White wine phenolics: what compounds are there and which ones cause problems? Technical manager for Laffort Australia, Alana Seabrook, highlights the main compounds that can negatively impact the quality of white wines. What are phenolic compounds seed and stems. Given where these in white wine? compounds are located in the grape, the quantity present in the must will Phenolic compounds are a broad be heavily influenced by viticultural spectrum of compounds found in nature, and winemaking practices. Phenolic characterised by a 6-carbon benzene compounds develop and change over ring. Their presence in white must and time, are influenced by pH, ageing wine is influenced by many of factors conditions, oxygen and temperature. ranging from vineyard and varietal to Importantly, their presence influences winemaking practices. In wine there the sensory attributes of the resulting are two sources of phenolic compounds, wine, from a taste and tactile point of oak or grapes. Phenolic compounds view as well as aroma. from grapes can be broken down into Which compounds do I need to 2 Flavonoids and non-flavonoids . know about for the production Non-flavonoid compounds are mainly of white wine? located in the pulp, while flavonoid For the purpose of this discussion, the compounds are localised in the skin, focus will be placed on hydroxycinamates
Table 1. Overview of the principal white wine phenolics (phenolic compounds produced and accumulated in grape berries (sourced from Teixeira A et al. 2013).
Principal White Wine Phenolics Group Non -flavonoids Flavonoids Hydroxycinnamates Flavan-3-ols
Hydroxycinnamic acids (mainly caftaric acid) (+)-Catechin, (-)-Epicatechin Localisation
Skin ++ ++
Seed ++ +++
Flesh +++ +
Quantity (mg/L)5 130 10-50
Factors affecting Varietal dependant1 Pressing cycles, rotation and max pressure1 quantity
Can they do good? In combination with SO2 these compounds can N/A help scavenge oxygen and prevent the oxidation of key aroma compounds6 Sensory No perceivable bitterness or astringency in wine 5 Bitterness and astringency in simple form, astringency in bound form5 Negatives Can contribute significantly to juice browning in Oxidation of these compounds can form a highly white wines reactive quinone able to scavange oxygen from anything, including important aroma compounds like thiols. Bottom Line Potential browning Trap for aroma compounds!
Very abundant compound (+++) to less abundant (+)
January 2019 – Issue 660 www.winetitles.com.au Grapegrower & Winemaker 53 winemaking
(non-flavonoids) and flavan-3-oles (GRP) and reduce the browning. SO2 (flavonoids) as these have the greatest will also hinder the grape enzymes from 10 Take home points contribution to white wine production. oxidising in the must but won’t hinder Oxidation of both flavan-3-ols and chemical oxidation over time if the 1. Combinations of fining agents hydroxycinnamates can occur either hydroxycinnamates are still available. have a better chance of attacking enzymatically in fresh must or chemically In their oxidised form they can bind a broader spectrum of oxidisable thiols as flavan-3-ols can, but they aren’t over time. In their oxidised form both and oxidised phenolics than a single as good at it and as a consequence, are can cause sensorial defects in terms of fining agent alone. less of a concern with regards to aroma binding to aroma compounds (thiols) scavenging3. In fact, in combination 2. The must/wine will oxidise if 2 and browning . Enzymatic oxidation with SO , the presence of Caffeic acid can oxidisable compounds remain in the 2 is very fast by grape oxidation enzymes actually slow the scavenging of volatile wine with insufficient protection. (Tyrosinase), but can be controlled with aroma compounds and be beneficial4,6. 10 3. Concentration and range of SO2 and settling strategies . Enzymatic In the levels found in wine there is no oxidisable and oxidised phenolic oxidation is more destructive in perceived bitterness or astringency8,9. compounds will vary depending on botritised fruit as the enzyme in question References varietal, harvesting technique, time (Laccase) is more resistant to SO2 and before and during pressing, inert more suited to wine pH10. 