Twin-screw extrusion extraction

The aim of this work was therefore to develop a process for direct atomization of the extract by considering twin-screw extrusion and ultrafiltration. Twin-screw extrusion is very efficient for alkali impregnation of wheat bran, but xylan extraction is achieved only with large L/S ratios. As xylan extraction from wheat bran alone is not feasible by twin-screw extmsion widi a low L/S, the coextrusion of wheat bran and wheat straw was investigated. 

The ultrafiltrated solutions were obtained under optimal conditions for stirred extraction (L/S = 25, bran/alkali = 1, 2 hour at 35°C) and twin-screw extrusion (L/S = 25, bran/alkali =1,1 hours at 35°C) and are described in table II. 

Ultrafiltration can be introduced in the polysaccharide purification scheme instead of the alcohol precipitation step, but the membranes must be optimized in each case. Fouling of the membrane can completely change its selectivity, and the fouling properties of the solution depend on the way the extract is obtained. Twin-screw extrusion or stirred extraction leads to solutions with partially hydrolyzed molecules. The size spectra of the molecules is very large, and the small molecules produced can be a source of membrane fouling. The best performance is obtained with the 50 kDa membrane for the stirred extract and with the 10 kDa, as concerns the twin-screw extract. It can be concluded from these results that the twin-screw extract contains smaller molecules than the stirred extract. 

The twin-screw extruder has proved to be a versatile tool for continuous treatment of vegetable matter, either for food or non-food applications. Trials for the direct alkaline extraction of xylans from wheat bran in a twin-screw extruder were unsuccessful. Bran impregnation with sodium hydroxide in the twin-screw extruder was very efficient, but it was necessaiy to make the separation between the hemicellulosic gel and the lignocellulosic matrix in another apparatus and remained difficult without a dilution to a L/S ratio of 50. Bran and straw co-extrusion was therefore investigated to be able to reduce the L/S ratio. Straw fibres form a dynamic plug in the restrictive elements of the screw profile just after the filtration zone. The pressure induced in the extruder sheath by the cellulosic fibres enabled the liquid/solid separation. 

Xylans from batch extraction are also good film formers even though the tensile properties of these films differ slightly from the twin-screw extracts. The difference might be a result of the compostion in minor compounds liable to oxidative cross linking in the twin-screw extract, thus leading to a more rigid film. Combined extrusion of wheat straw and bran leads to an extract witli a... 

Extraction Extrusion n An extrusion operation in which a volatile component present in the feedstock is removed by flash vaporization through a vent coimected to a vacuum pump. The volatile component is typically a small amount of water, but may be monomer or solvent. In a two-stage, single-screw extruder, the vent is located over the deep extraction section that begins the second stage of the screw. A few double-vented (three-stage) machines have been made. Some twin-screw machines have greater capacity for removal of volatiles. 

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