Characterization of Acetyl Starch by Means of NMR Spectroscopy and SEC/MALLS in Comparison with Hydroxyethyl Starch

The properties of starch derivatives which may be used as plasma substitutes, are dependent upon the molecular structure. Seven acetyl starch (AS) samples were determined and compared with results from hydroxyethyl starch (HES) samples. The molar masses and their distributions were determined with the combination of size exclusion chromatography and light scattering. Slightly asymmetric distributions were determined with a polydispersity M_w/M_n = 2.4 and weight-average molar masses of M_w = 250,000–300,000 g/mol for six AS samples and M_w/M_n = 3.6 and a weight-average molar mass of 766,000 g/mol for one AS sample. The average degrees of substitution (DS) and the substitution pattern were determined by high resolution NMR spectroscopy. The AS samples investigated had a DS of 0.42 to 0.81, comparable to HES, but the regioselective substitution pattern revealed differences. While for HES the position C-2 is preferred and the position C-3 has nearly no substituent, for AS both positions, C-2 and C-3, are substituted likewise. Degradability by α-amylase was tested in the laboratory for AS as well as for HES having nearly the same degree of substitution and molar mass, but C-2/C-6 = 2 for AS and C-2/C-6 = 1.4 for HES. An exponential decrease in the molar mass was observed over time, down to a limiting molar mass M_w = 50,000 g/mol for AS and M_w = 30,000 g/mol for HES, the degradation of AS occurred more slowly.     

The effect of long‐term alkali treatment on the molecular characteristics of native and extruded starches at 35°C

In this study the molecular characteristics of native and extruded starches of wheat, corn, and potato were observed under long‐term alkali treatment (0.5 M NaOH, 7 days, 35°C). The molar mass distribution of samples determined by means of calibrated semi‐analytical size exclusion chromatography (SEC) showed different profiles (double and single peak distribution) depending on the dissolution status of the starch polysaccharides with simultaneous loss of supermolecular structures because of disintegration of the aggregates. The molecular range was between 4 × 103 and 2 × 106 g/mol and their calculated average molar masses were M_w = 1.7 × 105–4.1 × 105 and M_n = 2.5 × 104–1.1 × 105 g/mol. The reduction value and the value of intrinsic viscosity were additional indicators for the dissolving status of starch polysaccharides as well as for the stability of starch molecules in alkaline solution.     

Molecular characteristics of native sago starch and isolated fractions determined using asymmetrical flow field‐flow fractionation

The apparent average molar masses (M_w,app), apparent average radii of gyration (R_g,app), of native sago starch and fractions were determined using asymmetrical flow field‐flow fractionation coupled with multi‐angle light scattering and refractive index detectors (AF4/MALS/RI). Amylose‐type (Fraction A) and amylopectin‐type (Fraction B) were chemically separated from native sago starch. Native sago starch and Fractions (A–B) were dissolved in 1M KSCN using a high pressure microwave vessel. The effect of varying cross flow rates at a fixed channel flow rate upon the M_w,app and R_g,app distributions of native sago starch and Fractions (A–B) were investigated. The average M_w,app values of native sago starch, Fraction (A) and Fraction (B) were 60 × 10⁶, 1.5 × 10⁶ and 60 × 10⁶ g/mol, respectively, with average R_g,app values of 142, 75 and 127 nm, respectively. The sphere‐equivalent hydrodynamic radii (R_h) values for native sago starch and fractions were determined from AF4 experimental parameters.     

Rice Starch Molecular Size and its Relationship with Amylose Content

The composition and starch molecular structure of eight rice varieties were studied. Waxy and non‐waxy (long‐, medium‐, and short‐grain) rice varieties from California and Texas were used. The amylose contents were measured using the Concanavalin A method and were found to be related to the type of rice: waxy ≈ 1.0%, short and medium grain 8.7–15.4%, and long grain 17.1–19.9%. The weight‐average molar masses (M_w) of the starches varied from 0.52 to 1.96×10⁸ g/mol. As would be expected, a higher M_w of rice starch correlated to lower amylose content. The range of M_w of amylopectin was 0.82 to 2.50 ×10⁸ g/mol, and there was also a negative correlation of amylopectin M_w with amylose content. Amylose M_w ranged from 2.20 to 8.31×10⁵ g/mol. After debranching the amylopectin with isoamylase, the weight‐average degree of polymerization (DP_w) for the long‐chain fraction correlated positively with a higher amylose content. California and Texas varieties were significantly different in their amylose content, starch M_w (short‐ and medium‐grain only), and amylopectin M_w (p < 0.05).     

