What Causes a Beer's Final Gravity to be Too High?

Discover why your final gravity is too high when brewing beer and what you can do to rectify the issue


Mat Stuckey

9/13/20234 min read

What Causes a Beer's Final Gravity to be Too High?

A beer's final gravity that is higher than expected can be caused by several factors in the brewing process. The main reasons for a high final gravity are:

  • Mash temperatures that are too low

  • Poor mash efficiency

  • Under attenuated fermentation

  • Adding too much unfermentable sugars or malt extract

  • Contamination with wild yeast or bacteria

Understanding these potential causes can help troubleshoot and prevent high final gravity issues in future brews. Let's explore each of these factors in more detail:

How Does Mash Temperature Affect Final Gravity?

The mash temperature is one of the most critical factors in determining the fermentability of the wort. Lower mash temperatures lead to higher final gravities for a few reasons:

  • Reduced enzyme activity - The enzymes that convert starches to fermentable sugars (like amylase) work best at certain temperature ranges. Too low, and they are less effective at creating fermentables.

  • More unfermentable sugars - Lower mash temperatures increase the production of sugars that cannot be fermented by brewer's yeast, like dextrins. This leaves more residual sweetness and body.

  • Starch haze - With inadequate starch conversion, the beer is more likely to have starch haze or a "homebrew twang".

As a guide, mash temperatures should be in the 64-68°C (147-154°F) range for most beer styles. For drier beers, a mash rest closer to 68°C is preferable.

How Does Mash Efficiency Impact Final Gravity?

The mash efficiency measures how much of the potential sugar content in the grains is successfully extracted into the wort. With low mash efficiency, less fermentable sugars make it into the kettle.

Some common reasons for poor mash efficiency include:

  • Insufficient water to grist ratio - Using too little water prevents optimal sugar extraction.

  • Improper ph levels - The ideal mash pH is 5.2-5.6. Values higher or lower can reduce enzyme activity.

  • Poor lautering - Improper sparging or a stuck mash impedes sugar extraction.

  • Mill gap too large - The grains need to be finely crushed to expose the starches.

  • Grain not fully mixed - Ensure even water distribution and avoid dry clumps.

Monitoring gravities throughout the mash and lautering process can identify efficiency issues. Adding rice hulls can also help create a more porous grain bed for better extraction.

How Does Fermentation Temperature Affect Final Gravity?

The fermentation temperature impacts the yeast's ability to fully attenuate and ferment all available sugars. Temperatures that are too low can lead to under attenuated beers with a high final gravity.

Some key points on fermentation temperature control:

  • Within yeast range - Ale yeasts perform best between 18-22°C. Lagers between 8-15°C.

  • Avoid large temperature swings - Fluctuating more than a few degrees stresses yeast.

  • Consider beer style - Some styles like saisons can tolerate higher temps up to 25°C.

  • Adjust for high gravities - For big beers, start cool and slowly ramp up temps as fermentation progresses.

  • Give time - Yeast often need a week or longer to fully attenuate, be patient.

With proper temp control, yeast can better chew through all the available sugars, leaving a lower final gravity.

How Do Added Sugars Impact Final Gravity?

Another potential factor in high final gravities is adding fermentable or unfermentable sugars that increase the starting and finishing gravity.

Common sugar additions like:

  • Honey

  • Maple syrup

  • Lactose

  • Dextrin malt

These can be great to boost flavour and body. But overdoing sugars without accounting for the gravity impact can lead to under attenuated, sweet beers.

When adding sugars:

  • Use restraint on amounts added

  • Substitute some base malt with simple sugars

  • Check the fermentability of speciality sugars

  • Consider the intended final gravity and attenuation

Added sugars are best incorporated during the last 5 minutes of the boil or during fermentation. This avoids caramelizing the sugars and maximizing fermentation.

How Does Contamination Cause High Final Gravity?

Wild yeasts and bacteria can often result in beers finishing sweeter than expected. Contaminants usually come from:

  • Poor sanitation of equipment

  • Exposing fermenter to open air

  • Reusing old yeast slurry with bacteria

Common beer contaminants like lactobacillus, pediococcus, and brettanomyces can continue to slowly ferment residual sugars. Their complex flavour compounds can sometimes make the beer seem sweeter due to reduced bitterness.

Preventing contamination requires:

  • Thorough cleaning and sanitizing

  • Using fresh yeast

  • Limiting oxygen exposure after fermentation

  • Proper cleaning of yeast harvesting equipment

How to Adjust and Prevent High Final Gravity

If faced with a beer that has stalled out early or finished too sweet, there are some ways to try adjusting the final gravity:

  • Repitch more yeast - Yeast nutrients can help stress yeast finish the job.

  • Raise fermentation temperature - Increase a few degrees within yeast tolerance.

  • Give it more time - Wait another week or two for the yeast to attenuate further.

  • Add amyloglucosidase enzyme - Helps break down unfermentable dextrins.

  • Dilute with water - Lowering volume can reduce gravity.

  • Blend with drier beer - Mix sweeter beer with a fully attenuated beer.

  • Referment with champagne yeast - Will further dry out the beer.

For future batches, prevent high FG by:

  • Hitting optimal mash temperature

  • Using a yeast starter for healthy fermentation

  • Controlling fermentation temperatures

  • Monitoring OG and FG to check attenuation

  • Adjusting water chemistry to enhance enzyme activity

  • Ensuring proper milling gap and mash efficiency

Proper troubleshooting and adjusting processes can help get your final gravities back in line.

Conclusion: Achieving the Target Final Gravity for Your Beer

Getting the final gravity right for your intended beer style and flavour profile is an essential part of brewing. While a high finishing gravity may occasionally be desirable for very strong or sweet beers, it is usually a sign that something needs adjustment in your process.

By understanding the various factors that can cause a high final gravity, you can isolate and address the potential issues in your brewhouse. Tightening up your temperatures, improving efficiencies, controlling fermentation, and managing sugars will help get your beers attenuating down to the expected range.

Making gravity measurements throughout the brew day is also key to staying on top of your numbers. Comparing your OG, FG, and apparent attenuation to the intended style guidelines will let you know if you've hit the target. Being prepared to troubleshoot and make adjustments will ensure you achieve the ideal final gravity for each batch.

If you want to learn more tips and techniques for dialling in your system and hitting your target numbers every brew, be sure to check out the Brewpedia blog for lots more helpful brewing advice!