Yeast is put into bread dough in order to produce carbon dioxide gas to lighten the whole mass. It is studied in the chapter on preservation of fruit, but in bread making we need to foster its growth instead of destroying it as, you will recall, we found necessary in preserving. You can easily reason out for yourself just how this should be done, especially after performing the experiments with yeast.
If yeast is in good condition, it has little effect on the flavor of bread, even if used in a rather large quantity to hasten the process.
The best forms of yeast now in use are the compressed and dried cakes, the former needing to be fresh, the latter keeping the vitality of the yeast cells for a long time. Liquid yeast may be made at home, but it is somewhat uncertain unless made with great exactness, and less easy to manage on the whole than the other forms.