A great many foods are mixtures. When a question arises about one, halacha frequently does not analyse every component — it identifies the ikkar, the primary ingredient, and treats the rest as tafel, secondary to it.
Knowing how that identification is made explains a good deal of otherwise puzzling rulings.
The principle
In deciding whether a concern applies to a product made of many ingredients, only the primary ingredient is considered. The secondary ones follow it rather than being assessed on their own.
This is why a question can attach to one food and not to a dish containing it, without anything inconsistent having happened.
How the primary ingredient is identified
Two tests, applied in order.
Does one ingredient serve the other? Where ingredient A is there to enhance ingredient B — to season it, to bind it, to improve it — then B is the ikkar and A is the tafel. Purpose decides, not quantity.
That is the primary test, and it is intuitive once stated: a spice in a stew is there for the stew.
Where neither serves the other, volume decides. If two ingredients are blended until they become one indistinguishable mass, and neither is enhancing the other, then whichever has the greater volume is the ikkar.
Note that this is volume, not weight — a distinction that matters more often than people expect, since ingredients differ considerably in density and labels are written by weight.
What it does not do
This principle decides which ingredient a question is asked about. It does not make a forbidden ingredient disappear — that is the separate question of bitul, with its own rules and its own exceptions.
The two get conflated constantly. "It's only a small part of the dish" could be an ikkar-and-tafel argument or a bitul argument, and they work differently and have different limits.
Where you meet it
Most often in questions about bishul akum, where whether a prepared food is affected turns on what its primary ingredient is rather than on a full audit of the recipe.
It also underlies questions about which blessing a mixed food takes — the same principle, applied to a different question.
Which ingredient is primary in a given dish is not always obvious, and it is the sort of thing to ask about rather than assume.
Where this comes from
- Shulchan Aruch, Orach Chaim 212, on ikkar and tafel
- Rulings of Rav Schachter and Rav Belsky on mixtures, recorded in a summary of piskei halacha