Purity: Purification of Impure Food Particles Between Teeth
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 166
With regard to impure bits of food that have remained in between the teeth, if water (albeit qalīl water) is gargled and it reaches all the bits of impure food, they become pure.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)