The secret ingredient in secret ingredient soup is "nothing".
The soups quality comes from a belief that it's special, not from any hidden ingredient.
This is a concept that originates from the Kung Fu panda Move, in which Mr. Ping reveals to his son Po that there's no secret ingredient in his noodle soup.
However a secret ingredient that is added to some soups is vinegar.
In some soup recipes, vinegar is often an added ingredient.
vinegar is a flavor enhancer and helps add umami flavor.
Due to the vinegar being a flavor enhancer, the vinegar is often used in cooking of soups, sauces and also in salad dressings.
The "secret" to making a good soup lies in building layers of flavor by using a high-quality homemade stock, browning meat and sautéing aromatics (like onion, garlic, carrots, and celery) at the start, and using fresh, high-quality ingredients.
And finishing the soup with a touch of acid, a sprinkle of salt and pepper, and fresh herbs just before serving also enhances its taste.
A flavorful stock or broth is often considered the foundational element for many soups, providing depth and richness.
Other vital ingredients for a good soup can include vegetables, beans, lentils, potatoes, and a balancing addition of acid, herbs, or spices to enhance the overall flavor profile and texture.