The pineapple (Ananas comosus (L.) Merrill) is the third most abundant tropical fruit cultivated in Brazil. Of the plagues which attack its plantations, gummosis is the most harmful because the production costs are increased and the quality of the fruit is reduced. These are caused by Fusarium subglutinans, which produces an external gum. The polysaccharide present in this gum is now studied. The gum was collected from the variety "Pérola", which is more susceptible to attack of the pathogen. It was dissolved in water and after with 0.5% aqueous KOH at room temperature. The solutions were treated with excess ethanol (the last after neutralization with 50% HOAc) to give precipitated fractions (APA and APK), which were dialyzed and freeze-dried. APK was hydrolyzed with 2M TFA and its monosaccharide composition was Ara, Xyl, Gal, Glc and uronic acid (31:18:13:7:31 molar ratio), as shown by GC-MS of derived alditol acetates. The ¹³C-NMR spectra of APA and APK were similar and very complex with many signals in the C-1 region, from d 96.4 to 109.0. HPSEC analysis showed a heterogeneous profile, and when APK was treated with aq. 2.5% cetylpyridinium chloride, a precipitate was formed (PAPK), which was homogeneous by HPSEC. The monosaccharide composition showed Ara, Xyl, Gal, Glc and uronic acid (25:18:14:14:29). A mild acid hydrolysis of PAPK (0.1M TFA, 3h, 100ºC) was carried out and the resulting polysaccharide had a ¹³C-NMR spectrum simpler than the original fraction and it monosaccharide composition was Ara, Xyl, Gal, Glc and uronic acid (1:28:5:32:35). In order to degrade the polysaccharide two controlled Smith degradations were carried out and a decrease of Ara, Gal and Glc with and increase of Xyl proportions was observed, indicating that these Xyl units are present in the main chain. From the ethanolic supernatant of aqueous extraction were isolated two oligosaccharides that will help elucidated the polysaccharide structure. Methylation analyses will then show the linkage between monosaccharide units.

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