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Tannin Management in the Vineyard

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Tannin Management in the Vineyard Fact Sheet MaY 2010 tannin Management in the Vineyard Author: Dr Mark Downey Group Leader, Plant Production Sciences, Mildura Senior Research Scientist, Viticulture & Oenology cwrdc GRAPE AND WINE RESEARCH AND DEVELOPMENT CORPORA TION c 210mm tannin ManaGeMent in the VineYaRd Fact Sheet MaY 2010 What is tannin? extension subunits can be any of the flavan-3-ols, but in most studies of grapes, catechin is the common terminal subunit and tannins are bitter and astringent compounds found in most epicatechin is the most common extension subunit. Published plants. they are polyphenolic research suggests that grape compounds that fall into two polymers range in length up to 30- classes, hydrolysable and condensed 40 subunits. tannins. there are many hundreds of There are TWO classes of in grapes, condensed tannins are individual compounds, but they have tannins, HYDROLYSABLE present in high concentration in the a few things in common. all tannins seeds and skins, but are also present bind protein, which is the basis of and CONDENSED tannins in the leaves and stems. tannins are their role in tanning hides for leather also present in wood, but become and gave rise to their name. this bound to cell walls and other phenolics during lignification and property of binding tannin also gives rise to the term astringent are not easily extracted. from the Latin, ad astringere, “to bind”. Roles of tannin Measurement of tannin tannins play two defensive roles, one against micro-organisms there are many methods for measuring tannin. Most methods and the other against herbivores. in both cases tannins bind measure phenolics, rather than tannins. the most common to protein making it unavailable as a nitrogen source. tannins phenolic measure is absorbance at 280nm (a280). as a also bind to cell walls and other phenolics to create a defensive surrogate for tannins it is quite poor because it assumes that barrier when cell integrity is lost. Being bitter and astringent, most of what was measured by a280 is tannin, which is often their taste is a deterrent to herbivores. not the case. all of the small non-tannin phenolic compounds absorb at 280nm, as do anthocyanins, flavonols, proteins, and RNA & DNA. a number of methods are based on stains that hydrolysable tannins react with tannins with varying degrees of specificity. these hydrolysable tannins include: dMACA (dimethylcinnamaldehyde), vanillin-HCL, Folin- include gallotannins denis, Folin-ciocalteau and iron chloride (Fecl3). Some of these and ellagitannins. methods are quite reproducible, but they assume non-tannin Gallotannins are made material is either insignificant or at least constant; there isn’t up of gallic acid bound good evidence for either assumption. to a sugar molecule, eg. the French method and the Butanol-HCL method rely on boiling pentagalloyl-glucose. the tannin in acid to produce the coloured anthocyanidin ellagitannins are precursors to anthocyanins. this approach is useful because it complexes of gallic acid. will release bound tannin. however, the lack of standards make hydrolysable tannins hydrolysable tannins, showing gallic acid, it difficult to quantify. are primarily derived and pentagalloyl glucose (gallotannin) as from wood. well as ellagic acid and an ellagitannin Protein precipitation assays are well established and robust. polymeric complex. Some, like haeme analysis that required blood have lost favour, but gelatine and albumin assays are still common. currently in condensed tannin grape and wine research, precipitation with BSa (bovine serum condensed tannins, also know as proanthocyanidins, are albumin) is widely used in research and numerous wineries polymeric compounds made up of flavan-3-ols subunits. the in the USa. Because tannin will also bind polysaccharides, flavan-3-ols are based on a three ring structure, the flavan precipitation with methylcellulose has also been quite extensively skeleton, and have an alcohol (Oh) at position 3 on ring c used by the australian Wine Research institute (aWRi). of the flavan. Further modifications of the flavan give rise to thin-layer chromatography (tLc) has also been used, but different flavan-3-ols. requires tannin extraction and purification prior to separation. in grapes, these include the method is good for monomers, although these are in low catechin, epicatechin, abundance relative to polymers, which appear as a blob or epicatechin-gallate and smear depending on solvent choice. the separated spots require epigallocatechin. the visualisation by a reagent such as DAMCA or Fecl and can be biosynthesis of flavan- 3 difficult to quantify. 