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Attenuation and degree of fermentation - where dry or sweet beer comes from

Why is one beer dry, lean and refreshing, and another sweet, thick and full-bodied, even though both were made from similar ingredients? The answer hides in one concept: attenuation, that is the degree of fermentation. It is a measure of how much of the sugar from the wort the yeast actually turned into alcohol and carbon dioxide. The more they ate, the drier, stronger and lighter in body the beer; the less, the sweeter and fuller. Attenuation thus decides the dryness, body, strength and character of a beer all at once. It is one of the most important concepts for understanding why beers differ so much. Here is a guide to attenuation: what it is, how apparent differs from real and how it shapes flavour.

What attenuation is

Attenuation is the degree to which the yeast has processed the sugars present in the wort. Wort, the sweet liquid before fermentation, contains sugars extracted from the malt. The yeast eats some of them, turning them into alcohol and carbon dioxide, and leaves the rest. Attenuation measures precisely this loss: how large a part of the available sugars was fermented. The higher the attenuation, the more sugar disappeared, and the drier and stronger the beer. The lower it is, the more sugar remained, giving a sweeter, fuller beer. This concept ties together four things: dryness, body, alcohol content and character. Understanding that attenuation is simply a measure of eaten sugars is the starting point for the rest. It is measured by comparing the density of the wort before and after fermentation. We cover the work of yeast more in beer yeast.

How it is measured: density

Attenuation is measured through the density of the liquid, that is how much dissolved sugar is in it. A hydrometer or refractometer serves for this. The brewer records the original gravity of the wort (before fermentation), called OG, and the final gravity of the finished beer (FG). Sugars are heavier than water, so the more of them remained, the higher the density. As the yeast eats the sugars, the density drops. The difference between OG and FG therefore says how much sugar disappeared - that is what the attenuation was. It is the basic measurement in every brewery, because it lets you track the course of fermentation and predict the strength and character of the beer. From these two numbers the degree of fermentation and the alcohol content are calculated. Density is the window through which the brewer watches what the yeast is doing. We cover the numbers on the label more in beer numbers.

Apparent versus real

Here an important distinction appears: apparent and real attenuation. Apparent is simply the percentage drop in density from OG to FG, calculated directly from the hydrometer reading. It is called apparent because it is distorted by alcohol: ethanol is lighter than water, so it pulls the density reading down, making the hydrometer show less sugar than actually remained. Real attenuation corrects this error, taking into account the density-lowering effect of alcohol, and says how much sugar the yeast really ate. Roughly, the real is about 0.8 of the apparent - if a beer has 75 percent apparent attenuation, the real is about 61 percent. In practice brewers usually quote apparent attenuation, because it is easy to measure. But it is worth knowing that the yeast really ate less than the simple reading suggests. It is a nuance, but an important one for understanding the numbers.

A table: apparent versus real

Let us gather the difference in one place:

The table shows the heart of it: apparent is easier to measure but overstated by the alcohol; real is more accurate but needs a correction. Both describe the same phenomenon from two sides.

What decides attenuation

Attenuation is not fixed - it depends on several factors. The most important is the yeast strain: different strains have a different ability to process sugars, that is a different attenuativeness. Typical ale and lager yeasts usually give from 65 to 80 percent apparent attenuation. The second factor is the fermentability of the wort, that is how much of its sugar can be eaten at all - this depends on the temperature of mashing the malt. A lower mash temperature gives more simple sugars, easy to eat, so a higher attenuation and a drier beer; a higher one leaves more complex sugars, giving a lower attenuation and a fuller beer. The pitching rate and the conditions of fermentation also have an effect. So the brewer steers attenuation deliberately, choosing the yeast and the way of mashing for the style. It is a tool for designing the character of a beer.

Attenuation, body and strength

Attenuation directly shapes two key features: the body and the strength of a beer. The higher the attenuation, the more sugar was turned into alcohol, so the beer is stronger, but also lighter in body and drier - less residual sugar is left to give sweetness and fullness. The lower the attenuation, the more sugar remains, so the beer is sweeter, fuller and more full-bodied, but usually a little weaker. This is why high-attenuation yeasts give a drier and lighter beer, and low-attenuation ones a sweeter one with more body. The brewer balances these features when designing a style: a refreshing, dry lager needs high attenuation, and a thick, sweetish dessert beer a lower one. Understanding this link explains why strength and body do not always go hand in hand. We cover the balance of flavour more in the flavour axis of beer.

Dry versus sweet styles

Attenuation explains the difference between dry and sweet beers. Dry, refreshing beers - like many light, summer beers or dry lagers - have to ferment more strongly than average, to be as dry and drinkable as possible. High attenuation removes the residual sugar, giving a clean, dry drinkability. On the other hand strong, sweet beers, like some imperial stouts or Belgian beers, deliberately leave more sugar for fullness and sweetness. Importantly: in strong beers proper attenuation is key, because under-attenuated beers can be cloyingly sweet and heavy. This is why the brewer must match attenuation to the style - too low spoils a dry beer, too high strips a sweet one of character. It is one of the key dials of designing a beer. Dryness or sweetness is largely a decision about attenuation.

How to sense it in the mug

Attenuation is easy to sense once you know what to look for. A high-attenuation beer is dry, lean, light in body and refreshing, often with a clean, short finish - it leaves the palate dry, not coated with sugar. A low-attenuation beer is sweeter, fuller, more full-bodied and soothing, with a longer, sweetish aftertaste. If a beer seems thick and sweet, it probably has a lower attenuation; if dry and light - a higher one. It is worth comparing a dry lager and a sweetish, dark beer, to feel how attenuation changes the impression. Notice the finish: dry and clean betrays high attenuation, sticky and sweet a low one. Over time you will start to read the degree of fermentation directly from the impression of dryness and body. It is one of the most practical concepts for a taster.

The essentials in brief

Let us gather it up. Attenuation is the degree of fermentation - a measure of how much of the sugar from the wort the yeast ate. It is measured through density: the difference between the original gravity (OG) and the final gravity (FG) shows the loss of sugar. Apparent attenuation is the simple density drop, overstated by the light alcohol; real corrects this error and is about 0.8 of the apparent. It depends mainly on the yeast strain (typically 65-80 percent) and the mash temperature. High attenuation gives a dry, light and stronger beer, low one a sweeter, fuller beer with more body. This is why dry lagers have to ferment strongly, and sweet beers leave sugar. Now you know where dry or sweet beer comes from and why attenuation is the key to its character.

Note every beer in GustoNote - including the dryness and body you sense. Over time you will start to recognise the degree of fermentation by flavour and better understand why beers differ so much.