Mashing is soaking malt grains (often mixed with a smaller quantity of other grains) in hot water for an hour or so - it really is a simple as that. During this time the starch in the malt (which isn't fermentable by brewer's yeast) is converted into sugar (which is). This is known as 'starch conversion' and is the key process in grain brewing.
The trick is getting the conditions just right for this reaction to take place correctly.
For starch conversion to take place, the mash needs to be at the right temperature, acidity (actually a measurement called 'pH') and dilution (mash 'thickness'). Don't panic - this is easier than it sounds! The ideal conditions are as follows: -
Aim for 67 º C. Anything between 62 º C and 68 º C will work, but the actual temperature affects the 'fermentabilily' of the finished wort (i.e. the liquor that is drained from the grains and boiled with the hops).
Lower mash temperatures create a more fermentable wort, higher mash temperatures a less fermentable wort. While it may seem that a less fermentable wort would produce a sweeter beer (less of the sugar is fermentable by the yeast, so more will be left in the finished beer) in practice the kind of sugars left unfermented tend to be 'dextrins' which aren't all that sweet, so a more noticeable effect is that the finished beer has more 'body' or 'mouth-feel'.
pH is a measure of the acidity or alkalinity of a liquid, and the scale runs from 0 to 14. pH 0 is the strongest acid you can get, pH 7 is neutral (neither acidic nor alkaline) and pH 14 is the strongest alkali you can get. Just remember that 'lower pH' equals 'more acidic', and that all solutions produced when brewing are slightly acidic (unless something goes drastically wrong!) so we'll always be dealing with pH values of less than 7 in brewing. pH can be measured using pH indicator papers obtainable from home brew suppliers (pH meters are also available but good ones are expensive and can be complicated to use).
Aim for pH 5.3 for English style beers. Conversion will happen between pH 4.5 and pH 5.6, but the fermentability of the wort will change - higher pH gets you less fermentable, lower pH gets you more fermentable. Fermentability, though, is best adjusted by varying the mash temperature, as varying the pH can lead to problems (e.g. slow conversion or hazes in the finished beer).
Mash pH is affected by a number of things. Recipes which use a lot of dark roasted malt will tend to produce a lower mash pH, and those containing mainly pale malts a higher mash pH, all other things being equal.. The main balancing factor in all this is the alkalinity of your brewing water. Note that this 'alkalinity' is NOT the same as the pH value of the water coming out of your tap, but a measure of the 'buffering power' of your water and results from its mineral content. The alkalinity of brewing water can be measured and adjusted to get the correct mash pH regardless of the 'bias' caused by the grain bill in any given recipe - see the water treatment pages in 'Extras', and particularly Graham Wheeler's water treatment calculator for the full gory details!
However, most domestic water supplies will work well enough to get a few brews under your belt before you worry too much about that - in fact when you think about it, your water's native alkalinity is bound to be perfect for at least one beer style!
This is simply the ratio of grain to water, which you can vary between certain limits to suit your purposes. Although mash thickness can affect wort fermentability, the effect is, for our purposes, negligible. You really just need a grain-water ratio that allows the water to penetrate the grain and leach out the sugars produced, and that will still allow the whole mash to fit in your mash tun. A ratio of 3 litres of water per kilogram of grain is a good target, with the practical range being from about 2 to 4 litres/kg.
The mashing procedure itself is pretty straight-forward: -
Now we can move on to the next step - Sparging.
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