How are sulfurous gas formed contained by the intestines?

Flatulence is typically caused by the fermentation of dietary fibers (which are polysaccharides) surrounded by the colon with the formation of short-chain fatty acids and gas (carbon dioxide, methane, hydrogen). Then why does flatulence (e.g. from beans) sometimes result in the formation of massive amounts of odorous sulfurous gas (like hydrogen sulfide, organic thiols) when the source of the flatulence are the undigested polysaccharides which do not contain any sulfur?How are sulfurous gas formed contained by the intestines?
The microbes in the intestines produce the sulfur by working on the undigested stuff.
They aren't pure polysaccharides. There are also proteins, and that includes sulfur-containing amino acids that the microbes digest.