Tannin seems to be one of the more confusing concepts to new wine learners. The word gets thrown around a lot, with a myriad of different descriptors applied to it, but it is rarely explained further than that. Tasting notes classify them as soft, firm, hearty, rich and even cascading. So what exactly are tannins?
First of all, before we get into too much science or specifics, one of the most important things to know about tannins is what they do to your mouth; they dry it out. If you are a tea drinker and you have ever left a black tea to steep too long in the cup you come back to find a tea that is bitter, a little harsh, and that quickly saps your mouth of all of your saliva. That is the tannins. And that is exactly what you experience with the tannin in wines as well. One of the first things that you need to know when you are trying to wrap your mind around tannins, they are not a taste they are a feeling.
To get a little more technical tannins are phenols and they are naturally occurring in almost all plant life, though you rarely find them in fruit (clearly grapes are one big exception). In wine the tannins mostly come from the skin of the grapes but there are also good concentrations of tannins in the seeds, stems and even the barrels that the wine is eventually aged in.
When looking at why some wine are more tannic than others you have to look at the grape itself. Cabernet Sauvignon grapes are very small and dark and very thick skinned. Which mean that there is a higher concentration of juice to skin contact. Which means more tannin. Whereas a grape like Pinot Noir, which tends to have larger grapes with much thinner skins (one of the reasons Pinot Noir is such a difficult and delicate grape) is going to have a lower ratio of skin to juice. Therefore pinot is a much less tannic wine. Now that doesn’t mean they aren’t there. Tannins are in all grapes, including white grapes. The difference in perception of them has to do with the original concentration and how the wine is made. With white wine the reasons tannins are never really part of the conversation is because white wines are not fermented with their skins the way that red wines are. That means that even though tannins are still present in the skin, seeds, and stems of white varietals they are never really in contact with the juice for any considerable period of time.
So why do we care about tannins and why are they such a big part of the wine conversation? Because tannins are what provide structure, complexity, and balance to wine. Tannins are also a natural preservative, which is why red wines have the ability to age longer than white wines and why highly tannic varietal like Cabernet Sauvignon and Nebbiolo can at times be aged for decades.