For most homebrews, the slightly low level of fermentability is either not a problem or actually a good thing. You can make a lot of all-malt, full-bodied beers with malt extract with no problem. And, as discussed yesterday, if you want more body in your beer, a beer that finishes at a higher final specific gravity (FG), you can add or swap in ingredients that contain unfermentable sugars.
On the other hand, if you want to raise the fermentability of a wort — so that it ferments to a lower FG and the resulting beer has a drier character — you need to decrease the amount of unfermentable carbohydrates in it. You can do that this either by swapping some of your malt extract for a complete fermentable sugar, or by actually altering some of the carbohydrates from the malt extract.
If you swap out a percentage of your malt extract and swap in some plain sugar — either table sugar (sucrose) or corn sugar (glucose) — you are essentially replacing a portion of the unfermentable carbohydrates with (completely fermentable) simple sugars. If you need to dry out an extract beer, replace up to about 20% of the grain bill with sugar. This will result in a lower FG, but not have any major flavor consequences. You will lose a little flavor and color from the malt extract you left out, but this can compensated for by adding more or slightly darker specialty grains to your recipe. (Some recipe calculators show the color of the beer and you can judge by this if you need to bother.) Swapping 10% sugar into an otherwise all-malt extract beer will lighten the body slightly, but not make a dry beer; at 20% it will be noticeably lighter in body, but will still be in the moderate body range.
If you exceed 20% sugar, you’ll likely need to add some yeast nutrients to help the yeast. Ramping up the sugar to 30 or 40% will get you dry beers, on par with American Pilsners.
Another way to raise the fermentability of your wort is to degrade some of the unfermentable carbohydrates in the malt extract. The best way to do this is to add a partial mash to your extract recipe. The partial mash will help somewhat by simply replacing extract wort with all-grain wort, but you can also employ the enzymes in the base malt to act on the malt extract. To do this, you can run off your partial mash wort into your brewpot and dissolve some or all of your malt extract in it. Hold the mixture of extract and all-grain wort at 148–152 °F (64–67 °C) for about 5 minutes and then proceed as normal. (For a drier beer, you could extend this hold, up to a couple hours if you want a bone dry beer.) For a 5.0-gallon (19-L) batch of beer, making a partial mash with your specialty grains and enough base malt to make 3 or 4 pounds (1.4–1.8 kg) of grist would give you enough enzymatic power to do this, and would also benefit the aroma of your extract beer.
Another way you can do this is to dissolve some or all of your malt extract into your brewpot, and steep the grains (performing the partial mash) in the extract wort. Steep (mash) the specialty grains and base malt for a full hour in wort less than 8 °Plato (SG 1.032) and your wort will be more fermentable than the dissolved extract would have been. And you can control how fermentable by selecting the mash temperature. Low temperatures — 148–152 °F (64–67 °C) — yield more fermentable worts. High temperatures — 158–162 °F (70–72 °C) — perhaps coupled with less mash time (as short as 30 minutes), yield less fermentable worts.
A final possibility is to add prepared enzymes. Many homebrew shops sell amylase enzymes and most drug stores sell digestive aids such as Beano. The drawback to using prepared enzymes is they may do too good of a job. Even tiny amounts of purified enzymes will go a long way. You could end up with a beer with little body to speak of. With a partial mash, you’ll have less enzymatic power and therefore better control over the results.
If you are shooting for a dry beer, be sure to aerate well and pitch an adequate amount of yeast. You should also choose a yeast strain that can reach the FG you are aiming for. Check the apparent attenuation levels reported for the yeast strain and see if it can do the job.
Extract brewing has come a long way since “the good old days” of early homebrewing. Advanced extract brewers can manipulate the fermentability of their worts to get the amount of body they desire, regardless of the fermentability of their malt extract.