Michael P. asks: Does tapping on the top of a shaken pop can really prevent it from fizzing up?
For the uninitiated, it’s commonly held that when opening a recently shaken can of soda, you can avoid, or at least reduce, the inevitable shower of sugary carbonated liquid by simply tapping the top or side of the can briskly with your finger a couple dozen times. So does tapping the can actually do anything?
Well, we looked far and wide for a definitive answer to this question and came up empty, so decided to run an experiment ourselves to find out the answer.
Now, to be clear, certainly there are many otherwise reputable sources throwing out an opinion on this one on both sides of the argument, but, outside of Snopes, nobody seems to have bothered to experimentally test whether tapping the can does anything. And as for Snopes, while they technically did do an experiment, this was a reported sample-size of just three runs (although they do allude to “a variety of experiments” not reported).
It is possible one doesn’t need a large sample size here to get meaningful results, so perhaps three runs is a perfectly sufficient sample. However, Snopes gives no hard quantitative data (only anecdotal observations) on this one, and did not necessarily shake each of the cans the exact same way (though did time it and presumably the shaking was approximately the same if they had the same person shaking each time). But needless to say, while Snopes’ conclusion may end up being perfectly correct, we weren’t really comfortable stating it as a definitive answer here given the way the experiment was conducted and lack of hard data.
But if you’re curious, their results indicated that tapping the side of the can produced slightly less foam than simply waiting to open it, but otherwise from a practical standpoint didn’t really make a difference.
As for expert opinions, these also were conflicting, though perhaps the best such source in Cornell University biochemist, and one of the world’s leading beverage foam experts, Karl J. Siebert, rang in on the side that at best tapping the can does nothing in his opinion, and even potentially makes the problem worse. As Dr. Siebert states, by tapping the can, “you risk creating more bubbles.”
Despite this, many otherwise reputable sources claim that tapping the can does actually help. Why? As you’re probably aware, when you shake the can, the agitation causes some of the dissolved carbon dioxide in the container to form bubbles at various nucleation sites on the inner surface. It’s also further widely held that some of the bubbles formed will stick to the inside of the container at these various nucleation sites, rather than rising to the top. When the can is opened and the high pressure thus released, these bubbles rapidly expand and shoot to the top of the container, pushing out some of the liquid they were surrounded with previously with them.
Thus, the hypothesis is that by tapping on the can, you can dislodge these bubbles and cause them to float to the top before opening the container, so that when you do open the can, the gas can expand and escape without taking any liquid with it.
So does this actually work?
To begin with in our little experiment, we needed a device that could shake our soda cans exactly the same every single time. The Shakenator T-3000 we made to do this works such that with each button press, it shakes the can exactly 10 times with a stroke length of 1.125 inches or 2.9 cm. Through a bit of experimentation, we ultimately found that at our coldest measured temperatures about 150 shakes (at about 8.8 complete shakes per second) was around the point where we started getting very good, measurable results with Coke cans, so went with that for the number shakes.
Because temperature is a big factor in how much foam is produced, the device also reports the temperature after each run, along with the number of shakes and the time it took for the run to complete.
Now to the experiment. There are a variety of ways we could have done this, but as we’re far more interested in the amount of foam coming out, rather than the amount of carbon dioxide, we’re choosing to measure the liquid that comes out of a can, glass bottle, and plastic bottle when the respective containers are: shaken and then tapped on the side, shaken and then tapped on the top, shaken but not tapped- simply waiting the same time interval as if we tapped it, and then shaken, and quickly opened upon removal from the machine.
This latter one is particularly of interest as one alternate hypothesis often put forth on why tapping the can does increase your odds of avoiding a fizzy bath has nothing to do with the tapping itself, but is because people tapping the can wait a short interval before opening it, giving some of the carbon dioxide time to re-dissolve into the liquid and any formed bubbles to rise to the top where they won’t push any liquid out.
Also, just because we were curious, we ran an additional experiment shaking several cans and then opening them at intervals to see how long it would take for no more liquid to be pushed out. Obviously the results here will vary for other shaking scenarios based on a variety of factors, but we were really just curious at about broad ballpark numbers here.
So what were our results?
Well, it turns out that the actual tapping of the can does nothing.
However, we were very surprised to note that the seemingly insignificant time interval here of 20 seconds from shaking to open actually did make a huge difference in the amount of foam produced. And, in fact, on the runs when we opened the can as fast as possible after being shaken, even just a change of a few seconds appears to have made a big difference in foam output, as you’ll see from the results which show that that portion, which was the only one not precisely timed, is the only place we really saw a large variance in resulting foam, even though every open in that case was within a few seconds of each other.
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