thats only partially true , about 5-15 % of tannins added prior to fermentation precipitate out . the rest form long chain bonds with the wines natural tannins and color elements and become a structural part of the wine. they don't fall out with the lees but become an integral part of the wine. So only a small amount is sacrificial .
in addition to bonding with the grape skin tannins and preserving colour , fermentation stage tannins also bond with any seed and stem tanins that may be in the must. this is important to know as if you have any green seeds or stem pieces in the must , the harsh short chain green tannins that will make your wine sharp and bitter , can be mitigated by binding with your fermentation stage addition.
discussed at length in this tread.
http://www.winemakingtalk.com/forum/f86/tannins-know-when-why-add-them-42039/
the yeast fermentation stage tannins are not truely sacraficial but they do add a lot of structure to the wine. there is some bonding with proteins from the added tannins that don't make it into the final wine, I guess this could be called sacraficial but this is actualy the minority of the impact. more of a side action than principle benefit. and not all products have a strong action or sacraficial effect.
yeast fermentation stage tannin additions bind with the two types of grape tannins naturally present , the skin and seed tannins and also the pigments of the grapes.
natural tannins from skins and seeds tend to have short molecular chains , they are unstable and want to form longer chains to become stable . this can happen naturaly , some natural binding happens durring ferment and some can happen as the wine ages , thats why ten year old reds are smoother than 1 year old reds.
but adding tannin in the yeast fermentation allows all the short chain tannins in the ferment to find a partner early and form long chains , long chains equals smooth and stable.
with the skin tannins the longer chains that are formed are similar to polimerisation , this improves tannin stability so they don't drop out later as the wine ages , ever had a filtered bottle of red wine later form a uniform coating inside the bottle not just on the bottom. this is tannin instability . very common with super ripe central valley grapes. syrah , zinfandel and cab sauv are particularly vulnerable at high levels of ripeness and matching high PH.
these longer chain tannin formations also improve mouth feel and reduce sharpness especialy if the fruit is a little green because of being under ripe or young vine or hybrid . vegital flavours are also surpressed, this is important if you get any green stem or leaf peices in the must, most basic crussher stemmers leave a little stem jacks in the must. nice brown jacks is ok in moderation but green ones can increase harshness , its almost impossible to pick all these out but tannin or oak dust can mitigate the impact.
the formation of these longer chain bonds are also very important late in yeast ferment as the alcohol level rises and seed tannins start to extract . seed tannins , especially if the seeds are at all green can make a wine very harsh . a tannin addition to reduce the unbound short chain natural tannins followed by delestage seed removal is a key strategy in reduceing rough character .
a tannin addition during yeast fermentation also help fix color and improve color stability , this is essential when also using an enzyme or heat spike to increase the colour and flavour density of the wine. you add the enzymes at crush to release the flood of color and aromatic compounds and then add tannins before 1/4 of the way into the ferment to lock in those color and flavour compounds.
its all about creating a wine with smooth mouthfeel , good body and stable color .
yeast fermentation stage tannins are the most critical , durring active ferment is when you have the greatest chance to influence what the wine will be . once primary is done , the horse is out of the gate and later tweaks become an attempt to cover flaws or shortcomings of the wine it can be like putting lipstick on a monkey . but fermentation stage additions , be they enzyme , tannin or SIY are like impacting the DNA before birth.
the impacts are better integrated and less obvious, especialy if you are a little heavy handed , which most homewinemakers are when it comes to additions.
I'd go as far as saying that if you can ,try to always add tannins during yeast fermentation stage and avoid the later stage additions . this can be hard for homewine makers to do since you often don't know your fruit that well but region , variety and must numbers can inform you decisions .
Central Valley , hot region , high crop loads , high brix high PH , syrah , cab , zin , sangiovese , would be no brainers.
as would any red hybrid or very cold finger lakes type region.
after that , experience and taste will guide you , if you know you like a wine with round, integrated full tannins add them durring fermentation
