From another thread, my understanding is that after you buy the
[Secret Gourmet Goods] suitcase with the silver key, the suitcase opens and there are secret ingredients available for purchase.
It costs one bronze (?) key to purchase the secret ingredients in the [Secret Gourmet Goods] suitcase. It doesn't appear that you have a choice of what ingredients you receive. You use the bronze key and get six ingredients which will automatically be added to your pantry.
You can choose not to use the key to buy the ingredients or you can wait until you have enough keys to buy the ingredients AND cook the recipes.
The gourmet recipes need certain secret ingredients AND keys to cook them. So if you have the ingredients and not
the required key you cannot cook the recipe.