兩支酒的異想世界

身為偽Riesling大使的雪莉，6月初上課時被學生們問到，為何所有白葡萄酒中，獨獨Riesling的汽油味如此明顯，藉著這篇文章也來幫大家科普一下啦！Riesling的汽油味來自於一種名為TDN的化學物質，這個物質究竟如何影響葡萄酒風味，且讓我們一起看下去吧！

令人又愛又恨的汽/煤油味-Riesling

何謂TDN?

TDN，由葡萄中前趨物所形成。所謂的前趨物，是存在於葡萄中一稱為類胡蘿蔔素的化合物。這些化合物在葡萄中至少有十幾種，大致可以分為兩類：胡蘿蔔素和葉黃素。隨著葡萄成熟，這類化合物在葡萄中的含量不斷增加，並在發酵過程中經過化學反應後逐漸釋放TDN。葡萄中類胡蘿蔔素含量越高，意味著葡萄酒中的 TDN 也越高。在葡萄酒裝瓶後，葡萄酒中亦會存在大量前趨物，這些前趨物隨著陳釀過程慢慢釋放 TDN。

紐約康乃爾大學的Gavin Sacks 和其同事們於2012年發表一篇關於TDN濃度的研究。這個研究發現，Riesling的TDN明顯高於其它品種所釀的葡萄酒。值得一提的是，年輕的Riesling即便有TDN所帶來的汽油味，但豐富的果香往往會覆蓋這個味道，這也是讓品飲者覺得年輕Riesling的汽油味較不明顯的主因。若比較澳洲和舊世界Riesling，澳洲Riesling TDN的含量通常較高。數據顯示，歐洲Riesling的典型含量為 1-50 微克/升，但澳洲Riesling TDN的含量可高達 250 微克/升，有時甚至更高。

影響TDN濃度的因子

陽光暴晒和水分脅迫是導致前趨物濃度升高的兩個主要因素。

陽光暴晒是關鍵因素：研究指出，任何高於 20% 的葡萄全日照照射都會提高葡萄酒中的 TDN。此外，TDN 前趨物濃度還取決於葡萄樹管理方式，例如葡萄串落葉、土壤施肥、灌溉和葡萄樹無性繁殖母株的選擇。由於全球氣候變遷，使得平均氣溫升高，以及增加葡萄樹暴露在陽光下的時間，這些因素都可能加劇Riesling葡萄酒中 TDN 的形成。葡萄酒裝瓶後，其他因素包括瓶中陳年時間、儲存溫度和 pH 值(較高的酸度)均會影響葡萄酒中TDN濃度。當葡萄酒酸度較高時，可為TDN 糖基化前體的酸水解提供更酸的環境；而較高的溫度則提高了反應速率。一些研究還發現，酵母菌株的選擇與葡萄栽培相結合可能會產生一定的影響。有趣的是，研究人員也發現，使用螺旋蓋會增加TDN濃度。根據 Tarasov 及其同事的研究發現，軟木塞和合成瓶蓋在裝瓶後的頭兩年內會消耗超過 50% 的 TDN。換句話說，通過選擇瓶蓋其實可以管理葡萄酒中的TDN 濃度呢！

TDN對當前釀酒業的影響

目前TDN 是德國釀酒師最關心的問題。釀酒師們一直嘗試不同的樹冠管理方法，藉此降低TDN濃度過高的風險。過去他們會摘除結果區所有葉子，以降低疾病壓力(例如：發霉)。現在他們改為分階投進行摘葉，嘗試留下一些外部葉子作為保護，並採摘內部葉子，以產生氣流，同時保留一些陰涼處。

Petrol/Kerosene Aroma of Riesling

Introduction of TDN

TDN, (1,1,6,-trimethyl-1,2-dihydronapthalene), a group of compounds called C13-norisoprenoids. It is formed from precursors in the grapes, which are a group of compounds present in grapes called carotenoids.

