Tannins.

One word: Tannins.  I cannot count the number of times that I have been told, in almost a folkloric way, that a Mayacamas Vineyards Cabernet sauvignon when young is so tannic that it is almost impossible to enjoy.  It is advised that one ages this wine for 20 to 25 years before drinking it.  (Hmm, that would take a lot of forethought.)  I have tasted a few vintages of this winery's Cabernet sauvignons (CS) in the past and, yes, I found them to be tannic.  I have also tried a barrel sample of their CS (and Chardonnay) when I visited the winery for a hillside viticulture class (as part of my degree programme at Napa Valley College) and it was, of course, very tannic.  So I was very curious to try this, not quite, 26 year old bottling from one of the classic wineries and much lauded producers of CS in California.
Tannins are naturally occurring phenolic compounds (technically, they are plant derived polyphenols) found in wine-grape skins, seeds and stems.  Tannins in wine are felt, not tasted - they are the textural component of a wine that has that astringent, tooth enamel stripping effect on the mouth.  CS as a wine-grape variety is inherently high in tannins.  And this particular wine is made from mountain fruit, so tannin extraction is elevated.  In addition to contributing texture, tannins act as a preservative enabling the cellaring of wine for an extended period.  So how tannic was this 2½ decades old wine?  Drum roll, please.
The 1994 Mayacamas Cabernet Sauvignon was simply fabulous.  The cork was a little dry and broke on removal, usually not a good sign.  The colour was amazing, a deep garnet, showing very little age in the meniscus.  On the nose, cedar, blackberries, blueberries and an appealing savouriness.  On the palate, cedar again, woodsy, red currants, black fruits and vanilla.  Amazingly long finish.  Amazing!  Velvety and silky, but firm and precise structured tannins, pepper and other spices.  Delicious.  Despite how tannic this wine may have been upon release, right now it is elegant, luscious, classy, refined and mind blowing.  I'd predict that the 1994 still has many years of ageing ahead of it.  Thank goodness for polyphenols.
Mayacamas oozes tannins.