Marta S. Izydorczyk

Cereal arabinoxylans: advances in structure and physicochemical properties

Abstract Arabinoxylans constitute a major fraction of cereal cell wall polysaccharides. They consist of a linear β-(1 → 4) linked xylan backbone to which α- l -arabinofuranose units are attached as side residues via α-(1 → 3) and/or α-(1 → 2) linkages. Several structural models have been put forward based on enzymic degradation studies and structure elucidation of oligosaccharides by NMR, methylation, and periodate oxidation techniques. These tentative models present different substitution patterns of arabinoses along the xylan chain. Cereal arabinoxylans exhibit a great deal of structural heterogeneity with respect to ratio of Ara f Xyl p , substitution pattern of arabinoses, content of feruloyl groups and molecular size. The conformation and physicochemical properties (viscosity, gelation potential, intermolecular association) of arabinoxylans in aqueous solutions are dependent on the molecular features of these polysaccharides; specific structure-property relationships have been established in model and actual food systems. Wheat and rye arabinoxylans are important functional ingredients in baked products affecting the mechanical properties of dough, as well as the texture and other end-product quality characteristics.

Effect of arabinoxylans on bread-making quality of wheat flours

Two highly purified arabinoxylan preparations of different molecular weight (HMW, [η] = 5.48 dl/g and LMW, [η] = 3.69 dl/g) were used to study the effect of these water-soluble polysaccharides on the bread-making quality of two wheat flours: a Canada western red spring (CWRS) composite sample and a Canada prairie spring (CPS), cv. HY 368. Both preparations significantly increased the farinograph water absorption and the dough development time for the two flours; the HMW arabinoxylans exerted significantly (P < 0.01) greater effects than the LMW preparation. The HMW arabinoxylans maximally improved the loaf volume of CWRS and CPS breads at a level of 0.5% (w/w) supplementation. The LMW arabinoxylans maximally increased the loaf volume of CWRS and CPS breads at 0.7 and 1.1% (w/w), respectively. Breads containing HMW arabinoxylans retained significantly (P < 0.01) more water than the breads with LMW polymers at 0.5-0.9% (w/w) supplementation levels. Because of their higher moisture content the arabinoxylan-fortified breads exhibited a greater rate of starch retrogradation as assessed by calorimetry. Nevertheless, these breads had softer breadcrumbs than the controls; breads fortified (0.3-0.9% w/w) with HMW arabinoxylans were significantly (P < 0.01) less firm than those with LMW polymers over the 7-day storage period.

Structure and physicochemical properties of barley non-starch polysaccharides — I. Water-extractable β-glucans and arabinoxylans☆

Water-soluble non-starch polysaccharides were extracted from a Canadian malting barley (cv. Harrington) by sequential treatment with water at 40 °C (WE40) and 65 °C (WE65). The yields were 1.4 and 1.3% (w/w), respectively, of the dry barley grist. The WE40 extract was composed of 82.5% glucose, 8.9% xylose, and 7.0% arabiose residues, whereas WE65 contained 93.3% Glc, 3.3% Xyl, and 2.5% Ara. Only minute amounts of mannose and galactose residues were found in either fraction. Both extracts were further fractionated by stepwise (NH4)2SO4 precipitation into several polysaccharide populations. Subfractions from both extracts, obtained up to 45% saturation with (NH4)2SO4, contained mostly β-glucans, whereas subfractions precipitated at increasing saturation levels of (NH4)2SO4 (45–100%) contained progressively more arabinoxylans and less β-glucans. Compared to WE40, the WE65 extract was enriched in β-glucan populations with higher molecular size, higher limiting viscosity values, and higher content of β-(1 → 4) linkages. The ratio of tri-/ tetrasaccharide oligomers was also higher in β-glucans extracted at 65 °C than those extracted at 40 °C. Arabinoxylans in both extracts, WE40 and WE65, were highly substituted and contained large proportions of doubly substituted xylose residues.

Oxidative gelation studies of water-soluble pentosans from wheat

The gel formation potential of purified wheat water-soluble pentosans and their fractions, arabinoxylan and arabinogalactan, in the presence of H 2 O 2 /peroxidase was investigated by small amplitude oscillatory rheological measurements. Gelation proceeded in two stages: a rapid increase in gel rigidity ( G ´) during the first hour, followed by a much lower rate of gel development thereafter. Disappearance of feruloyl groups coincided with the first stage of the gelation process. Gel permeation chromatography on 8epharose CL-4B and Sephacryl 8-300 revealed that only arabinoxylan actively participated in the network. The rate and extent of gel rigidity development was dependent on polymer and oxidant concentration. Temperature had a negative effect on the gelation process. Gel network development with FeCl 3 was similar to that with H 2 O 2 /peroxidase, while (NH 4 ) 2 S 2 O 8 induced gelation at a much lower rate. The cross-linked pentosans and arabinoxylan held up to 100 g of H 2 O per g of polymer.

