Fabienne Guillon

Structural and physical properties of dietary fibres, and consequences of processing on human physiology

Abstract Dietary fibre encompasses very diverse macromolecules exhibiting a large variety of physico-chemical properties. They might be naturally present in the food (in cell walls of vegetables and fruits, for instance) or introduced in the food to improve their nutritional properties (e.g. bran in bread products) or their physical characteristics (e.g. gelling agents or improved resistance to storage). The properties that are nutritionally relevant are mainly the particle size and bulk volume, the surface area characteristics, the hydration and rheological properties, and the adsorption or entrapment of minerals and organic molecules. Amongst these properties, the viscosity and ion exchange capacity are the main contributors to metabolic effects (glucose and lipid metabolisms) whereas fermentation pattern, bulking effect and particle size are strongly involved in effects on colonic function. Technological treatments can modify the physico-chemical properties of the fibre. This can be further exploited to optimise both their techno-functional and physiological properties.

Carbohydrate fractions of legumes: uses in human nutrition and potential for health

Starch and fibre can be extracted, using wet or dry processes, from a variety of grain legumes and used as ingredients for food. alpha-Galactosides can be isolated during wet processes from the soluble extract. Starch isolates or concentrates are mostly produced from peas, whereas dietary fibre fractions from peas and soyabean are commercially available. The physico-chemical characteristics of fibre fractions very much depend on their origin, outer fibres being very cellulosic whereas inner fibres contain a majority of pectic substances. Inner fibres are often used as texturing agents whereas outer fibres find their main uses in bakery and extruded products, where they can be introduced to increase the fibre content of the food. Most investigations on impacts on health have been performed on soyabean fibres. When positive observations were made on lipaemia, glucose tolerance or faecal excretion, they were unfortunately often obtained after non-realistic daily doses of fibres. Legume starches contain a higher amount of amylose than most cereal or tuber starches. This confers these starches a lower bioavailability than that of most starches, when raw or retrograded. Their low glycaemic index can be considered as beneficial for health and especially for the prevention of diseases related to insulin resistance. When partly retrograded, these starches can provide significant amount of butyrate to the colonic epithelium and may help in colon cancer prevention. alpha-Galactosides are usually considered as responsible for flatus but their apparent prebiotic effects may be an opportunity to valorize these oligosaccharides.

Physical and chemical transformations of cereal food during oral digestion in human subjects

Chemical and physical transformations of solid food begin in the mouth, but the oral phase of digestion has rarely been studied. In the present study, twelve healthy volunteers masticated mouthfuls of either bread or spaghetti for a physiologically-determined time, and the levels of particle degradation and starch digestion before swallowing were compared for each food. The amounts of saliva moistening bread and spaghetti before swallowing were, respectively, 220 (SEM 12) v. 39 (SEM 6) g/kg fresh matter. Particle size reduction also differed since bread particles were highly degraded, showing a loss of structure, whereas spaghetti retained its physical structure, with rough and incomplete reduction of particle size. Starch hydrolysis was twice as high for bread as for spaghetti, mainly because of the release of high-molecular-mass alpha-glucans. The production of oligosaccharides was similar after mastication of the two foods, respectively 125 (SEM 8) and 92 (SEM 7) g/kg total starch. Starch hydrolysis, which clearly began in the mouth, depended on the initial structure of the food, as in the breakdown of solid food. These significant physical and chemical degradations of solid foods during oral digestion may influence the entire digestive process.

GDP‐d‐mannose 3,5‐epimerase (GME) plays a key role at the intersection of ascorbate and non‐cellulosic cell‐wall biosynthesis in tomato

