By David Doucette
All grain brewing covers a large scope of brewers and brewing setups. Brew in a bag all the way to fully electric 3 vessel systems. Sometimes you don’t always have the space or equipment you need, and have to do a partial boil. Let’s go over the difference between full and partial wort boils.
In a full boil, the entire volume of wort is collected prior to bringing it to a boil. In full boils, the brewer must calculate how much mash and sparge water they need to have a given volume after the boil has ended. They also need to account for the amount of boil-off they will have. Because of this, a full wort boil may start around 6 or 7 gallons and boil down to 5 to go into the fermenter. A partial boil is a little more relaxed. In a partial boil, you mash and sparge to get the sugars you need out of the grain, but the volume in the kettle isn’t as important. This is because clean water is added after the boil to give you your desired volume in the fermenter. A brewer may choose to do a partial boil for several reasons. They may not have enough room for a full boil in the kettle they bought. Many beginner brew kits have the brewer do a partial boil as well as save extract for after the boil (to prevent any undissolved extract from caramelizing from adding it to a boil).
The advantage of a partial boil is you can chill faster, as you’ll be topping up with cold purified water. You will also use less propane (if you brew outside) bringing 3-5 gallons to a boil versus 6-7 gallons. A partial boil also allows you to do 5 gallon batches on weaker stovetop systems instead of 3 gallon ones. However, there are some downsides. Hop utilization drops when doing a partial boil. This means you will need to add more hops to create the same IBUs as you would in a full boil. The amount you need to adjust for increases with the amount of water you add at the end of the boil. You will darken the wort more than you would have in a full boil. So if you are brewing a style of beer that needs to be light (like a Pilsner or Kolsch), you’ll want to strive for a full boil. The higher concentration of sugars in partial boils also increases the risk of kettle caramelization. This (while uncommon) is when the sugar created from the mash is heated to a point where the sugar starts to become unfermentable. The result of kettle caramelization in a finished beer is a lower than anticipated ABV and higher final gravity. It is less common in a full boil because typically, the thinner the liquid, the lower the boiling point. In candy making, caramelization is ideal and the thick sugary liquids can heat up to above 300F (water boils around 212F based on elevation). As I stated before the advantages of a full boil allow you to better predict the hops you’ll need, and the SRM you’re achieving. You also have less of a contamination risk doing a full boil. This is because the entire wort is pasteurized during the boil. In a partial boil, you are adding new water, which carries the small additional chance of infecting your beer. With that said, a healthy pitch of yeast will likely out-compete anything dwelling in your water supply. If your source of water has chlorine in it, a boil will help remove it. However you can also treat your water with 1 campden tablet per ten gallons 24 hours before brewday to do the same thing.
You can use smaller and less powerful equipment for a partial boil. Doing a boil separated out between kettles on a stovetop isn’t unheard of. It’s also much easier to do 3 gallon full boils or 5 gallon partial boils on a stovetop if you don’t have the ability to brew outside. A full boil will require a much larger kettle than a partial boil. This is because you have to account for your boil off in the starting volume, so a full boil for 5 gallons may start at 6 to 7 gallons. On top of that, you want extra room in your kettle to prevent boil overs. On the other hand, a 7 gallon pot can handle 3 gallon full boils and 5 gallon partial boils readily. After the boil is done, best practices include cooling the wort as quickly as possible. A partial boil can survive without a wort chiller as you’re adding cold water to the wort. But cooling a full boil wort doesn’t have that luxury, so a wort chiller of some kind is recommended. At the end of the day, a full boil is a better option for the reasons above. But if you find yourself needing to partial boils, you can still create exceptional award winning beer.
Resources
Boil picture from HomebrewTalk user Redbeard_41. Image retrieved from: http://www.homebrewtalk.com/photo/boil-time-64847.html
Second Boil picture from HomebrewTalk user mscg4u. Image retrieved from: http://www.homebrewtalk.com/photo/boil-56845.html
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