Riesling aroma intensity is highly correlated with the terpenes concentration in the wine. A few other varities are also defined by the terpene concentration such as Muscat or Gewurztraminer (though Muscat and Gewurztraminer are much richer in terpenes). Terpenes are found mainly in the berry skin and tends to accumulate during the course of the ripening season. One major problems for winemakers is that the terpenes are principally chemically bound to grape sugars and are ordorless in that form. To reveal terpene aromas, the sugar-terpenes molecule as to be broken in some fashion and the terpene as to be release, free of some sort. In Riesling only one quarter of total terpenes are freed up during the winemaking process. Activities enhancing the amount of free terpene in a wine include fermentation, time on the lees and bottle aging (the choice of closure is also critical as natural cork and synthetic corks absorb terpenes). During fermentation, yeasts enzymes can release terpenes but unfortunately they do no work very well at low pH which is often the case with Riesling. Some companies sell commercial enzymes to supplement natural yeast activities but such enzymes are not always easy to use (nor very natural) and can lead to an overextracted, almost tacky wine. Lees aging can also promote free terpene content by the release of those enzymes in the wine. Finally, it has been proven that enzymatic activity is possible many years after bottling and generally a Riesling can have increases in free terpene content two or three years after bottling. Unfortunately most Rieslings are consumed well before that. The major terpenes found in Riesling are: Linalool (Rose aroma), Alpha-terpineol (lilac), Citronellol (citronella), Nerol (mandarin), Geraniol (grapefruit) and Hotrienol (Lime).