The GDP-D-mannose 3,5-epimerase (GME, EC 5.1.3.18), which converts GDP-d-mannose to GDP-l-galactose, is generally considered to be a central enzyme of the major ascorbate biosynthesis pathway in higher plants, but experimental evidence for its role in planta is lacking. Using transgenic tomato lines that were RNAi-silenced for GME, we confirmed that GME does indeed play a key role in the regulation of ascorbate biosynthesis in plants. In addition, the transgenic tomato lines exhibited growth defects affecting both cell division and cell expansion. A further remarkable feature of the transgenic plants was their fragility and loss of fruit firmness. Analysis of the cell-wall composition of leaves and developing fruit revealed that the cell-wall monosaccharide content was altered in the transgenic lines, especially those directly linked to GME activity, such as mannose and galactose. In agreement with this, immunocytochemical analyses showed an increase of mannan labelling in stem and fruit walls and of rhamnogalacturonan labelling in the stem alone. The results of MALDI-TOF fingerprinting of mannanase cleavage products of the cell wall suggested synthesis of specific mannan structures with modified degrees of substitution by acetate in the transgenic lines. When considered together, these findings indicate an intimate linkage between ascorbate and non-cellulosic cell-wall polysaccharide biosynthesis in plants, a fact that helps to explain the common factors in seemingly unrelated traits such as fruit firmness and ascorbate content.

Enzymic hydrolysis of the “hairy” fragments of sugar-beet pectins

Abstract The “hairy” fragments of acid-soluble (HP) and alkali-soluble (OHP) beet pectins have been treated with an arabinofuranosidase, an endoarabinanase, a β- d -galactosidase, and an endogalactanase from Aspergillus niger separately, in sequence, in combination, and prior to mild hydrolysis by 0.05 m trifluoroacetic acid. The products were analysed by h.p.l.c. and by gel-permeation chromatography on Bio-Gel P-2 and Sepharose CL-6B. The side-chains occur as branched structures attached to the rhamnogalacturonan backbone. They consist mainly of (1→5)-linked α-arabinans with branches attached to positions 3 randomly distributed along the main core, (1→4)-linked β-galactans of low d.p., and highly branched (1→3,6)-linked galactans. Of the feruloyl groups, 20–30% are carried by the arabinans and are probably responsible for their limited degradation by arabinofuranosidase. The remaining feruloyl groups are attached to the galactose residues and may contribute to the low activity of the galactanases. Some of the feruloyl groups and part of the galactose are not released by a more drastic treatment by acid. A tentative structure for the “hairy” fragments from sugar-beet pectins is presented.

Down-Regulation of the CSLF6 Gene Results in Decreased (1,3;1,4)-β-d-Glucan in Endosperm of Wheat

(1,3;1,4)-β-d-Glucan (β-glucan) accounts for 20% of the total cell walls in the starchy endosperm of wheat (Triticum aestivum) and is an important source of dietary fiber for human nutrition with potential health benefits. Bioinformatic and array analyses of gene expression profiles in developing caryopses identified the CELLULOSE SYNTHASE-LIKE F6 (CSLF6) gene as encoding a putative β-glucan synthase. RNA interference constructs were therefore designed to down-regulate CSLF6 gene expression and expressed in transgenic wheat under the control of a starchy endosperm-specific HMW subunit gene promoter. Analysis of wholemeal flours using an enzyme-based kit and by high-performance anion-exchange chromatography after digestion with lichenase showed decreases in total β-glucan of between 30% and 52% and between 36% and 53%, respectively, in five transgenic lines compared to three control lines. The content of water-extractable β-glucan was also reduced by about 50% in the transgenic lines, and the Mr distribution of the fraction was decreased from an average of 79 to 85 × 104 g/mol in the controls and 36 to 57 × 104 g/mol in the transgenics. Immunolocalization of β-glucan in semithin sections of mature and developing grains confirmed that the impact of the transgene was confined to the starchy endosperm with little or no effect on the aleurone or outer layers of the grain. The results confirm that the CSLF6 gene of wheat encodes a β-glucan synthase and indicate that transgenic manipulation can be used to enhance the health benefits of wheat products.

