Dr. Carien Coetzee
When it comes to Sauvignon Blanc aroma, the possibilities for creating wines with different aromatic profiles are almost endless. Aroma groups such as the volatile thiols and methoxypyrazines are known to be typical to Sauvignon Blanc and are considered to be impact compounds which impart potent aromatic attributes due to the extremely low sensory perception thresholds. However, Sauvignon Blanc aroma certainly does not start and finish with thiols and pyrazines. Other aroma groups such as acetate esters, ethyl esters, alcohols, acids and terpenes can contribute significantly to Sauvignon Blanc aromatic composition and complexity.
Esters represent the largest and one of the most important groups of aroma compounds produced during fermentation. They are particularly important in young, fruit-driven Sauvignon Blanc wines and are typically associated with a diverse range of pleasant fruity flavours. Attributes such as banana and pineapple are often associated with esters, however, other flavours such as citrus and floral can also occur due to the presence of esters.
Formation of esters
- The vast majority of esters in wine are created during fermentation7. Esters are produced by the condensation of an alcohol and a coenzyme-A-activated acid8. This reaction is governed by an enzyme, alcohol acetyl transferase8. With the many different acids and alcohols found in wine, there is considerable potential for the formation of a wide range of esters. Yeasts are therefore the main contributor to ester formation. However, yeasts do not only produce esters, but it can also produce enzymes that degrade esters, called esterases9. The balance between ester synthesis and hydrolysis by yeast enzymes is important for the net rate of ester accumulation in wine.
- Lactic acid bacteria can also contribute significantly to the ester content in wine. The aroma compound that is mostly produced during malolactic fermentation is ethyl lactate, which characteristically contributes attributes described as milky, soapy, buttery and fruity. The production of this ester is coupled to lactic acid formation and its synthesis can be correlated with the percentage of degradation of malic acid10.
- Esters can also form naturally through chemical reactions, however, these chemical reactions are slow and contribute little to wines. Chemical hydrolyses of esters can, however, lead to significant losses in ester content over time.
Types of esters
Acetate esters of the higher alcohols and the ethyl esters of straight-chain, saturated fatty acids are the most significant esters produced in wine8. A list of some of the important wine esters is shown in Table 111.
- The acetate esters comprise of an acid group (short-chain acetic acid) and a longer-chain alcohol group (fusel alcohols), mostly complex alcohols derived from amino acid metabolism. These are low molecular weight esters often termed “fruit” esters and have a distinctly fruit-like fragrance. It is also these esters that are considered to give wine much of its vinous fragrance. Examples of acetate esters are isoamyl acetate, with aromas of banana and fruits; and isobutyl acetate, with fruity and apple flavours.
- The ethyl esters comprise a short-chain alcohol group (ethanol) and a longer-chain acid group (such as medium- to long-chain fatty acids). The ethyl esters have weaker odours and, in general, does not appear to be aromatically significant in wine and will likely only add to the wines complexity at low concentrations. Higher concentrations would result in unwanted aroma characteristics. Examples of ethyl esters include ethyl hexanoate, with reported aroma characteristics that include fruity, strawberry, green apple and anise; ethyl octanoate, with sweet, fruity, ripe fruit, burned and beer characteristics; and ethyl decanoate, which imparts an oily, fruity and floral character.
Table 1. List of some esters found in wine, aroma perception thresholds and attributes used to describe the various odours.11
Ethyl acetate is quantitatively the most prominent ester in wine, due to its spontaneous or enzymatic formation from ethanol and acetic acid. It is therefore both an acetate and ethyl ester. At low concentrations it may give desirable and fruity character to the wine; however, at higher concentrations, it can impart a solvent or nail varnish aroma, and contribute to the perception of volatile acidity (VA).
Even though grape cultivars can differ considerably in terms of ester content, the production of esters is not cultivar specific, meaning that significant amounts of esters can form in all wine varieties and that the presence of these compounds will most likely not be the differentiating factor between the wines’ aromatic compositions. Studies have shown that the amount of volatiles (esters, higher alcohols and fatty acids) present in Sauvignon Blanc and Chardonnay wines were not significantly different except for hexyl acetate, hexanoic acid, decanoic and octanoic acid occurring in higher concentrations in Sauvignon Blanc wines12.
