“Give an offering of bread from your dough, as teruma – like the teruma of the granary, that is how you shall offer it” [Bamidbar 15:20].
Setting aside “challah” is one of the mitzvot connected to the “five species of grain” defined as a special group. This group plays a special role from the point of view of halacha as compared to other grains. The blessings for bread from the five grains are “hamotzi” and the Grace after Meals, these grains can become chametz and are therefore prohibited on Pesach, and they can be used to bake the matza used for the mitzvot of Pesach. Produce from the “five grains” may not be harvested before the Omer Sacrifice is brought, and it may not be eaten before the Omer Sacrifice.
The five species are: wheat, barley, buckwheat, rye, and oats. Other grains used for making bread that are mentioned in the traditional literature include rice, millet, and cow-wheat. These are of a lower status, and they are treated in the halacha in a different way than the five main grains.
The identities of various grains, except for wheat and barley, were the subject of detailed discussions by commentators and researchers (see a long summary by Z. Amar, “Five Species of Grain”). Many factors are taken into account in the attempt to identify the five grains. One of these is their suitability for baking high-quality bread. The importance of this factor can be seen from the fact that even within the group itself the grains are rated according to this criterion. Thus, the blessing is recited first for wheat bread and only afterwards for bread made from barley. We can also add another halacha, the preference as a basis for an eiruv to link together several courtyards on Shabbat. The criterion is that the material used for the eiruv must be suitable for making bread. An eiruv can be made with bread made from the five grains or from rice, but not with millet, which shows the low status of this grain even as compared to rice (Eiruvin 81a).
In order to judge if a grain is more or less suited for baking bread, we must know what factors establish the quality of the resulting bread and if they exist for the species being studied. In order to put our study on a firm basis, we can compare the properties of the “five grains” to rice and millet, which are not included in the special set of five even though they can be used to make a type of bread.
The Fluffy Texture
The most important factor is the fluffiness of the baked bread, which is a result of the rising of the dough. The rising stems from bubbles of carbon dioxide which are produced by the cells of yeast during the process. The yeast cells make use of the glucose in the dough to produce energy, and in addition to the gas they also produce alcohol, which evaporates during the baking. While wheat is mainly starch, the action of the enzyme β-Amylase converts it into sugar. Another condition for the rising of the dough is that the carbon dioxide must be trapped by a complex net of thin fibers which are produced by the protein gluten.
Bread that is Chametz
Making the dough into chametz: The leavening that was used to make the dough rise, before the invention of industrial yeast, includes bacteria of the type lactobacillus in addition to yeast. These bacteria produce lactic acid, which gives the bread a desirable odor and a long shelf life. (The same bacteria are used in various fermentation processes, such as the preparation of yogurt and pickled vegetables.) This acidic taste is the source of the word “chametz.” It may be that the leavening which is forbidden on Pesach includes not only the rising of the dough which is relevant for industrial yeast but also the accumulation of acid, which gives the leavened bread its typical taste.
The fact that the “five species of grain” can become chametz and therefore they are the only grains suitable for the matzot is not an arbitrary ruling. Rather, it stems from the fact that the ability to become chametz and to rise are necessary conditions for the definition of a product that can be called bread. “Bread” that is made from rice, millet, or legumes does not rise because it does not contain gluten and a sufficient amount of the enzyme β-Amylase, and it is therefore of a lower quality than bread made from the five grains. And just as the bread mentioned with respect to Pesach must be able to become matza or chametz, so the bread that is relevant for “challah” must be able to form matza or chametz. If you study the matter, you will find that this is restricted only to the five special grains. Other materials do not form into matza or chametz, rather they “spoil if left for too long a time” [Talmud Yerushalmi Challah 1:1].
Challah from Rice Bread
The biological analysis of the five species of grain allows us to understand the explanation given by the Talmud Yerushalmi as to why there is an obligation to give challah from dough that was prepared from a mixture containing mostly rice, with a small amount of wheat. It is written in the Mishna, “If one takes leavening from dough made from flour and puts it into dough made from rice – If it has the taste of grain there is an obligation of challah, otherwise there is no obligation…” (Challah 1:1).
The discussion in the Babylonian Talmud is focused on concepts of “cancelling” the prohibition based on the relative proportions of wheat and rice. In the Yerushalmi, on the other hand, a more “practical” explanation is given: “Rabbi Hila says in the name of Rish Lakish: This applies only to making dough from wheat and rice, where the wheat takes control over the mix.”
The Ramban writes in Hilchot Challah 4 (31a in the Vilna Talmud): “However, when it is mixed with grain and takes on its taste the grain ‘takes control’ of the dough to make bread, and it is therefore necessary to set aside challah from it.” This implies that mixing grain into the rice dough causes it not only to produce chametz but also to make true bread. According to our analysis above, “taking control” is a process of fermenting which can take place in rice that is under the influence of the enzymes found in the wheat. Since these enzymes are active at very low concentrations, the enzymes in the wheat can operate on the starch in the rice even though amount of the enzyme is no more what is needed to “provide a taste” of the wheat (usually taken as one-sixtieth of the amount of the main ingredient).