Red fermented bean curd is red because of the addition of

Red bean curd is red because of the addition of red yeast rice.

Red yeast rice, or red yeast for short, is a kind of rice that is inoculated with Monascus bacteria on steamed rice and fermented. The rice is purple-red or brown-red and is used for fermentation, preservation, and food coloring. It has other functions and is a natural food additive.

There are also other names such as __, 福_, red yeast rice, red yeast rice, and red rice. China has a long historical tradition of making red yeast rice.

Red yeast rice can be used to make red yeast wine and red lees.