Partial Boils
Posted Monday, January 19, 2009
Along with several bug fixes and other enhancements, today's update to Brewtility includes functionality for specifying a partial boil when creating a recipe.
So, when creating a partial mash or extract recipe you'll now see a field labeled "Boil Size." (Shown below)
Setting the boil size equal to the batch size would indicate a full boil and the rest of the recipe will be unchanged. However, if you're an extract or partial mash brewer and you only boil, say, 3 gallons of liquid and then top off to 5 gallons after the boil, you'd set the Batch Size to 5 gallons and the Boil Size to 3 gallons.
So what does this really mean?
Bascially, when boiling a given amount of extract in a smaller volume of liquid, the specific gravity of the wort in the boil is much higher than if it were boiled in the full and final volume because the sugars are more conectrated. There are several effects of boiling this more concentrated wort that you need to be aware of. One effect is that there is usually more carmelization of the sugars and thus more darkening of the wort and potentially more unfermentable sugars that are created as a result. Also, hop utilization goes down as the specific gravity of the wort during the boil goes up. This is the effect that matters to us as we're formulating recipes. Since hop utilization goes down, the quantity of hops needed to attain a given number of IBU's goes up.
So, when you set your boil size to an amount less than the final batch size, you'll see the estimated IBU's go down. Once you've got your batch and boil sizes set for a recipe, you can simply increase the amount of hops until you get to the IBU number that you're shooting for.
Happy Brewing! 
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