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I'm in the process of sizing down my 5 gallon brew day to a 3 gallon, no sparge, BIAB (on the stop-top). I took an American Wheat recipe for 5 gallons and scaled it by 3/5. It called for 44% 2-row and 44% wheat malt. I was calculating for 0.12 gallons/lb of absorption. With a grain bill of 5.6 lbs, I figured it'd b 0.672 gallons. However, I measured only about half that was absorbed when I was done.
So I was just trying to find out if maybe wheat absorbs water differently than malted barley?
For a very precise answer, there is actually a paper published in the American Society of Agricultural and Biological Engineers journal ('Water Absorbtion Characteristics of Wheat and Barley During Soaking' Transactions of the ASABE. Vol. 46(2): 361–366 . (doi: 10.13031/2013.12916) @2003) The abstract reads:
Water absorption characteristics of wheat and barley during soaking were measured at five temperatures ranging from 10.C to 50.C. From the water absorption characteristic curves, it was determined that water absorption of wheat and barley was in the second falling rate period. Using the measured data, a non–linear least squares method was applied to an approximate solution of the diffusion equation MR = B1 exp(–Kt). Values of parameter B1 for the diffusion model were estimated to be about 0.6 for wheat and about 0.8 for barley. Therefore, the measured data were fitted to the exact solution for the sphere diffusion model for wheat and for the infinite plane sheet diffusion model for barley by a non–linear least squares method. The measured results agreed well with the calculated results. The values determined for the diffusion coefficients were 1.1 . 10–12 to 1.0 . 10–11 (m2/s) for wheat and 3.5 . 10–12 to 3.9 . 10–11 (m2/s) for barley. An Arrhenius–type equation was used to relate the diffusion coefficient of wheat and barley to temperature (T), and the energy of activation (E) for wheat and barley was estimated. The values determined were 44.0 kJ/mol for wheat and 45.9 kJ/mol for barley.
A hopefully more useful answer for those above who don't understand what the above really all means is that wheat (and rye for that matter) don't have the same fibrous husks that Barley have, and therefore absorb less water. I've heard numbers (unsourced, from memory) of about 0.07 gallons / lb of wheat or rye malt.
I'm not sure about the absorption, per se, however, if I don't use rice hulls along with wheat, my liquid volumes are low and my efficiency drops as much as 5-10%, especially if there is 50% or more wheat in the grain bill. It's kind of like a stuck sparge, but not quite.
