Starting out, homebrewing can sometimes seem an exercise in crisis management. Until you’re secure with the process, any deviation from expectations can start a cycle of worry. Since hydrometer readings are so precise, it’s easy to get hung up on those numbers. If the recipe indicates an OG of 1.057, but your measured gravity is different, how much should you care? What if your beer’s final gravity is out of line with the prediction? Is there anything you can do about it?
In general, it’s a matter of degree. If you’re within 5 points (around 10 percent or so), it’s worth some analysis, but it’s not that big of a deal. You should examine larger variations to understand what’s going on. Likely explanations and mitigation will depend on whether your issue is with the starting gravity or finishing gravity.
Eliminate the Obvious
Before you launch a full investigation, validate your measurements and your tool. Many brewers forget that hydrometer readings are temperature dependent. Each one has a reference temperature (usually 60°F/15.5°C or 68°/20°C); if your sample is significantly warmer or colder, your reading will be off. For example, wort at 100°F (38°C) will read as 1.056 on a 60°F hydrometer when it’s actually 1.062. Remember to use a temperature adjustment table or calculator if necessary. In addition, you should check that your hydrometer is correctly calibrated. It’s not unusual for the paper scale to slip slightly and skew your measurements.
Original-Gravity Problems
Whether you’re brewing all-grain or extract, you should always stir your wort well before measuring the gravity. If the wort isn’t thoroughly mixed, it can be layered. A sparge naturally starts out with high gravity and then tapers off, while extract can settle to the bottom without being fully diluted. As a result, the wort in the bottom of your kettle can read significantly higher than the thinner top layer. This can explain some very large gravity discrepancies in either direction.
Mis-measured ingredients can also be a contributing factor to missing your original gravity target. Losing count and skipping a pound (454 g) of malt can drop your gravity by 3 or 4 points. But the most likely measuring mistake isn’t malt or malt extract—it’s water. Overshoot your volume and your gravity plummets; too little water and it’s higher than expected. That’s why it’s good to have a measuring stick for your brewpot, or you can get fancy and etch volume markers into your kettle. The evaporation rate might also be an issue because it will affect the final volume, just like using the wrong amount of water.
All-Grain Specifics
All-grain recipes bring in additional process issues that can affect original gravity readings. Improper pH, adverse water chemistry, grist quality, mash time, and temperature can all degrade your mash and lautering efficiency. Assuming your ingredient measurements were all good, if you were well under your target gravity, then you should look at how to improve your mash and sparge process. If your gravity was higher than expected, your efficiency may be better than the recipe writer’s.
Fixes
The best option may be to work with what you have. You may not match the original gravity in the recipe, but you could still end up with a good beer. Of course, if your original gravity is unexpectedly lower than 1.040 or higher than 1.080, you should make some adjustments. Fortunately, this is prior to fermentation, so you can add some dry malt extract to bring up lighter-gravity worts (boil the DME in a small amount of water). Adding malt extract is preferable to extending an already completed boil, because hops character would likely suffer. Or if your reading is too high, you can dilute the wort by adding water.
Final-Gravity Woes and Stuck Fermentation
If your final gravity is much higher than expected, make sure that the beer has actually finished fermenting. Give it some more time, then check the gravity again. If it stays the same, then you should look more closely.
Final-gravity problems tend to be tied to yeast health and fermentation issues—the gravity remains high because the yeast cells were not as effective at processing the sugar. Once again, minor problems can be accepted; however, in the future, you should make sure you’re pitching enough yeast and that your wort is aerated properly. In the worst cases, though, you may have a stuck fermentation in which the yeast cells have given up, but there’s still plenty of food for them. Fortunately, you still have some options. Rousing the yeast or warming things up may help, or you can always repitch.
All-Grain Effects
Temperature issues can show up and affect the final gravity in all-grain batches. Higher mash temperatures will yield more unfermentable sugars, while lower ones will sacrifice body for yeast food. If you miss your final gravity reading, before you brew your next batch, you should make sure your thermometer is calibrated and double check your mash temperature. Assuming that your mash is thoroughly mixed when you check the temperature, if your final gravities have been lower than expected, make sure your mash tun is insulated to hold temperature. If they’ve been higher, give the mash time for the temperature to settle then recheck to make sure you didn’t overshoot.
But What If It’s Low?
If the final gravity is very low and mash temperature doesn’t seem a likely culprit, then you have to let the beer tell you what’s going on. Sour, medicinal, or plastic flavors indicate an infection, while a big fruity or solventy character points to a hot fermentation. In either case, you should adjust your process in the future. If the beer is clean, you may just have a very attenuative yeast (champagne, maybe?). Try a different yeast the next time you try the same recipe.
Aside from addressing unfinished or stuck fermentations, there’s not much you can do to adjust a beer’s gravity after it’s done. So learn what you can for the next time.