Enzymic modification of cellulose - xyloglucan networks : implications for fruit juice processing = Enzymatische modificatie van cellulose - xyloglucaan netwerken

<br/>Xyloglucans play an important role in connecting cellulose microfibrils in the primary coli wall of plants, and the resulting cellulose-xyloglucan network is thought to determine the strength of these walls. Xyloglucans were isolated from apple fruit and potato tuber cell wall material by alkaline extraction and their primary structures were determined. Major differences between these two polysaccharides were their degree of backbone branching and the presence of fucosyl and arabinosyl residues.<p>The substrate specificity of three ondoglucanases from <em>Trichoderma viride</em> (endol, endolV and endoV) was investigated. The target substrate of endol is cellulose, that of endolV xyloglucan, whereas endoV is the most versatile endoglucanase having the ability of degrading both substrates. EndolV and endoV differ in their mode of action towards potato xyloglucan. Further, strong indications were obtained that xyloglucanase activity is related to a long array of substrate-binding sites.<p>The degradation of the cellulose-xyloglucan network in isolated coli wall material from apple fruit involves several glucanase activities. Xyloglucanase activity is important to make cell wall embedded cellulose more accessible to true cellulolytic enzymes such as endol and cellobiohydrolase. Extensive degradation is required because xyloglucan fragments having a backbone of 20 glucosyl residues (five building units) still bind to cellulose surfaces. These results might explain why fungi excrete so many different kinds of endoglucanases.<p>When the cell walls of living apple fruit tissue were treated with pectin lyase and a mixture of glucanases from <em>Trichoderma viride</em> (liquefaction), the ease with which the apple tissue disintegrated, seemed to depend on the maturity of the fruit. The disintegration of apple fruit tissue during liquefaction correlates to the level of (ripeningrelated) xyloglucan endotransglycosylase activity in apple fruit. An hypothesis for the synergism of fungal and plant glucanases is put forward. Under certain circumstances (controlled liquefaction) the cellulose-xyloglucan network can be modified in such a way, that a stable cellulose-based cloud is formed in the resulting apple juice. The significance of these observations for juice manufacturing is discussed