Some people do, some people don't, but here's what Heinz says.
Some disagreements in life are better left unsettled. I, for instance, stand firmly in the camp of refrigerating peanut butter. My boyfriend, at any given time, has two opened jars of Skippy sitting in the pantry. We've been happily keeping our PB rations separate since 2011.
But the Great Ketchup Debate over keeping the popular condiment in the pantry or refrigerator is one we're both on the same page: Once it's opened,to the fridge it goes. As it turns out, ketchup giant Heinz, agrees. “Because of its natural acidity, Heinz Ketchup is shelf-stable. However, its stability after opening can be affected by storage conditions. We recommend that this product be refrigerated after opening to maintain the best product quality," Nicole Kulwicki, director of brand building for Heinz Ketchup, says on ReadersDigest.com.
As for why restaurants seemingly store the stuff at room temp on tables? "We portion each order of ketchup that will be served that day,” says chef Luke Venner of Elm Restaurant in New Canaan, Connecticut, in the same Reader's Digest article. “We always return the larger container immediately back to a refrigerator. Any unused portions are discarded at the end of the shift.” Also, since ketchup maintains its taste at room temperature for about a month, notes the Huffington Post, the quick ketchup consumption rates at diners and restaurants makes keeping ketchup out a non-issue if an establishment chooses to do so. (The germs on said ketchup bottle, by contrast...)
Well, now that that's all cleared up, if only I had the guts to google "Should you refrigerate peanut butter?"