There are at least a dozen of them in grapes and they fall into two categories: carotenes and xanthophylls. Levels of these compounds build up in the grapes and then after veraison they break down to form the C13-norisoprenoids. This group includes the precursors to TDN, and they undergo a chemical modification known as glycosylaytion, which involves adding a sugar. This creates a reservoir of bound compounds in the grape juice that can then release TDN during fermentation. Higher levels of caretonoids in grapes means more TDN in the wine. Then, after the wine is bottled, there is a pool of precursors in the wine that can later release TDN during ageing.

In a study published in 2012, Gavin Sacks and colleagues from Cornell University in New York State looked at the thresholds of tasters to TDN, and also measured concentrations in a number of wines. They found the TDN concentrations were significantly higher in Riesling than in other varietal wines, at 6.4 μg/L, compared with an average of 1.3 μg/L for the other wines. Peoples’ thresholds were 2 μg/L, which is ten times lower than the previously reported odour threshold. So this meant that 27 of the 28 Riesling wines (aged 1-3 years) were over this threshold, while only 7 of the 69 wines from other varieties were over this level.

Detection threshold (4 μg/L): the lowest level at which a stimulus can be detected but not recognized

Recognition threshold (10-12 μg/L): it can be detected and recognized

Rejection threshold (71-82 μg/L): where the wine is rejected as faulty

This study indicates that half the population should be able to perceive TDN in most Riesling wines. However, the fruity characteristics of young wines can mask TDN, so that even if you can perceive it, you might not recognize it. Australian Riesling can often have higher levels even in young wines. Typical levels in European Rieslings would be 1-50 μg/l, but Australian Riesling can have levels as high as 250 μg/l, and sometimes even higher.

Influent Factors of the TDN concentration

Sun exposure & water stress are the two main factors which lead to higher levels of precursors. Sun exposure is the key factor: anything higher than 20% full sun exposure of the grapes raises TDN in the wine. The quantity of TDN precursors also depends on the viticulture practices such as grape clusters defoliation, soil fertilization, water irrigation, and the selection of vine clones. Global climate change, warmer temperatures, and higher sun exposure of the grapes may intensify formation of TDN in the succeeding Riesling wines. Once the wine is bottled, other factors can raise the level, including bottle age, storage temperature and pH value (higher acidity). When the wine has higher acidity, TDN develops more quickly. The possible explanation of such effects may be that lower pH provides a more acidic environment for acid hydrolysis of glycosylated precursors of TDN, and warmer temperature increases the rate of the reaction. Some studies also found that selection of yeast strains may have certain effects in combination with viticultural practices. Interestingly, using screwcaps can result in higher TDN levels because studies from Tarasov and colleagues have found that cork and synthetic closures scalp more than 50% of the TDN in the first two years after bottling. On the other hand, through selection of bottle closures can manage the TDN level in wine well.

TDN is a major concern to winemakers in Germany at this moment, because they have been experiencing warmer, drier summers. Karsten Peter, at Gut Hermannsberg in the Nahe, says they have been experimenting with different approaches to leaf plucking as a way to lower the risk of TDN, which they are very concered about. Typically they will remove all the leaves in the fruiting zone to keep disease pressure low. Now, they do a staged leaf pluck. And they are trialling leaving some outer leaves as a sort of umbrella, but picking just the inside leaves, to create airflow but keep some shade.

Reference:

[1] Introducing TDN, a compound responsible for petrol aromas in Riesling and other white wines – wineanorak.com

[2] 1,1,6-Trimethyl-1,2-dihydronaphthalene (TDN) Sensory Thresholds in Riesling Wine - PMC (nih.gov)

[3] TDN, (1,1,6,-trimethyl-1,2-dihydronapthalene) | Waterhouse Lab (ucdavis.edu)

[4] What do we know about the kerosene-petrol aroma in Riesling wines (infowine.com)