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Odd Thermal Reaction of Coffee Grounds

  1. Oct 9, 2014 #1
    While on an elk hunting trip in the mountains we awoke to roughly 2 feet of snow. Everyone wanted coffee and nobody had brought any conventional coffee tools. A cowboy boiled water in an aluminum pot just dumped coffee grounds in the water. Naturally a lot of those grounds floated near the surface. He then simply touched the bottom of the pot to the snow and almost instantaneously the grounds, all of them, shot to the bottom. He was then able to pour off the liquid with almost no grounds in it as long as the pour was slow and smooth.

    Was this simply some sort of thermal reaction or was there an electrical component?
  2. jcsd
  3. Oct 9, 2014 #2


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    O.k., I'll give it a shot, but by no means I'm certain it's the case.

    I'd say the pot, after taking off the stove/fire, cools mainly through evaporation on the liquid/air interface, with the conductive cooling through the container walls being much lower in magnitude. This should create temperature gradient in the liquid with top being the coolest and bottom the hottest. As a result, convection currents would keep circulating(just as during boiling, but less vigorously), pushing all sorts of debris to the top.
    Once you touch the bottom of the (presumably metal) container to the snow, the conductive cooling through the bottom goes up, leaking more heat through the bottom than the top, thus shutting down the convection currents and allowing the beans to sink.(or maybe even reversing the currents?)
    Last edited: Oct 9, 2014
  4. Oct 9, 2014 #3


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    This seems a bit like what happens in the classic coffee brewing method in a jug. Hot water is added and the grounds all rise to the top. You give a very light stir with a spoon and they fall down. I always thought this was just air being released from the grounds but I think Bandersnatch's idea of cooling could be related. If the grounds trap a thin layer of hotter water and insulate it they would stay suspended (average density of hot water plus coffee ground). A gentle stir (or a tap on the ground) could cool the grounds and increase their density. It could be an easy experiment for someone who makes coffee this way - just put a thermometer in amongst the grounds and stir.
  5. Oct 10, 2014 #4
    Thanks for replying and of course I had considered the simply thermal possibilities and the replies do firm that part up some. The reason for my post and still a cause for a bit of consternation is that the grounds did not just "settle down". They went like a shot! extremely rapid and on a vector essentially straight down. Is it likely that mere thermal difference could attain this degree of change?
  6. Oct 10, 2014 #5


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    It sounds unlikely to me too but there really aren't any other forces around - except buoyancy, due to air bubbles on the grounds. It would then be due to the 'bump' on the ground, rather than any thermal shock from the snow. (What does that guy do in the summer, I wonder? lol)
    I cannot remember seeing any 'froth' left at the top of the coffee - which sort of lends doubt to the bubble explanation.
    This is interesting. . .
  7. Oct 10, 2014 #6
    Hi enorbet,

    If you can reproduce this effect in your kitchen and post it on youtube, I'd really like to see it. If you do, please use a glass coffee pot so that the grounds can be seen to shoot down. Thanks
  8. Oct 10, 2014 #7
    If it is an aluminum pot, how can you tell that the grounds went like a shot or even that they went? If the mixture was so transparent, why bother anyway? :)

    I make coffee like this almost every day (it is the so called "Turkish" or "Syrian" coffee, popular in Eastern Europe). If the water is hot enough, after mixing the coffee with the sugar and water only very few grounds remain on the surface anyway. Not enough to bother you. On the surface you have mostly the foam (which is desirable). It is said that a few drops of cold water will make any grounds left on the surface to go down but i did not find it very effective.
  9. Oct 10, 2014 #8
    I was standing over his shoulder as were the others because we all wanted to see this in action. Apparently, even though we were all locals (I was a new local but the rest were born at least nearby) he was the only one who knew this "trick". In all honesty I can't say how hard he placed it in the snow because it was fresh Rocky Mountain powder, quite fluffy, so only he holding the pot could have felt how quickly and how solidly compression and the temperature "battle" resisted. I do recall that the pot was roughly 5 inches deep and it sunk to the handle at the rim. I have no way of knowing if it stopped on it's own due to compacting or if he just didn't lower it any further. It is quite possible however that a bump occurred and that contributed. All of us were rather amazed and of course the cowboy doing the dunking tried not to show how pleased he was that everyone was grinning in marvel.

    That's a bit of a tall order since I don't have a Pyrex pot that size and the lady of the house may chase me out of the kitchen with a broom, but I will indeed try.

    Very observant. :) In truth, 2 things affected this.

    1) One of the cowboys had stomach ulcers and had asked that it not be overly strong, especially since the very same coffee making fellow had doused the previous evening's potatoes with a huge amount of black pepper.

    2) I have no idea if it was due to the temperature near the surface, but the mixture, though uncommonly weak for my taste (but we did crave something hot more than a pick me up... we were VERY awake! We were at 8800ft altitude and it was COLD!) was darkly translucent near the bottom and nearly transparent at the top. He had taken care not to disturb it and moved from the fire to the snowbank very carefully. My memory fails me if I could see the grounds actually hit bottom, but I am quite certain the grounds shot down for at least 2-3 inches of the 5. More than that I cannot safely say.

    That said, because of the reaction we all shared (except for the maker) we were all rather astounded and looked at each other wide-eyed and grinning, and this says something about a quick surprise, not a lazy spiral down. The reaction was extremely fast, both the grounds and ours.

