Have fatty acids and glycerol undergone istiḥāla? And is istiḥāla a Sharʿi concept by scholarly consensus?

Transmutation (Istihalah): It is the change of a substance itself and the inversion of its reality into another reality. It includes every chemical reaction that transforms a substance into another compound, such as the transformation of oils and fats into soap. Transmutation purifies the substance and permits its utilization, according to the majority of the Hanafis and Malikis, and a group of scholars like Ibn Taymiyyah and Ibn al-Qayyim.

Thus, an intrinsically impure substance becomes pure through transmutation according to the Hanafis and Malikis. Examples include wine when it transmutes into vinegar, and salt that was once a donkey or a pig. Their fundamental principle is that "the Sacred Law assigned the attribute of impurity to that specific reality, so it ceases with its cessation," and "the 'alaqah (clot of blood) is impure, but when it transforms into a mudghah (lump of flesh), it becomes pure."

As for the Shafi'is and Hanbalis, according to the apparent view of their madhhabs, their opinion is that an intrinsically impure substance does not become pure through transmutation, with the exception of wine if it spontaneously turns into vinegar.

Ibn al-Qayyim argued that the purity of wine through transmutation is in accordance with analogy. If the attribute that necessitates impurity vanishes, then the necessitating factor vanishes. He then analogized all other impurities to this, should they undergo transmutation. He mentioned examples including the Prophet (peace be upon him) exhuming the graves of the polytheists from the site of his mosque without removing the soil, and that Allah Almighty informed us about milk that it "comes out from between excrement and blood." And an animal fed impure substances, then pure ones, becomes permissible. And crops and fruits, if irrigated with impure water then pure water, become permissible; due to the transmutation of the attribute of impurity. Therefore, if impure fats undergo transmutation, they become pure and permissible to consume.