1Nikolantonaki, Maria (2010) These doctorat conditions and press fraction. (L’universite de Bordeaux). Incidence de Flavan-3-ols (Flavonoids) l’oxydation des composés phénoliques sur 4. SO is not enough to prevent oxidation la composante aromatique des vins blancs 2 Flavan-3-ols comprising principally of of key phenolic compounds. 2 (+)-catechins and (-)-epicatechins are Du Toit WJ, Marais J, Pretorius IS, Du Toit 5. Understand the volatiles that need important in white wines as they are M. Oxygen in must and wine: A review. South highly reactive towards oxygen. The African Journal of Enology and Viticulture. protecting in that varietal and whether 2017 Mar 1;27(1):76-94. they are susceptible to oxidation: amount in which they are present is highly dependant on grape processing 3Nikolantonaki M, Jourdes M, Shinoda • thiols in Sauvignon Blanc – very methods (harvesting method, press cycle K, Teissedre PL, Quideau S, Darriet P. susceptible to oxidation, yeast both cycles and maximum pressure) Identification of adducts between an derived as these are located more in the skins odoriferous volatile thiol and oxidized and seeds than the pulp7. It is well grape phenolic compounds: kinetic study of adduct formation under chemical and • esters – less susceptible to documented that in their oxidised form oxidation, yeast derived enzymatic oxidation conditions. Journal of they are highly reactive towards oxygen agricultural and food chemistry. 2012 Mar • terpenes in Muscat– volatile and can irreversibly bind to thiols which 1;60(10):2647-56. have been produced by yeast in high overtime but not highly 4 thiol varieties (i.e. Sauvignon Blanc) 1,2. Roussis IG, Lambropoulos I, Tzimas P. oxidisable, grape derived. Protection of volatiles in a wine with low Once bound the thiols are no longer sulfur dioxide by caffeic acid or glutathione. 6. Understanding press cycles: aromatically available. In the presence of American Journal of Enology and Viticulture. glutathione the oxidised (+)-catechines 2007 Jun 1;58(2):274-8. • impact on the phenolic load of and (-)-epicatechins will bind and leave 5 the wine the aromatic thiols to be available. Teixeira A, Eiras-Dias J, Castellarin SD, Gerós H. Berry phenolics of grapevine under • where are the ingresses of Hydroxycinnamates challenging environments. International oxygen? (Increasing pressure and (Non-Flavonoids) Journal of Molecular Sciences. 2013 Sep minimal rotations will minimise 11;14(9):18711-39. Hydroxycinnamic acids are not only phenolic compound extraction). 6Cilliers, J. J. L. & Singleton, V. L. (1990). the principal form of non-flavonoid in Caffeic acid autoxidation and the effects of 7. Fining during alcoholic fermentation the grape, but they are responsible for thiols. J Agric Food Chem 38, 1789-1796. is a highly effective way of utilising 80% of white wine phenolic compounds 7 the kinetics of fermentation to in white wine which has not had any Singleton, V. L. & Trousdale, E. (1983). White wine phenolics: Varietal and processing maximise fining-phenolic contact. skin contact. They are localised in differences as shown by HPLC. Am J Enol the grape pulp and exist as esters of Vitic 34, 27-34. 8. Fining after alcoholic fermentation is tartaric acid (Caftaric acid is Caffeic much more impactful on the sensory acid esterified with Tartaric acid). They 8Waterhouse AL. Wine phenolics. Annals profile of the wine. are important in white wine as they are of the New York Academy of Sciences. 2002 the first to oxidise. Hydroxycinnamates May 1;957(1):21-36. 9. Oxidation in the grape must happens play a key role in juice browning; in 9Robichaud JL, Noble AC. Astringency very quickly due to enzymatic juice, enzymes are released which and bitterness of selected phenolics in activity from the grapes. oxidise these compounds very quickly. wine. Journal of the Science of Food and Agriculture. 1990;53(3):343-53. 10. Chemical oxidation happens over In the presence of glutathione (GSH), time at a much slower rate. the oxidised hydroxycinnamate will 10Schneider, V., 1998. Must hyperoxidation: a form a colourless grape reaction product review. Am. J. Enol. Vitic. 49, 65-73
54 Grapegrower & Winemaker www.winetitles.com.au January 2019 – Issue 660