A soluble mannan from ripe banana fruit

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Virginia Polytechnic Institute and State University


Soluble and insoluble non-starch polysaccharide (NSP) fractions have been isolated from the pulp of ripe banana fruit.

Attempts to determine the composition of the insoluble NSP indicated the fraction to be exceptionally complex and difficult to analyze. Only about 44% of the dry weight could be accounted for as NSP, the protein content was high (26%) and about 22% of the dry weight appeared to be a complex mixture of phenolic and other unknown components. One interesting property of the insoluble NSP was its unusually high water-holding capacity.

The soluble NSP (isolated by extraction from fresh pulp with water at 25°C and not dried prior to analysis) contained about 48% pectin, 20% hemicellulose, 21% partially hydrolyzable polysaccharides and 11% protein. The hemicellulose represented about 0.05% of the banana fresh weight or about 0.2% of dry weight. Hydrolysis of the hemicellulose and analysis of the resulting sugars via gas chromatography revealed the hemicellulose to be made up predominantly (84%) of mannose, the remainder being 12% arabinose and 4% xylose. After removal of most of the pectin via treatment with anion exchange resin, the major hemicellulose component was isolated by gel filtration. The isolated hemicellulose was a homomannan as demonstrated by gas chromatography and high performance liquid chromatography of acid hydrolysates.

The banana homomannan appears to be unique among plant mannans. All previously isolated plant homomannans were reported to be insoluble in water. Further studies will be required to establish the exact structure of the banana mannan. The molecular weight (MW) of the mannan in solution appears to be exceptionally high, the peak MW being about 5 million, as determined by gel filtration.