WINETECH Technical Yearbook 2019

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 (Obreque-Slíer et al ., 2009). 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 be tween 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. Depend i ng on the mo l ecu l ar we i ght 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 (Poncet-Legrand et al ., 2007). Higher molecular weight mannoprotein products can precipitate tannin. Another study showed t hat approx imate l y 70 kDa mannoproteins can prevent anthocyanin adduct precipitation and thereby promote colour stabilisation (Alcalde-Eon et al ., 2014). SIGNIFICANCE OF THE STUDY The study indicates the diversity of offerings between suppliers of products marketed as oenotann i ns and mannoprote i ns . Winemakers cannot simply just “chop and change” between supp l i ers , 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 Li, S., Wilkinson, K.L. & Bindon, K.A., 2018.

Compositional variability in commercial tann i n and mannoprote i n produc t s . American Journal of Enology and Viticulture 69(2): 176-181. Harbertson, J.F., Parpinell, G.P., Heymann, H. & Downet, M.O., 2012. Impact of exogenous tannin addi tions on wine chemistry and wine sensory character. Food Chemistry 131: 999-1008. 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. 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. 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.