3-ols is well-studied, but the mechanism of high-performance liquid chromatography (hPLc) is widely polymer formation is used as a research tool and some wineries also employ this unknown. the flavan- technology. the equipment is costly and requires a skilled 3-ols can be combined technician to operate and in-depth knowledge of tannins a generic condensed tannin structure to interpret the data. hPLc methods are either based on in different sequences showing terminal an extension subunits and and proportions in the ring labelling and carbon numbering separating monomers from the tannin “blob” similar to tLc or tannin polymers. convention for the flavan skeleton rely on depolymerising the tannin with acid and separating the (uppermost extension subunit). individual subunits. there are also some hPLc methods that condensed tannin separate polymers based on size. polymers have a terminal subunit and a series of extension subunits linked by an More recently, rapid analytical methods that don’t require inter-flavan bond; a bond between flavan units. the terminal and chemical analysis have been developed, but are not yet ready for 210mm widespread adoption. For industry, the BSa and methylcellulose acids (eg, caffeic acid, resveratrol). this pathway also makes the precipitation methods are most accessible. precursors for lignin (wood) production. during berry development the main phase of tannin tannins in wine accumulation is between flowering and veraison. Studies conducted in tannins contribute to the mouthfeel McLaren Vale (South australia) on and colour stability of wine. the colour The most accessible Shiraz skin showed a high level of of grapes and of young red wine is method for Australian tannin at flowering followed by a due to the presence of anthocyanins. period of tannin synthesis towards anthocyanins are structurally similar to wineries to measure tannin veraison, after which tannin levels tannins, but are relatively unstable. in declined towards harvest. From wine they breakdown or interact with is the methylcellulose season to season there was variation other elements to form new compounds, method offered by AWRI. in the timing of the peak in tannin some of which are coloured. accumulation, but generally this the largest single class of the coloured was accepted as the typical pattern of tannin accumulation compounds in older wines are the pigmented polymers. in grape skin. however, samples collected from the Sunraysia Polymeric pigments are tannin polymers with anthocyanins region of northwest attached. there is limited information on the structure of Victoria showed a very pigmented polymers or their formation in wine. Pigmented different pattern with polymers are only formed during winemaking not in the grape. the peak in tannin occurring at fruit-set Because tannins are bitter and astringent they contribute to the and declining from mouthfeel of the wine. tannins precipitate the salivary proteins then on. this pattern that lubricate the mouth and make it easy to chew and swallow was seen in Shiraz, food. this causes the drying and puckering sensation associated cabernet Sauvignon with drinking red wine. in addition to protein, tannins also and chardonnay skin interact with polysaccharides in saliva as well as cells in the oral over several seasons cavity. these interactions are impacted upon by the alcohol and indicates a strong the pattern of tannin accumulation in skin of concentration and viscosity of the wine and can be masked by Shiraz berries from two regions, Sunraysia (nW regional effect on residual sugar and other flavour and aroma characters, such as Victoria) and McLaren Vale (South australia) tannin accumulation fruit intensity. the perception of mouthfeel is also affected by during berry development. in grapes. tannin concentration, polymer length and polymer composition. We know that epicatechin-gallate is perceived as coarser than epicatechin, while epigallocatechin is perceived as smoother, Variation between varieties however the sensory character of oligomers and polymers anecdotal evidence indicates significant differences in tannin and combinations of tannins and other elements such as levels between grape polysaccharides is unknown. varieties, with some varieties considered tannins added to wine high in tannins, eg. tannat and nebbiolo, While winemakers recognise there are tannins in grapes, often and others low in the level of extracted from the grapes is not sufficient for a tannin, eg. Merlot particular wine style, so it is common practice to add tannin to and Barbera. wine during fermentation or aging. the main reasons for adding tannin are to stabilise colour or to alter mouthfeel. a study of grape Variation in total skin tannins (mg/g FWt skin) skin tannins from 36 the tannins that are added to the wine can come from oak between varieties. cultivars showed that barrels, chips and staves (hydrolysable tannin), or can come many of the cultivars
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