Ask Mr. Wizard

How do hops affect the final gravity of a beer?


Mike Seward, Barrington, Rhode Island asks,

How do hops affect the final gravity of a beer? I recently brewed a new England IPA with 12 ounces of hops for 5 gallons (19 l). The final gravity was 1.018 and i expected 1.014 with the Gigayeast Vermont IPA yeast. So to me it tastes stronger than the 5.5% ABV that i calculated, and i don’t understand why the hop content Doesn’t make the hydrometer
float higher.


Hops do not have a significant impact on wort or beer density because the concentration of hop soluble compounds is simply too low to make much of an impact on density. In order to increase the density of 1 liter of wort by 1 ˚Plato, 0.01 kg or 10,000 mg of soluble solids must be added. A beer that contains 100 mg of iso-alpha-acids per liter of beer (100 IBUs) adds sufficient “extract” to boost the gravity by 0.1 ˚Plato, which is within the error range of the typical brewery hydrometer. Hops do lend compounds to beer other than iso-alpha-acids, such as non-isomerized alpha acids, beta acids, and hop oils, but these are minor players in comparison to the iso-alpha-acids. Furthermore, hop oils are not brought into solution by dissolution and do not affect gravity like dissolved solids. When miscible liquids are added to an aqueous solution, the effect on density is a weighted average of densities. For example, if you mix 100 mL of water (density = 1.000 g/mL) with 100 mL ethanol (density = 0.789 g/mL) you end up with a 50% (volume/volume) ethanol solution with a density of 0.895 g/mL.

So let’s dig a bit deeper into what you observed. For starters, you ended up with a gravity that was higher than what you expected. I take a pretty cynical view about expected finish gravities (FG). Is the expected FG based upon what the recipe states? Is it based on the yeast strain used for fermentation? Is the expectation based on the malts used in the mash? Has the mash profile been factored into the predicted outcome? Or is the expectation based on a forced fermentation of the wort using the same yeast strain as the actual brew? The only thing that really matters is the latter; all the other questions just give information to help formulate an educated guess. Kind of like a brewer’s parlor trick borrowed from magicians who can intuit so much from basic observations. So if your expectation of 1.014 was based on any method other than a forced fermentation, I would argue that your expectation, not the finished beer gravity, seems off! I am going to come back to this subject.

You also think the beer tastes stronger than your calculated strength. This is one of those observations that can be a warning flag. Not all alcohols have the same perception of strength, and not all beers containing the same concentration of alcohols are perceived the same. When I taste a beer with a higher-than-expected FG that seems stronger than expected, my first thought is a fermentation problem that resulted in the production of more high-molecular weight alcohols (fusels). The primary factors that tend to increase higher alcohol production during fermentation include high fermentation temperature, low pitching rate, increased cell growth, increased wort amino nitrogen, yeast strain, and higher-than-normal wort aeration levels. In your particular case, you may be associating higher strength with higher levels of higher alcohols.

The corollary to the strength discussion is two beers that seem to be different in strength, yet have the same alcohol composition. In this case, a drier beer with lower background flavor intensity usually seems stronger than a beer with more flavor and bigger body. Not what you observed, but something that may be perceived. A contemporary example of this is how many craft beer drinkers are surprised by the strength of an IPA because the flavor intensity masks the strength.

Now it is time for a variant on the topic; how can a brewer actively influence the final gravity of a beer? The basic answer to the beginning of this detour is that the wort fermentability is pushed in the desired direction. The first thing that brewers need to keep in mind is that many of the classic brewing textbooks are becoming out-of-date with respect to current trends in barley and malt. Today’s malt tends to contain a more potent enzyme package in comparison to malts that were common when some of these books were written. Today’s malts, even those made in Europe, tend to be very well modified, and much of the discussions about using mashing to compensate for undermodified malt is not relevant to most of the malt being produced around the brewing world.

One strategy to control fermentability is to either preserve or actively reduce the potency of the enzyme package that comes in with malt. This is not something that many brewers consider because enzymes are invisible, they cannot be tasted in malt, and they don’t have an obvious effect on finished beer flavor. Since enzyme concentration is directly related to reaction rate, more enzymes means less time required for mashing. But look at contemporary brewing recipes and begin looking back in time. You will probably discover that mash times and temperatures cited in recipes have not changed over the last 30 years. During this same 30-year period malts most certainly have changed.

One idea is to figure out how to give the modern mash a chill pill. Shortening the mash is one way to harness these very “hot” (highly enzymatic) modern malts. A related idea, and one that I did not pursue with my answer to Dave Allen’s BIAB question, is to use mash thickness to cool off these hot malts. Since enzymes are more prone to thermal denaturation when not latched onto their substrates, thinning the mash out makes mash enzymes more sensitive to temperature. Diluting the enzyme package with unmalted grains (adjuncts) is another possibility. Brewers can also source malts that are lower in enzymes, but this is not so easy these days.

The use of special malts is another way to exert control over finished beer gravity. Crystal and roasted malts are changed in the process such that malt enzymes are not able to convert much, if any, of the extract from these malts into fermentable sugars. So using more or less of these ingredients is yet another tool in the brewer’s tool box that can be used to influence beer.

Yeast selection can also be used to influence FG. Some strains are able to ferment a wider array of carbohydrates than others, specifically some strains can ferment maltotriose (a polysaccharide containing three glucose monomers) while others cannot. And some yeast strains tend to exit the scene early and leave residual extract in the beer. I am not a big fan of the latter method of FG “control” because it is not exactly precise, and leaves the beer prone to uncontrolled re-fermentation in the bottle. Then there are the yeasts that secrete enzymes, so-called diastatic yeast strains that will finish off what was left unconverted in the mash. Brettanomyces and some saison yeasts are known to be diastatic; these critters need to be used with care because they will continue fermenting high FG beers in the bottle.

The take home message here is that the world of brewing is constantly changing. Bumping present materials and methods against older references is a useful and head-scratching exercise. Many a brewer has solved a brewing problem by noting the differences in the beer-time continuum. Brew long and prosper!

Response by Ashton Lewis.