# Why don't the raisins in Raisin Bran all wind up at the bottom of the box? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [April 15, 2006, 9:03am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772 "2006-04-15T09:03:57Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Packer](https://avatars.discourse-cdn.com/v4/letter/p/4da419/32.png) [@Packer](https://boards.straightdope.com/u/Packer)\ **Post date:** [April 15, 2006, 9:03am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/1 "2006-04-15T09:03:57Z")
Apologies if I am posting in the wrong place. The classic article “Why don’t the raisins in Raisin Bran all wind up at the bottom of the box?” [http://www.straightdope.com/classics/a3\_090.html](http://www.straightdope.com/classics/a3_090.html) was in the email newsletter, and the link there “[Comment on this answer]” brought me here. I don’t buy the logic of: "First, while a crumb weighs less than a big piece, the crumbs and chunks in aggregate weigh more per unit of volume. That’s because the big pieces have lots of space between them and the crumbs don’t. " The bulk of crumbs may be more or less dense, but that is because of relative particle shape, not because of relative particle size. The aggregate weigh per unit of volume or bulk density depends on the packing geometry, which depends primarily on the particle shape, so if the crumbs are somehow fluffy, spiky or hollow in shape when compared to the big pieces, they will have a lower bulk density. If they are solid and more spherical, they will have a higher bulk density. They do, however, seem to wind up at the bottom of the jar, regardless. Imagine two similar containers each part-filled with same-size polished steel bearing balls, the balls in one container being many times the diameter of those in the other, and the containers having been vibrated to cause the balls to pack down as far as possible. If the balls are small compared to the containers, boundary effects can be ignored. The balls in both containers will settle to the same close pack [http://en.wikipedia.org/wiki/Sphere\_packing#Sphere\_packing](http://en.wikipedia.org/wiki/Sphere_packing#Sphere_packing) with a bulk density of about 74% of the density of the balls, despite there being much bigger spaces between the larger balls. It is about shape, not size. Concerning “the mechanics of sifting”, one can visualise taking a handful of the tiny balls and dropping them into the container of much larger balls where they would run down through the large gaps to the bottom of the container. What happens in the opposite case, i.e. taking a large ball and putting it into a vibrated small-ball container is not obvious to me. I think that if the vibration causes behaviour to approximate frictionlessness, the large ball should submerge and sink to the bottom, but that is just by analogy with a solid ball in a less dense fluid. So I am not sure that “big pieces don’t make much headway sinking into the densely packed small stuff”. Comments? As to the flip reply to the initial question, observation of Raisin Bran in milk shows that the raisins sink whereas the alleged “bran” shows some floating, some neutral buoyancy and some sinking. Having no real evidence at all, I would guess the non-migration of raisins is due more to friction effects than density differences. ---
**Author:** ![lowkey\_but\_high\_locks](https://avatars.discourse-cdn.com/v4/letter/l/59ef9b/32.png) [@lowkey\_but\_high\_locks](https://boards.straightdope.com/u/lowkey_but_high_locks)\ **Post date:** [April 15, 2006, 6:01pm UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/2 "2006-04-15T18:01:11Z")
I don’t have any raison bran handy, but a good way to test it would be to take some raison bran and shake it for awhile. ---
**Author:** ![pinkfreud](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pinkfreud/32/17603_2.png) [@pinkfreud](https://boards.straightdope.com/u/pinkfreud)\ **Post date:** [April 16, 2006, 12:13am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/3 "2006-04-16T00:13:49Z")
I found this on an [Ask the Experts](http://72.14.203.104/search?q=cache:gfiibGrkCTEJ:bloopdiary.com/viewentry.php%3Fid%3D6342%26num%3D39+%22) site: Q: Why don’t the raisins in Raisin Bran fall to the bottom of the box? A: “Raisins are added to boxes only after more than half of the cereal has already been packed. The cereal thus has a chance to settle and condense. During average shipping conditions, boxes get jostled a bit … so the raisins actually sift and become evenly distributed throughout the box.” (From Why Do Clocks Run Clockwise, and Other Imponderables, by David Feldman) ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 16, 2006, 6:35am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/4 "2006-04-16T06:35:08Z")
> [@Packer](#): > > What happens in the opposite case, i.e. taking a large ball and putting it into a vibrated small-ball container is not obvious to me. I think that if the vibration causes behaviour to approximate frictionlessness, the large ball should submerge and sink to the bottom, but that is just by analogy with a solid ball in a less dense fluid. So I am not sure that “big pieces don’t make much headway sinking into the densely packed small stuff”. Comments? This is actually a well-understood phenomenon, and it’s exactly the opposite of your intuition. In granular media, larger grains “migrate” to the surface. Really it’s that smaller grains get to the bottom faster – imagine which is going to find an opportunity to fall a little lower first. As for it approximating frictionless conditions, it’s also well-understood that granular media are _much_ different from fluid media, if that’s what you’re thinking. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [April 17, 2006, 1:51pm UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/5 "2006-04-17T13:51:55Z")
