Fate of yeast and grape pectic polysaccharides of a young red wine in the cross-flow microfiltration process

Abstract

Cross-flow microfiltration of a young red wine through a mineral membrane of zirconium oxide (average pore size 0.2 μm) laid over a support of agglomerated microporous carbon reduced by 44 % the concentration of the starting wine in soluble polysaccharides. These carbohydrate polymers were mainly constituted of mannose, arabinose, galactose and galacturonic acid associated with minor amounts of rhamnose, glucose, xylose and fucose. The polysaccharides from starting wine and final permeate were separated by gel filtration on Ultrogel AcA 34 (exclusion limit for globular proteins 750,000) in at least four fractions (I-IV) of respective Kav 0.22, 0.50, 0.75 and 0.90. Each polysaccharidic population contained various proportions of yeast mannans, while grape polysaccharides were unequally distributed, fraction I containing neutral type II arabinogalactans and fractions II to IV being complex mixtures of type II arabinogalactans, arabinans and degraded forms of acidic rhamnogalactumnans (pectins). Losses due to microfiltration were positively correlated to hydrodynamic volume (molecular weight) of molecules: (I) "" 79 %, (II) "" 58 %, (III) "" 38 % and (IV) no loss. Yeast and grape polysaccharides coexisting in a given fraction (having the same Kav) were not equally affected by the microfiltration process, yeast mannans passing preferentially the membrane, while grape polymers were more retained. This differential retention was only observed in fractions of high molecular weights (I and II) and was discussed in relation with possible modifications at the molecular level (size and shape of polysaccharides) occurring in the concentration polarisation layer. Application of a back-flush pulse destined to unplug the membrane resulted in a reenrichment of the permeate in the polysaccharides present in the starting wine at a 82 % level.