The difference between brewing using a partial mash or by extract with the addition of specialty grains is subtle but something that many beginner brewers may be confused by. In fact the processes probably have more in common than things that are different.
The major change in the process between the two is time. When brewing with a partial mash, the grains need to be submerged for a longer period of time in order to extract and convert the fermentable sugars from the grains. Beyond this much of the processes are the same, both involve submerging your grain for a period of time at a defined temperature and then using the liquid that is produced to start your wort.
The only other significant difference between the two homebrewing methods is the grains used. The various malted grains used in brewing beer are produced in a variety of ways. This means that some just need to be soaked in warm water while others require a little more consideration.
A partial mash is just a longer steep
If your brewing process already includes using a different pot for steeping specialty grains than the one that is used for your boil, you may have already been mashing and not known. If not, it is not a huge transition and for many will just involve leaving the grains to steep for longer.
Your grains could be soaked in the water directly but because it makes the clean-up process far easier, I suggest using a grain bag.
The actual processes for steeping specialty grains or to mash grains and base malts are almost identical. For both the grains are soaked in water at 150-155°F / 65-68°C for the defined period of time to make the start of the wort. For mashing this is normally done with about 2qts/lb or 4l/kg of grain.
Steeping specialty grains is very similar to making a large pot of tea and is a bit more forgiving in regards to both temperature and water volume. For best results the amount of water used shouldn’t be more than about 1 gallon / 3.5 liters.
What is happening during the mash?
With a small number of exceptions, all grains used in homebrewing are malted by a professional ‘maltster’ prior to being sold.
During the malting process the grains used in brewing are encouraged to start growing (germinating) to produce the starches and carbohydrates that the plant would otherwise use to grow. The grains are then kiln dried to halt this process while still retaining the sugars that will be used to later brew into beer.
The process of mashing grains in order to convert these potential sugars, involves simply soaking them in hot water for a period of time long enough for the conversion to occur. This process is generally complete after around 45 minutes but 60 minutes is the general standard as there is no negative impact to allowing this additional time.
As the grains are soaking, the crushed grain is hydrated by the hot water. This gelatinizes the starches and releases enzymes which would otherwise be used by the grain to create the energy required to grow. These enzymes then work to convert the released starches into the fermentable sugars used to brew beer.
The grains make all the difference
Apart from the time required to complete the process, the largest difference between partial mash brewing and using extract with specialty grains is the actual grains that are used.
The roasting, kilning and stewing processes that are used to create the various specialty grains also work in different degrees to release the sugar potential within the grains. As a result, some (such as crystal/caramel malts) will only need to be soaked in hot water to release their wanted flavour and color characteristics.
Other malts such as base malts like Pale, Munich or Vienna need to be mashed in order to release their fermentable sugars. It is the use of these base malts that serves to define a partial mash. The mashed base malts retain the ability to convert starches into fermentable sugars through mashing. This conversion also then works on the other grains that are mashed with them.
Although it is possible to just steep (and not mash) any grain, the results from doing this may vary. Some grains may still release the desired flavors into the water by steeping. As the mashing process has only half completed though, the starch and enzymes released into the water may add an unwanted haze to your beer. The mashing process will transform these starches into fermentable sugars which will in turn add additional alcohol and more flavour to your beer.
Advantages of Partial Mash
The advantages found through partial mash brewing are similar to those seen when by adding specialty grain to extract brews. There is a range of additional malts and grains that can be used to further modify and improve your homebrew. Brewing a partial mash allows much of the control associated with all grain brewing but without the need for the required equipment or space.
Photo by pierre-alain dorange