The Australian Wine Research Institute

Background

Tannins are a sub-class of phenolics that can precipitate proteins. They contribute to wine texture, particularly astringency. When grapes are crushed, the tannins present in grape skin and seeds begin to be extracted into the grape must. Those from skins tend to be more easily extracted than those from seeds. Once extracted, the grape tannins begin to chemically rearrange, turning into wine tannins, which can be significantly different in structure from the original grape tannins.

Addition, rather than extraction of tannin, has been reviewed by Versari et al. (2013). Key findings were that oenotannins had little impact on colour stabilisation regardless of timing of addition, the dose, oenotannin type, grape cultivar or maturity. Occasionally colour impacts can be observed soon after completion of fermentation, but they are generally not maintained through ageing (Smith et al. 2015). Impacts of oenotannin addition on astringency are often reported, but as the composition of commercially available oenotannins is highly variable the sensory results can also be variable.

Application and helpful hints

Rates of addition vary from 50 to 1,000 mg/L, and in the 2018 Cabernet trial a fairly conservative addition of 300 mg/L was used. The tannin chosen was TANIN VR SUPRA® ÉLÉGANCE, which was added to the must after crushing.

The cell walls from grape skins and pulp in grape must can affect the concentration and composition of added oenotannins, and thus oenotannins should be added to wine when the cell wall material suspended in the must is low (e.g. at the end of alcoholic fermentation or after pressing). Oenotannins with a smaller proportion of high molecular mass tannins (skin tannin) should be used to reduce the loss of tannins.