Fermentation Management on Gluten-Free Wort
The site already covers which strain to pick and how to feed it. This page is about running the fermentation: the schedule, the numbers, and what they should look like on the way down.
The one thing that changes everything
Rule 9 on this site is that fermentation starts in the mash. That is not a slogan. An external-enzyme mash — particularly one carrying glucoamylase — produces a sugar spectrum with far less unfermentable dextrin than a barley congress-style mash leaves behind. The yeast is handed a wort it can take much further down.
Two consequences, and they pull in opposite directions:
- Attenuation runs high. The beer finishes drier and thinner than the same original gravity would give you on barley. If you designed the recipe expecting residual sweetness to arrive on its own, it will not.
- Nutrition runs short. Sorghum wort carries roughly half the free amino nitrogen of barley wort. More fermentable sugar and less nitrogen is a combination that produces stress character if you pitch and walk away.
The mash decides the first. The nutrient plan decides the second. Neither is fixable once fermentation is running.
What production actually ran
From Bard's production recipe records, on the flagship golden lager — a 10.5 °P wort finishing near 4.7% ABV:
| Stage | Setting |
|---|---|
| Primary | 20.5 °C / 69 °F, American ale yeast |
| Diacetyl rest | at 5 °P, raised to 22 °C / 72 °F |
| Crash | 1 °C / 34 °F |
Where a British ale strain was used instead, it was run at the cool bottom of its stated range rather than the middle — on a thin, dry base there is less malt character to absorb ester and fusel production, so the strain was deliberately kept quiet.
Note the diacetyl rest is triggered by gravity, not by the clock. Raising temperature at 5 °P means raising it while there is still active fermentation to scrub the diacetyl — a rest started after the yeast has finished and flocculated is a warm-conditioning step, not a diacetyl rest.
Production brewed strong and cut to package strength. The four-year QA log shows original gravity near 16.5 °P through the early stages, dropping to about 11 °P between stages — the dilution is visible in the raw data. If you are reading those records, the gravity discontinuity is process, not error.
Reading the numbers
Two measures of "how far did it go" get used interchangeably and should not be. Both appear in our records.
Apparent attenuation compares original gravity to apparent final gravity — measured with a hydrometer, in a liquid that now contains alcohol. Alcohol is less dense than water, so the reading sits lower than the true sugar content and the percentage reads high.
Real degree of fermentation (RDF) compares original extract to real extract, corrected for the alcohol. It is the honest number, and it is always lower.
Our own data shows both, and they do not describe the same batch:
| Source | Measure | Value |
|---|---|---|
| Production QA log, 2014–2017 | RDF, four years | 65–68%, against a published spec of RDF 68 ± 1 |
| Independent laboratory panel, Feb 2019 | Apparent attenuation | ~90% — apparent extract ~1.0 °P, real extract ~2.8 °P, SG 1.004, ABV 4.87% |
Those are different measures on different samples years apart, and we are not going to force them into agreement. What they agree on is direction: this wort ferments hard and finishes dry. A 1.004 finishing gravity on a beer under 5% is not a stuck fermentation to be rescued — on a glucoamylase-driven sorghum wort it is the expected outcome.
The useful takeaway is that RDF holding 65–68% across four years is the consistency claim, demonstrated rather than asserted. If your RDF wanders batch to batch on the same recipe and system, the variable is upstream — enzyme dose, mash temperature, or grist — not the yeast.
Where it actually goes wrong
Under-pitching into a low-nitrogen wort. The strain that ferments barley wort cleanly at a given pitch rate may throw stress character on sorghum wort at the same rate. Production carried a nutrient addition as standard practice, not as a rescue.
Treating a dry finish as a fault. Chasing residual sweetness with a warmer rest or a fresh pitch after the beer has finished at 1.004 does not add body back. Body on this base is a recipe and mash decision — see Grist Design and The Sugar Bible.
Running the strain warm. A thin, low-residual base gives ester and fusel character nowhere to hide. Where barley brewing tolerates the middle of a strain's range, this base rewards the bottom of it.
Ending the diacetyl rest by the calendar. Confirm it by taste or measurement, not by elapsed days.
Forgetting the fermenter is a gluten-free control point. The production log carries twenty-one rows reading "Gliadin Negative" — recorded at fermentation, filtration, and final stages, as routine. Fermentation is not where a gluten-free claim is established (that is pre-fermentation evidence), but it is where a batch record either continues the chain or breaks it.
What to record
Fermentation is where batch records earn their keep, because a mash problem often only becomes visible here. At minimum, per batch:
- Pitch rate, yeast source, generation, and the gluten-free statement on file
- Nutrient addition — amount and timing, as planned
- Temperature actually held, not the setpoint
- Gravity trajectory with dates, not just start and finish
- The gravity at which the diacetyl rest was started, and how its end was confirmed
- Final real and apparent extract, and RDF
- Anything that deviated
Limits of this page
The schedule above is one company's practice on one flagship beer with two strain families, recorded across a specific production era. It is a documented starting point, not a specification for your beer, your strain, or your system. We have no controlled comparison of fermentation temperature against sorghum wort — nobody has published one that we have found — so treat the temperatures as where a working brewery landed, and change one variable at a time from there.
Related
- Yeast Selection — which strain, and the gluten pathway in propagation media
- Yeast Nutrition — the nutrient plan this page assumes
- Hops and Yeast for Sorghum Malt — what production used and why
- The Sugar Bible — why the wort is this fermentable
- Batch Records — the record this page feeds
- Production QA Log 2014–2017 — the four years of numbers behind it
Source and Validation Notes
The temperature schedule, strain families, and the 10.5 °P / 4.7% ABV / 18 IBU flagship target come from Bard's production recipe records, already published on Hops and Yeast for Sorghum Malt. RDF 65–68% across four years, the RDF 68 ± 1 published spec, the high-gravity dilution, and the twenty-one gliadin-negative rows come from the Production QA Log 2014–2017, 155 stage-by-stage records. The February 2019 figures — apparent attenuation ~90%, apparent extract ~1.0 °P, real extract ~2.8 °P, SG 1.004, ABV 4.87% — come from an independent brewing QC laboratory panel on the finished flagship, held in the source archive.
The two attenuation figures are different measures taken on different samples years apart and are presented as such rather than reconciled. The claim that sorghum wort carries roughly half the free amino nitrogen of barley wort is carried on Yeast Nutrition and is not re-derived here. No controlled comparison of fermentation temperature on sorghum wort exists in our records or in the literature we have surveyed; the guidance to run strains at the cool end of their range is production practice and reasoning from a low-residual base, stated as such and not as a finding.