It is easily done, you mash in expecting to hit your desired mashing temperature and when you measure the actual temperature of your mash it is either too cold or too hot. The question is does this really matter and what can be done to achieve the correct mashing temperature when mashing in? The key is the mashing strike temperature a little know but very useful concept which allows you to hit the correct mashing temperature every time.
What is the problem if the mashing temperature is not correct?
The mashing temperature is critical in achieving your desired final beer. For example if we consider the fermentability of wort this is directly affected by mash temperature due to the heat stability of the two amylases alpha and beta-amylase. It is beta-amylase which is critical to the fermentability of your wort as it breaks large unfermentable carbohydrates, known as dextrins, into fermentable maltose and it just so happens that it is quite sensitive to temperature. Therefore if your mashing temperature is just a few degrees out, say 68ºC or 62ºC, rather than the 65ºC you are aiming for, fermentability can be quite different. At the lower temperature of 62ºC alpha-amylase is active, but perhaps not at its optimum, so is cheerfully breaking down starch into smaller dextrins. Beta-amylase is also active but it is at its temperature optimum so is busily liberating fermentable maltose from the dextrins. The resultant wort is highly fermentable so will produce a dry beer with not much body and a higher alcohol content. However, at the higher temperature of 68ºC alpha-amylase is active, and working at its optimum, so is still merrily breaking down starch into smaller unfermentable dextrins. However, beta-amylase is not so active, in fact much of the beta-amylase activity would have been lost as it will be de-natured at these higher temperatures, so the level of maltose in the wort will drop significantly. The resultant wort is not as fermentable, due to the lower levels of maltose, and so will produce a beer which is fuller in body and lower in alcohol. Mashing at 65ºC is a good compromise as it favours the action of both enzymes and so will produce a relatively balanced beer. Therefore it should be clear that you can create very different types of beer just by manipulating the mashing temperature. This brings us back to how to hit the correct mashing temperature at mashing in. The critical element here is the mashing strike temperature of your mashing liquor.
So what do we mean by mashing strike temperature?
The mashing strike temperature is the temperature of the mashing liquor before being mixed with the grist. Usually when mixed with grist at a cooler temperature the mashing liquor temperature falls as the heat is utilised to increase the temperature of the grist. For example if you mashed in 10 kg of malt with 26 litres of mashing liquor at a temperature of 65ºC and the grist being mashed was at room temperature (20ºC) the actual temperature of the mash when mashing in was complete would be 59ºC. Unless you have the ability to heat your mash you will have significant problems with mash conversion giving a slow wort run-off and poorly fermenting wort. To compensate for this drop in temperature and to guarantee that the desired mashing temperature is achieved the mashing or strike temperature of the mashing liquor needs to be elevated. The strike temperature can be calculated using the following equation.
(Desired mash temperature x (litres of water + (0.4 x kg malt)) – (0.4 x kg malt x malt temperature)/litres of water
Therefore the strike temperature, if you are mashing in 10 kg malt at 20ºC with 26 litres of water to achieve a mash temperature of 65ºC, is calculated as follows:
((65 x (26 + (0.4 x 10)) – (0.4 x 10 x 20))/26
= ((65 x (26 + 4)) – (4 x 20))/26
= ((65 x 30) – 80)/26
= (1950 – 80)/26
This calculation for working out the mashing strike temperature has been adapted from the version shown in the excellent book Brewing by Lewis and Young.
You can use this equation as a good place to start for gaining better control of your mashing in temperature and therefore be in a better position to brew the beers that you want to brew.