Once the mash is completed, you could just open the tap on your mash tun and allow the sweet liquid to drain off the grains. However if you did nothing else, not only would you not have enough liquid to make your planned brew length (volume) but you would also leave behind a significant amount of extract in the grain husks. To avoid this, the grains need to be rinsed with hot water to leach out the remaining sugars.
There are two popular methods of sparging, and both of them are carried out with the grain still in the mash tun. This is obviously desirable, as it saves moving hot grain about.
The first method, Fly Sparging, is the traditional way and is still reputed to be the most efficient. It involves sprinkling hot water slowly over the grains while at the same time running off sweet wort from the tap of the mash tun, through the false bottom or filter. This washes out the the vast majority of remaining sugars (though no sparging system will get 100% of it out). To fly sparge you will need some kind of sprinkler system and a container to hold the hot water and provide a head of pressure.
The second method, Batch Sparging, was developed as a home brewing alternative, and is reputed to be simpler, faster and require less equipment. After the mash, you top up with hot water to fill the mash tun, run off as much wort as you can, then refill the mash tun with more hot water and repeat. This will rinse out most of the remaining sugar (though not as efficiently as fly sparging will).
Fly sparging is the method described here, but there is a detailed article on Batch Sparging in 'Extras'.
To avoid grain debris from the mash ending up in the boiler, the usual practice is to collect the first few litres of wort from the mash tun tap and pour it back over the grain bed (being careful not to disturb it too much). Most of the tiny particles will be contained in these first runnings, after which the grain bed will settle and act as a fine filter to stop any more coming through. Obviously, this filter is not 100% effective, and some particulate will always get into the boiler - luckily, this isn't a problem.
The usual arrangement for fly sparging is shown in the picture to the right. The equipment is set up on 3 levels: -
The temperature of the water hitting the grain bed should ideally be in the region of 72 º C to 74 º C, but because of the inevitable drop in temperature from the hot liquor tun to the sprinkler, you should heat the bulk of the water to around 80 º C at least. Stick a thermometer in the top of the grain bed to check the temperature (you don't need to do this every time you brew - just until you get to know your equipment). Too cool will mean poor extraction, too hot (e.g. over 75 º C) will cause extraction of undesirable tannins from the grain husks resulting in an astringent flavour in the finished beer.
The flow rate should be quite slow to allow effective extraction of the remaining sugar. The video below will give you some idea of the flow rate, though some people like to go even slower than shown here (as you'll see from the comments if you look at the video on YouTube!).
The total process usually takes about half an hour to collect a full batch of wort, and that's slow enough in my opinion. Grain is relatively cheap, and adding a bit more is far more effective use of your time and effort than chasing the last few percent of efficiency out of your sparge.
Ideally, you should sparge until you have collected the correct volume of sweet wort for your brew size (allowing for some evaporation during the next phase, which is the boil). In practice, as you continue to sparge, the dilution of the liquid in the mash tun will increase. The effect of this is to increase pH (increase alkalinity) which can, if you go too far, extract tannins from the grains, again causing astringency in the finished beer.
This means that in practice it's best to stop once the specific gravity of a sample of the wort coming out of the mash tun falls below about 1.008. You'll need to use a hydrometer temperature correction table to get the correct reading, or alternatively just taste the liquid coming out - you can soon tell when there's no more sugar coming out and that it's time to stop. If at this stage you haven't collected the full quantity of wort, add water to the boiler straight from the hot liquor tun. As mentioned earlier, trying to chase the last ounce of extract from the grains is unnecessary and is counterproductive if doing so puts the quality of your beer at risk.
Next - we move on to the Boil!
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