Paltrinieri discusses the issue of using natural yeast

Azienda Paltrinieri, a Lambrusco producer, on using indigenous and selected yeasts in winemaking.

There are useful types of yeast and non-useful types, the latter meaning not useful for fermentation or even dangerous. If you don't select the yeast, you don't know what kind of chemical prevails in the yeast.

Paltrinieri has picked the safer option of using selected yeasts for the first fermentation of their sparkling Lambrusco wines. But they also value their roots so they let the second fermentation occur naturally in the bottle, a natural refermentation.