results from X-ray powder diffraction and differential scanning calorimetry that showed a rapid increase of crystallinity and enthalpy for HLCS during retrogradation, but negligible changes for sample HSCS. Therefore, retrogradation was associated with SDS content for group I samples, but not for group II samples. Analysis of amylopectin fine structure, SDS content, retrogradation enthalpy, SDS material debranching profile, and hydrolysis pattern demonstrated, for group I samples, that linear branched chains of DP 9-30 of amylopectin may act as anchor points to slow the digestion of branced-chain fractions of DP > 30, which constitute the major slowly digestible portion, whereas for group II samples, it is the inherent molecular structure of amylopectin with a higher amount of branches and shorter chains that is not favorable for rapid enzyme digestion. The concept of a slowly digestible starch state (SDS state) that could be a chemical or physical entity is proposed to better describe the mechanistic underpinning of the slow digestion property of starches.     

Rheological and Thermal Properties of Aged Starch Pastes from Three Waxy Maize Genotypes

The rheological and thermal properties of aged starch gels (15:85 starch-water) from three waxy maize genotypes (wx, wx sh1, and du wx) during storage (4°C for up to 25 days) were studied. After storage, changes of storage modulus (G′) and phase angle (δ) of the gels as a function of temperature were measured using oscillatory rheometry. For the du wx samples, G′ at 25°C increased rapidly during the first four days of storage at 4°C, compared to the gradual increases over the 25-day storage period for the wx and wx sh1 samples. A peak in G′ at 45°C was observed during heating for the du wx samples after 10 days of storage and for the wx sample stored for 25 days. The G′ peak may have been due to syneresis in the gels. Retrogradation of amylopectin of the aged starch samples was examined using differential scanning calorimetry. The du wx starch had greater retrogradation enthalpies than the other two samples (which showed similar retrogradation behavior) throughout the storage. The retrogradation enthalpy of the du wx samples increased rapidly during the first seven days, followed by a slower increase through the rest of storage. For the wx and wx sh1 samples, no endotherm was observed during the first four days of storage, after which the enthalpy increased steadily as a function of storage time. Addition of sucrose delayed the formation of gel networks for all three starches. The greater tendency for gelling and retrogradation of the du wx starch might be attributed to the greater proportion of DP20–30 chains of the amylopectin.     

Structural and Retrogradation Properties of Rice Endosperm Starch Affect Enzyme Digestibility of Steamed Milled-Rice Grains Used in Sake Production

Structural and physicochemical characteristics of endosperm starch from milled rice grains of seven Japanese cultivars used in sake production were examined. Amylose content was 15.2–20.2%, number-average degree of polymerization (DP_n) of amylose was 900–1,400, and the ratio of short-to-long chain amylopectin was 2.7–3.5, respectively. The degree of retrogradation of purified starch stored for seven days at 4°C after gelatinization was 20–31%. The degree of retrogradation correlated negatively with the ratio of short-to-long chain amylopectin. The effect of holding time after steaming on enzyme digestibility and starch retrogradation of steamed rice grains was investigated. The longer the holding time after steaming, the greater the extent of retrogradation, and the less the degree of enzymatic digestibility. The decreased rate of enzyme digestibility correlated with amylopectin chain length distribution. Samples with short-chain amylopectin exhibited a slow decrease in enzyme digestibility. It was determined that the structure and retrogradation properties of endosperm starch in Japanese rice cultivars affect the decreasing rate of enzyme digestibility of the steamed, milled rice grains.     

In vitro starch digestibility and predicted glycemic index of corn tortilla, black beans, and tortilla-bean mixture: effect of cold storage

People in the rural areas of Mexico consume corn tortillas and beans as basic components of their diet. However, little is known about the nutritionally relevant features of starch present in such combined meals. The objective of the present study was to evaluate the in vitro bioavailability of starch in tortilla-bean mixtures stored at 4 degrees C for different times, as compared to that of corn tortilla and boiled black beans kept separately under the same conditions. Available starch (AS), resistant starch (RS), and retrograded resistant starch (RRS) contents were measured. The in vitro starch hydrolysis indices (HI) of freshly cooked and cold-stored samples were evaluated using a chewing/dialysis digestion protocol. HIs were used to predict glycemic indices (pGI) of the samples. AS in tortilla and beans decreased between 3 and 6% after 48-72 h, whereas values in the mixture fell by 3% after 48 h, with no further change by 72 h. Only minor rises in RS contents (1.5-1.6%) were recorded for tortilla and beans after 72 h of storage, and a lower increase (0.4%) was recorded in the mixture. Judging from RRS values, an important proportion of RS is due to starch retrogradation. The HI and pGI were higher in tortilla than in bean and the mixture. Hydrolysis rate values decreased in the stored samples, a pattern that corresponded with RS and RRS changes. The slow digestion features of common beans are largely retained by the legume-tortilla combination. Data support the perceived health beneficial properties of starch in this traditional cereal-legume food.