Introduction and Concepts
One of the most cost-effective steps you can make in advancing your homebrewing skills, while still producing great beer, is to start partial mashing. Partial mashing is an intermediate step between extract brewing and all-grain brewing. It uses all the steps of all-grain brewing, but is smaller in scale and still relies on malt extract to provide the majority of the fermentables. Partial mashing requires less equipment than all-grain brewing - less space and money - and offers higher flexibility than steeping for extract brewing.
For containers 2 gallons in size, this method allows you to use about 4 pounds of grain; 3 gallons allows you to go to 6 pounds of grain. Grain gives you fewer points of gravity per pound than malt extract, but is significantly cheaper. The cost of upgrading to partial mashing will be recouped over the course of a few batches due to this balance. Beyond this, some grains offer significantly greater character when mashed than when steeped.
Steeping and mashing, by the way, are fundamentally different. When you steep grains, you don't need to worry so much about the temperature of the process because the desired result doesn't require enzymatic action. Mashing implies that you want enzymes acting on the starch molecules in the grain, thus you have to pay attention to the temperature to make sure you are in the proper range for the enzymes you want to be doing the majority of the work. Denaturing enzymes is also a risk and will happen if you raise the temperature too far.
Fermentability is affected by the enzymatic activity. By using a lower temperature mash, you can make the wort produced by it more fermentable; using a hotter mash, you can make it less fermentable. Extract brewers sometimes have attenuation issues (wherein the yeast do not ferment out as much of the sugar available as the brewer expects them to), so being able to drive up the fermentability of the wort will allow better attenuation numbers.
Partial mashing doesn't require much in the way of equipment. You might have all of it laying around already, actually, and can source it very cheaply if you don't. There are generally three different types of partial mashing, one of which could be converted to all-grain brewing equipment if you're so inclined. All of them are, technically, Brew in a Bag methods, but don't concern yourself with that too much unless you're doing a bit of research. Basically, you need a container or two capable of holding the grist (the grain and water), a mesh bag that fills out the volume of your container, a drain valve, and a pitcher or large pyrex measuring cup.
Cooler setup: If you opt for this method, you will need a drink cooler that's either 2 or 3 gallons. Drink coolers can go up to 10 gallons, but stick with something small to start with. This is the way I chose to go.
Bucket setup: You need a bottling bucket. A cinch. Though I don't use this system, there are benefits to it: It uses equipment you probably already have, costs a bit less and is very easy to scale to larger volumes and/or allows you to use thinner mash thicknesses, which are a bit more than this particular node intends to go into.
Two kettles: It may or may not be more of an investment, but if you have two large kettles around, it will work. One of them should be your boil kettle; both should be a minimum of 8 quarts, 12 quarts would be better.
So now that you've got your equipment, there are two more things. Before we change process, you need to refactor your recipe. When you change over, you'll need to figure out a few things. Recipes are pretty easily divided into two categories and they need to be addressed separately.
Before you start either process, you will need to determine how much grain you can mash in your vessel. Go to Green Bay Rackers' calculator page, scroll down to, "Can I mash it?" and change the grain weight downward until you reach your mashtun's volume. You can fiddle with the mash thickness: On the low end, 1.0qt/lb is do-able; on the high end, you will want to use a maximum of 2.0qts/lb.
All Grain to Partial Mash: More complicated than going from extract to partial mashing, but much more doable than going from all grain to extract. Before you start, check that the recipe you are converting does not have more specialty grain than your vessel can hold; preferably, you want about half of the grains being used to be specialty grains and half to be base malts.
Convert the base malt in the recipe into malt extract. If the recipe uses American 2 Row, find the plainest malt extract; Pilsener malt is replaced with Pilsener extract; etc. I would suggest search around online to find recommendations for replacement.
After you've done that, proceed as below and act like you're converting extract to partial mash.
Extract to Partial Mash: First, determine the proper base malt to replace malt extract. If you're using American light malt extract, 2 Row is the best choice; higher color malt extracts require additions of specialty grains to match the color. Pilsener malt extract is replaced by Pilsener malt; British malt extract with Marris Otter (anecdote: other homebrewers have suggested that a few ounces of Victory/Biscuit malt added to a large amount of 2 Row will simulate Marris Otter).
Once you've figured out what malts to replace your extract with, you need to figure out how much to replace. Mass of base malt to mash = Maximum grain mass - Mass of specialty grains. Example: If you are able to mash 6lbs of grain and need to use 2lbs of specialty grain, you will be using 4lbs of base malt.
Skippable explanation: If you want to wind up with a beer that is close in specific gravity to the recipe, you will need to remove some malt extract. Dried malt extract generally contributes around 45-47 points of gravity per pound per gallon (hereafter, pppg); liquid malt extract contributes about 35-37pppg. Base malts can contribute around 35pppg, but will only actually give you a percentage of that. Assume you will only get 60% of the sugar out of it, so 21pppg. Ignore the gallon term and think, "It cancels out."
