The lactone in tofu curd refers to gluconolactone.

Gluconolactone is a non-toxic chemical substance that is white crystalline and easily soluble in water. It is often used as a preservative, quality improver and preservative in the food industry. When making tofu curd, the principle of using gluconolactone is to mix it with water, and when dissolved, gluconic acid will be generated. Gluconic acid can cause acid coagulation of the protein in soy milk, thereby forming the coagulation structure of tofu curd. Using gluconolactone to make tofu can preserve freshness and improve the taste. Gluconolactone is generally not harmful to the human body when consumed in moderation.