J.-F. Thibault

Cell wall polysaccharide interactions in maize bran

Sequential extractions with alkali have been carried out in order to study the nature of linkages which hold heteroxylans in maize bran cell walls. Treatment with 0.5 m sodium hydroxide at 30 °C for 2 h released all the phenolic acids (p-coumaric, ferulic, and diferulic) but extracted only ~30% of heteroxylans (S1); further treatment with 1.5 m potassium hydroxide at 100 °C for 2 h released the remaining heteroxylans (S2). The heteroxylans from S1 and S2 had a similar neutral sugar composition and structure, but their weight average molecular weights were 270 kDa (MwMn = 2) and 370 kDa (MwMn = 2.8), respectively. Proteins (5%) are found with polysaccharides from S1 and S2, with different amino acid composition. The results suggest that covalent linkages through phenolic acids are only partly responsible for the associations of maize bran heteroxylans in the cell wall and that linkages to structural cell wall proteins were probably the main cause of heteroxylan insolubility.

Influence of extrusion-cooking on the physico-chemical properties of wheat bran

Wheat bran was extruded under various conditions with a Clextral BC 45 twin screw extruder. Water (6·1–17·5 % of dry solid feed rate) and feed rate (20–32 kg/h) were varied in order to obtain treatments of different severities. These were monitored by the specific mechanical energy measured by the electrical consumption of the main motor drive. The water solubility of the extruded products increased from 20 to 40 % and the soluble fibre content from 8 to 16 %. These values were well correlated to the specific mechanical energy (230–380 kWh/t). Analysis of the sugar composition showed a slight increase in the solubility of glucuronoarabinoxylans arising from the external pericarp layer and a marked increase in the solubility of arabinoxylans and β-glucans probably arising from the aleurone and endosperm cell walls. Water absorption capacity, as measured by the Baumann method, increased from 270 to 375 % for the products processed under low severity treatments.

Alkaline extraction and characterisation of heteroxylans from maize bran

Abstract Response surface methodology was used to study the alkaline extraction of heteroxylans from maize bran. The ratio volume of alkali/weight of bran and the particle size had no effect on extraction yield, whereas the yield increased significantly with the temperature and time of extraction and the concentration of the alkali. The variation in the yield depended on the nature of the alkali, and empirical second-order models were built to fit the results obtained by extraction with KOH and Ca(OH)2. Comparison of the compositional and structural features of the heteroxylans obtained by extraction with 0·8 m KOH at 85 °C, saturated Ca(OH)2 at 95 °C and 0·5 m KOH at 65 °C with one obtained by industrial lime-cooking of maize kernels showed that all four samples were very similar and that a very high extraction yield (87 %) was achieved.

Studies on apple protopectin: I. Extraction of insoluble pectin by chemical means

Abstract The pectic material from Golden Delicious apples, strongly bound in the cell walls of parenchymatous tissues (not extractible with Cyclohexane-Diamino-Tetracetic Acid (CyDTA)) was extracted by: (i) successively, hot buffer, hot acid and cold sodium hydroxide, (ii) sodium carbonate (4°C then 20°C) and (iii) chlorite. The pectin extracts were analysed for uronide, neutral sugars and protein contents, and degrees of methylation and acetylation. They were also subjected to gel-filtration and ion-exchange chromatography. Subfractions obtained by ion-exchange were also analysed. Approximately 55% of the pectic material was not extracted by the CyDTA. Hot buffer, hot acid and chlorite extracted small amounts of relatively highly methoxylated and acetylated pectins. The most efficient extractants were the cold carbonate and the cold sodium hydroxide. They extracted saponified pectins that exhibited similar behaviour on gel-filtration and ion-exchange chromatography, showing two subfractions, of which one was rich in neutral sugars and had a high molecular weight. There was no evidence of phenolic interlinkages.

Studies on apple protopectin V: Structural studies on enzymatically extracted pectins

Abstract Apple insoluble pectins were extracted by pectolytic enzymes alone and combined with side-chain degrading enzymes or endo -glucanase. The highest yield of pectic material was obtained with the combination pectinlyase plus endo -glucanase, showing the existence of a connection between rhamnogalacturonan and xyloglucan. The extracts were fractionated on Sephacryl S 500 and DEAE Sepharose CL-6B. Methylation analysis of high-molecular-weight material coming from the ‘hairy’ regions showed the presence of highly branched arabinans and arabinogalactans of type I and II side-chains, and of terminal xylose residues linked to the rhamnogalacturonan backbone.

Characterisation and oxidative crosslinking of sugar-beet pectins extracted from cossettes and pulps under different conditions

Abstract Pectins have been extracted from sugar-beet cossettes by a sequential treatment with water, oxalate, hot dilute acid and cold dilute alkali, and from pulp with different conditions of pH and temperature. All these samples have been chemically characterised and tested for their gelling capacity with persulphate. Pectins from cossettes have characteristics very close to those previously extracted in the same way but are not able to give gels by addition of persulphate. On the other hand, some, but not all, of the pectins from pulp can gel under these conditions. No simple relationship was found between the gelling ability and the chemical characteristics of the pectins.

