After some discussion with folks about the impacts of maceration and, more specifically, the effects of stems on juice chemistry and winemaking, this topic is far from clear.

Stems clearly contribute considerable polyphenols, and more specifically, what is generally referred to as bad tannins, which impart a bitter taste, when macerated with grape skins and juice. However, stems with their pH hovering over 4.0 reduce (NOT increase) acidity, and slightly increase pH.