Posted Thu Dec 13, 2012, 9:40am Subject: Re: Clever Coffee Dripper - Help me understand
I take it from your post that you are letting all the initial brew water drain, then adding the second half of the brew water?
How's this for a theory?
The resistance to flow and hence drain rate is limited by the finest particles of coffee, particularly when they get caught in the filter paper, or collect together in the bed. Initially in the filter bed, the finer grounds are mixed in with the coarser grounds. In either case, the flow of water through the bed will draw finer grounds towards the filter paper, creating a layer that is more resistive to flow. In the initial stage of the 2-stage brew, the filter bed will be more resistive to flow than a brew with more water, as the fines are spread over a smaller area. Stirring will redistribute the coarse grounds that are sitting on top of the filter bed, but might not shift the smaller fines that are sitting in/on the surface of the porous filter paper, and will help finer grounds stuck higher up the filter bed to migrate downwards.
In any case, letting the bed drain in the 2 stage brew will draw fines suspended in the water and trapped higher up in the bed down to the base of the filter bed, settling the bed, and making the bed much more resistive to flow with the second batch of water.
If you dissect a filter bed after a single phase brewing, I think you would see a collection of fines on top that were collected from suspension in the brew water by the overall filter bed, plus a layer of fines near the filter paper. After a second phase, I think those fines will be distributed further down into the filter bed, and possibly with a thicker layer near the paper.
Tests that could be done on this theory: The latter point about the second batch through the grounds being slower can be tested with any size of batch, with any initial drain rate. The effect of stirring on the drain time could be measured. You could sift the grounds to remove more or less finer particles. You could plot the drain rate vs. batch size, with and without stirring. You can vary the porosity of the surface of the filter paper - using e.g. Melitta dotted filter papers vs. cloth, vs. flat filtropia papers or bamboo filters.
Some or all of those might help show differences between the two methods.
But that's all a WAG from this astrophysicist... ;)
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