Yeast is a single-celled fungus that eats the sugar extracted from malt and turns most of it into alcohol and carbon dioxide, and nearly everything else about a beer, from the strength in the glass to the banana note in a wheat ale to the buttery taste in a bad one, comes from what it does with the small remainder. Four things decide whether a fermentation goes well: the temperature the strain is kept at, how many healthy cells went in, how much oxygen it was given at the start and none afterwards, and what the mash left for it to eat. Manage those and an ale is done in a week. Ignore them and no amount of expensive hops will make the finished beer taste like the recipe was supposed to.
What Decides A Good Fermentation
- Watch the temperature the strain is kept at, because an ale that runs warm trades fruit for solvent and the damage cannot be aged out.
- Give the yeast a healthy cell count for the size of the batch, since too few cells sits behind a great deal of stalled, sweet or harsh beer.
- Add oxygen at pitching and none afterwards, because air once fermentation has begun makes a fresh beer taste stale.
- Take a gravity reading on three days in a row rather than trusting the airlock, because bubbles stop for reasons that have nothing to do with completion.
What Changes Chemically
The core reaction is short enough to write on your hand. One molecule of a fermentable sugar such as glucose breaks down into two molecules of ethanol and two of carbon dioxide, and heat is given off at the same time. That is where the alcohol comes from, where the gas that carbonates the beer comes from, and where the warmth a fermenting bucket gives off comes from. Of the sugar a healthy yeast actually ferments, a little under half the weight leaves the beer as alcohol, about the same again goes out as carbon dioxide, and the remainder is built into new cells.
This is why brewers measure gravity rather than alcohol. A hydrometer tells you how much sugar went in and how much came out, and the difference between the two is a close estimate of the strength, which is why the arithmetic of a recipe is a subtraction rather than a lookup. It also explains a rule that surprises people the first time they brew: yeast cannot make alcohol out of a sugar it cannot eat, so the body and the strength of a beer are both decided before the yeast ever arrives.
How Long It Took Anyone to Notice Yeast Was Alive
Brewers and bakers had been working with yeast for thousands of years without knowing it was a living thing. The first sighting of the cells belongs to Antonie van Leeuwenhoek in the seventeenth century, who looked at beer through a ground lens and drew little round bodies with no idea what they were doing. The explanation came two centuries later from Louis Pasteur, who showed from the 1850s onward that alcohol fermentation is the metabolism of a living organism rather than a chemical breakdown, and that the souring of wine and beer is the work of different organisms entirely. The last piece fell into place in the 1890s, when the German chemist Eduard Buchner ground yeast into a pulp with sand and found that the liquid still fermented with no whole cells left in it, a discovery that put the process somewhere between biology and chemistry and earned him a Nobel Prize in 1907.
Two Species and the Temperatures They Want
| Ale yeast | Lager yeast | |
|---|---|---|
| Species | Saccharomyces cerevisiae | Saccharomyces pastorianus, a hybrid with a cold-tolerant parent |
| Where it works in the vessel | Rises to the top as a krausen | Works through the liquid and drops to the bottom |
| Typical range | 18 to 22 C | 8 to 14 C |
| What it throws off | Fruit esters and spicy phenolics | Very little, plus sulphur that fades |
| Time to finish | Three to seven days | One to three weeks, then weeks cold |
The difference in flavour between the two is not a matter of one being better behaved. Ale yeast works at temperatures where its metabolism is busy enough to leak side products into the beer, and those side products, mostly esters, are what people mean when they call a beer fruity. Lager yeast works cold, makes fewer of them, and lets the malt and hops stand exposed, which is why a poorly made lager has nowhere to hide and why the style is harder to brew well at home than almost any ale.
What the Wort Has to Give the Yeast
Yeast needs sugar, and it also needs nitrogen to build the proteins it uses to reproduce, which wort supplies as amino acids under the general label of free amino nitrogen. It needs a few minerals and vitamins, with zinc the one most often short in a grain bill. And it needs oxygen in the first hours, because a yeast cell that is about to divide has to make the sterols and unsaturated fatty acids that hold its cell wall together, and it cannot do that without air. This is why wort is splashed, stirred or pumped with fine bubbles at pitching and why the same vessel is sealed and airlocked from that point on. Oxygen after fermentation has begun is a fault rather than a nutrient, since it reacts with the compounds yeast has made and turns a fresh beer stale and cardboard-like within weeks.
