After the start of imsāk time (إِمْسَاك) — the time marking the beginning of the Fajr (dawn) prayer — if food particles remain in the mouth and are smaller than the size of a chickpea, swallowing them does not break the fast.
However, if the remnants are equal to or larger than a chickpea when gathered, swallowing them breaks the fast, and making up (qaḍāʾ) that day’s fast becomes obligatory.