"major ingredient" is correct and usable in written English.
You can use it to refer to a key element or component of something, such as a recipe or a process. For example, "Salt is a major ingredient in this recipe."
Each major ingredient is treated separately.
They are also a major ingredient in smog.
Sheer sound color is a major ingredient here.
They are a major ingredient in building a great company.
Another major ingredient for growth has been Atlantic-branded conferences.
After water, sugar is, of course, the major ingredient in its product.
Ethanol tends to evaporate easily, and unburned evaporated fuel is a major ingredient of smog.
PETN is a major ingredient of the plastic explosive Semtex and is used in detonation cables.
Aluminum oxide is the major ingredient in the commercial chemicals known as aluminas.
Acrobatics and farcical scenes were a major ingredient of the phlyax.
Ferrite crystals are a major ingredient in snooper paint, which makes stealth airplanes undetectable by radar.
