I usually belch out what I’ve eaten during fasting after the call of salat, though I try to swallow back my spit so that the food will not pass through my throat and I do not really take much food, I will like to know if it invalidates my fast?
If one who is fasting belches out something and it comes into the mouth, if he swallows it again, it will invalidate his fast no matter how little it is and he has to make up for it and also make kaffarah (either fast 60 days or feed 60 poor people the way mentioned in related books). Also, if what he has swallowed is something that is haram to eat (when not fasting), such as blood, he will have to do kaffarah jam’ (meaning that he has to both fast 60 days and feed 60 poor people). All if this is if he swallowed it intentionally. None of what was mentioned applies if it was unintentional though and the fast is valid.
Question: If a fasting person belches and swallows intentionally that which comes in his mouth, does he have give Kaffara in addition to the qadha?
Answer: If a fasting person belches and swallows intentionally that which comes in his mouth, his fast becomes void, and he should give its qadha and Kaffarah also. And if the thing which comes to his mouth is haraam to consume, like, blood or some food which no more looks like food, and he swallows it intentionally, he will give the qadha of that fast, and as a recommended precaution, give all the three Kaffarah.
 Tawdihul-Masa’el (annotated by Imam Khomeini), vol. 1, pg. 933.