What "Stuck" Really Means

Few things deflate a brew day like opening the fermenting vessel three days later to find a flat, silent beer. Before you panic, be clear about what you are looking at. Airlock activity is not evidence of anything — a well-sealed bucket can sit quiet while a steady ferment ticks along underneath, and a poor seal can bubble merrily from CO2 escaping around the rim. The only reliable measure is gravity.

Take a sample with a sanitised wine thief or a clean turkey baster, cool it to around 20 °C, and read it with your hydrometer. Then do exactly the same thing 48 to 72 hours later. If the number has not shifted at all, you have a genuinely stuck fermentation. If it has dropped a point or two, you simply have a slow one, and patience plus warmth is usually all that is needed.

It also helps to know where you expected to land. A beer that has dropped from 1.048 to 1.020 with a British ale strain is often not stuck at all — it is finished. Slightly sweet, full-bodied bitters and milds are entirely normal, and a 75% apparent attenuation is a perfectly respectable result.

Rule Out the Simple Things First

• Check the hydrometer. Float it in water at 20 °C; it should read 1.000. Remember your sample temperature matters — a warm sample reads low, and correction tables exist for a reason.
• Look for the yeast cake. A thick, compacted layer with a clear beer above it means the yeast has flocculated and dropped out of suspension before finishing the job.
• Consider the weather. A cold snap in an unheated garage or a shed in February will knock an ale yeast sideways. A stick-on thermometer on the side of the vessel tells you the truth.
• Think about your pitch. Old liquid yeast, no starter, or a single packet pitched into a 1.065 beer is a recipe for a sluggish, incomplete ferment.
• Suspect unfermentable sugars. A mash rested at 68–70 °C, a heavy hand with crystal or roasted malts, or the addition of lactose will all leave you with a higher final gravity by design.

Rousing the Yeast Gently

If the gravity truly has not moved, your first move should be the gentlest one. Leave the airlock in place, sanitise the outside of the vessel, and simply rock or swirl it for thirty seconds to lift some of that settled yeast back into suspension. Do this once or twice a day for two days and take another reading.

Two warnings here. First, do not splash. Once fermentation is underway the beer is vulnerable to oxygen, and aeration at this stage will stale it quickly. Second, resist the urge to stir with a long spoon — every implement that goes in carries an infection risk, and a vigorous stir drags in air along with it. A firm, steady swirl is all you need.

Raising the Temperature

Warmth is the single most effective remedy for a stalled ale. Nudge the temperature up by one or two degrees a day, aiming for 20–22 °C at the beer itself, not the room. Many classic British strains will happily chew through the last few points at 20 °C but grind to a halt at 14 °C.

Stop at 24 °C. Pushing beyond that risks fusel alcohols and a solventy, headache-inducing pint that no amount of conditioning will fix. In winter, a brew belt, heat mat, or simply standing the fermenter in a water bath with an aquarium heater works wonders. An old duvet wrapped around the vessel is a free and surprisingly effective option, and the airing cupboard has rescued many a stalled brew.

Pitching a Fresh Active Starter

When rousing and warming have failed — or when you are dealing with a big beer that has been stuck for more than five days — it is time to add more yeast. Build a one to two litre starter from a neutral dried strain, or from the same strain you used originally, and pitch it at high krausen when it is visibly active. Pitching at 20 °C into an equally warm beer gives it the best possible start.

Add yeast nutrient at the same time. A teaspoon of a balanced nutrient, or even a tablespoon of boiled bread yeast, gives the newcomers the trace elements and free amino nitrogen they need to finish the job. Rehydrate dried yeast properly rather than sprinkling it dry onto an already hostile beer.

A word of caution about champagne yeast. It is a persistent homebrew myth that it will rescue any stuck beer. It cannot ferment maltotriose, so it will only nibble at the simple sugars and leave the bulk untouched — and any surviving cells may carry on working in the bottle months later, producing gushers. If you use it, be aware of what you are getting.

Knowing When to Stop and Bottle

Sometimes a beer is simply done, and the kindest thing you can do is accept it. If the gravity has held steady for a week, the beer tastes balanced rather than cloying, and you are within a few points of your target, call it finished. Rack it, condition it, and enjoy a slightly fuller-bodied pint than planned.

Never bottle a stalled beer with a normal dose of priming sugar. If there are fermentable sugars left and viable yeast still present, you are building bottle bombs. If you must package, prime lightly, use a fresh bottling strain, store the bottles somewhere cool, and keep them in a lidded plastic crate. Open one after a week to check carbonation before trusting the rest. A little caution now saves a sticky afternoon clearing up the garage later.