Characterisation of normal corn starch using asymmetrical flow field‐flow fractionation

The apparent average molar masses (M_w,app), apparent average radii of gyration (R_g,app), diffusion co‐efficients (D_T), and hydrodynamic radii (R_h) of normal corn (maize) starch and fractions were determined using asymmetrical flow field‐flow fractionation coupled with multi‐angle light scattering and refractive index detectors (AF4/MALS/RI). AM‐type (Fraction A) and AP‐type (Fraction B) were chemically separated from normal corn starch. Normal corn starch and Fractions (A–B) were dissolved in 1 M KSCN using a high pressure microwave vessel. The effect of varying cross flow rates at a fixed channel flow rate upon the M_w,app and R_g,app distributions of normal corn starch and Fractions (A–B) were investigated. The average M_w,app of normal corn starch, Fractions (A) and Fraction (B) were 41 × 10⁶, 1.4 × 10⁶ and 39 × 10⁶ g/mol, respectively, with R_g,app values of 129, 60 and 129 nm, respectively.     

Determination of molar mass distribution of tapioca starch using asymmetrical flow field flow fractionation

The apparent average molar masses (M_w,app) and apparent average radii of gyration (R_g,app) of native tapioca starch and fractions were determined using asymmetrical flow field flow fractionation (AF4) coupled with multi‐angle light scattering and RI detectors (AF4/MALS/RI). AM‐type (Fraction A) and AP‐type (Fraction B) were chemically separated from native tapioca starch. Native tapioca starch and Fractions (A and B) were dissolved in 1 M KSCN using a high pressure microwave vessel. The effect of varying cross flow rates at a fixed channel flow rate upon the M_w,app and R_g,app distributions of native tapioca starch and Fractions (A and B) were investigated. The average M_w,app values for native tapioca starch, Fraction (A) and Fraction (B) were 59 × 10⁶, 2.1 × 10⁶ and 19 × 10⁶ g/mol, respectively, with average R_g,app values of 165, 73 and 87 nm, respectively. Hydrodynamic radii (R_h) values for native tapioca starch and fractions were determined directly from AF4 experimental parameters.     

Determination of the mass‐specific distribution of the substituents in cationic starch derivatives

The mass‐specific charge distribution in molar mass fractions of cationic starch derivatives was investigated. The molar mass fractions were produced by semi‐preparative SEC (SP‐SEC). The derivatives were cut into an amylopectin‐rich fraction F1, an intermediate fraction F2, containing low molar mass amylopectin and high molar mass amylose, and an amylose‐rich fraction F3. The weight‐average molar mass (M_w) and molar mass distribution (MMD) of the fractions were determined by SEC with multi‐angle laser light scattering (SEC‐MALLS). The mass‐specific charge of each fraction was calculated from the consumption of anionic titrant solution using polyelectrolyte titration (PET) in combination with particle charge detection (PCD). The difference in substituent distribution between the fractions was tested by the Student's t‐test. The weight‐average molar mass of the starch derivatives was not dependent on the degree of substitution (DS) or the derivatization process. Depending on the DS value or derivatization process, different substituent distributions were observed. The results for the mass‐specific charge distribution in different molar mass fractions of cationic starch derivatives with graded DS between 0.015 and 0.130 from the slurry process were discussed. The heterogeneity of substituent distribution decreased with increasing DS of the starch derivative. This was the case for samples from both the slurry and semi‐dry processes. The heterogeneity of derivatization was highest for low DS samples up to DS 0.03, with the amylopectin‐rich fraction incorporating more charges than the amylose‐rich fraction. This was more pronounced for the sample from the slurry process than from the semi‐dry process.     

