Why lemon juice make harden beans?

Lemon juice can cause beans to harden during cooking due to its acidity. The acid in lemon juice can affect the pectin in the beans' cell walls, which can result in a firmer texture. Additionally, if beans are cooked in acidic conditions before they have softened, they may not fully hydrate and can remain tough. To ensure beans cook properly, it's best to add acidic ingredients like lemon juice after they have softened.