Liquid soy lecithin vs sunflower lecithin I'm currently reading Modernist Cuisine at Home and one ingredient that keeps coming up is "liquid soy lecithin". I'm pretty familiar with the powdered variant for making light, airy foams, but the book is adamant that the powder cannot substitute for the liquid.

So I go on Amazon to try to find some, and I simply cannot! All I see is liquid sunflower lecithin. Does the source of the lecithin matter? Can I safely use sunflower lecithin in the exact same way I would otherwise have used soy lecithin?
As long as you know the lecithin content (in the liquid) you can substitute sunflower lecithin with soy lecithin. 
It’s more or less similar molecules. I use them interchangeably. (Even make mine at home from sunflower seeds ;))