Studies on apple protopectin. II: Apple cell wall degradation by pure polysaccharidases and their combinations

Plant cell walls are complex associations of polymers, the interconnections of which can be studied by sequential enzyme degradation. Partially depectinated apple cell wall material was degraded by pure polysaccharidases (pectin-lyase, polygalacturonase, pectinesterase, endo-β-(1,4)-glucanases, exo-β-(1,4)-glucanase, endo-α-(1,5)-arabinanase, arabinofuranosidase, endo-β-(1,4)-galactanase and endo-β-(1,4)-xylanase) and combinations of these enzymes. Amounts of extracted material and composition of the residues were determined. Principal component analysis permitted an objective assessment of these results. Extraction of significant amounts of uronides was only possible with enzyme formulations active on highly methylated pectins. Addition of endo-β-(1,4)-glucanase enhanced cell wall degradation by enzyme formulations active on highly esterified pectins, but arabinanases and galactanase did not increase the extraction of uronides. Principal component analysis indicated that the effects of the endo-β-(1,4)-glucanases were related to degradation of a fucogalactoxyloglucan, and emphasized the specificity of the action of one of the endo-β-(1,4)-glucanases.

Studies on extraction of pectins from citrus peels, apple marks and sugar-beet pulps with arabinanase and galactanase.

Abstract Pectin was extracted from citrus, apple and sugar-beet pulps, with an enzyme preparation from Bacillus subtilis . The preparation contained endo -arabinanase, endo -galactanase and residual endo -pectate lyase. Extraction conditions were 30°C and 0·03 m sodium acetate buffer, pH 5·0, so that pectate lyase activity was minimised. Under these conditions, an appreciable amount of pectin (4·9% by the buffer and 10·8% by the enzymes) could be extracted from citrus pulp, but very little from apple and sugar-beet pulps. The citrus pectin had a relatively low molecular weight (47 000) compared to acid-extracted pectin (82 000). The breakdown may be due to either enzymatic or chemical β-elimination. Significant amounts of pectin (12·9%) can also be extracted from citrus pulp with water and chelating agents. It was concluded that pectins cannot efficiently be extracted with these enzymes.

Efferent projections from the retrochiasmatic area to the median eminence and to the pars nervosa of the hypophysis with special reference to the A15 dopaminergic cell group in the sheep.

Anterograde tracers, viz. Phaseolus vulgaris leucoagglutinin and fluorescein dextran, were used in conjunction with tyrosine hydroxylase immunohisto-chemistry to study the projections of the A15 dopaminergic cell group towards the median eminence and pituitary in sheep. After injection of the tracers in the retrochiasmatic area, which contains the cell group A15, fibres containing anterograde tracer were observed in the internal zone of the median eminence and in the pars nervosa of the pituitary. Numerous tyrosine hydroxylase immunoreactive fibers were present in the external zone of the median eminence and in the pars intermedia and the pars nervosa of the pituitary, with characteristic patterns of organisation in each area. Most tyrosine hydroxylase-immunoreactive fibres containing fluorescein dextran were located in the pars nervosa, whereas only a few were observed in the internal zone of the median eminence. It was concluded that at least part of the dopaminergic innervation of the pars nervosa originated from the A15 group. These results provide morphological evidence for (1) the role of dopaminergic neurons of the A15 cell group in the seasonal control of prolactin secretion via the release of dopamine in the pars nervosa, and (2) putative physiological interactions between dopamine and the secretion of neurohypophysial hormones in sheep.

Viscometric and potentiometric study of high-methoxyl pectins in the presence of sucrose

The polyelectrolytic behaviour of high-methoxyl pectins (degree of esterification = 72·9) was studied in the presence of sucrose. Potentiometric measurements were carried out on the acidic form by titration with NaOH. Variations of the apparent dissociation constant pKa as a function of the degree of dissociation α were studied in salt-free solutions on a range of sucrose contents (0–60% w/v). Experimental plots could be described by Lifson-Katchalskys (LK) treatment up to a 20% sucrose content. The intrinsic apparent dissociation constant pK0 value was 3·1 ± 0·1 in agreement with previous data on pectinic acid. Above the 20% sucrose content, the LK theory was not appropriate owing to aggregation phenomena of macromolecular chains. Viscometric measurement were performed on the sodium form (α = 1) and for three sucrose contents (0%, 40% and 60%). By carrying out isoionic dilutions, the effect of ionic strength (It) on the intrinsic viscosity [η] was studied. The same infinite limit value [η]∞ was obtained for each sucrose content and linear variations of [η] as a function of It−12 allowed an estimate of the relative stiffness of the macromolecular chain. This stiffness was slightly increased by sucrose addition. From these data, it could be concluded that a high sucrose content did not significantly change the macromolecular size (for α = 1), as shown by viscometry. However, when α decreased, potentiometric investigations with sucrose showed a higher value of pKa and a down-curvature of pKa(α) which could be interpreted by assuming aggregation. These occurred at about 40% sucrose and above which was a much lower concentration than the gelling conditions (about 60%). Therefore aggregation phenomena could be present for relatively low sucrose contents if α < 0·5. It was postulated that interchain associations could occur progressively (from 40 to 60%) and involve aggregate formation with a critical size for gelation.