Compositional variability in commercial tannin and mannoprotein products


By Karien O'Kennedy

The aim of the study was to evaluate commercial tannin and mannoprotein products in terms of their composition and molecular size distribution.

Project layout

14 grape-based enotannins and eight mannoprotein products commonly used in the Australian wine industry were evaluated.

Enotannins:
·         Tannin products were labelled skin, seed, or skin and seed, according to the information on the product’s origin given by its supplier.
·         All enotannins were dissolved in a model alcoholic solution at 3 g/L and the total tannin concentration measured by the MCPT assay.
·         Subsequently the tannins were purified and subunit composition, mean degree of polymerisation (mDP) and molecular mass was determined.
·         The un-purified tannins were also analysed to determine the concentrations of monomeric phenolics such as catechin, epicatechin and gallic acid.
·         Anthocyanins were also measured.

Mannoproteins:
·         Powdered and liquid mannoproteins were reconstituted in model wine at 1 g/L.
·         Total polysaccharide content was measured.
·         After hydrolysis, released monosaccharides were measured.
·         Total nitrogen content was measured.


Main results

Enotannins:
·         There was a big difference in actual tannin content between the products, ranging from as low as 16% to as high as 90%.
·         The monomeric phenolic compounds: catechin, epicatechin and gallic acid made up between 1 – 10% of the enotannin product weights.
·         One product had a substantial proportion of anthocyanins: 12%.
·         All the enotannins contained tannins from ripe skins or seeds or both.
·         Only some of the products contained tannins from the origin claimed on the packaging (skin, seed, or skin and seed).
·         In some cases the analytical methods used could not determine the origin of the tannins. It is possible that these enotannin products may contain tannins from plant origin, not declared on the packaging. 
·         In some cases different products from the same supplier, claiming different effects, had similar compositions.

Mannoproteins:
·         Mannoprotein content as a percentage of the product ranged from 60 – 100%.
·         Compositional differences in terms of hydrolytically released monosaccharides were obtained and can be attributed to the different manufacturing methods of different suppliers. Saccharomyces cerevisiae contains two types of mannoproteins, which have different monosaccharide ratios.
·         Three of the products contained arabinogalactan-proteins (AGPs), which are not derived from yeast (can come from grapes or gum arabic).
·         The protein content varied between 10 and 50%.

Significance of the study

There is a big diversity amongst commercial tannins and mannoproteins and as a result, different effects can be obtained in the winemaking process. Some products showed good agreement in terms of origin and chemical composition in terms of what was declared on the packaging and some products did not. It is therefore recommended that winemakers do trials first to ensure that the claimed effect is obtained before making additions to wines.

Reference

Sijing Li, Kerry L. Wilkinson, Keren A. Bindon (2018). Compositional Variability in Commercial Tannin and Mannoprotein Products. Am J Enol Vitic. 69:176-181; published ahead of print December 15, 2017; DOI: 10.5344/ajev.2017.17057

Image: Shutterstock

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