Understanding and measuring tannins in red wines

The importance of phenolic compounds on wine quality require methods that accurately measure the tannin content.

Tannins are phenolic compounds that play an important role in the astringency perception of red wines. Tannins also are very complex molecules with different sizes and conformations. The decrease in the astringency intensity during ageing is due to different phenomena, including cleavage reactions (appearance of smaller less astringent molecules), precipitation from solution, due to insolubility situations, conformational arrangements (shape of the tannins molecule) and interactions with other components (such as anthocyanins). Precipitation based methods (BSA and MCP tannins assays) are highly suitable for routine tannin analysis. Both methods positively correlate with each other and also with the astringency intensities measured by a sensory panel.

Introduction

Tannins are phenolic compounds that are involved in red wine mouthfeel attributes. They are also thought to play a very important role in the astringency perception. When drinking red wine, the tannin compounds interact with the salivary proteins, creating a macromolecular complex that precipitates from solution and causes a drying and puckering mouthfeel sensation, also known as astringency.¹ The intensity of this feeling depends on many factors. The size and conformation of the molecules, the combination with other wine components and the levels found in wine define the astringency intensity.

It is also known that the wine astringency decreases over the ageing process.^2,3 This behaviour that was initially attributed to a decrease in the total tannin content present in the wine, is nowadays ascribed to different phenomena. Starting with the assumptions that tannins polymerise during ageing and that the ability of the tannins to elicit astringency increases with tannin size (i.e. the bigger the molecule, and the higher the number of sites available to interact with the salivary proteins, the higher the ability to combine and precipitate proteins)² other phenomena that explains the decrease in the astringency intensity needs also to be playing role. First of all, cleavage reactions, which means large tannin molecules break down giving rise to smaller, less astringent tannins, have been proposed by some researchers.⁴ Moreover, molecular conformational arrangements have also been identified as a possible reason.⁵ Bigger and larger tannin molecules can also be too bulky (which means that due to the molecular conformation, the active binding sites are not available to interact with the salivary proteins). In this specific scenario larger tannin won’t give rise to an increased astringency perception. It is also well accepted that anthocyanins play an indirect role in wine astringency. The anthocyanin-tannin molecules cause a reduction on the ability of the newly formed polymeric pigment to interact with salivary protein thus reducing astringency (phenomenon that could also be related to the abovementioned conformation rearrangement scenario).⁶ The later phenomena together with the precipitation of tannins from solution, due to insolubility conditions, may explain why the wine astringency softens during ageing.

Tannin measurement by acid hydrolysis

The quantification of tannins has been challenging researchers over the past years as these compounds are of a very diverse nature, a fact that makes it difficult to estimate their concentrations. However, a number of methodologies are currently available for the measurement of the wine tannin levels. A method that has been commonly used for a long time exploits the ability of the tannin molecules to break down in a heated acid environment (acid hydrolysis method).⁷ The individual molecules show a red coloration after the heating process and can then be measured by quantifying the intensity of the red tonality using a conventional spectrophotometer. This method that is used worldwide presents a number of limitations. It does not take into account the structure of the tannin pool and it also does not consider other components (anthocyanins) that can interfere in the reaction and measurement. Due to this, the tannin concentration in wine is often overestimated and it is common to observe an increase in the wine total tannin content during ageing. Nevertheless, the method also has some advantages as the ease of implementation and reliability.

Tannin measurement by precipitation

Once understood that the astringency perception is caused by the precipitation of the salivary proteins after the interaction with the tannin molecules, the following question comes into our minds: Why not using the same principle that occurs naturally in our mouth to measure tannins? Based on this reasoning two new methods for tannin analysis were recently developed. The first one relies on the interaction of tannins with an animal protein. This method uses bovine protein and is known as the bovine serum albumin protein or BSA method.^8,9 The first step of the method consists of the precipitation of the tannin compounds after interaction with the BSA protein. However, a further step is required as the protein shows similar spectral properties than the tannin compounds and cannot therefore be quantified at the maximum absorption band of the tannin molecules. The reaction of the tannin complex with ferric chloride, that gives rise to blue coloured compounds, is thus measured.

The second method that simulates the precipitation of tannins by proteins (precipitation based methods) uses a different precipitation agent. A methyl cellulose polymer is in this case used to interact with the tannins, creating a macromolecular complex that precipitates from solution. This method is known as the methyl cellulose precipitable tannins assay (MCP tannin assay)¹⁰ and its main advantage relies on the possibility to quantify tannins at the maximum absorption band characteristic of this compounds, as in this case the polymer does not interfere with the spectral measurement. The quantification is done by comparing the absorbance of a control sample and that of a tannin precipitated sample.

Both methods have been adapted to high throughput formats and are thus highly suitable for routine analytical procedures.^11,12 Wineries and small labs only need a conventional spectrophotometer, a centrifuge and a limited number of reagents. On the other hand, the comparison of both methods has been extensively studied the past few years. It is commonly accepted that both methods are able to precipitate the same amount of tannin material, however, the composition of the tannin pool will depend on the ability of both precipitating agents (BSA and MCP) to interact with the tannin molecules.

