The kitchen sink drain, disposal and connecting pipe are often
overlooked, but they should be sanitized periodically by pouring down
the sink a solution of 1 teaspoon (5 milliliters) of chlorine bleach in
1 quart (about 1 liter) of water or a solution of commercial kitchen
cleaning agent made according to product directions. Food particles get
trapped in the drain and disposal and, along with the moistness, create
an ideal environment for bacterial growth.
Source: Excerpted from FDA Consumer, November 1996 update: Can Your Kitchen Pass the Food Safety Test?