Have you ever been in the brewhouse on brew day and you found it difficult to hit your target gravity for your beer on a consistent basis? Your original gravity is really important because basically it sets out your alcohol, which is essentially the consistency of your product. And your consumers demand consistent product, it’s what helps to grow your brand. But there’s so many variables on brew day that can really clutter and create a lot of noise when you’re trying to brew consistent wort. There’s things like your mash efficiency, your runoff rate, you boil off in the kettle, even the weather can play its part sometimes. So how do you make sure when you’re on the brew house that you send your wort to the fermentor and that it’s on spec every single time?
And I’ll even measure my running’s as I go and I can pretty much calculate how much would I should have in the kettle at the start of the boil. Because if I run too much wort off into the kettle, I’m likely to undershoot my gravity and if that happens, the only way to fix that is to lengthen the boil and boil off more water out of that wort so that I hit spec. That’s not really efficient in terms of time or energy and it’s going to mess with your hop utilization and then send your IBUs out of spec.
Early in my brewing career, I was taught this really neat trick on how to check your wort gravity in the kettle or whirlpool before you send it out to the fermentor to make sure that it’s okay, because once it’s in the fermentor, you cannot really change it.
Here’s how you do it. Run the mash off into the kettle as normal. When you’re writing your beer recipe for whatever brewhouse you happen to be brewing on, make sure you set your into boil gravity to be about 10 to 20% over what your product spec should be. Run your boil program as per normal. Now, don’t worry that you gravity is slightly high and its potential effect on hop utilization. When you start out doing this, it’s pretty much negligible and it’s something that you can dial in later with a little bit of lab work. In the last five to 10 minutes of the boil, carefully grab a sample from the kettle. Take a gravity rating of your wort sample from the kettle, and then run it through the dilution calculation. I’ll come back in a minute and tell you about that. Then dilute you wort with hot liquor in the kettle or the whirlpool down to your target specification. The dilution calculation will tell you how much water to add. Double check your gravity again to make sure that it’s right. Once you’re comfortable with it and the wort’s in the whirlpool and it’s all ready to go, knock it out to the fermentor as you normally would.
So what’s the dilution calculation I hear you ask? Well, it’s a really neat little cross multiplication formula that helps you to understand what volume you’ve got and what gravity you’ve got and where you should be in order to hit your target gravity. Because I’ll let you in on a little bit of a secret, gravity is just a ratio of water and sugar in your wort. That’s it. So when you think about wort as a ratio of water to sugar, then a simple cross multiplication formula will help you to work out how much hot liquor to dilute your wort with at the end of the boil in order to hit your target gravity.
And the beauty of this formula is it doesn’t really matter what unit of measure you use. So if you’re Australian like me, a normal person using the metric system, I use specific gravity and hectolitres in the brewhouse. If you’re one of my imperial system using American compatriots, you’re going to use BBLs and Plato and that sort of thing. But here’s the go. You can actually use firkins, litres, hectolitres, specific gravity, Plato, Baume, Brix. As long as you’re consistent in units of measure and how accurate you can measure volume and gravity, then this will work out for you.
To make this calculation, you need to know the following. You need to know the current volume in your kettle, you need to know the current gravity in your kettle, and naturally you need to know your target OG for your product.
As you can see whether you use barrels and Plato or SG and litres or hectolitres, it doesn’t matter. It’s worked out the same. What we’ve done is we’ve added a small amount of hot liquor into our wort at the end of the boil and we’ve wound up hitting our target gravity. And we can be 100% sure that when we knock out and send that wort to the fermentor that it’s going to be on spec.