Answered by Shaykh Faraz Rabbani
What is the minimum amount of food that can go down the throat during salat without breaking the salat? For example, if food is lodged in between the teeth, and comes loose during the salat and then is swallowed, will this break the salat? And what if it is not swallowed, but kept in the mouth until the end of the salat?
1. If one swallows less than a dried chickpea worth of food from within the mouth, it does NOT break the prayer. More than that does.
2. If one swallows even a little food or liquid from outside the mouth, it invalidates the prayer.
3. If food remains within the mouth, it does not affect the validity of the prayer.
[from: Maraqi, Durr]