For liquid malt extract: Mass of LME to remove = (Mass of base malt * 21 points)/35 points.
For dried malt extract: Mass of DME to remove = (Mass of base malt * 21 points)/45 points
Example: 1.5lbs of 2 row * 21 points = ~35 points / 35 points = 1lb of LME.
Next, determine the amount of wort you will get out of your mash. That wort is called the first runnings. Grain absorbs about .5 quarts of water per pound.
Wort volume = (Mash thickness in quarts per pound * Pounds of grain) - (Pounds of grain * .5 quarts).
Example: First runnings volume = (1.25qts/lb * 6lbs) - (6lbs * .5qts/lb) = 4.5qts
Determining the strike water volume and sparge water volume is even simpler: Strike/sparge water volume = Mash thickness * Mass of grains.
Final, simple step: Determining the foundation water. Determine how much wort you want in your kettle, then subtract the volume of the first runnings and the sparge water. The foundation water can be heated in your boil kettle before the mash is finished, to make the whole process a little faster.
Once you have your recipe redesigned and ingredients purchased (don't forget to mill your malts!), you're ready to brew. It's going to be somewhat similar to an extract brew day, but with some important differences. Make sure the valve (if one is present) on your mashing vessel (hereafter, the MLT or mash/lauter tun) is closed. To begin with, put some water on to boil in a small pot; in a larger kettle, bring 1.25 quarts of water per pound of grain to ~166degF (hereafter, this is your strike water). Once the smaller pot of water is at a boil, pour it in the MLT. The reason for doing this is that you have to account for thermal mass within the system; by preheating the MLT, you'll eliminate it from the equation. Anyways, cap the MLT until the strike water is ready to go.
Once the strike water is ready to go, empty the now-preheated MLT of water. Pour in the strike water. Put the mesh bag into it, making sure it doesn't collapse. Pour the milled grain into the grain bag and mix it around in the water very thoroughly. It should not have any dry spots. Check the temperature, making sure it is about 150-155degF. (The grains will suck about 11degF out of the water to come up to temperature.) If it is below that, add boiling water to bring it up to about 152degF. Mix thoroughly. After this, cap up the vessel and set a timer for 45 minutes.
At 45 minutes, you will need to start heating the sparge water (whose volume you determined earlier). It should be at 165degF or so, but don't worry too much. If you have to use any foundation water, begin heating it in your boil kettle.
Once your 60 minute timer goes off, begin to draw off the wort from the mash. If you are using buckets or a cooler, open the valve and run about 1 quart of the wort into your pyrex measuring cup or pitcher, then pour it on top of the mash gently. (Don't just dump it in there; pour it as if you were pouring a cup of tea.) Do this two more times. This is vorlaufing, recirculation of wort to increase clarity. It should cut down on the amount of grain bits coming out of the mash.
After you've finished vorlaufing, drain the wort from the MLT into the boil kettle (with any foundation water you might have been heating). Set it on your burner and start heating it to a boil. After you've done that, pour the sparge water into your MLT and mix the grain into it. Cover the MLT and allow it to sit for about 10 minutes. In the meantime, stare at your boil kettle meaningfully.
Once you've resigned yourself to your fate, begin to vorlauf the wort in the MLT again. After recirculating a couple of times, pour the wort into the boil kettle to join the rest of your wort.
For those using a two-kettle setup: Rather than draining the wort from the kettle (assuming it lacks a ball valve), remove the grain bag from the first kettle and allow it to empty itself of wort. Then, dunk the bag into the second kettle (the one with the sparge water) and mix up the grains within the bag to expose to water. Allow it to sit ten minutes, remove the bag and allow it to drain into the kettle. Once it finishes draining, put the bag elsewhere, then pour the contents of the second kettle into the other kettle.
From here on out, proceed as you would in an extract brewday. Add extract, boil, add hops, chill, pitch yeast.
There are a few variables which I assumed certain values for. You can modify most of them with fairly predictable results. There are ranges you shouldn't go beyond. I'm not going to go into too much depth on what will happen, as things get complicated very quickly.
Mash temperature: Changing the mash temperature will change the fermentability of the wort. 155degF is a bit above the middle of the useful range and should give you wort that's roughly as fermentable as what extract is. Temperatures from 140degF to 160degF are usable, with lower temperatures producing more fermentable wort.
Mash thickness: By lowering the mash thickness (increasing the amount of water per pound of grain), you produce a thinner mash. Thinning the mash can increase efficiency of the mash. This is useful if you're getting lower-than-expected efficiency. Mashing as thinly as you can is Values from 1.0qts/lb to 2.0qts/lb comprise the standard range of thicknesses. I would suggest mashing as thinly as possible while still being able to fit the wort from the strike water and the sparge water into your boil kettle.
Mash length: Longer mashes generally produce a more fermentable wort. Conversion of starches is usually finished within 30 minutes or so, but the wort is significantly less fermentable at this point than at 60 minutes. Generally, recipes call for a 60 minute mash.