The ĥametz that the Torah forbids is comprised of flour and water. If flour was kneaded with fruit juice – even if the dough sits a full day and rises – it is not considered ĥametz since rising of this kind is different from the type forbidden by the Torah. The category of “fruit juice” (“mei peirot”) includes wine, honey, milk, oil, and egg, in addition to all juices squeezed from a fruit, like apple or berry juice. Since fruit juice does not cause dough to become ĥametz, one may knead, bake, and eat such dough on Pesaĥ. Nevertheless, one would not fulfill the mitzva of matza on the first night of Pesaĥ with it, because the Torah calls matza “leĥem oni” (“poor man’s bread”), and matza made from fruit juice is “matza ashira” (“rich matza” – colloquially known in English as “egg matza”), since it possesses more than the taste of just flour and water.
If a drop of water gets mixed in with the fruit juice, it can cause the dough to become ĥametz. Moreover, according to many poskim, the combination of water and fruit juice actually expedites the leavening process. Thus, in order to avoid such doubts, the Sages prohibited kneading dough with a mixture of fruit juice and water during Pesaĥ (SA 462:1-3).
The Ashkenazic custom is to avoid eating anything made of dough kneaded with fruit juice out of concern that water mixed with the fruit juice causing the dough to become ĥametz. Furthermore, it takes into account the opinion of Rashi, who disagrees with most Rishonim and maintains that fruit juice alone can cause something to become ĥametz on the rabbinic level. Although in principle it is possible to follow the lenient ruling of the vast majority of poskim, Ashkenazic custom, which should not be altered, is nevertheless strict, except in the case of the elderly and ill, where the custom is to be lenient (Rema 462:4). Nowadays, many Sephardic poskim also rule stringently, because it is known that water and other ingredients are generally added to fruit juice, increasing the likelihood that the egg matza became ĥametz (R. Mordechai Eliyahu).
. There are two issues here: The first is whether flour mixed with fruit juice can become ĥametz, and the second is whether, in a case where water was added to the dough, one may knead the dough while being careful that it does not become ĥametz.
The first issue is summarized in Birur Halakha on Pesaĥim 35a, s.v. “mei peirot ein maĥmitzin.” According to Rashi and those who agree with him, a flour and fruit juice mixture can become ĥametz nuksheh (see above 2:5), which is rabbinically forbidden. Conversely, most Rishonim maintain that flour mixed with fruit juice alone never becomes ĥametz, but if any water got into the mixture, the mixture can become ĥametz. There is a dispute about the severity of this ĥametz: According to Rambam, it is ĥametz gamur, and according to Rabbeinu Tam it is ĥametz nuksheh. Pri Megadim states that all agree it becomes ĥametz gamur when the majority of liquid in the mixture is water (BHL 462:2 s.v. “memaharim”). See the book Matza Ashira 5:7-8.
The second issue is explained in Birur Halakha on Pesaĥim 36a, which cites a tannaitic dispute about matza ashira on Pesaĥ. According to Rif and Rambam, the halakha follows Rabbi Akiva that one is permitted to knead flour in a mix of fruit juice and water provided that one takes caution to prevent the mixture from becoming ĥametz, just as one would with a standard flour-water mixture. This is also the opinion of R. Natronai Gaon and Me’iri. On the other hand, many Rishonim maintain that one should not knead flour with fruit juice and water, since it turns into ĥametz faster than does flour and water. There is a dispute about what to do be-di’avad. According to R. Hai Gaon and Behag, the halakha follows Rabban Gamliel that one is required to burn the kneaded dough. According to Rabbeinu Ĥananel, Ritz Gi’at, and Rosh, the halakha follows the Sages that if one bakes the dough very quickly, he is permitted to eat it. This is the opinion that SA accepts (462:2). The Ashkenazic custom is to be strict out of concern for Rashi’s opinion that flour and fruit juice alone can become ĥametz, or out of concern that some water will mix with the fruit juice.
The parameters of what is considered water and what is considered fruit juice are discussed in SA 462:3 and 7 and §466. The entire topic is summarized in detail in Encyclopedia Talmudit entry “Ĥametz” pp. 89-99.
Regarding the Sephardic custom, R. Mordechai Eliyahu had a long-standing opposition to certifying matza ashira as kosher for Pesaĥ out of concern that the fruit juice has been mixed with water or a leavening agent. (There are also grounds to forbid this because it generates activity that mimics the process of becoming ĥametz, as explained by Maharam Halawa on Pesaĥim 28a and Matza Ashira p. 178.) Over time, it became apparent that this concern was justified, and water and occasionally leavening agents are added to fruit juice. Nevertheless, some authorities rule leniently; according to them, these leavening agents do not cause the dough to become ĥametz. This ruling appears in Yabi’a Omer 9:42 and Shema Shlomo 4:13-17. However, according to many authorities these leavening agents are forbidden by the Torah, or at least rabbinically, and therefore they prohibit Sephardim from eating factory-produced matza ashira on Pesaĥ.