Analysis of polysaccharide and proteinaceous macromolecules in beer using asymmetrical flow field‐flow fractionation

This paper demonstrates the potential of asymmetrical flow field flow fractionation coupled with online multi‐angle light scattering, differential refractive index and UV detection for the fractionation and analysis of macromolecules in beer regarding their composition, molar mass (M) and relative concentration. The macromolecules in the liquid and foam of two types of beer, light lager and porter, were analysed in their native state with minimal sample preparation. The results showed the presence of three major populations of macromolecules. In lager beer liquid, the early eluting population has an average M of 2 × 10⁴ g/mol and an intense UV absorbance at 280 nm suggesting the presence of proteinaceous macromolecules. The second and the third populations, which elute at consecutively longer retention times, have M ranging from 10⁵ to 10⁷ g/mol. They are not UV‐active at 280 nm, suggesting the elution of polysaccharides. The second population was identified as β‐glucans as a result of β‐glucanase treatment. The third population was not identified in the present study. The results show that similar populations are present in lager beer foam and that the macromolecules appear to be present in a more aggregated state. The M range of macromolecules in porter beer liquid ranged from 10⁵ to 10⁸ g/mol. A fraction of macromolecules eluting at longer retention times is highly UV‐active, which shows that there are great variations in the macromolecular profile of lager and porter beer. Copyright © 2015 The Institute of Brewing & Distilling     

Effects of molecular characteristics of on konjac glucomannan glass transitions of potato amylose, amylopection and their mixtures

BACKGROUND: The purpose of this study was to explore further the functions of konjac glucomannan (KGM) in starch‐based foods. Experiments were carried out using the mixed amylose/amylopectin/KGM system as a model. High‐speed differential scanning calorimetry (hyper‐DSC) with the support of high‐performance size exclusion chromatography (HPSEC) equipped with multi‐angle laser light scattering (MALLS) and differential refractive index (RI), X‐ray diffractometry (XRD) and viscosimetry was used to investigate the effects of KGM on glass transition temperatures (T_gs) of mixtures with different amylose/amylopectin ratios. RESULTS: Hyper‐DSC results showed that the T_gs of amylose, amylopection and their mixtures decreased with increasing concentration of KGM. Based on the molecular characteristics of KGM, HPSEC‐MALLS‐RI, viscosimetry and XRD results showed that the molar masses of KGM ranged from 1.023 × 10⁶ to 1.329 × 10⁶ g mol^?1; the root mean square (RMS) radii were distributed from 110.5 to 129.6 nm, and M_w/M_n was 1.017. KGM was a linear molecule with random‐coil conformation in solution and the crystallinity was 0.00%. CONCLUSION: It is suggested that the addition of KGM has plasticizing effects on the structures of amylose and amylopectin, which can increase free volume and molecular movement of amylose and amylopectin chains, resulting in a decrease in their T_gs. Copyright © 2010 Society of Chemical Industry     

Investigation of molecular weight distribution of LBG galactomannan for flours prepared from individual seeds, mixtures, and commercial samples

The molecular weight distribution has been determined for the galactomannan solubilized from three types of locust bean gum (LBG) flours: single carob seeds, mixtures, and a range of commercial products. To prepare crude endosperm flours from carob seeds with minimal galactomannan degradation, a new extraction and milling method was developed. The method consists of applying a brief thermal shock to the seeds, followed by an extended 3-day swelling period, and manual separation of endosperms; particle size reduction to a flour is accomplished on hydrated endosperms using a centrifugal mill. This method was optimized so that redissolved LBG flours produced solutions with the highest possible viscosity and the least amount of galactomannan degradation as determined by SEC. For the three samples types, the molecular weight distribution, w(M), was found to be unimodal, appearing as a sharply defined main peak (M_p≈1.1×10⁶ g/mol) with a small high molecular weight tail (up to 2.0×10⁶ g/mol) and broad low molecular weight tail (down to 0.01×10⁶ g/mol); polydispersities (M_w/M_n) were estimated to be 1.5–1.8. Variations in M_w and [η] for galactomannans extracted from individual seeds originating from the same carob tree were minimal and nearly indistinguishable from a bulk mixture (6 seeds, M_w=0.96–1.1×10⁶ g/mol, [η]=14.2–15.1 dl/g). There was a higher variability in these molecular parameters for galactomannans solubilized from commercial LBG flours, which generally exhibited lower M_w and [η], broader distributions, and reduced solubilities (M_w=0.86–1.0×10⁶ g/mol, [η]=12.4–13.6 dl/g). These side-effects were attributed to damage caused by industrial scale seed processing. The near constancy of M_p for the three sample types suggests that the average molecular size of LBG galactomannan varies only slightly due to natural or biological causes.     

