Dear Mr. Wizard,
I have started to drain my mash tun completely before sparging to my desired volume. So far, I have not experienced a stuck mash and have increased my efficiency from 70 to 75 percent. Besides the increased risk of lautering problems, are there any other problems that I am likely to encounter, such as increased tannin being leached from the grain?
Moreno Valley, California
Mr. Wizard replies: There are really two common methods of sparging: continuous sparging and batch sparging. The method you have switched to is batch sparging. The most significant problem that I am aware of with this method is the potential for slower wort collection rates after the first sparge. The reason for a decline in wort flow rate is the presence of oxygen — when the bed is exposed to air, proteins begin to crosslink and effectively increase in size. Much of the tannish or gray “teig” or “dough” seen in a lauter bed is crosslinked proteins.
The other problem that can accompany this method is cloudy wort following each drain cycle of the bed. Cloudy wort is primarily known for high lipid levels and can contain higher levels of tannins, although high tannin levels are typically associated with low-gravity, high-pH last runnings (“glattwasser” in German). Most commercial brewers who batch sparge add the next batch of water, underlet their bed and recirculate the wort for a few minutes (“vorlauf”) after each drain cycle. The main goals of this method are to improve wort clarity and to lift the grain bed off of the false bottom to improve run-off rates between cycles.
One of the reasons that brewers use batch sparging is for an increase in yield and it looks like this method has significantly increased the yield of your system. There is no magic behind the increase in yield if you think about what happens when you drain the grain bed.
Gravity drains much of the free liquid contained in the little grain bits when the bed runs dry. This extract-rich liquid collects on the bottom of your lauter tun and runs into your kettle. And the second addition of sparge water then leaches even more extract from the grain.
This is different than continuous sparging because, with continuous sparging, the only physical phenomenon driving the extract out of the grain bits is the difference in concentration between the liquid around the grain and the liquid in the grain.
Within the grain bits are concentration gradients, where the highest concentration of extract is found in the center of the grain bit and the lowest concentration is found on the outside of the grain bit. Diffusion is the term used to describe the movement of molecules from an area of high concentration to an area of low concentration. This concept is ubiquitous in biology, chemistry and physics. The rate of diffusion attaches a time factor to this migration of molecules.
The concentration gradient of extract — coupled with the rate of diffusion — has real practical implications on both continuous and batch sparging. Diffusion rates slow down when resistance is introduced. Big pieces of grain will release their extract slower than small pieces of grain and this explains why finely-milled malt typically has better extract efficiencies than coarsely milled malt.
This concept also explains why very fast wort collection rates can reduce extract yield — the sparge water zips through the bed faster than the extract can completely diffuse into the liquid. And it explains why batch sparging produces a better yield — the liquid around the grain is drained and replaced with water and the goodies inside the grain are driven by the increase in concentration gradient.
If your beer tastes fine, looks fine and smells fine coupled with an increase in extract yield, I say go for it, Andrew! By the way, most commercial breweries are equipped with lauter tuns running a continuous sparge.
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