One plant that can be used for a rennet is stinging nettle (urtica gracilis). To make, boil a pound of stinging nettle in just enough water to cover for twenty to thirty minutes. Strain off the liquid and add as much salt as will dissolve with agitation. With home-brewed extracts, it's difficult to know how much of the infusion to use because of the variability of its strength and the milk's acidity. A good starting point would be one-half cup of nettle infusion per gallon of milk.
Stinging Nettles as a Natural Rennet for Cheese MakingAnother little trick is to use stinging nettles to set your cheese. Take a gallon of any kind of milk. Then take some herbs you like, and set them in some hot water, like you were making tea from them. Set that aside. Only use enough water to cover plus a tad bit more.Now heat your milk to about 160°F and hold. Add the herbs and maintain the temp of 160°F. Let cool to 80°F add 1 cup cultured butter milk stir and wait a moment, then add a hand full or about 1/2 cup of stinging nettle. Your cheese will set.Now just cut the curds (use only stainless steel when working with cheese aluminum will kill you) After your curds are cut, keep on the heat at 100°F a bit more for just a few minutes. Then hang over night in cheese cloth and enjoy.This also can be made with any kind of milk. This cheese can also be pressed to make a hard cheese. It is a very versatile base that has limitless possibilities. It is soft or hard, spiced or not, it is what ever you want it to be.