Influence of the Cross‐linking Agent on the Gel Structure of Starch Derivatives

Hydrogels were synthesized through cross‐linking of carboxymethyl starch (CMS; Degree of Substitution DS = 0.45) using polyfunctional carboxylic acids (malic, tartaric, citric, malonic, succinic, glutaric and adipic acid). The syntheses used a cross‐linking agent ratio (ratio of the number of cross‐linking agent molecules to the number of monomer units constituting the polymer) of F_Z = 0.05. After cross‐linking the gels were dried, ground and then hydrogels of a polymer concentration of 4 mass‐% were produced. These CMS‐hydrogels were then rheologically characterized using dynamic oscillatory measurements. From measurements of the plateau region storage modulus G'_P, the network parameters molar mass between two entanglement points M_e (M_e ranging from 9.318 (citric acid) to 281.397 g/mol (tartaric acid)), the cross‐link density ν_e and the distance between two entanglement points ξ were calculated. Using carboxylic acids without other functional groups, a maximum in gel sturdiness is found at a spacer length of two CH₂‐groups. The evaluation of the loss factor tan δ for the CMS‐hydrogels showed that values of tan δ = 0.2 varied only slightly with the frequency ω. Flow curves showed a pseudopIastic flow behavior for all CMS‐hydrogels (the shear viscosity η declining over five decades in the range of the shear rate γ of 10⁻³ to 10³ s⁻¹) The different polyfunctional carboxylic acids have a strong influence on the sturdiness of the synthesized CMS‐hydrogels.     

Molecular Background of Technological Properties of Selected Starches

Selected starches, i.e. waxy maize, amaranth, quinoa, wheat, millet and buckwheat starches, were investigated with respect to their technological properties such as gelatinization, stability to mechanical stress, resistance to conditions and stability in continuous freeze/thaw cycles. Technological properties are correlated with molecular features such as branching characteristics in terms of iodine-complexing potential, molar mass, occupied glucan-coil volume, packing density of glucan coils and rheological properties. Waxy maize and amaranth starches were found to be amylopectin-type short-chain branched (scb) glucans with weight average molar masses M_w = 17 × 10⁶ g/mol and 12 × 10⁶ g/mol, respectively. Waxy maize starch had a high gelatinization potential, high viscosity at 95 °C (340 mPas) low stability at acidic conditions, average stability to shearing and good freeze/thaw stability. For amaranth starch a viscosity of 122 mPas at 95 °C, low resistance to acid, but high stability to applied shearing and even high freeze/thaw stability was determined. Investigated quinoa starch was classified as scb-type glucan, however, the branches are significantly longer than those of waxy maize and amaranth. With a M_w = 11 × 10⁶ g/mol and a viscosity of 187 mPas at 95 °C, this sample is comparably resistant to acidic conditions and to shearing, but instable in freeze/thaw experiments. Wheat, millet and buckwheat starches contain significant percentages of amylose-type long-chain branched (lcb) glucans (22.1, 32.1 and 24.3 %, respectively) with M_w values of 5 × 10⁶ g/mol, 12 × 10⁶ g/mol and 15 × 10⁶ g/mol, respectively. Wheat starch, with a viscosity of 107 mPas at 95 °C, shows low stability under acidic conditions, but high stability to shearing. Wheat and millet starches, but not buckwheat starch, form weak gels in the course of subsequent freeze/thaw cycles. Millet starch, with a viscosity of 101 mPas at 95 °C was found to be moderately stable under acidic conditions and to shearing. Buckwheat starch with a viscosity of 230 mPas at 95 °C shows no acid resistance and is instable upon shearing but performs very well in freeze/thaw experiments.     

