Cool compounds: pepper and raspberry in Shiraz wines

This festive time of the year, being summer time, one tends to drink more white wine. However, spending Christmas in CAW (Cold and Wet George), I will probably open a bottle of Shiraz to go with the cold meats tomorrow. Shiraz is a very versatile cultivar, making wines that can be either spicy, more fruity/flowery or a combination of these.

The spicy, pepper aroma in Shiraz is caused by a sesquiterpene called rotundone. This compound is chemically related to monoterpenes. I have heard that having oily hair is currently quite a fashion thing in Cape Town due to the water restrictions, but those of you who still uses shampoo on a regular basis will know terpenes, as these are often used in shampoos among others for a pleasant aroma.

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Rotundone gives the peppery aroma to black pepper, in grapes and other types of plants such as herbs. Most of us can start smelling rotundone at 16 nanograms per litre in wine. However, around 20% of us cannot perceive this compound, so don’t feel bad if you do not pick it up in wine (or on your eggs for that matter!). Levels of 145 ng/L has been found in Australian wines, including Shiraz, Mouvedre and Durif. Even higher levels have been found in Schioppettino and Vespolina red wines and in Gruener Veltliner white wines from Europe. Rotundone levels can vary greatly (like the Springboks scoreboard these days) depending on the vintage, where the grapes are grown and according to the spatial distribution within a vineyard.  Experiments have shown that grapes grown in cooler areas often have higher rotundone levels. Rotundone levels in grapes normally increases after veraison until harvest, but temperatures exceeding 25 °C in the fruiting zone during this time can lead to drastic reductions in rotundone levels. Removing leaves at veraison can also lower rotundone levels with up more than 60%. The topography of the vineyard also plays an important role, with grapes coming from vines from cooler areas in the same vineyard having up to 15 times more rotondune levels than those from warmer areas. Investigations in the distribution of  rotundone levels in a bunch itself found that the berries from the top part of bunches also contains much more rotundone, probably due to the lower shading of leaves and higher solar radiation in the bottom berries, leading to higher temperatures and thus lower levels of rotundone in these.

It seems that during winemaking only a small percentage of rotunonde are extracted from the skins into the grapes, but these tend to stay relatively constant during bottle ageing. Rotundone also occurs in the stems of the grapes and these are sometimes added with the fermenting wines to increase the spiciness.

The other compound that contributes to the flowery, fruity aroma of Shiraz wines is called ionone. Other compounds, such as esters, also plays an important role the fruity flavours in red wines. Ionone, coming from the grapes as well, normally contribute to the raspberry or violet aromas in wines. We have the pure ionone compound in my lab, and it always reminds me of the pink sweets one used to find in Lucky Packets. Ionone is often used in the perfume industry, as it also gives off a potent rose aroma.

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Ionone can occur up to 1 mg/L in the grapes, but levels are normally much lower in wine. Some experiments have shown that its levels are not that much influenced by leaf removal as with rotundone and it can also play an important role in Pinot noir, Cabernet Sauvignon and Merlot wine’s fruitiness. In Shiraz wines originating from warmer areas ionone would probably play a more important role.

One should remember that wine’s aroma is like a symphony, not only one or two instruments lead to a master peace. The same applies to wine, as hundreds of compounds contribute to the overall aroma of excellent wine, but some compounds like rotundone and ionone can have an important role in deciding if you will have violets/raspberries or pepper in your glass for Christmas!

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