Doing a full boil with all of your extract may cause the extract to darken and ruin the appearance of your beer. The amount of darkening depends on the boil time and the concentration of the wort. The longer the boil and the more concentrated the wort, the darker it will be. To reduce darkening, a partial boil with a late extract addition is a better choice. Adding extract late in the boil ensures that it's sterilized while minimizing darkening.
Malt extract doesn't need a long boil, as it's already been boiled during manufacture. A 90-minute boil with an extract based beer may cause it to feel thin or watery (due to protein breakdown). Extract beers usually benefit from a 60-minute or 45-minute boil. I've even heard of 20-minute boils being acceptable for the lightest styles. Regardless of the boil length, a late extract addition can improve the finished beer's body and head retention.
When doing a late extract addition, it's important to first turn off the heat and make sure that the extract has fully dissolved before turning the heat back up. Otherwise, the extract may accumulate on the bottom of the kettle and cause it to scorch. This could easily run the color and flavor of the beer.
Doing a late extract addition isn't as simple as just holding back a quantity of the extract to add late into the boil. The lighter gravity of the wort prior to the extract addition can result in a greater extraction of bittering compounds from the hops, and a much bitter beer. You'll need to adjust the amount of hops to avoid getting a more-bitter beer than you intended.
If you're using software like BeerSmith or Beer Tools Pro, the bitterness calculations should happen automatically for you as you input the recipe. If you're working this out without such a tool, there are online calculators that can help. You basically want to treat the boil as two different boils at different gravities. If you are doing a 45-minute boil with a single hops addition, with extract added 10 minutes prior to the end of the boil, here's what you'd do.
- Calculate the bitterness of the wort based on the gravity at the start of the boil and running for 35 minutes.
- Calculate the gravity after the 35 minute mark when the last of the extract is added, and determine the bitterness imparted during the last 10 minutes of tbe boil based on that gravity.
- If you're doing multiple hops additions and extract additions, you'll need to calculate the hops quantity, wort gravity, and boil time at each step to best estimate bitterness.
How much does this really mater? A beer which calculates to an IBU level of 30 IBUs with a full boil and full early extract addition could demonstrate a bitterness level of 50 IBUs by changing only when extract is added to it. A beer done with less than a full boil, multiple hops additions, and late extract addition could see an even greater difference.