Hairy regions

Comparison of the MHR with non-modified pectic hairy regions of apple cell wall, isolated in a mild and defined way, revealed great resemblance indicating that the modifications of the MHR during enzymic liquefaction were only minor. Analogous MHR fractions could be isolated from potato fibre, pear, carrot, leek, and onion tissue. 

Finally, an adapted model is presented for the prevailing population of apple MHR having the highest molecular weight. The universal validity of this model for pectic hairy regions from other plant sources is discussed. 

Other polysaccharides of primary cell walls.-A complex mixture of enzymes including endopolygalacturonase, pectin methylesterase, and/or pectin lyase solubilizes a mixture of polysaccharides from the primary cell walls of fruits [57-64]. Food scientists have referred for some 15 years to this mixture of polysaccharides as the hairy region to describe the highly branched character of the polysaccharides in the fraction and to emphasize the contrast to unbranched homogalacturonan. The recent discovery of rhamnogalacturonan hydrolase [65,66], which selectively cleaves the backbone of RG-I, led to the realization that the hairy... 

The polysaccharide composition and structure of pectic polysaccharides present in fruit-derived products have been the aim of numerous studies [14-19]. However, the attention of scientists was focused on the analysis of rhamnogalacturonan I or so-called pectic hairy regions. As far as we know, the presence of RG-II in fruit juices has not been reported. 

Schols, H.A., Vierhuis, E., Bakx, E.J., and Voragen, A.G.J. (1995) Different populations of pectic hairy regions occur in apple cell walls. CarbohydrRes. 275 343-360. 

The galacturonic acids of a plant cell wall mainly belong to smooth chains of homopolygalacturonic acid (PGA) and to hairy regions of rhamnogalacturonan I (RGI). In green plants, other uronic acids can be found in hemicelluloses. Provided they are not methylesterified, all these carboxylic acids deprotonate at the more or less acidic pH of wall water. The electrostatic charges of these polyanions are then compensated by cations ultimately derived from the environment. 

Subsequently, a clear Juice is obtained by ultrafiltration. A serious problem in this process is the fouling of the ultrafiltration membrane, causing a reduced flux rate. For apple processing, the material responsible for this effect has been isolated and extensively characterized [2-4]. It appeared to consist mainly of ramified pectic hairy regions (MHR), which were not degraded by the pectolytic enzymes present in the technical pectinase preparation. 

H.A. Schols, Structural characterization of pectic hairy regions isolated from apple cell walls. Thesis Wageningen Agricultural University (1995). 

Schols HA, Geraeds CCJM, Searle-van Leeuwen MF, Kormelink FJM, Voragen AGJ (1990a) Rhamnogalacturonase a novel enzyme that degrades the hairy regions of pectins. Carbohydr Res 206 105-115... 

Schols HA, Posthumus MA, Voragen AGJ (1990b) Structural features of hairy regions of pectins isolated from apple juice produced by the liquefaction process. Carbohydr Res 206 117-129... 

Schols HA, Voragen AGJ, Colquhoun IJ (1994) Isolation and characterization of rhamnogalacturonan-oligomers, liberated during degradation of pectic hairy regions by rhamnogalacturonase. Carbohydr Res 256 97-111... 

One peak is eluted at the void volume of the column and contained the bulk of neutral sugars and few galacturonic acid residues while the peak at the total volume contained almost pure oligogalacturonides. Therefore the same model of "hairy" regions and "smooth" regions also applied to these pectins. 

When comparing this result with that obtained with the acid-extracted pectins, the same two populations were separated but the amount of "hairy" regions is lower for the acid-extracted pectins (less than 10 % of the pectins) than for the pectins (=15 %). This result confirmed that extrusion-cooking is less degradative towards the side-chains that the acid treatment it also shows that both pectins have large "smooth" regions. 

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