Once the enzymes have finished doing their thing and the starches are converted, the brewer has to separate the now sweet liquid from the grains (lautering) and the remaining sugars rinsed free (sparging). In large scale brewhouses the mash and lauter tuns are separate vessels, but in most home breweries it is one. The most common home set ups are either a large stainless pot or a cooler with a tap and false bottom/screen installed (to keep the grains from running out). The pot can be directly heated, but is a more expensive setup, where the cooler is a relatively cheap alternative, but temperature steps must be accomplished using hot water additions or decoctions.

Initially, the runoff from the mash tun is cloudy. The grain bed isn’t set and there are particles and proteins still floating around. The first few liters (or quarts) need to be run off and gently poured back on top of the grains (called vorlauf). This has to be done as not to disturb the grains that are acting as a filter. The liquid (wort) will start to run fairly clear and then it can be run into the kettle. The run off from has to be done slowly so that the grain bed doesn’t become compacted. The grain bed is acting as a filter and if the wort runs out too fast the grains will settle tightly and the runoff will stop. This is called a stuck mash. To fix it, the runoff is stopped and the grains mixed up, sometimes hot water is added, and the process is started over. It happens to the best of us at times.

Sparging is the act of rinsing the grains to get the maximum amount of sugars into the kettle. There are two basic ways this is done: Batch sparging, and Fly sparging. In both cases the temperature of the sparge water is about 77 C (170 F) – any more and it there is a risk of getting tannins form the grains, any lower and there is a risk of a stuck mash.

Batch Sparging is done by running out the wort into the kettle then adding back more hot water, mixing it up, vorlaufing again and running it out. This is pretty straightforward and doesn’t require extra equipment. The key to preforming a batch sparge is to make sure that the wort is completely run off before doing the sparge. If the wort isn’t fully run out before doing the batch sparge some of the sugars are going to be diluted.

Once the slow trickle of wort is finished running out, add the batch of hot water that is equal to what is needed to get to the pre-boil volume of liquid. A second batch may be needed if the mash tun is too small for the volume needed—not a problem, just repeat the process. Once the first runnings are out whatever water is added will be equal to what is going to be available to get out since all the dead space in the mash tun is full from the original wort and the grains are saturated.

Here’s the quick batch sparge math:

Pre-boil volume
Expected wort in the fermenter + boil-off + trub (the crud left in the kettle after the boil) loss

For example, if the end amount expected is 20L (5 gal), the boil-off rate for the kettle is 4L/hour (1 gal/hr) and its a 1-hour boil, and trub is estimated at 2L (0.5 gal), then

20L + 4L + 2L = 26L pre-boil volume
[5 gal + 1 gal + 0.5 gal = 7.5 gal]

Batch Sparge amount
Pre-boil volume – first-runnings

For example
The pre-boil volume was 26L (7.5 gal) and the first-runnings were 13L (3.75 gal), then

26L – 13L = 13L batch size.
[7.5 gal – 3.75 gal = 3.75 gal]

13L (3.75 gal) of hot water would need to be added back to the mash tun, mixed up, vorlaufed, and run out into the kettle.

Of course, these are just estimates and adjustments will be needed to fit each system.

Fly Sparging is done by continuously rinsing the grains as the wort is running into the kettle. Once the vorlauf is complete and the wort is slowly being run into the kettle, hot water is slowly added to the top of the grains. The key is to keep the grains covered by about an inch of water or so and to match the sparge and run-off rates. If the grains go dry there’s a risk the mash will become stuck. The sparge must be added evenly over the top of the grains and not disturb the grains. If the sparge water starts to dig into the grain bed then it may create a channel through the grains and not allow for proper rinsing—called channeling. Slow and steady is the way to go. The sparge is continued until the desired volume is in the kettle or the specific gravity of the runoff drops to 1.008 or less (at this point tannins can start to leech into the wort).

To achieve the proper flow some extra equipment is needed. There needs to be a hot liquor tank (a cooler or pot of hot water) that sits above the mash tun so it can be gravity fed (or a pump is required) to get the sparge water to the mash tun, and the mash tun needs to sit above the kettle for run-off (or pump is required). This is called a tiered system. The other option is to manually add the sparge water over the top of the grains, but the pouring action brings the risk of channelling, though the use of a colander, or other device to disperse the water as its being poured, and paying careful attention can work, but this can become a lot of work for the brewer.

Fly sparging doesn’t require any special math and typically results in high yield and efficiency from the mash, but requires the brewer to pay close attention and specialized equipment is needed.