Yesterday we finally pressed the Sangiovese after more than three weeks of slow and steady fermentation. I'm really impressed with the way the clay amphora kept the fermentation temperature predictable and cool, and also appreciative of the fact we didn't have to worry about the must overheating. The resultant product tastes too good to be a young wine and I'm really hoping it gets rougher after a few weeks of settling.
Glad you asked! The answer involves some phenolic chemistry but I'll try and present it in a way that assumes you don't care one bit about phenolic chemistry. Shortly after fermentation a young red wine should be a soup of tannins, anthrocyanins (color phenols), questionable odors (sulfides), and fine solids (lees). The tannins and anthrocyanins come primarily from the grape skins although some tannin comes from the seeds and we also throw in untoasted oak chips to add even more tannin to the soup. The odorous sulfides are a result of the fermentation process and are essentially yeast burps for lack of a better term. The lees are the spent yeast and other solids that precipitate from the wine over time. The focus of the early stages in a young wine are the tannins and anthrocyanins. The sulfides and lees will play a part later.
Early tannins are short monomers that are often referred to as "green." They are harsh and aggressive and are sensed on the tip of the tongue. Over time, these monomers will polymerize or link up until they form long polymer chains. As the tannins chain up their perceived mouthfeel softens, but if they are allowed to get too long they create what are referred to as "dry" tannins. Dry tannins are felt throughout the mouth but are usually concentrated in the cheeks and under the tongue and are associated with the drying sensation one feels as the saliva is stripped from their mouths. This is where anthrocyanins come into play. The anthrocyanin has the ability in the presence of oxygen to attach itself to the ends of a tannin polymer which prevents that polymer from linking up to other tannins. In effect, color stops the polymerization process. The reaction for anyone who is interested is known as the vicinal diphenol cascade. It's important because it accomplishes two things - it freezes the tannins in their soft state and it stabilizes color. Anthrocyanins are very unstable and after the first year in the cellar if they haven't found a tannin to link up to they can precipitate out of solution making the wine lighter and allowing the tannins to become coarse. The key component to the process is the presence of small amounts of oxygen and it's the reason why we now use breathable plastic tanks to store our wine. A micro-oxygenation machine can also be purchased for this task, but we don't have the money and I kind of like the "passive" approach. Also, as mentioned above we add untoasted oak chips to the fermentation which add no flavor but tons of tannin. If done correctly, we can preserve all of the color Sangiovese is famous for not having while also creating a soft, velvety wine in your mouth. Tannins can also be added by throwing in a small percentage of white grapes whose skins are very high in tannin, but since we don't currently grow any we'll continue to use the oak chips. This is now a very long post so I'll let y'all chew on it for a while.