Lemon in cooking beans: a balance of flavor
A slice of lemon in the cooking water of the beans balances their intense, earthy flavors while helping them maintain their texture.
The citric acid in lemons increases the acidity of the cooking water, which affects some components of the bean skins. For this reason, the timing of its addition during cooking is important. Too much acid when the beans are still hard can delay their softening; it's best to add it when they are already quite soft.
During cooking, the pods of dried legumes sometimes separate or become too soft and fall apart. The acid in lemon juice can help reduce this disintegration by altering the texture of the legumes, making them appear more compact. However, this effect varies depending on the type of legume, the soaking time, and the cooking method, so lemon juice alone does not completely prevent legumes from falling apart.
Beans cooked with tomato paste, onion, and oil can have a rather intense flavor. The slightly acidic aroma of lemon balances this intensity, resulting in a more vibrant taste. The goal isn't for the dish to have an overpowering lemon flavor, but rather to add a subtle freshness. For this reason, it's best to leave a slice of lemon in the cooking water throughout the cooking process, which can be removed at the end to check the dish's flavor.