Why does boiling cause some foods to float?

November 10, 2007 2:49 PM

Why does boiling cause some foods to float?

Ok, so I have a really stupid and basic question. Why is it that after boiling, food (hot dogs, broccoli, won tons, etc) start to float? Does boiling really change their density? I'm assuming that they're not losing mass but maybe they're gaining volume from the water? And I don't think it's that the boiling water is somehow creating enough force to push them up - when I put cold hot dogs in already boiling water, it sinks. Am I just being dense about this? (har har.)

posted by be11e to Food & Drink (13 answers total)

Heat expands the food item, causing it to be less dense over a larger volume.
posted by stavrogin at 3:04 PM on November 10, 2007

Things float because they are (or become) less dense that the water they are in. As Stavrogin says, expanding through heat effectively spreads the mass over a bigger volume and reduces the density, and so it floats.
posted by Brockles at 3:12 PM on November 10, 2007

Best answer: It's not so much that heat expands the item at the molecular level (or this would only apply to a small subset of foods, those that were more dense than water at room temperature and less dense at 212ºF), but that moisture trapped in the food becomes trapped pockets of gas, increasing its buoyancy.
posted by nicwolff at 3:21 PM on November 10, 2007 [1 favorite]

As others have said, heat causes them to expand. Gaining volume from water wouldn't make them float because the added volume would also come with the added mass of water; it is only the expansion due to heat.
posted by ssg at 3:22 PM on November 10, 2007

listen to nicwolff
posted by caddis at 4:23 PM on November 10, 2007

air trapped in the food will also expand, apart from the cintribution of water vapor.
posted by Good Brain at 5:18 PM on November 10, 2007

The phase change of fats from solid to liquid can also significantly alter boyancy (the sperm whale, if I am remembering right, uses this trick to control its boyancy), this is probably a big contributor to the boyancy of cooked hot dogs.
posted by Good Brain at 5:22 PM on November 10, 2007

Actually, the biggest reason is simpler than any mentioned so far. The food is being brought to the surface by rising bubbles of steam. Especially because steam bubbles are forming on the surface of the food which, being uneven, provides lots of nucleation sites.
posted by randomstriker at 9:57 PM on November 10, 2007

Good point randomstriker, I like that answer.
posted by nicwolff at 11:13 PM on November 10, 2007

Response by poster: thanks for all the good answers, everyone. i actually like nicwolff's explanation the best, because i can understand the fact that there's lots of moisture trapped. i can't imagine that there's so much air trapped as to be the major source of buoyancy, though i guess it can contribute in a smaller way. randomstriker, if the food is being brought up by rising steam bubbles, wouldn't i see food start to float as soon as i dropped it in boiling water? there's a lag time between that drop and the food floating.

i wonder if this could be solved by somehow dehydrating food, then heating it up without water, and seeing if it floats in both cold and boiling water. that would probably differentiate between nicwolff's explanation and other people's.
posted by be11e at 10:14 AM on November 11, 2007

I don't think my explanation is inconsistent with your observations. When you drop the food in, it's cooler than the the water. Thus water in direct contact with the food will not vaporize. When the food reaches 100 C, nucleation will occur.
posted by randomstriker at 11:08 AM on November 11, 2007

The water in food will have some small amount of gas dissolved in it at room temp, too, and as it heats up the solubility of the dissolved gases goes down and they are forced out as gases.
posted by oats at 4:51 PM on November 11, 2007

Response by poster: ah, i missed that point before, randomstriker. is it known which action contributes the most to this process? would that be the nucleation?
posted by be11e at 2:58 PM on November 12, 2007