I think there are two parts of it: Increased acidity certainly makes tannins more pronounced because of how they interact with the saliva in your mouth at lower pHs–and that’s the effect when you’re blending red and white. Also, where co-fermentation is concerned, white grapes often have really high tannin loads and hard tannins at that. I think that’s largely because white grapes weren’t bred for their tannin qualities over hundreds of years, whereas most red grapes were selected (at least somewhat) for textural traits. I’ve worked with quite a bit of Pinot Gris as a skin-contact wine, and it really can veer towards bitterness if you aren’t careful, and the tannins are definitely much less refined as Pinot Noir, for example.
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