If the dough be baked during the alcoholic and carbonic-acid stage of fermentation, the gas will render the loaf light and porous. The alcohol will be dissipated by the heat during the baking, or evaporated shortly afterward, provided the baking be thorough. If the fermentation is allowed to proceed until the acetous fermentation has begun, the loaf, when baked, will be "sad" and heavy, since there is no longer any gas to puff it up. If, however, during the first or alcoholic stage of fermentation, new material be added, the same kind of fermentation will continue for a certain period longer. These facts serve to show that great care and attention are necessary to produce good bread by a fermentative process. If the fermentation has not been allowed to proceed far enough to generate a sufficient amount of gas to permeate the whole mass, the result will be a heavy loaf; and if allowed to proceed too far, acid fermentation begins, the gas escapes, and we have sour as well as heavy bread. It is not enough, however, to prevent bread from reaching the acetous or sour stage of fermentation. Bread may be over-fermented when there is no appreciable sourness developed. Fermentation may be carried so far as to destroy much of the richness and sweetness of the loaf, and yet be arrested by the baking process just before the acetous stage begins, so that it will be light and porous, but decidedly lacking in flavor and substance. Over-fermentation also develops in the bread various bitter substances which obscure the natural sweetness of the bread and give to it an unpleasant flavor. Many of these substances are more or less harmful in character, and include many poisons known as ptomaines, a class of chemical compounds produced by germs whenever fermentation or decomposition of organic matter takes place. Much skill is required to determine at what point to arrest the fermentation, in order to save the sweetness and richness of the bread.