Rehydration of Freeze‐Dried and Convective Dried Boletus edulis Mushrooms: Effect on Some Quality Parameters

ABSTRACT: Quality of rehydrated products is a key aspect linked to rehydration conditions. To assess the effect of rehydration temperature on some quality parameters, experiments at 20 and 70 °C were performed with convective dried and freeze‐dried Boletus edulis mushrooms. Rehydration characteristics (through Peleg's parameter, k₁, and equilibrium moisture, W_e), texture (Kramer), and microstructure (Cryo‐Scanning Electron Microscopy) were evaluated. Freeze‐dried samples absorbed water more quickly and attained higher W_e values than convective dried ones. Convective dehydrated samples rehydrated at 20 °C showed significantly lower textural values (11.9 ± 3.3 N/g) than those rehydrated at 70 °C (15.7 ± 1.2 N/g). For the freeze‐dried Boletus edulis, the textural values also exhibited significant differences, being 8.2 ± 1.3 and 10.5 ± 2.3 N/g for 20 and 70 °C, respectively. Freeze‐dried samples showed a porous structure that allows rehydration to take place mainly at the extracellular level. This explains the fact that, regardless of temperature, freeze‐dried mushrooms absorbed water more quickly and reached higher W_e values than convective dried ones. Whatever the dehydration technique used, rehydration at 70 °C produced a structural damage that hindered water absorption; consequently lower W_e values and higher textural values were attained than when rehydrating at 20 °C.     

Storage stability of the natural colourant from Justicia spicigera microencapsulated in protective colloids blends by spray‐drying

Muitle (Justicia spicigera), a Mexican native plant, produces a purple aqueous extract (MAE) because of its anthocyanin content. The aim of this work was to microencapsulate MAE by spray‐drying in two different protective colloids blends in a 1:1 weight ratio: gum Arabic‐mesquite gum (GA50‐MD50) and mesquite gum‐maltodextrin DE10 (MG50‐MD50), yielding the microcapsules M_GA50‐MD50 and M_MG50‐MD50. The minimum integral entropy of the microcapsules was determined at 20, 35 and 40 °C, and the resulting water activities (a_W) were 0.555, 0.592, 0.627 for M_GA50‐MD50 and 0.581, 0.587, 0.648 for M_MG50‐MD50, respectively. These a_W temperature sets were considered as the most adequate conditions for achieving maximum storage stability of the microcapsules. Total anthocyanin content (TAC) and total colour change (ΔE) suffered considerable degradation at all storage conditions, but that degradation was significantly inhibited by encapsulating MAE in the biopolymers blends especially that made up by MG50‐MD50.     

Effect of Molar Substitution on Rheological Properties of Hydroxypropylated Rice Starch Pastes

The steady and dynamic shear rheological properties of hydroxypropylated rice starch pastes (5%, w/w) were evaluated at different molar substitution (MS, 0.030‐0.142). The swelling power (35.5‐52.8 g/g) and solubility (8.19‐10.7%) values of the hydroxyproylated rice starches were higher than those of native rice starch (26.6 g/g and 7.78%) and increased with an increase in MS. The hydroxypropylated starch pastes at 25°C showed a pronounced shear‐thinning behavior (n = 0.33‐0.40) with Casson yield stress (σ_oc = 15.9‐31.7 Pa). The consistency index (K) and yield stress (σ_oc) values of the hydroxypropylated starch pastes were lower than those of the native starch, and increased progressively with an increase in MS. The apparent viscosity (η_a,500) obeyed the Arrhenius temperature relationship over the temperature range of 10‐55°C; the activation energies (E_a) of the hydroxypropylated starch pastes were in the range of 14.8‐18.5 kJ/mol, i.e. higher than that (14.1 kJ/mol) of the native starch. Storage (G′) and loss moduli (G′′) of hydroxypropylated starch pastes increased with an increase in MS, while tan δ (G′′/G′) values decreased, indicating that G′ rose more strongly than G′′ with increased MS.     

