Session beers allow beer drinkers to enjoy “a few” without becoming overly intoxicated. They are low-gravity, low-alcohol beers for extended drinking sessions. Brewers can — and, of course, have — differed over exactly how low in alcohol a beer needs to be in order to qualify as a session beer. For the sake of this article, let’s say session beers are those under 4.5% ABV. (And outside of this article, feel free to apply your own definition.)
Just because these are “little” beers, however, doesn’t mean that they don’t require our full attention. One of the biggest potential problems when brewing a session beer is oversparging. With a smaller malt bill, collecting your full pre-boil wort volume may mean you’ve sparged past the point that tannins become much more soluble. Oversparging in this manner results in beer with an astringent mouthfeel. This is frequently described as the puckering, bitter-like feeling one experiences when drinking tea.
The solubility of tannins increases as pH increases. When fly sparging, the pH of wort in the grain bed rises as the sugars are progressively rinsed out. Thus, the longer you continue to sparge, the higher the probability of extracting excess tannins from the grain hulls. In most cases, if you discontinue collecting wort when the pH of the runnings rises to 5.8, or the density of the runnings drops below about 2.5 °Plato (SG 1.010), you will be fine.
When brewing a normal-strength beer — around 5% ABV — the volume of pre-boil wort required for a 60–90 minutes boil is generally right on the edge of the limit to which it can be sparged without extracting tannins. The exact point depends on your water chemistry (and hence mash pH), extract efficiency, and how effective your lautering is. For smaller beers, you need to cut sparging short and add water to your kettle to reach your pre-boil volume.
When fly sparging, it’s easy to know when to stop. If you have a pH meter, check the wort coming off the grain bed before it enters your kettle. Stop sparging when the pH of these runnings climbs to 5.8. Alternately, check the gravity of the runnings and stop when they drop to SG 1.010 — this usually corresponds to wort with a pH of 5.8, assuming your water chemistry is in the right ballpark.
In either case, the amount of wort you collect depends on the weight of your grain bill. More grains means more wort can be collected before the cutoff point. For whatever it’s worth, on my system, I can collect between 0.65 and 0.68 gallons of wort per pound of grain (X–Y L/kg) before I need to stop sparging.
At this point in brewing a session beer, you will likely have less wort than you need to yield your full batch volume after the boil. As such, as water to make up the deficit in volume.
When batch sparging, you can’t quit sparging at a given pH level because the wort is collected in batches. Aim to collect a volume of wort equal to what you would have collected fly sparging. One way to do this is to fly sparge the first time you brew a recipe, note the volume of wort you collect, and then batch sparge when you brew the beer again.
A simpler approximation is to collect two worts of equal volume, with the mash thickness for each being between 1.25 qts./lb. and 1.38 qts./lb. (2.6–2.9 L/kg). This way, as the size of your grain bed increases, the volume of wort you collect increases proportionally. And, you’ll be in the right ballpark for the amount of wort to collect. As with fly sparging, you will likely need to add water to your kettle to reach your target pre-boil wort volume.
When brewing a session beer, a no-sparge approach can work well. Mash the grains at a mash thickness around 1.25 qts./lb. (2.6 L/kg). When the mash is over, add water to your mash tun such that the volume doubles. Stir the mash, then let it sit for 5 minutes. Then recirculate and run off the wort as quickly as you can manage. As with the other sparging options, the amount of wort collected is tied to the weight of the grain bed. Add water, if needed, to make you full pre-boil wort volume.
Collecting the right amount of wort will give you the highest possible extract efficiency possible without extracting excess tannins. When brewing a session beer, you will likely need to collect less wort than your full pre-boil wort volume. Otherwise, you’ll end up with an astringent brew. You can choose between lautering methods, but always base the volume of wort you collect on the weight of the grain bed. Make up any required volume in the kettle with water.
Of the lautering methods, fly sparging will give you the best extract efficiency, followed by batch sparging, then no sparge. However, the time required to lauter is greatest with fly sparging, followed by batch sparging. No sparge is the quickest. As such, you can choose your method based on your own preferences.