Application Of Oenological Tannin For Colour And Mouthfeel: A Riverland Shiraz Case Study

After establishing a new micro-winery in Adelaide, Rocco Longo Ph.D. & the team at Winequip SA conducted a trial focusing on the application of oenological tannin for colour and mouthfeel on Riverland Shiraz. Read the below Case Study to find out more.

micro-winery development

Before proceeding with the trial of tannin for colour and mouthfeel, the micro-winery had to be set up at Winequip Dudley Park (SA). The tanks used, designed by Letina, are 100L conical variable capacity tanks that include dimple jackets for heating and cooling, sample tap and thermometer. Further, Kreyer Glycol Chillers and Fermflex units were used for temperature control.

introduction

Oenological tannins are extensively employed in the winemaking process to increase the colour intensity, body, and astringency of red wines. They are also used as fining agents in white wines because of their ability to precipitate proteins (Chen et al., 2016; Versari et al., 2013). 

Commercially available tannins are generally classified into three groups: (a) hydrolysable; (b) condensed (or proanthocyanidins); and (c) a mixture of both types of tannins (Obreque-Slíer et al., 2009). They are sourced from a variety of botanical species and, from high to low prices, are typically arranged in the following order: grape skins > grape seeds > grape stems ≥ Quebracho ≥ other vegetal origins. Skin and seed tannins are extracted from the grape pomace and include both non-flavonoid (benzoic and hydroxycinnamics acids) and flavonoid (anthocyanins, flavanols, and flovonols) compounds. Oak wood tannins include phenolic acids (gallic and ellagic), gallotannins, ellagitannins, and some flavanoles and cumarins. Other sources include two species of Quebracho, namely Schinopsis balansae and Schinopsis lorentzii. They are typically grown in South America, particularly in northern Argentina and eastern Paraguay (Obreque-Slíer et al., 2009). 

Tannins are employed for different winemaking applications. They inhibit laccase, a fungal enzyme associated with Botrytis rot on grapes that is responsible for early browning of bottled white wine and colour deterioration in red wine. Proanthocyanidins can help to stabilise the colour of red wine. They can also improve mouthfeel (oral-tactile stimulations such as astringency and fullness) when interacting with the oral components (Harbertson et al., 2012).

Given some scientific and empiric evidence exists that suggests there might be an impact of tannin addition on wine mouthfeel and colour, we added a number of commercially available hydrolysable and condensed tannins to wines made with Shiraz grapes grown in the Riverland region (Waikerie) of South Australia. Exogenous tannins were added at equivalent amounts at the crusher and at 1/3 through alcoholic fermentation. Colour analyses were conducted at bottling, as well as a number of blind sensory tests using professional winemakers.

experimental

Grapes. Shiraz grapes were donated by the Growers Wine Group (Waikerie, South Australia) and were sourced from the Riverland region of South Australia in 2022. The vineyard was own-rooted, drip irrigated and trellised to vertical shoot position. A total of 360 kg of grapes were machine harvested on April 6 and transported to the Winequip’s micro-winery in Adelaide. The grapes were well mixed to minimise within-vineyard differences in TSS, separated in four batches of 80 kg, and analysed for basic chemical composition with results presented in Table 1.

Winemaking. The four × 80 kg lots of grapes were individually vinified. Each fruit lot was destemmed and crushed with the addition of 30 mg/kg of potassium metabisulfite and Viscozyme L (30 mL/t; Novozymes, Denmark). The resulting must was pumped into a 100-L stainless steel vessel and inoculated with the Saccharomyces cerevisiae Okay (Lallemand, Australia) at a rate of 300 mg/L together with 300 mg/L GoFerm Protect (Lallemand) as a nutritional supplement. Prior to inoculation with dry yeast, the acidity of each trial (average pH 3.8) was adjusted to pH 3.6 by addition of tartaric acid at a rate of 2 g/L. The ferments were maintained at 24.5 ± 1°C, and caps were punched down for five minutes twice daily. Tannin additions were made to the four VCTs at the crusher and during 1/3 of the alcoholic fermentation as follows (tannins were mixed with distilled water before addition per manufacture’s recommendations).