Enzymatic synthesis of ethyl hexanoate by transesterification

Summary Enzymatic synthesis of ethyl hexanoate by means of an acyl transfer reaction has been studied by using an immobilized Rhizomucor miehei lipase (RML). The effect of reaction parameters on ester synthesis has been investigated. Rhizomucor miehei lipase showed more specificity than other lipases when ethyl hexanoate was synthesized in n‐hexane. Maximum ester synthesis was obtained by using a 0.5 m substrate concentration (equimolar ratio). Temperatures in the range of 45–55 °C were found to be optimum and at higher temperatures (>60 °C) deactivation of enzyme was observed. Higher molar concentrations of hexanoic acid inhibited RML, but no inhibitory effect of ethyl caprate, even at higher molar concentrations, was observed. Apparent kinetic parameters have been determined. The values are as follows: K_M (ester), 0.0135 m ; K_M (acid), 0.08466; K_i (ester), 3.07 m ; K_i (acid), 0.550 m ; V_max, 1.861 µmol min^?1 mg^?1 enzyme.     

Production and characterisation of alginate from Azotobacter vinelandii

The alginate produced from glucose by Azotobacter vinelandii DSM 576 in the shaken‐flask (SF) scale was characterised in terms of chemical composition and average molecular mass (M_n). Its chemical composition was practically independent of the fermentation time with an average content in uronic acids of 0.66±0.09 g/g, a high proportion of mannuronic acid (M =0.75±0.04) and a mean acetylation degree of 18.5±7.9%. On the contrary, M_n reached a maximum value of ca. 220 kDa after 40–42 h fermentation and then decreased to 60 kDa in the following 20–22 hours. The fermentation process was scaled‐up in a laboratory fermenter (LF) and the kinetics of alginate production was classified of the mixed‐growth ‐associated product formation type with a contribution of non‐growth associated production of the order of 70 or 80% depending on the LF or SF scale used. However, the rate of M_n increase was found to be strictly growth‐associated, since alginate depolymerisation was observed as soon as the exponential cell growth rate stopped and this coincided with ammoniac nitrogen levels smaller than 10–15 g m ⁻³. © 1999 Society of Chemical Industry     

Thermal and rheological properties of the supersaturated sucrose solution in the presence of different molecular weight fractions and concentrations of dextran

In our current research work, we investigated the effects of molecular weight (M _w) and the concentration of dextran presence during cane sugar manufacturing on the rheological and glass transition properties of supersaturated sucrose solution. Three dextrans of various M _w, namely 100,000 g/mol (T ₁₀₀), 500,000 g/mol (T ₅₀₀) and 2,000,000 g/mol (T ₂₀₀₀), were admixed in concentrations between 1,000 and 10,000 ppm with 65 and 75% w/w sucrose solution. The results indicated that both the apparent viscosity and dynamic modulus increased with an increase in dextran concentrations and they demonstrated strong dependence on its M _w. Glass transition temperature (T _g) of the samples was measured by differential scanning calorimetry, and their dependence on dextran M _w and concentration was analyzed by the Fox and expanded Gordon–Taylor mathematical models. It was found that the higher the M _w and concentration of the dextran, the greater the increase in T _g. The expanded Gordon–Taylor equation has proved useful in predicting the T _g of the sucrose solution in the presence of polymer.     

