What color is tofu? White. What do we associate with white? Purity. A symbol of cleansing the soul. The tofu represents a new beginning: now that the criminal has paid their debt to society and is starting to begin their life again, they are trying to once again be as pure as the white tofu they consume.
White is also the symbol of death in East Asia (both literal and metaphorical), meaning the death of one life and the start of another, much like how Japanese brides wear white and how winter represents the end of a year.
There’s also a book called Dooboo (Korean for tofu) and in the synopsis of the book, this explanation is given: "A life behind the bars is said to be ‘eating boiled beans.’ A tofu has been released from beans but can never return to beans. Thus a feeding a tofu might mean a wish that the person would never go back to prison."