Sensorial interaction between different esters as well as between esters and non-ester compounds can alter the sensory experience of the wine. For instance, the presence of one ester might amplify or suppress the aroma perception of another. The combination of certain esters can also result in a completely different aroma from what they smell like individually.
One of the factors that make Sauvignon Blanc wines unique is the diverse range of aromatic compounds that occur. Some compounds have very low perception thresholds and a significant impact on the wine aroma, while others might only play a (crucial) supportive role.
Esters are known to contribute pleasant fruity attributes to wine, a property which is desired by most Sauvignon Blanc producers. In this Part 1 of the Esters 101 series, the formation and types of esters in wine are briefly discussed. In Part 2, a quick overview of factors affecting the ester content in wines will be addressed.
(1) Benkwitz, F.; Tominaga, T.; Kilmartin, P. a.; Lund, C.; Wohlers, M.; Nicolau, L. Identifying the Chemical Composition Related to the Distinct Aroma Characteristics of New Zealand Sauvignon Blanc Wines. Am. J. Enol. Vitic. 2012, 63 (1), 62–72. https://doi.org/10.5344/ajev.2011.10074.
(2) Guth, H. Identification of Character Impact Odorants of Different White Wine Varieties. J. Agric. Food Chem. 1997, 45 (8), 3022–3026. https://doi.org/10.1021/jf9608433.
(3) Escudero, A.; Gogorza, B.; Melús, M. A.; Ortín, N.; Cacho, J.; Ferreira, V. Characterization of the Aroma of a Wine from Maccabeo. Key Role Played by Compounds with Low Odor Activity Values. J. Agric. Food Chem. 2004, 52 (11), 3516–3524. https://doi.org/10.1021/jf035341l.
(4) Valero, E.; Moyano, L.; Millan, M. C.; Medina, M.; Ortega, J. M. Higher Alcohols and Esters Production by Saccharomyces Cerevisiae. Influence of the Initial Oxygenation of the Grape Must. Food Chem. 2002, 78 (1), 57–61. https://doi.org/10.1016/S0308-8146(01)00361-2.
(5) Ferreira, V.; López, R.; Cacho, J. F. Quantitative Determination of the Odorants of Young Red Wines from Different Grape Varieties. J. Sci. Food Agric. 2000, 80 (11), 1659–1667. https://doi.org/10.1002/1097-0010(20000901)80:11<1659::AID-JSFA693>3.0.CO;2-6.
(6) Peinado, R. a.; Moreno, J.; Bueno, J. E.; Moreno, J. a.; Mauricio, J. C. Comparative Study of Aromatic Compounds in Two Young White Wines Subjected to Pre-Fermentative Cryomaceration. Food Chem. 2004, 84 (4), 585–590. https://doi.org/10.1016/S0308-8146(03)00282-6.
(7) Nykänen, L. Formation and Occurrence of Flavor Compounds in Wine and Distilled Alcoholic Beverages. Am. J. Enol. Vitic. 1986, 37 (1), 84–96.
(8) Lambrechts, M. G.; Pretorius, I. S. Yeast and Its Importance to Wine Aroma – A Review. South African J. Enol. Vitic. 2000, 21 (Special Issue), 97–129.
(9) Lilly, M.; Bauer, F. F.; Lambrechts, M. G.; Swiegers, J. H.; Cozzolino, D.; Pretorius, I. S. The Effect of Increased Yeast Alcohol Acetyltransferase and Esterase Activity on the Flavour Profiles of Wine and Distillates. Yeast 2006, 23 (9), 641–659. https://doi.org/10.1002/yea.1382.
(10) Heyns, E. Esters – Wine’s Own Perfume. Winel. Mag. 2014.
(11) Coetzee, C. Oxidation Treatments Affecting Sauvignon Blanc Wine Sensory and Chemical Composition, Stellenbosch University: Stellenbosch, 2014, Vol. PhD Disser.
(12) Louw, L. Chemical Characterisation of South African Young Wines, University of Stellenbosch, 2007, Vol. M.Sc.