Arabinoxylan and (1→3),(1→4)-β-glucan deposition in cell walls during wheat endosperm development

Arabinoxylans (AX) and (1→3),(1→4)-β-glucans are major components of wheat endosperm cell walls. Their chemical heterogeneity has been described but little is known about the sequence of their deposition in cell walls during endosperm development. The time course and pattern of deposition of the (1→3) and (1→3),(1→4)-β-glucans and AX in the endosperm cell walls of wheat (Triticum aestivum L. cv. Recital) during grain development was studied using specific antibodies. At approximately 45°D (degree-days) after anthesis the developing walls contained (1→3)-β-glucans but not (1→3),(1→4)-β-glucans. In contrast, (1→3),(1→4)-β-glucans occurred widely in the walls of maternal tissues. At the end of the cellularization stage (72°D), (1→3)-β-glucan epitopes disappeared and (1→3),(1→4)-β-glucans were found equally distributed in all thin walls of wheat endosperm. The AX were detected at the beginning of differentiation (245°D) in wheat endosperm, but were missing in previous stages. However, epitopes related to AX were present in nucellar epidermis and cross cells surrounding endosperm at all stages but not detected in the maternal outer tissues. As soon as the differentiation was apparent, the cell walls exhibited a strong heterogeneity in the distribution of polysaccharides within the endosperm.

Methylation analysis and mild acid hydrolysis of the hairy fragments of sugar-beet pectins

Abstract The fragments of the rhamnogalacturonan backbone carrying neutral side-chains ( “hairy” fragments) of sugar-beet pectins have been subjected to methylation analysis and mild acid treatment. The fragments have similar structures which consist of a rhamnogalacturonan backbone carrying highly branched (1→5)-linked arabinans and linear (1→4)-linked galactans of low d.p. together with (1→3,6)-linked galactans in low proportions. Of the feruloyl groups, 50–60% are ester-linked to the neutral side-chains and are removed together with arabinose or galactose residues.

Viscous soluble dietary fibers alter emulsification and lipolysis of triacylglycerols in duodenal medium in vitro

Abstract The present in vitro study was designed to test the hypothesis that soluble dietary fibers can alter the process of lipid emulsification, and possibly, subsequent triglyceride lipolysis. Three guar gums, two pectins, and gum arabic were dissolved in reconstituted duodenal medium in the range 0.3 to 2.0% ( w v ). Viscosities of solutions were measured. Emulsification of a lipid mixture (triolein/phospholipids/cholesterol) was performed under mild conditions in the presence of increasing concentrations of soluble fibers. The amount of emulsifled lipids was reduced and the size of emulsified droplets was increased by raising the concentration of viscous fibers only. The extent of emulsification (r = −0.79), the droplet size (r = 0.88), and the overall droplet surface area (r = −0.59) were strongly correlated to the medium viscosity in the range 0–20 mPa.s. Addition of solutions of viscous fibers to a preformed standard emulsion did not change the lag time and initial velocity of pancreatic lipase reaction. Conversely, when incubating emulsions prepared in the presence of fibers (with different droplet sizes) with excess enzyme for 2 hours, the high- and medium-viscosity guar gums significantly reduced the extent of triglyceride lipolysis only. Thus, reducing emulsification of dietary lipids is a mechanisms by which soluble viscous fibers can alter lipid assimilation.

Brachypodium distachyon grain: characterization of endosperm cell walls

The wild grass Brachypodium distachyon has been proposed as an alternative model species for temperate cereals. The present paper reports on the characterization of B. distachyon grain, placing emphasis on endosperm cell walls. Brachypodium distachyon is notable for its high cell wall polysaccharide content that accounts for ∼52% (w/w) of the endosperm in comparison with 2-7% (w/w) in other cereals. Starch, the typical storage polysaccharide, is low [<10% (w/w)] in the endosperm where the main polysaccharide is (1-3) (1-4)-β-glucan [40% (w/w) of the endosperm], which in all likelihood plays a role as a storage compound. In addition to (1-3) (1-4)-β-glucan, endosperm cells contain cellulose and xylan in significant amounts. Interestingly, the ratio of ferulic acid to arabinoxylan is higher in B. distachyon grain than in other investigated cereals. Feruloylated arabinoxylan is mainly found in the middle lamella and cell junction zones of the storage endosperm, suggesting a potential role in cell-cell adhesion. The present results indicate that B. distachyon grains contain all the cell wall polysaccharides encountered in other cereal grains. Thus, due to its fully sequenced genome, its short life cycle, and the genetic tools available for mutagenesis/transformation, B. distachyon is a good model to investigate cell wall polysaccharide synthesis and function in cereal grains.