I am a young brewer and have been wondering for some time about gravity of my lauter. It is common knowledge that you stop collecting wort into the boil kettle when the gravity reads below 1.008 – 1.010. However is this gravity being corrected for its elevated temperature or do you take it “as is”? In other words, if I take a sample of wort toward the end of my lautering process and it reads 1.002 at 160 degrees Fahrenheit, is this the point to stop? Or do I correct for temperature which results in an actual gravity of 1.024. To be safe I have stopped lautering when I fall below the specified range, but my efficiency takes a bit of a hit. Just wondering if maybe I am being to careful.
It’s pretty well documented that you should stop lauter running’s at 2 deg Plato (SG 1.008) to avoid absorbing astringent tannins from the grain into the wort. Your hydrometer is calibrated to read at a certain temperature which should be stamped or labeled on the stem, usually 60 deg F or 68 deg F, that is the temperature where the reading will be accurate. The hydrometer measurement is based on the density of the wort, the warmer the liquid the less dense it is so it will read lower at a warm temperature than it will at a cooler temperature. If you are reading the hydrometer at 160 deg F you are reading it at a very low density and it will give you an inaccurately low reading. In your example you are taking a measurement at 160 deg F and getting a reading of SG 1.002 (0.5 deg Plato) with a corrected reading of SG 1.024 (6 deg Plato), the latter is the accurate reading and you should continue to run off until you get a corrected reading of 2-2.5 deg Plato (SG 1.008-1.010). The reason why you are losing efficiency in extracting from the mash is because you are cutting your run-off too early.
To cool the sample quickly in the brewhouse you can use a small stainless cup for the sample sitting in a water bath with cold water running through or ice. Stir the wort sample in the cup with a bi-metal or digital thermometer (never mercury!!) until the desire temperature is reached then take your hydrometer measurement. Invest in a good quality calibrated hydrometer with temperature compensation to get the most accurate results, these are available through commercial brewing supply or laboratory supply websites. The most accurate will come in concise ranges, ie 0-12 deg Plato and 9-21 deg Plato.