Functional food carbohydrates

Cereal -Glucans: Structure, Physical Properties, and Physiological Functions, A. Lazaridou, C. G. Biliaderis, and M. S. Izydorczyk Resistant Starch, Donald B. Thompson Konjac Glucomannan, K. Nishinari and S. Gao Seed Polysaccharide Gums, S. W. Cui, S. Ikeda, and M.N.A. Eskin Microbial Polysaccharides, I. Giavasis and C. G. Biliaderis Chitosan as a Dietary Supplement and a Food Technology Agent, R.A.A. Muzzarelli and C. Muzzarelli Arabinoxylans: Technologically and Nutritionally Functional Plant Polysaccharides, M. S. Izydorczyk and C. G. Biliaderis Carbohydrates and the Risk of Cardiovascular Disease, G. Onning Carbohydrates and Obesity, G. Woodward-Lopez, D. E. Gerstein, L. D. Ritchie, and S.E. Fleming Dietary Carbohydrates and Risk of Cancer, J. Slavin The Role of Carbohydrates in the Prevention and Management of Type 2 Diabetes, K. Poutanen, D. Laaksonen, K. Autio, H. Mykkanen, and L. Niskanen Carbohydrates and Mineral Metabolism, D.D. Kitts. Dietary Carbohydrates as Mood and Performance Modulators, L. Christensen Carbohydrates and Gastrointestinal Tract Function, B.O. Schneeman Probiotics, Prebiotics, and Synbiotics: Functional Ingredients for Microbial Management Strategies, G.C.M. Rouzaud Potential Use of Carbohydrates as Stabilizers and Delivery Vehicles of Bioactive Substances in Foods, P. Forssell, P. Myllarinen, P. Hakala, and K. Poutanen Food Regulations: Health Claims for Foods Fortified with Carbohydrates or Other Nutraceuticals, J. Zawistowski

Structural characteristics and rheological properties of locust bean galactomannans: a comparison of samples from different carob tree populations

Water-soluble galactomannans were isolated from the seeds of 12 carob tree populations grown in different regions of southern Greece. Their structures, molecular weight distributions and rheological properties were examined. The intrinsic viscosity values and estimated average molecular weights varied in the ranges 8.15–13.02 dl g−1 and (2.3–3.9) × 106 respectively. Molecular sieve chromatography showed large variations in the molecular size distributions. The Manp/Galp ratio varied between 3.08 and 3.82, as determined by GC analysis of the alditol acetate derivatives of the monomeric constituents. There were also differences in the relative distribution of the free (Manp-Manp), monosubstituted (Manp-Manp(Galp)) and disubstituted (Manp(Galp)-Manp(Galp)) dyads of β-(14)-mannose, as revealed by 13C NMR spectroscopy of the native polysaccharides. The pseudoplastic behaviour of galactomannan solutions (10–20 g l−1) was confirmed by steady shear and dynamic rheological tests. Aqueous galactomannan (5 g l−1)–xanthan gum (5 g l−1) mixed systems gave gels on cooling (G′ ≫ G″). Galactomannans with high limiting viscosities, high Manp/Galp ratios and a low proportion of the Manp(Galp)-Manp(Galp) dyad yielded the strongest mixed gel network structures with xanthan, implying that interchain associations between the two polymers occur via the unsubstituted segments (‘smooth regions’) of the carob galactomannan chains. © 2000 Society of Chemical Industry

Studies on the structure of wheat-endosperm arabinoxylans

Water-soluble arabinoxylans from a Canadian western red spring flour (cv. Laura) were fractionated by a graded ammonium-sulphate-precipitation technique into five fractions obtained at 55% (F55), 60% (F60), 70% (F70), 80% (F80), and 100% (F100) saturation with (NH4)2SO4. The yields of the fractions (based on the total material recovered) were 37·5%, 22·3%, 23·6%, 11·9%, and 4·7%, respectively. The relative amount of Xylp residues doubly substituted at C-2 and C-3 with Araf increased from F55 to F100, whereas the amount of mono- and unsubstituted Xylp residues decreased in the same order. The presence of short Araf chains was more pronounced in F100 than in other samples. The content of ferulic acid was highest in F55. Fractions F55 and F100 were degraded with purified endo-β-xylanase from Trichoderma viride. The hydrolysate fragments were separated, quantified (Bio-Gel P-2, HP-SEC), and characterized (1H-NMR and methylation analysis). F55 appeared to be built up from three structurally different regions. The first region, I55 (15%), was of relatively high DP and was high in terminal arabinose doubly linked to xylose residues at C-3 and C-2. The second region, II55 (40%), contained high amounts of terminal arabinoses linked to xylose residues at C-3. The third region, III55 (45%), contained high amounts of contiguously unsubstituted and C-3 monosubstituted xylose residues. Fraction F100 was built up mainly from the highly substituted region I100 (75%), enriched in C-2,3 di- and C-2 monosubstituted xylose residues as well as in short arabinose side chains. Region III100 (18%) contained a high proportion of unsubstituted xylose residues; however, the amount of disubstituted xylose residues in this region was still higher than that of monosubstituted residues.

