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Australian researchers did an in depth study of the compositions of 14 commercial grape-derived tannins and eight mannoproteins obtained from five different suppliers in the Australian market.1 They found significant differences in the compositions of these commercial products and cautioned winemakers that significantly different effects can be obtained during winemaking, depending on the choice of product.

The use of commercial grape derived tannins (oenotannins) and mannoproteins is widespread amongst winemakers around the world. Oenotannins are added to juice or fermenting must to promote colour stability, increase tannin content in final wines, prevent oxidation or to mask wine faults. Mannoproteins are usually added to finished wines to increase mouthfeel, reduce astringency or to improve tartrate stability. Generally suppliers of commercial tannins and mannoproteins place emphasis on the proposed effect of these products and supply only limited compositional information on these products.

 

Experimental layout

Tannin products were labelled skin, seed, or skin and seed according to the information given by the suppliers. The total tannin concentration of all products was measured using the MCPT essay. All the oenotannin products were then purified to obtain only the tannin fractions in order to determine subunit composition, degree of polymerisation (mDP) and molecular mass. These three parameters determine the origin of the tannins, i.e. skin or seed. The unpurified oenotannin products were also analysed for monomeric phenolics (tannins are polymeric structures), such as catechin, epicathechin and gallic acid, in order to determine their contribution to the total product sold in the bag (or container in the case of the two liquid oenotannins analysed). Finally anthocyanin concentration was also measured.

Both powdered and liquid mannoproteins were reconstituted in a model wine solution at 1 g/L. After hydrolysis total polysaccharide composition was determined and the released monosaccharides were quantified. Total nitrogen content was also measured.

 

Results and discussion

There was a big difference in the actual tannin content between commercial products, ranging from as low as 16% to as high as 90%. This finding is in accordance with a previous study that found oenotannin products to contain only 12 – 48% of actual tannin.2 The monomeric phenolic content only accounted for 1 – 10 %, therefore not explaining what is filling the rest of the bags. One product contained 12% anthocyanin, most likely not declared by the supplier on the packaging. All the products analysed contained tannins from skins, or seeds, or both. Several products contained what was indicated on the packaging. In some cases the analytical methods could not determine whether the tannins were from skins or seeds, indicating the possibility that these oenotannins may contain tannins from stems, as stem tannins contain the same subunits as skin and seed tannins, interfering with origin identification using chemical analysis. Previous studies also reported that commercial tannins may contain tannins from plant origin not declared on the packaging.3 The implications here are that skin tannins are usually more expensive than other types of tannins and in this case winemakers who paid a premium are not receiving what they paid for. Lastly in the case of one supplier, three products had similar compositions, but were marketed as three distinct products.

As a percentage of the total product the actual mannoprotein concentrations ranged from 60 – 90%. Differences were observed in the hydrolytically released monosaccharides. Typically yeast mannoproteins contain mannose and glucose monosaccharides and depending on the mannose glucose ratio, one can determine if the origin of the mannoproteins are exocellular, i.e. secreted during fermentation, or if they are cell wall mannoproteins. Compositional differences were indeed observed between the products, indicating different production processes of manufacturers.

Three of the eight mannoprotein products contained significant amounts of galactose and arabinose. This is an indication of the presence of arabinogalactan-proteins (AGPs), which can only come from plants, e.g. grapes or gum arabic. In the case of one product the arabinose and galactose concentrations exceeded the mannose and glucose units indicating that this product sold as “mannoproteins” is indeed probably more gum arabic.

The protein content of the mannoproteins ranged between 10 and 50%. The significance of this is that proteins can bind to polyphenols, especially tannins, and then precipitate. The higher the protein content of mannoprotein products, the bigger the effect on reducing astringency. On the other hand polysaccharides and tannins form more loose aggregates that are more likely to stay in solution.

Depending on the molecular weight distribution that was observed for the different mannoproteins (5 – 600 kDa), these products can have differential effects on wine sensory qualities. One study showed that smaller molecular weight mannoproteins (50 – 60 kDa) can reduce tannin aggregation.4 Higher molecular weight mannoprotein products can precipitate tannin. Another study showed that approximately 70 kDa mannoproteins can prevent anthocyanin adduct precipitation and thereby promote colour stabilisation.5

 

Significance of the study

The study indicates the diversity of offerings between suppliers of products marketed as oenotannins and mannoproteins. Winemakers cannot simply just “chop and change” between suppliers, as vastly different effects can be obtained. Winemakers are advised to perform small scale trials first, in order to establish if the proposed effect of these type of products are observed in their wines, before using it on commercial scale.

 

References

  1. Li, S., Wilkinson, K.L. & Bindon, K.A., 2018. Compositional variability in commercial tannin and mannoprotein products. American Journal of Enology and Viticulture 69(2): 176 – 181.
  2. Harbertson, J.F., Parpinell, G.P., Heymann, H. & Downet, M.O., 2012. Impact of exogenous tannin additions on wine chemistry and wine sensory character. Food Chemistry 131: 999 – 1008.
  3. Obreque-Slíer, E., Peña-Neira, A., López-Solís, R., Ramírez-Escudero, C. & Zamora-Marín, F., 2009. Phenolic characterization of commercial enological tannins. European Food Research and Technology 229: 859 – 866.
  4. Poncet-Legrand, C., Doco, T., Williams, P. & Vernhet, A., 2007. Inhibition of grape seed tannin by wine mannoproteins: Effect of polysaccharide molecular weight. American Journal of Enology and Viticulture 58: 87 – 91.
  5. Alcalde-Eon, C., García-Estévez, I., Puente, V., Rivas-Gonzalo, J.C. & Escribano-Bailón, M.T., 2014. Color stabilization of red wines. A chemical and colloidal approach. Journal of Agricultural and Food Chemistry 62: 6984 – 6994.

 

– For more information, contact Karien O’Kennedy at karien@winetech.co.za.

 

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