Soft Matter, has short report  (3 pages) about the rheology of peanut butter, but from a rather odd perspective, that of a metallurgist . We all know that peanut butter has a yield point, a point (defined by either a stress or a strain) that the material needs to be deformed beyond in order for it to flow. Here the authors of this report go one step further and put a good deal of time into showing that peanut butter (creamy, not chunky) also shows work hardening - once the "flow" regime is reached, the material now has a higher yield point than it initially did.
I used the phrase "good deal of time" for a reason: they let the peanut butter sit in the rheometer for 2 hours before making any measurement. This was necessary to let any residual stress from the sample preparation completely relax.
I would imagine that chunky peanut butter would give similar results, but making the measurements would be much more challenging. Both creamy and chunky peanut butter have a base made up of a large amount (64 wt.% in this case) of fine peanut particles (3.9 μ), but the large chunks could be quite a hassle to work with in a rheometer - they sample thickness needs to be significantly larger than the largest chunk size so that it doesn't protrude from on either edge.
So next time you are making a PB & J sandwich for your significant other or your children (grilled ones, of course, are the best), be sure to tell them all about the work hardening that is occurring right in front of their eyes. They'll thank you for the knowledge and be even more impressed with your intellect, won't they?
 Open access until March 4, 2011, but only after free registration Tip of the hat to the Soft Matter blog for highlighting the article and providing open access. The Royal Society of Chemistry is doing this for all their journals, something that should be emulated on this side of the pond. It's another reason I am only too happy to review articles for the RSC.
 Given that I work with 3 metallurgists, I am perfectly entitled to say they have a rather odd perspective on rheology. They do!!