Moreover, it is also accepted that the values obtained with the MCP method are on average threefold higher than those observed with the BSA tannin assay.¹³ The explanation of this behaviour relies on the different quantification principles used. The BSA tannin, due to the interference conditions, are quantified after a reaction with a strong phenolic binding agent (ferric chloride), while the MCP tannins are directly measured at the maximum absorption band showed by these compounds. The highest tannin value reported for the BSA method goes up to 2 g/ℓ with an average value of 0.45 g/ℓ, while for the MCP tannin assay values can be as high as 4 g/ℓ with an average value of 1.34 g/ℓ. Additionally, a strong correlation (r² = 0.8) between the values observed for both methodologies have also been observed for a set of 240 South African red wines, meaning that values from both methods are correlated.¹³ Finally, one of the most important benefits of these methods deals with the fact that the tannin values obtained positively correlate with the astringency intensities. Logically as the quantification principle simulates the real scenario occurring in the mouth cavity one could expect a precipitation phenomenon similar to that caused by the human salivary proteins. The Department of Viticulture and Oenology at Stellenbosch University is currently busy with an extensive Winetech funded research programme on these methods and how they can be applied to the South African wine industry.

References

1. De Beer, D.; Harbertson, J.F.; Kilmartin, P.A.; Roginsky, V.; Barsukova, T.; Adams, D.O. & Waterhouse, A.L., 2004. Phenolics: A comparison of diverse analytical methods. Am. J. Enol. Vitic. 55(4), 389 – 400.
2. Cheynier, V.; Dueñas-Paton, M.; Salas, E.; Maury, C.; Souquet, J.M.; Sarni-Manchado, P. & Fulcrand, H., 2006. Structure and properties of wine pigments and tannins. Am. J. Enol. Vitic. 57(3), 298 – 305.
3. Monagas, M.; Bartolomé, B. & Gómez-Cordovés, C., 2005. Evolution of polyphenols in red wines from Vitis vinifera L. during aging in the bottle: IIII. Non-anthocyanin phenolic compounds. Eur. Food Res. Technol. 220 (3 – 4), 331 – 340.
4. Vidal, S.; Francis, L.; Guyot, S.; Marnet, N.; Kwiatkowski, M.; Gawel, R.; Cheynier, V. & Waters, E.J., 2003. The mouth-feel properties of grape and apple proanthocyanidins in a wine-like medium. J. Sci. Food Agric. 83(6), 564 – 573.
5. Scollary, G.; Pásti, G.; Kállay, M. & Blackman, J., 2012. Astringency response of red wines: Potential role of molecular assembly. Trends Food Sci.
6. Gómez-Plaza, E.; Gil-Muñoz, R.; López-Roca, J.M.; Martínez-Cutillas, A. & Fernández-Fernández, J.I., 2001. Phenolic compounds and color stability of red wines: Effect of skin maceration time. Am. J. Enol. Vitic. 52(3), 266 – 270.
7. Ribéreau-Gayon, P. & Stonestreet, E., 1965. Le dosage des anthocyannes dans Je vin rouge. Bull. Soc. Chim. Fr. 9, 2649 – 2652.
8. Harbertson, J.F.; Kennedy, J.A. & Adams, D.O., 2002. Tannin in skins and seeds of Cabernet Sauvignon, Syrah, and Pinot noir berries during ripening. Am. J. Enol. Vitic. 53(1), 54 – 59.
9. Harbertson, J.F.; Picciotto, E.A. & Adams, D.O., 2003. Measurement of polymeric pigments in grape berry extracts and wines using a protein precipitation assay combined with bisulfite bleaching. Am. J. Enol. Vitic. 54(4), 301 – 306.
10. Sarneckis, C.J.; Dambergs, R.G.; Jones, P.; Mercurio, M.; Herderich, M.J. & Smith, P.A., 2006. Quantification of condensed tannins by precipitation with methyl cellulose: Development and validation of an optimised tool for grape and wine analysis. Aust. J. Grape Wine Res. 12(1), 39 – 49.
11. Mercurio, M.D.; Dambergs, R.G.; Herderich, M.J. & Smith, P.A., 2007. High throughput analysis of red wine and grape phenolics: Adaptation and validation of methyl cellulose precipitable tannin assay and modified somers color assay to a rapid 96 well plate format. J. Agric. Food Chem. 55(12), 4651 – 4657.
12. Heredia, T.M.; Adams, D.O.; Fields, K.C.; Held, P.G. & Harbertson, J.F., 2006. Evaluation of a comprehensive red wine phenolics assay using a microplate reader. Am. J. Enol. Vitic. 57(4), 497 – 502.
13. Aleixandre-Tudo, J.L.; Nieuwoudt, H.; Aleixandre, J.L. & Du Toit, W.J., 2015. Robust ultraviolet-visible (UV-vis) partial least-squares (PLS) models for tannin quantification in red wine. J. Agric. Food Chem. 63(4), 1088 – 1098.

– For more information, contact Jose Luis Aleixandre-Tudo at joaltu@sun.ac.za or Wessel du Toit at wdutoit@sun.ac.za.