So, apparently, you can't burp in space! I don't know why Commander Hadfield or one of the many hundred (?) of current or former astronauts out there haven't shared this gem of information more widely.
It's the sort of thing that is obvious once pointed out. We burp when excess gas builds up in our stomachs above the no doubt awful looking mess that is made up of our food being slowly digested. And gravity dictates that the digesting food will at the bottom and the gas at the top - and that its easiest way out is back up through the oesophagus. But in the (effective) absence of gravity, things aren't so simple. As the commander explains in his tweet, the food, liquids and gases all float together in your stomach. If you burp, all you can do is expel a big chunky blob of that mix. Which can't be either pretty or pleasant.
Without Twitter, we'd never get to know these things!