> [@Mathochist](#): > > This is actually a well-understood phenomenon, and it’s exactly the opposite of your intuition. In granular media, larger grains “migrate” to the surface. Really it’s that smaller grains get to the bottom faster – imagine which is going to find an opportunity to fall a little lower first. > > As for it approximating frictionless conditions, it’s also well-understood that granular media are _much_ different from fluid media, if that’s what you’re thinking. Can you link to something talking about that phnomenon? I remember reading about it, indeed, I think I recall someone getting kudos for having demonstrated the actual process involved. When I read the article this morning at the website, I immediately thought of that and of you; not sure that’s what you hope for in life. 😛 ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 17, 2006, 4:59pm UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/6 "2006-04-17T16:59:25Z")
> [@DSYoungEsq](#): > > Can you link to something talking about that phnomenon? I remember reading about it, indeed, I think I recall someone getting kudos for having demonstrated the actual process involved. When I read the article this morning at the website, I immediately thought of that and of you; not sure that’s what you hope for in life. 😛 [This guy](http://roso.epfl.ch/dm/sigma.html) does computer simulations, including [this one](http://roso.epfl.ch/dm/S/seg.mpg) showing an example of large grains moving up. Beyond that, there’s a lot of information out there, but I’m not the expert to find specific cites. I’m wondering, though, why you thought of _me_. I’m a mathematician, not an engineer or materials scientist… ---
**Author:** ![foolsguinea](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foolsguinea/32/14751_2.png) [@foolsguinea](https://boards.straightdope.com/u/foolsguinea)\ **Post date:** [April 17, 2006, 10:43pm UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/7 "2006-04-17T22:43:55Z")
I used to eat a brand of raisin bran which tended to send the raisins to the bottom of the bag. It may be a packaging issue. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [April 18, 2006, 2:07am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/8 "2006-04-18T02:07:27Z")
It’s been many a year since I was in a house where someone purchased that vile so-called foodstuff. But it was definitely my experience that the raisins did settle to the bottom. (It just took time and normal shaking use.) A notable advantage when all I wanted to eat were the flakes. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [April 18, 2006, 2:25am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/9 "2006-04-18T02:25:13Z")
> [@Mathochist](#): > > [This guy](http://roso.epfl.ch/dm/sigma.html) does computer simulations, including [this one](http://roso.epfl.ch/dm/S/seg.mpg) showing an example of large grains moving up. Beyond that, there’s a lot of information out there, but I’m not the expert to find specific cites. > > I’m wondering, though, why you thought of _me_. I’m a mathematician, not an engineer or materials scientist… At its base, all such things are mathematical. N’est-ce pas? ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 18, 2006, 2:46am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/10 "2006-04-18T02:46:59Z")
> [@DSYoungEsq](#): > > At its base, all such things are mathematical. N’est-ce pas? There’s a sense in which _everything_ is mathematical at its base. That doesn’t mean it’s remotely tractable or comprehensible directly in terms of mathematics. ---
**Author:** ![Anhouten](https://avatars.discourse-cdn.com/v4/letter/a/71e660/32.png) [@Anhouten](https://boards.straightdope.com/u/Anhouten)\ **Post date:** [April 18, 2006, 4:03am UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/11 "2006-04-18T04:03:36Z")
The effect is known as Kinetic sieving. It is associated with debris flows, mixed nuts and any other hetrogenious mixes of solids. ---
**Author:** ![UncleBeer](https://avatars.discourse-cdn.com/v4/letter/u/977dab/32.png) [@UncleBeer](https://boards.straightdope.com/u/UncleBeer)\ **Post date:** [April 18, 2006, 8:42pm UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/12 "2006-04-18T20:42:27Z")
> [@Anhouten](#): > > The effect is known as Kinetic sieving. It is associated with debris flows, mixed nuts and any other hetrogenious mixes of solids. There was an article several years ago in a publication I subscribed to at that time - _Science News_ - which took up the current thinking about this problem. They called it the “Brazil Nut Effect.” The article is available on-line only for registered subscribers today. I’ll see if I can dig that issue this evening. An appetizer may be found at the all-pervasive Wikipedia site: [http://en.wikipedia.org/wiki/Brazil\_nut\_effect](http://en.wikipedia.org/wiki/Brazil_nut_effect) ---
**Author:** ![Packer](https://avatars.discourse-cdn.com/v4/letter/p/4da419/32.png) [@Packer](https://boards.straightdope.com/u/Packer)\ **Post date:** [April 19, 2006, 9:01pm UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/13 "2006-04-19T21:01:45Z")
> [@Mathochist](#): > > In granular media, larger grains “migrate” to the surface. Really it’s that smaller grains get to the bottom faster – imagine which is going to find an opportunity to fall a little lower first. > > As for it approximating frictionless conditions, it’s also well-understood that granular media are _much_ different from fluid media, if that’s what you’re thinking. Thanks. I had an intuition that my intuition was missing something 🙂 , that’s why I said it was not obvious to me. The links and what you wrote also clarifies the nuts bit of Cecil’s original article for me. ---
**Author:** ![UncleBeer](https://avatars.discourse-cdn.com/v4/letter/u/977dab/32.png) [@UncleBeer](https://boards.straightdope.com/u/UncleBeer)\ **Post date:** [April 20, 2006, 4:53pm UTC](https://boards.straightdope.com/t/why-dont-the-raisins-in-raisin-bran-all-wind-up-at-the-bottom-of-the-box/352772/14 "2006-04-20T16:53:51Z")
> [@UncleBeer](#): > > There was an article several years ago in a publication I subscribed to at that time - _Science News_ - which took up the current thinking about this problem. They called it the “Brazil Nut Effect.” The article is available on-line only for registered subscribers today. I’ll see if I can dig that issue this evening. Okay. I wasn’t able to find that Science News issue, but the link below from the PhysicsWeb, seems, to my memory, to be pretty well representative of the article: [http://physicsweb.org/articles/news/5/4/5](http://physicsweb.org/articles/news/5/4/5)