(This is an actual factual question, though it’s about food so…..)
Think about it. Heinz (or whoever) brews up the ketchup, pours it into a bottle, and seals it up with a little cardboard/foam/metallic foil disk underneath the screw-on dispenser (maybe) top. Said bottle is then packed into a carton and shipped off to a warehouse and then maybe a second distribution warehouse and then to a store where it might sit in a backroom before being put on a shelf and eventually bought and taken home.
This sequence has to take time. Maybe months in some cases. And yet, when the customer finally opens the bottle, what comes out the top is a perfectly mixed substance right from the first drop.
Then the customer puts the bottle in the fridge. … and from then on, you have to shake the bottle up before pouring ketchup because otherwise you get some pallid, watery mixture pouring out to make your bun instantly soggy.
Why?
I mean, clearly, it’s just the denser ‘solids’ in the mix settling down by gravity as you’d expect, leaving the stuff at the top too ‘rich’ in watery stuff (basically water itself) at the top of the bottle. What I want to know is WHy doesn’t this happen during the days/weeks/months between when that bottle of ketchup was sealed up and when the consumer opened it??? From then on, it starts separating right away, to the point there’s significant wateriness developing in just the gap between lunch and dinner sometimes. Why the change?
Is what looks like air above the ketchup not really air but some other gas?
Is it due to the special little sealing disk?
Is it … a magic spell? Maybe when you open the seal you’ve set the imprisoned genie free that otherwise keeps sneakily stirring the ketchup/water back together?
I believe it has to do with the fact that once the bottle is opened, you are exposing the contents to air, and that allows the various components to begin to separate. When it was originally made and sealed up, there was no air in the bottle, and therefore no reason for it to separate until the buyer finally opened it.
That’s a somewhat reasonable theory, except that a sealed ketchup bottle DOES have air )or some other colorless gas) inside it. At least, I’ve never seen a ketchup bottle that was filled from the bottom all the way to the cap. It’s especially clear on the bottles with a sorta stretched out neck shape bottle – there’s often a 2 or 3 inch gap between the top of the ketchup itself and the screw on cap.
Another idea I had was that the ketchup ‘sets’ somehow. Sort of like a jelly: it goes into the jar significantly hotter than normal room temperature and then as it cools down in the jar it forms a sort of gel, locking everything into place so the denser bits of tomato/spices can’t just fall down through the no-at-tje-moment liquid ketchup. And by inverting/shaking/squeezing the ketchup on first opening you disrupt that gel? Turning it back into a liquid that can let heavier bits fall down. Maybe? (At least that sounds a bit more plausible than imprisoned genies.)
It does. That’s why you get the effect where ketchup will refuse to come out of the bottle then suddenly squirt out making a mess; it sets into a sort of “soft crystal” that resists being squeezed out of the bottle until the crystal structure “shatters”, releasing the ketchup all at once. Preventable by carefully banging the bottle against a counter or something to break up the crystal before squeezing.
Ketchup is probably the best example of a thixotropic liquid - one that is a gel or semisolid at rest, and reverts to the liquid, pourable state when shaken up.
My WAG is that, for whatever reason, a film is created on the surface of the ketchup (at the ketchup-air interface), and this film is much more viscous (“thicker”) than the rest of the ketchup in the bottle. And this film sorta “holds back” the rest of the ketchup when you turn over the bottle. Shaking the ketchup destroys the film, which makes it flow more easily.
A web search seems to confirm my hunch, following along the theories being presented here, that the Ketchup bottles are sealed at the factory after purging the oxygen from the airspace. Removing the oxygen might be the key here, to prevent the formation of that film / membrane. Once opened, oxygen is admitted into the container’s head space and the mentioned issues start to occur.
Ketchup is a colloid, known to maintain its integrity in a suspended state. I believe mayo qualifies as that also, which is also why it maintains its integrity in a suspended state.
Ding! Ding! Ding! I think we have a winner! When you search on “ketchup” and “thixotropic’ several relevant articles appear and discuss this critical issue seriously.
I am so happy to have an answer – and a brand-new to me word! Wahoo! This is why I love the Dope.
Seriously. This is the thing that made me happiest yesterday, redeemed the whole day. Thank you!