Phenolic Compounds and Antioxidant Capacities of 10 Common Edible Flowers from China

The free and bound phenolic compounds in 10 common Chinese edible flowers were investigated using reversed phase high‐performance liquid chromatography. Their antioxidant capacities were evaluated using 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) radical‐scavenging activity, 2,2'‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulphonic acid) (ABTS) radical‐scavenging activity, oxygen radical absorption capacity (ORAC), ferric reducing antioxidant power (FRAP), and cellular antioxidant activity (CAA). Free factions were more prominent in phenolic content and antioxidant capacity than bound fractions. Paeonia suffruticosa and Flos lonicerae showed the highest total phenolic content (TPC) 235.5 mg chlorogenic acid equivalents/g of dry weight and total flavonoid content 89.38 mg rutin equivalents/g of dry weight. The major phenolic compounds identified were gallic acid, chlorogenic acid, and rutin. P. suffruticosa had the highest antioxidant capacity in the DPPH, ABTS, and ORAC assays, which were 1028, 2065, 990 μmol Trolox equivalents/g of dry weight, respectively, whereas Rosa chinensis had the highest FRAP value (2645 μmol Fe²⁺ equivalents /g of dry weight). The P. suffruticosa soluble phenolics had the highest CAA, with the median effective dose (EC₅₀) 26.7 and 153 μmol quercetin equivalents/100 g of dry weight in the phosphate buffered saline (PBS) and no PBS wash protocol, respectively. TPC was strongly correlated with antioxidant capacity (R = 0.8443 to 0.9978, P < 0.01), which indicated that phenolics were the major contributors to the antioxidant activity of the selected edible flowers.     

An SEC−MALLS Study of Molecular Features of Water‐soluble Amylopectin and Amylose of Tef [Eragrostis tef (Zucc.) Trotter] Starches

The molecular features of five tef starches along with those of commercial normal maize starch were investigated by size‐exclusion chromatography with multi‐angle laser light scattering‐differential refractive index detection (SEC/MALLS‐DRI) after solubilization in water by cooking in a household pressure cooker. The weight‐average molar mass ( ) and weight‐average root‐mean square radius of gyration (<R_g>_w) of the amylopectin (AP) of tef starches ranged from 10.1×10⁷ g/mol (156 nm) to 16.5×10⁷ g/mol (205 nm) with a mean of 13.9×10⁷ g/mol (186 nm). The AP of the tef starches was considerably smaller than that of maize starch ( = 19.6×10⁷ g/mol, <R_g>_w = 207 nm). These considerably smaller AP molecules in tef starches were most probably responsible for the low paste viscosity of tef starches as compared to maize starch. In most tef starches, the polydispersity index (PI) of the AP was broader than that of the AP of maize starch. The intermediate fraction (IN) 1.0−1.6, mean = 1.1) of most tef starches were similar to those of the IN of maize starch. The amylose (AM) (range 1.5×10⁶−3.0×10⁶ g/mol, mean = 2.2×10⁶ g/mol) and size (range 176−214 nm, mean = 191 nm) of most tef starches was also apparently similar to that of the maize starch ( = 2.3×10⁶ g/mol, <R_g>_w = 193 nm), but the polymer distribution was narrower. The AM−iodine complex of the tef starches had a λ_max range of 611−679 nm and the absorption shifted toward longer wavelengths by 8−14 nm as compared to the maize starch AM−iodine complex. The blue value (absorption at λ_max) for 1 mg/mL of tef AM had a range of 2.3−2.8 (mean = 2.5), whereas for the maize starch, the mean was 2.2. The branched nature of tef starches was also investigated by debranching with isoamylase and determination of chain lengths (DP_n) of the branches by size exclusion chromatography with refractive index detector (SEC‐RI). The AP in tef starches had a polymodal distribution with a periodicity similar to that of cereal starches. The branches had DP_n values of A = 11, B1 = 16, B2 = 46 (range 46−47), B3 = 70 (range 69−72) and B4 = 118 (range 113−123). The outer (A + B1) chains were shorter than those of maize starch AP with abundance (74%, w/w) only slightly less than that of the maize starch (75%, w/w). The slow rate of retrogradation, the slightly lower percent crystallinity, the lower gelatinization temperatures and the lower gelatinization enthalpy observed for tef starches (as compared to maize starch) are probably related to the shorter outer (A + B1) chain lengths of their amylopectin molecules, and may be the foundation of the comparably good keeping quality of tef injera, the main staple in the Ethiopian diet.