Juice and yeast
 

A little anecdotal evidence about native yeast. My wine making
partner and I have a tiny backyard vineyard east of San Diego. Our
mourvedre got out of control this year at over 27 brix. The native
yeast fermented it absolutely dry. I suppose that gave us more than
15.5 percent alcohol. We did not sulfite or even crush; we just
destemmed and waited.

The resulting flavor is rich and ripe. Most people love it. I find
it a little hot and a little overripe.