    It should be noted that this hunting trip was the first of it's kind for me (having horses, even pack animals, sleeping in tents in 2 feet of snow, etc. and hunting with 5 other people. I had always hunted alone, sometimes on snowshoes but never more than a 2-3 hours from home. Apparently the trip made a huge impression on me and involved a great number of neural connections because I can recall almost hour-by-hour much of the 2 day trip, but would be hard-pressed to recall events of the day before.

    The day after I mostly slept. It was very intense. We almost got caught in the wilderness in a bad snowstorm. We packed up when we saw the sky change but we thought we may have reacted too slowly as things got a bit bad while traversing a heavily wooded "bowled" valley. Thankfully we found the pass out and down and the snow turned to rain, and things went from life-threatening to merely miserable.
  10. Oct 11, 2014 #9


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    There's a similar trick to making tea leaves sink. When brewing tea in an open can hung by wire over a campfire, heat the water, throw in the tea leaves, remove from heat (optional: swing it 3 times in a vertical circle), then give the side of the can 2 or 3 sharp taps with a stout stick and almost all of the tea leaves still floating will immediately sink to the bottom of the can. Pour beverage into mugs, no strainer required.
  11. Oct 11, 2014 #10
    @NascentOxygen It is similar in the respect that the solid matter goes to the bottom but it would seem this method exerts considerable downward force on that matter. This was not the case with the coffee.
  12. Oct 11, 2014 #11
    Your trip is memorable. A "wow" and "awe" justs makes it much more so.

    I bet if you'al had woken up with no snow, these cowboys would probably have had other tricks in their bag to pull out.
    One easy one, but maybe not so cowboyish, is the floating coffee cup. For some reason the cowboy with the ulcers would have "lost" his cup. The brewing cowboy would have been generous and let him use his own, but amazingly he would have had an extra battered looking one in his saddle bag, his lucky cup handed down from his grandpapy that he carries around everywhere. After downing his coffee, he could have dazzled you by making it float in the thin mountain air.
  13. Oct 11, 2014 #12
    @256bits OK I'm lost. Care to expound?
  14. Oct 11, 2014 #13

    Doug Huffman

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    I think that @256bits is suggesting as gently as possible that your observation was anecdotal, without adequate information from which to explain. Read up on the narrative fallacy.

    A possibility is that OP did not see the surreptitious addition of a flocculant like the traditional egg albumin from the egg shells.
  15. Oct 11, 2014 #14


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    While you may see it like that, if the tea leaves are less dense than the water then they leaves are actually forced away from the base of the can---the denser water is pushing harder against the base of the can so lifts the leaves towards the water's surface. It's a simple centrifuge.

    In any case, this is irrelevant because, as I indicated, swinging the can is optional. It does not affect the outcome of tapping the can's side with a stick.
  16. Oct 11, 2014 #15
    The sinking coffee ground demonstration made your trip much more memorable, even if you did not get an elk, and you are sure to reflect upon it years to come on a snowy day just by drinking a hot coffee of your own. Hats off to the cowboys!
  17. Oct 11, 2014 #16
    But the cowboy didn't make a bean, bacon and egg breakfast!!:p
  18. Oct 12, 2014 #17
    Hello again
    Of course I was well aware that my posted question is anecdotal and I'm familiar with "narrative fallacy" but I didn't presuppose an answer or explanation. It is true that the coffee maker could have "played a trick" because I can't say for certain that my eyes never left the pot. My intent is posting the question is wide open whether it is pure Science or scientific trickery. I simply hoped someone else knew of a mechanism by which this admittedly uncontrolled observation could be explained. I will look into albumin from egg shells even though it might be a stretch to assume it would be possible to introduce that element in such cold weather without being seen, absent a shill to actively distract the "marks" and frankly the mood that morning was not particularly jovial as we were discussing the prospect that our bad luck made our camp choice a bad one and whether we should abandon it and try elsewhere. While drinking that coffee, one of us spotted a small herd of about 30 elk running across a large meadow on a distant hillside. The caffeine could not compare with adrenaline. The mood changed instantly :)

    Utterly tangential, we did get several elk and it contributed to the problem of beating out the storm as the pack animals were heavily burdened and slowed. By the time we reached the bowled valley it was snowing so hard we could not see even large trees beyond about 20 feet which was why it was difficult to see the path to the pass and safety.
  19. Oct 12, 2014 #18

    Doug Huffman

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    What did you do with your elks hides, add them to the twenty feet tall piles pre-existing?

    We took an elkhound with us that proved to be useless for not resisting the attraction of porcupines' soft underbelly. He tried about every other day, and every time got his nose filled with quills. Someone would have to sit with the miserable dog and, push or pull, remove the quills - so he could try the next day.

    And there were no fish for having walked above too many falls. But a beautiful trip - with an experienced trail chef.
  20. Oct 13, 2014 #19
    @Doug Huffman - Well I suppose since my OP questions whether the reaction was all physics or included some chemistry it isn't too far OT to answer your question (heheh). This all took place in Delta County, Colorado in the Uncompahgre Wilderness and the County Seat, Delta, had a tannery at the time despite complaints when the wind was just right or rather just wrong. The coffee maker basically hosted the trip even though it was all "just friends" by providing the pack horses and racks and he did this free in exchange for getting all the hides which he was careful to prepare well for a trip to the tannery, so no pile. Everyone was happy with this arrangement afaik since even 0.5 elk is considerably more than a freezerfull and some of the best meat one can ever hope to eat.

    PS - I wonder why that is that some dogs keep their distance from porcupines and some just can't resist?
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