Gas in Abdomen - Belching 

Belching or burping consists of swallowed air, which is largely nitrogen; therefore, burping rarely smells bad. Passing gas from the rectum is known as flatulence. It depends on the food consumed and the activity of the bacteria in the colon. Undigested food that reaches the large intestine is broken down by the bacteria that are normally found in the colon. Bacteria breakdown the food into hydrogen, carbon dioxide and methane. Any food that doesn't get absorbed well is likely to lead to gas formation, for example artificial sweetener like sorbitol which is present in sugar-free gums. Sorbitol is not absorbed directly. Bacteria decompose it and produce gas. Some people produce methane and others do not. Methane imparts foul smell to gases coming out of the rectum. The number of times gas is passed correlates well with the total amount ( volume) of passed gas. Therefore, in cases of flatulence, the number of times gas is passed should be counted for several days. It is normal to pass gas up to 20 times a day. 

Gas in Abdomen Belching
Causes Of Gas In Abdomen
Gas in Abdomen Repetitive Belching 
Treatment of Gas in Abdomen