The phrase "a primary ingredient" is correct and usable in written English.
You can use it when referring to the main or most important component of a recipe, product, or process.
Example: "Flour is a primary ingredient in making bread, providing the necessary structure and texture."
Alternatives: "a key component" or "a main element."
Chitosan is a primary ingredient in dietary weight-loss supplements.
James's departure is a primary ingredient in a blend of conspiring incidents.
Public space is a primary ingredient in urban environment; places of encounter where public life unfolds.
Ammonium perchlorate is a primary ingredient in rocket fuel, fertilizers, paints, and lubricants.
On Monday, readings of ozone, a primary ingredient in smog, were so high that residents in some neighborhoods were advised to stay indoors.
A primary ingredient in Listerine and several other commercial mouthwashes is eucalyptol, a natural oil that is also an active ingredient in botanical repellents.
And dishes with white wine as a primary ingredient well, surprise, white wine, likewise dishes cooked with red wines.
The choreography shows elsewhere that the nine admirable young men performing it are certainly capable of épaulement; I just wish that element of style were a more primary ingredient throughout.
Each chef must incorporate an exotic primary ingredient -- like beef tongue, pigeon or sea urchin -- into every course, from appetizer to dessert.
Those who want to drink soda like it used to taste buy Coke from Mexico where it's still made with sugar, not corn syrup, putting them in the odd position of paying a premium price for a product whose primary ingredient is Mexican water.
But the shrimp are covered in a wrapper whose primary ingredient is flour, which could be measured as a commodity or in its milled form.
