The foundational distinction is stated by Rava in the Gemara: a forbidden item mixed with food not of its own type is governed by taste, while a forbidden item mixed with food of its own type is governed by majority, and appearance governs where visible (Zevachim 79a:4), a rule echoed in Chullin 97b:1 and Chullin 99a:3.
The Rambam codifies this principle directly: a forbidden substance mixed with a different type is forbidden if its flavor is detectable, while mixed with its own type it is nullified by a simple majority, though the Sages nevertheless required that the forbidden component be lost within a majority so that no significant piece remains identifiable (Mishneh Torah, Forbidden Foods 15:1-5).
The Shulchan Arukh rules that when the forbidden and permitted foods differ in kind, a gentile's taste-test determines the outcome, and absent such a test, the permitted food must be sixty times the forbidden food, a measure also applied to dry mixtures of the same kind requiring the permitted portion to be double the forbidden (Shulchan Arukh, Yoreh De'ah 98:1, Shulchan Arukh, Yoreh De'ah 109:1).
The Gemara in Chullin illustrates how the sixty-fold measure is actually calculated in practice, discussing whether one measures the pot's absorption together with the meat when forbidden fat falls in (Chullin 98a:7), and the parallel mishna of a milk drop falling onto meat, where the piece itself can become forbidden and spread the prohibition further if the mixture is not nullified (Chullin 108a:2).
The Arukh HaShulchan explains that the sixty-measure is a rabbinic enactment grounded in an asmachta from the ram of the nazirite, since Scripture itself does not treat even a pungent forbidden ingredient like onion or radish as detectable in a larger quantity (Arukh HaShulchan, Yoreh De'ah 98:1-30), while elsewhere he affirms that dry-in-dry mixtures are nullified by simple majority even by Torah law (Arukh HaShulchan, Yoreh De'ah 109:1-20).
The Shulchan Arukh further limits nullification through the category of a 'significant piece' (chatichah ha-re'uyah l'hitkabed), which is treated like a distinct entity that cannot be nullified even a thousandfold (Shulchan Arukh, Yoreh De'ah 101:1), and similarly certain items that are commonly sold by count, such as nuts, pomegranates, and sealed barrels, are not subject to dissolution at all (Shulchan Arukh, Yoreh De'ah 110:1, Zevachim 72b:1).
Rav Eliezer Melamed's contemporary treatment reflects this same structure, describing the practical reliance on the sixty-fold measure rather than a gentile's taste on account of the unreliability of taste-testing (Peninei Halakhah, Kashrut 34:5), while also stressing that one may not deliberately create or increase a mixture in order to achieve nullification by rov or by sixty (Peninei Halakhah, Kashrut 34:13), and that the sixty-measure itself is a stringent rabbinic safeguard rather than a precise indicator of when taste is actually undetectable (Peninei Halakhah, Kashrut 35:5).
Among later authorities addressing the application of bitul b'rov versus bitul b'shishim to particular cases, Igros Moshe (Igros Moshe YD 4:14) holds that at least an equal proportion can warrant prohibition and that similar stringency should be applied to bitul b'rov in mixtures of the same kind, since the forbidden item was nevertheless consumed.
Igros Moshe (Igros Moshe YD 3:12) distinguishes the two modes of nullification further, explaining that in a mixture of unlike kinds the forbidden substance's very identity can vanish when its taste is undetectable even if it is intrinsically significant, whereas in mixtures of like kind only the legal status is nullified and an inherently significant item may remain forbidden.
Tzitz Eliezer (Tzitz Eliezer 9:43) examines a case of a forbidden ring mixed among many permitted rings, concluding that Torah law generally permits reliance on bitul b'rov, while also addressing whether the nullified item then takes on the character of the majority.