Different companies produce cornflakes in different ways, and this difference may affect the cornflakes’ status vis-ŕ-vis Berachot. Kellogg’s produces its cornflakes by smashing pieces of corn into a flake (after removing some portions of the corn) and then adding several additional ingredients. With respect to Berachot, then, Kellogg’s Cornflakes resembles mashed potatoes, which require the Beracha of "Ha’adama" just like the potato itself, since it is still a potato which has just undergone a change in form. Another example is a mashed apricot, which requires the Beracha of "Ha’etz" just like an ordinary apricot, for the same reason. Thus, according to all opinions, the Beracha for Kellogg’s Cornflakes is "Ha’adama."
Other companies, however, such as General Mills, produce cornflakes by grinding corn into flour and then reconstituting the flour into flakes. We might compare cornflakes produced in this fashion to Pringles potato chips, which are produced from potato flour. The Beracha over Pringles (as discussed in a previous edition of Daily Halacha) is "She’hakol," because many other ingredients are added to the potato flour, and the flour comprises less than half of the chip. The Halachic authorities discuss the status of cornflakes made from corn flour with respect to Berachot, and the general consensus is that one should recite "She’hakol," but if one recited "Ha’adama" he has fulfilled his requirement and does not recite a new Beracha.
If a person is served cornflakes and does not know whether it was made from corn flour or from whole corn, he must try to determine how it was produced. If this is not possible, then he should recite "She’hakol."
(Based on Rabbi Bitan’s discussion in Yalkut Yosef – Berachot, vol. 3, p. 63)
Summary: The Beracha over Kellogg’s Cornflakes is "Ha’adama," since they are made by mashing whole corn. Cornflakes produced from corn flour, however, require "She’hakol," though one who recited "Ha’adama" has nevertheless fulfilled his obligation and does not recite a new Beracha.
