As noted, there are five types of grain that can become ĥametz after touching water. However, if they are roasted in fire, they can no longer become ĥametz, and in principle they may be mixed with water. Nevertheless, the Sages were concerned lest the roasting not be thorough, and thus the grain would become ĥametz when coming into contact with water. Therefore, one must treat them just as one treats regular cereal grains. If they became wet and eighteen minutes passed, we suspect they may have become ĥametz, and it is forbidden to gain any benefit from them, and one must destroy them (SA 463:3; MB ad loc 7).
The above concerns kernels of grain that were roasted in fire, but if it was flour that was roasted, there are Rishonim who are lenient, maintaining that one need not suspect that the flour was not roasted well. Thus, it is permissible to mix such flour with water or in a cooked food without concern for ĥametz (Rashi, Rambam). However, many Rishonim hold that in the case of flour, as well, one must be concerned that it may have not been roasted thoroughly (Rabbeinu Yeruĥam, Hagahot Sefer Mitzvot Katan, Hagahot Maimoniyot, and others). The Aĥaronim rule that one must not mix roasted flour with water or in a cooked food, lest it become ĥametz. Nevertheless, if one did make such a mixture, even though it is forbidden to eat it, it is permissible to keep it until after Pesaĥ and to eat it then (MB 463:8; Kaf Ha-ĥayim ad loc. 13).
However, concerning matza that was properly baked, it is agreed that it cannot become leavened again. Accordingly, it is permissible to soak matza and matza meal in water, and, indeed, this is what most people do. Ĥasidim, however, customarily do not eat soaked matza (see below 8:2).
Scalding the grains or the flour in boiling water also destroys the capacity for becoming ĥametz. However, the Ge’onim prohibited doing so, for today no one knows how to do this scalding, and if the boiling does not destroy the potential for becoming ĥametz, an opposite process of rapid fermentation may be generated, as heat may hasten fermentation. Therefore, scalded grains or flour are treated just like ĥametz: it is forbidden to gain any benefit from them and one must burn them (SA 454:3; MB 13).
Flour on which water dripped, drop by drop, continuously, even all day long, does not become ĥametz, since the falling of the drops disturbs the flour and shakes it, and does not allow the leavening process to develop. Immediately upon cessation of the dripping, one should knead the dough and bake it. If there is doubt that some of the dripping may not have been continuous, then this is a doubt concerning a law of Torah, and one must relate to that flour as ĥametz and burn it (Pesaĥim 39b; SA 466:6).
Another way to prevent the dough from fermenting is by soaking it in cold water (Pesaĥim 46a; SA 457:2). Preferably, one should not do so, lest the water not be cold enough, allowing the dough to ferment (Rosh, MB 454:18).
Flour that was kneaded with fruit juice does not become leavened at all, but if even a little water was added to the mixture, then it will become leavened (matza ashira will be explained below [8:1]).
All of the processes that slow the leavening process are summarized in the Encyclopedia Talmudit entry “Ĥametz” pp. 83-89. And even though we generally are not lenient when it comes to scalding the dough, there are practical applications when dealing with a dangerously sick person, where it is better to minimize the number of prohibitions involved, as explained in MB 454:13.