Composition of flatus
Nitrogen, the main constituent of air, is the primary gas released during flatulence, along with carbon dioxide, which is present in higher quantities in those who drink carbonated beverages regularly. The lesser component gases methane and hydrogen are flammable, and so flatus containing adequate amounts of these, can be ignited. However, not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained archaea capable of producing methane.[1] Similar results are found in samples of gas obtained from within the rectum.
The gas released during a flatus event frequently has an unpleasant odor. For many years, this was thought to be due to skatole and indole, which are byproducts of the digestion of meat. However, gas chromatography testing in 1984 revealed that sulfur-containing compounds, such as methanethiol, hydrogen sulfide (rotten egg smell) and dimethyl sulfide, were also[2] responsible for the smell.[3]
The incidence of odoriferous compounds in flatulence emissions increases from herbivores, such as cattle, through omnivores to carnivorous species, such as cats.[citation needed] Such odor can also be caused by the presence of large numbers of microflora bacteria and/or the presence of feces in the rectum.
The major components of the flatus, which are odorless, by percentage are:[4]
* Nitrogen: 20–90%
* Hydrogen: 0–50%
* Carbon dioxide: 10–30%
* Oxygen: 0–10%
* Methane: 0–10%
Causes
Intestinal gas is composed of varying quantities of exogenous sources (air that is ingested through the nose and mouth) and endogenous sources (gas produced within the digestive tract). The exogenous gases are swallowed (aerophagia) when eating or drinking or increased swallowing during times of excessive salivation (as might occur when nauseated or as the result of gastroesophageal reflux disease). The endogenous gases are produced either as a by-product of digesting certain types of food, or of incomplete digestion. Anything that causes food to be incompletely digested by the stomach and/or small intestine may cause flatulence when the material arrives in the large intestine, due to fermentation by yeast or prokaryotes normally or abnormally present in the gastrointestinal tract.
Flatulence-producing foods are typically high in certain polysaccharides, (especially oligosaccharides such as inulin). Those foods include beans, lentils, dairy products, onions, garlic, scallions, leeks, turnips, rutabagas, radishes, sweet potatoes, potatoes, cashews, Jerusalem artichokes, oats, wheat, and yeast in breads. Cauliflower, broccoli, cabbage, Brussels sprouts and other cruciferous vegetables that belong to the genus Brassica are commonly reputed to not only increase flatulence, but to increase the pungency of the flatus.
In beans, endogenous gases seem to arise from complex oligosaccharide (carbohydrates) that are particularly resistant to digestion by mammals, but which are readily digestible by gut flora – microorganisms (methane-producing archaea; Methanobrevibacter smithii) that inhabit the digestive tract. These oligosaccharides pass through the upper intestine largely unchanged, and when they reach the lower intestine, bacteria feed on them, producing copious amounts of flatus.[6]
In the case of people who have lactose intolerance, intestinal bacteria feeding on lactose can give rise to excessive gas production when milk or lactose-containing substances have been consumed.
Interest in the causes of flatulence was spurred by high-altitude flight and the space program; the low atmospheric pressure, confined conditions, and stresses peculiar to those endeavours were cause for concern.[6] In the field of mountaineering, high altitude flatus expulsion was first noticed over two hundred years ago.
Some infections, such as giardiasis, are also associated with flatulence.[7]
Source:
http://en.wikipedia.org/wiki/Flatulence 
The problem here is that flatulence is a natural occurring phenomena that simply cannot be thwarted in a natural physical environment. The Bible makes it clear that the physical laws of the universe were in full operation before the fall, therefore even gross things like flatulence must have been in place before the fall. In other words, given the laws of physics, it is not possible to create an organism that does not fart.. Given the laws of physics and the nature of quantum events, it is not possible to design an animal that operates without giving off some gas. God can't stop it, much like cancer cannot be stopped since it also a random event. It's a price you have to pay when you mix chemicals in a physical world. You simply cannot escape it.. It's a natural evil or smell..