All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If food remains between the teeth, then this does not invalidate the prayer if the praying person did not swallow it during the prayer. If you mean that you swallowed it while praying, then if it is small and not a solid substance and it may go down with the saliva, then this does not invalidate the prayer as this is difficult to spit out.
On the other hand, if it has a dimension and the praying person swallowed it intentionally, and not out of forgetfulness, then the prayer becomes void because he ate (while praying). The jurists stated that if a praying person "takes out from between his teeth something that is not a solid substance and he swallows it, then his prayer becomes void. Their evidence is that it is not difficult to avoid that…"
Allaah Knows best.