The Mash Decides What the Yeast Can Eat
Two enzymes do most of the work of turning mashed grain starch into fermentable sugar, and they compete across a narrow band of temperature. The one that produces maltose works best at the cool end of the mash rest and falls apart as things get hotter, while the one that leaves longer, unfermentable sugar chains works best at the warm end. Mash at the low end and you hand the yeast a wort it can drain almost completely, giving a dry, higher-alcohol beer. Mash at the high end and you leave sugar behind that the yeast cannot touch, giving body and sweetness at the same gravity. Fermentation cannot undo a mash decision, which is why two brewers can pitch the same packet into worts from the same grain and end up with different beers.
Count the Cells Before You Pitch
Too few yeast cells for the volume and strength of the wort sits behind a great deal of stalled, sweet or harsh homebrew. The working rule most brewers use is roughly three-quarters of a million cells per millilitre of wort per degree Plato of original gravity for an ale, and double that for a lager, which is why one vial of liquid yeast is short for a normal batch and why a starter exists. A fresh packet of dry yeast is usually enough on its own, and a packet a year past its date is not, which is where most of the trouble starts. Underpitching forces the few cells present to reproduce hard under stress, and the byproducts of that are the solvent-like higher alcohols people call fusel, along with a sweet, unfinished taste from sugar that never got eaten. A starter, or a second pitch, costs almost nothing and removes the largest variable in the process.
The Four Phases of a Fermentation
- Lag phase runs for the first six to twenty-four hours, during which the yeast is taking up oxygen and nutrients and multiplying rather than making alcohol, and nothing visible happens in the vessel.
- The high krausen phase is the loud part, when a thick head of foam sits on top and bubbles come out of the airlock in a steady stream, and this is when most of the sugar is consumed.
- The finishing phase follows as the foam collapses and fermentation slows, and the yeast begins to clean up after itself by reabsorbing some of the compounds it threw into the beer.
- Conditioning is the last stage, warm rather than cold, and it is the one homebrewers cut short, since pulling a beer into the fridge early locks in the buttery and green-apple notes that the yeast would otherwise have removed.
What Yeast Leaves Behind
Alcohol is the overwhelming product by weight, and yet the flavour of a beer lives almost entirely in the trace leftovers. Esters give fruit, with isoamyl acetate the banana of a wheat beer and ethyl acetate the apple or solvent note of one that got too warm. Phenolics give clove and black pepper, and in the Belgian and German strains they are a feature rather than a fault. Higher alcohols are comfortable in trace amounts and harsh in quantity. Diacetyl is the slippery, buttery compound that every young beer contains and that healthy yeast reabsorbs if you let it, which is the reason for a warm rest at the end of a lager. Acetaldehyde, tasting of green apple, sits in the same category, and sulphur compounds fade in days. None of these is a sign of a broken recipe. They are all signs of a fermentation that was rushed.
Frequently Asked Questions
How do I know fermentation is actually finished?
Take a gravity reading rather than watching the airlock, because bubbles stop for reasons that have nothing to do with completion, including a leaky bucket lid or a temperature drop. Draw a sample on three consecutive days. If the number has not moved across all three, the yeast has eaten what it can reach and the beer is ready to be packaged, and if it has moved, it is not finished whatever the calendar says.
Can I reuse yeast from one batch in the next?
Yes, and most breweries do. Rinse the slurry off the bottom of a fermenter within a day or two of finishing, store it cold and sealed from air, and use it within a few weeks and within a handful of generations, because each reuse accumulates mutations and dead cell material. Go from a lighter beer to a darker one rather than the reverse, so the yeast is not carrying roast malt into a pale wort, and treat anything that smells off rather than yeasty as a reason to throw it out.
Does yeast change the flavour as much as hops do?
In several styles it changes more. Brew the same wort with a clean American ale strain, a banana-and-clove wheat strain and a peppery Belgian strain and you get three different beers before a single hop goes near them. The reason hop varieties get more attention is that their contribution is easier to describe, while the difference between two yeast strains is often a matter of degree, which is exactly the thing a drinker notices without being able to name.
What happens if the beer gets too warm?
Above roughly 25 C most ale strains start producing esters in excess and a noticeable amount of harsh higher alcohol, and the fruitiness stops reading as mango and starts reading as solvent. The damage is done while the fermentation is running and cannot be aged out completely, so the fix is preventive: keep the vessel in the low part of the strain's range during the loud phase, when the yeast is warmest from its own activity, and let it drift up only at the end.
The Yeast Decides More Than the Recipe Does
Every part of this process is cheap to control and expensive to ignore. A thermometer in the fermenter, a hydrometer read on three days instead of one, oxygen at pitching and none after, and enough cells for the size of the batch will move the quality of homebrew further than any upgrade to the equipment list. The chemistry underneath is the same one Pasteur worked out with a lens and a flask, and it has not become more forgiving since.