The wines were left to macerate on grape skins until the ferments reached dryness (RS ≤1 g/L). The ferments were then pressed, and the resulting wines were inoculated with the malolactic bacteria (VP41, Lallemand) and maintained at 22 ± 1°C until the malic acid concentration decreased to less than 0.05 g/L. Wines were racked off the gross lees and potassium metabisulfite was added to achieve 25-30 mg/L of free SO₂. Wines were adjusted to pH 3.6 with tartaric acid and racked two further times for clarification. Concentrations of free SO₂ were adjusted to give 0.5-0.8 molecular SO₂ prior to bottling in 750 mL glass bottles with screwcap Stelvin® closures. Bottles were stored at 15 ± 1°C from early June 2022 when the phenolic/colour analyses and sensory tastings analyses were carried out.

key tannins used

results & discussion

A preliminary colour/phenolic analysis was performed at the AWRI (Glen Osmond, South Australia) following bottling on June 2022. The most noticeable outcomes, as indicated in Table 2, included Trial 2’s increased colour density (a.u.) and free anthocyanins (mg/L). This shows that adding oak tannin (i.e., Tanifase Elevage) during the first third of the alcoholic fermentation at a 25 g/t addition rate may have been enough to provide a noticeable improvement. However, given that more tannins were added to Trials 1 and 2, greater overall tannin concentrations (reported as catechin eq. in g/L) were very much expected. Hue values were similar across the treatments which suggest that a longer period of in-bottle ageing (>1 month) is needed to have a significant impact.

A preliminary sensory analysis of the four wine samples was also conducted. Wines were served to several professional winemakers in separate occasions. The four samples (25 mL) were presented to each individual in 215 mL ISO/INAO clear wine glasses, labelled with a random number. Then, each winemaker was asked to rank their preferences from 1 to 4, with 1 being their top choice and 4 their least.

Following the collection and analysis of 45 individual tasting notes, about 90% of respondents named Trial 2 as their first-choice sample, followed by Trials 3 and 4. One of the most often heard comments was that Trial 2 had a more developed mouthfeel and a more complex aroma profile. Another observation was that Trial 2 would be perfect for in-oak ageing to allow the wine more time to mature, while Trials 3 and 4 would be perfect for early release labels providing a flavour profile often desired by younger customer segments.

“…Preliminary trials conducted at Growers Wine Group using Fullcolor tannin on a commercial scale, and those performed by Winequip showed promising results for the retention and stability of colour, and building of structure in Riverland Shiraz…”

Sue Franke

Senior Winemaker,
Growers Wine Group

Colour and phenolic analyses will be repeated on all samples to determine how long each wine will last and whether both colour and flavour traits will change over the following 12 months.

acknowledgement

Thankyou to Growers Wine Group for contributing the fruit used for the Riverland Shiraz, and in particular to Sue Franke for actively participating in the trials and providing the team at Winequip SA with feedback.

references

• Chen, K., Escott, C., Loira, I., del Fresno, J., Morata, A., Tesfaye, W., Calderon, F., Benito, S., SuárezLepe, J., 2016. The Effects of Pre-Fermentative Addition of Oenological Tannins on Wine Components and Sensorial Qualities of Red Wine. Molecules 21, 1445. https://doi.org/10.3390/molecules21111445
• Harbertson, J.F., Parpinello, G.P., Heymann, H. and Downey, M.O. (2012) Impact of exogenous tannin additions on wine chemistry and wine sensory character. Food Chemistry, 131, 999–1008.
• Obreque-Slíer, E., Peña-Neira, A., López-Solís, R., Ramírez-Escudero, C., & Zamora Marín, F. (2009). Phenolic characterization of commercial enological tannins. European Food Research and Technology, 229, 859–866.
• Versari, A.; du Toit, W.; Parpinello, G.P. (2013). Oenological tannins: A review. Australian Journal of Grape and Wine Research, 19, 1–10