Tapping a Can of Pop

Cecil's Columns/Staff Reports
1

I’ve been waiting for Cecil to re-run this column for some time, as I’ve been itching to comment on it.

I notice that in Cecil’s experiments, and in related experiments, the experimenters shook the subject cans vigorously. However, as a soft drink lover myself, I can tell you that I only employ the tapping method when a particular can is shaken moderately. For example: said can falls out of my hand as I remove it from the fridge, or said can falls through the innards of the vending machine.

In such cases of slight to moderate shaking, opening immediately (without tapping) will produce, at a minimum, sticky hands. Tapping, however, works every time.

And the link to the column is http://www.straightdope.com/classics/a4_198.html - Jill
[Note: This message has been edited by JillGat]

2

I have been a tapper for some time, mostly because a tapped can rarely is a problem. However, I must admit that doing something simply because it SEEMS to work isn’t an indication it DOES work.

In the absence of a scientific explanation of why tapping WOULD work, I have to wonder if it does indeed do anything worthwhile. As the article notes, it may do nothing more significant than make you wait longer to open the can. :slight_smile:

3

Don’t underestimate the placebo effect.

4

This isn’t the placebo effect. I believe we have two things happening here (I think Cecil mentioned them both, but since the OP neglected to post a link I’m not going to look it up):

  1. Tapping the can takes time. If instead of tapping for 30 seconds, you just waited for 30 seconds you’d get the same results.

  2. Everytime you tap the can and it works, you think “It didn’t froth over because I tapped it” everytime you tap the can and it doesn’t work, you think “I didn’t tap enough, it must have been really shook up”. This reinforcing system of thought is common to practically every single irrational idea - from palm reading to soda tapping.

Also, the entire theory behind this is ridiculous. The problem with the carbonation is due to agitation - how could more agitation solve it?

5

Note: I’m not a tapping believer.

How would you setup an experiment?

My ideas are, shake a group of cans on a paint mixer, or some other automated shaking. When this is done, take two cans. Set them down, tap half, don’t touch the rest. Open after a set period of time. Repeat until satisfied.

Perhaps tapping could be automated, if people could agree on the correct motion and spot on the can…
I don’t think people even know why tapping would work. The only reason I can think of is that tapping might reduce foam, while maintaining pressure. Opening a can of pressurized but foam-free pop wouldn’t be as messy, even though the pressure equalization would be the same.

Perhaps this could be tested with plastic pop bottles. They’re thin enough to deform when tapped, and should function in the same way.

Shake a bottle and try to get rid of the foam. The visual feedback should make this fairly easy. Try tapping, rolling, letting sit, squeezing, and any other method you can think of. When you’ve got a method that seems to work, try testing it on cans using some sort of automated shaking and either side-by-side comparisons of two identically shaken cans, or blind procedures where the shaking is heard, but the experimenter doesn’t know if the can they’re given is the shaken one or not.

6

I too am a tapper. And some casual experiments seems to confirm that it works.

I believe that when you shake a can, some liquid accumulates on the top of the can, waiting to be expelled out when the can is opened.

Tapping simply knocks the liquid down.

7

In Penn & Teller’s book about playing with food, there’s a bit about the trick of transferring carbonation from one can to another. It’s based on the fact that along with the change to aluminum cans, there was also a change to very little airspace at the top of a can. Thus, after 20 secs, even a vigorously shaken can will calm down.
Here’s the trick, paraphrased:

  1. Furiously shake one of two cans of carbonated beverage, leaving the other unshaken.
  2. Set down the shaken can, and launch into a dramatic mumbojumbo about being the God of Carbonation, who has the power to move pressure at will from one can to another.
  3. After you’ve blathered for 20 secs, pick up the shaken can and open it. No spray.
  4. Pick up the unshaken can. Just as you open it, squeeze the soft aluminum sides. The spray will drench your amazed friends. With a little practice, you can learn to make a tremendous mess.
    –Nott, the large
8

Tapping on a can IS a placebo - a pet peeve of mine. Do these people also tap on two-liter bottles, and if so for how long? Do they tap on champagne bottles, too?
Does seltzer require more or less tapping than a cola?

Working with the technical nature of foamy fluids, known as two-phase flow, I can assure you the tapping just delays one from opening the bottle prematurely. Thus, it just gives one a false sense of security - working only by coincidence.

It does not serve to re-dissolve the CO2 back into the liquid! The benefit is nil. Do you think there’s a little guy tapping on water at the bottling plant to make carbinated water?

[Think of the bigger picture: For the chemical industry, foam and/or bubbles present many a major headache! If tapping DID work, the chemical industry would be mighty greatful since this is a major nuisance when pumping fluids around, preparing batches, etc., etc., etc.]

When a soda can “explodes”, we are witnessing the fact that the gas is moving fast enough to entrain the liquid at the surface along with it. Sea spray is the same effect - the air moving along the surface of the water entrains droplets of water along with it.
Fisherman should be able to relate, here.

Beware that a can (or bottle) doesn’t even have to be agitated for this to occur. A warm soda can is a prime candidate to “explode” since gasses are less soluable as temp. increases. At the extreme, an unopened fizzy beverage trapped in a car on a hot day can self-destruct!

Also, although rare, in airplanes, it has been witnessed where pressure differential between cabin pressure and inside the soda caused an entire serving cart of soda cans to erupt spontaneously!

…Just hum “Tiny Bubbles” without tapping!

9

I tap the can for the same reason some people go to church. I was told to a long time ago, and now it’s a habit.

10

Hey! I got a theory here! I think I know where this tapping habit comes from!

Tapping on an OPEN container can help in bubble breakdown, and the gas can escape nicely. This is true for baby bottles (without the top screwed on) and syringes as well (usually a flicking action).

However, with soda pop, tapping may assist in some liquid-gas separation, but:
a) Re-dissolving gas into liquid is nil
b) Separated gas above the liquid level escapes with enough velocity to entrain the liquid at the surface.

Ok, I should explain that there are two things going on when a can explodes. First, liquid at the surface is entrained by the escaping gas. Now, when I say surface, I mean the top of the liquid level…which may drop dramatically during a violent eruption.

Next, gas bubbles on the bottom and walls of the can (below liquid level) displace liquid upwards prior to and during release. This is known as “liquid swell” and contributes to the overall eruption.

I have found that, for some reason, 7-Up products are most sensitive to agitation. WAG: A more highly carbinated product?

11

Yes, 7-Up has twice the carbination of standard soda pop . . . this goes back to its origin as a psuedo-medical beverage . . . the extra carbination is supposed to help with upset stomach . . . in my experence, 7-Up does help, although I don’t have the slightest idea if the extra carbination is what does it . . .

Shadz