Structural and functional aspects of cereal arabinoxylans and β-glucans

Publisher Summary Cereal grains, like many other foods of plant origin, contain numerous constituents with a varying degree of structural and functional complexity. Proteins and starches, being the major components in cereals, have been extensively studied to characterize their molecular structures and improve our understanding about structure–function relationships. As a result, the functional role of these biopolymers in cereal-based products has been irrefutably demonstrated. Moreover, some minor grain components, especially the non-starch polysaccharides, which constitute the structural polymers of cell walls in plant tissues, have begun to attract scientific attention as evidenced by the growing number of investigations undertaken in the recent past. This upsurge of interest in cereal cell-wall polysaccharides stems from the realization that these minor components play various roles in the human diet and substantially affect the physicochemical properties, the processing behavior, and the stability shelf life of cereal-based foods in storage. Among the several non-starch polysaccharides that build the cell walls of cereal grains, β-glucans and arabinoxylans have attracted the most attention. Advances in the elucidation of the fine structure of non-starch cereal polysaccharides have been possible via developments of appropriate analytical methods.

Production and characterization of pullulan from beet molasses using a nonpigmented strain of Aureobasidium pullulans in batch culture

The production of pullulan from beet molasses by a pigment-free strain of Aureobasidium pullulans on shake-flask culture was investigated. Combined pretreatment of molasses with sulfuric acid and activated carbon to remove potential fermentation inhibitors present in molasses resulted in a maximum pullulan concentration of 24 g/L, a biomass dry wt of 14 g/L, a pullulan yield of 52.5%, and a sugar utilization of 92% with optimum fermentation conditions (initial sugar concentration of 50 g/L and initial pH of 7.0). The addition of other nutrients as carbon and nitrogen supplements (olive oil, ammonium sulfate, yeast extract) did not further improve the production of the exopolysaccharides. Structural characterization of the isolated polysaccharides from the fermentation broths by 13C-nuclear magnetic resonance spectroscopy and pullulanase digestion combined with size-exclusion chromatography confirmed the identity of pullulan and the homogeneity (>93% dry basis) of the elaborated polysaccharides by the microorganism. Using multiangle laser light scattering and refractive index detectors in conjunction with high-performance size-exclusion chromatography molecular size distributions and estimates of the molecular weight (Mw=2.1−4.1×105), root mean square of the radius of gyration (Rg=30−38 nm), and polydispersity index (Mw/Mn=1.4−2.4) were obtained. The fermentation products of molasses pretreated with sulfuric acid and/or activated carbon were more homogeneous and free of contaminating proteins. In the concentration range of 2.8−10.0 (w/v), the solution’s rheologic behavior of the isolated pullulans was almost Newtonian (within 1 and 1200 s−1 at 20°C); a slight shear thinning was observed at 10.0 (w/v) for the high molecular weight samples. Overall, beet molasses pretreated with sulfuric acid and activated carbon appears as an attractive fermentation medium for the production of pullulan by A. pullulans.

Sequential solvent extraction and structural characterization of polysaccharides from the endosperm cell walls of barley grown in different environments.

The objective of this study was to examine the composition and molecular structure of the endosperm cell walls (CW) derived from barley grain grown in three environments in Canada, and differing in grain hardness, protein, and total β-glucan contents. The endosperm CW were isolated from barley, cv. Metcalfe, grown in Davidson, SK (Sample A), Hythe, AB (sample B), and Hamiota, MB (sample C). The CW were sequentially extracted with water at 65(o)C, saturated Ba(OH)2, again with water at 25(o)C, and 1M NaOH, resulting in fractions designated WE65, BaE, Ba/WE, and NaE, respectively. The monosaccharide analysis indicated the presence of β-glucans, arabinoxylans, and small amounts of arabinogalactans, glucomannans, and xyloglucans. Cellulose was detected in the CW remnants. The CW of sample A, exhibiting a lower grain hardness than sample B, contained the lowest amount of β-glucans, but the highest amount of arabinoxylans and the mannose-containing polysaccharides. The CW of sample C, characterized by very high protein content in the grain, contained the highest amount of β-glucans and the lowest amount of other polysaccharides. Polysaccharides in the CW of sample B, exhibiting the highest grain hardness, were characterized by the highest weight average molecular weights (Mw). β-Glucans in the CW of Sample B showed the highest ratio of DP3/DP4 and the longest cellulosic fragments in the polymeric chains. Of the three barley samples, arabinoxylans in the endosperm CW of sample A exhibited the lowest degree of branching, the highest amount of unsubstituted Xyl residues, and the highest ratio of singly to doubly substituted Xylp. The highest water solubility of the CW of sample C was associated with the highest concentration of β-glucans, the lowest DP3/DP4 ratio, and the lowest Mw of the polymeric constituents. Arabinoxylans with the lowest amount of doubly substituted but the highest amount of unsubstituted xylose residues and long sequences of unsubstituted xylan regions were found in the NaE fractions. The NaE fractions showed a high ratio of →4)-Glcp-(1→ to →3)-Glcp-(1→ linkages and some →4)-Manp-(1→ linkages, indicating a high level of long cellulosic regions in β-glucan chains and the presence of glucomannans.