There is a saying in the computer reliability biz that the number of rat hairs per bushel of wheat isn't ZERO. This is intended to be a rather graphic way of explaining that almost nothing is really perfect, and that small defects exist almost everywhere.
While attempting to make my lectures more authentic (and wanting to have a
little "fun" from time to time) I decided to look up the number. It
turns out that this isn't the actual metric used, but the USDA has the grain
inspection manuals on line. The table of interest is on page 13-2 of:
http://www.usda.gov/gipsa/reference-library/handbooks/grain-insp/grbook2/bk2ch13.pdf
which says that the specification for wheat being unacceptable is "2 or
more rodent pellets, bird droppings, or equivalent quantity of animal filth per
1,000 grams of wheat."
So we start with the values:
http://grainstore.com/Wheat.cfm claims that the endosperm (the white flour-producing part) is 83% of the kernel, although it's not clear if that is by weight or by volume. But if we say that perhaps 75% of the raw wheat (which includes other stuff too that is sorted out before refinement) makes it into refined flour, that's probably about right.
From a cookbook, 1 pound of refined flour is about 4 cups sifted, which makes two standard (one-pound) loaves of bread.
This gives:
http://www.doyourownpestcontrol.com/rodentpic.htm has a picture of rodent droppings. The parent page also says that each such pellet is about 1/4" to 1/2" inch in length. I'm not inclined to find samples and weigh them myself, but I'd have to say (having seen such things while growing up on the farm) that about 10 hairs are roughly "equivalent" to a rodent pellet. A weight comparison would of course depend on freshness of the pellet, so I'm going by volume of compressed-together hairs. (We're going to assume that this estimate also includes any potential further contamination during flour storage and in the baking facility.)
So, you can rest easy, because the US Department of Agriculture is working hard to ensure that each and every loaf of bread you eat has NO MORE THAN a half dozen rat hairs in it (or 0.6 rodent fecal pellets, or an "equivalent quantity of animal filth").
Bon Apetite!
-- Phil Koopman
(In fairness, as we all know the reality is that many loaves of bread have no rat hairs, and the above is purposefully slanted toward the absolute worst case in many respects. But it is sometimes interesting to consider the reality of having to deal with an imperfect world.)
Note: None of the web sites referenced have endorsed this analysis, nor are even aware of it. This site is not intended to have a high degree of scientific accuracy, and certainly no substantive conclusions should be drawn from it. That being said, if you have suggestions for improvement beyond nit-picking,please drop me a line at koopman@cmu.edu.