Before you head to a psychic to have your charts done or make any deals you might later regret, here are a few things you should consider. Low yields can be tracked to three general areas: milling, mashing technique and wort separation method. Typically, decoction mashing gives the highest yield, followed by temperature profile and infusion mashing methods. The difference in yield among these mashing styles may be about five percent – really not a huge difference. Wort separation method is also an important consideration, but this is mainly a commercial brewery issue because homebrewers don’t have too many options with respect to lauter tun design. This leaves milling.
There is no doubt in my mind that your problem is a direct result of coarse grist. In order for malt starch to be converted to sugar by amylase enzymes, it first needs to dissolve. Generally, most malts have a laboratory extract yield of about 80 percent. This means that, under laboratory conditions, 80 percent of the weight of the malt will end up in the wort as extract. Laboratory conditions are quite different from brewery conditions because wort clarity and ease of wort recovery are not important – the laboratory yield is simply the maximum yield possible. Most systems can easily yield about 90 percent of the laboratory number. This number is most commonly referred to among brewers as the “brewhouse yield.”
Using a more finely milled grist can increase brewhouse yield. I like to use the finest grist possible without creating cloudy wort or slowing wort collection. This is simply a matter of gradually reducing the gap on your mill until you get to the point where the grist is too fine and then backing up a step. This is where your problem could get a bit more complicated.
You may have found that the only way to produce clear, easy- flowing wort is by using a very coarse grist. If this is the case, your mill may be chewing up the malt husk or the design of your false bottom may be inadequate.
Experiment by milling samples while using different mill gaps and visually examining the grist. Ideally, you should see chunks of white endosperm (malt starch) and nice, big, fluffy pieces of husk. If you don’t see nice pieces of husk you may have an inadequate mill. If that’s the case, I suggest finding a homebrew supply store that has a good mill and buying milled malt or, if you like milling your own grain, then you should buy a different mill.
Your mill, however, may not be the problem. If you have a false bottom with large holes or slits, you will be forced to use a very coarse grist to produce a clear wort. Changing your false bottom will address this problem. I have always liked the copper manifold wort collection device that can be assembled in the bottom of a cooler. The width of a hacksaw blade produces a narrow gap that works well with relatively fine grist. This system produces very bright wort, gives a good flow rate and has an excellent brewhouse yield.
I don’t think your problem has any supernatural cause and am confident that you will be able to get your yields back on track after making these adjustments.