Does Beer Need to Ferment in the Dark?

It has long been suspected that the quality of beer was affected by exposure to sunlight when kept outdoors, but it was only in 2001 that the negative impact of light on beer was scientifically proven.

Beer quality is affected by exposure to ultraviolet rays or fluorescent light, through the formation of chemical compounds that generate undesirable flavors and aromas in beer, which are both permanent and irreversible.

How does light affect beer quality?

Both ultraviolet light and artificial light (fluorescent) can generate undesirable flavors in beer, called off-flavors, which are often caused by the interaction of light with the hop components, and to a lesser extent by the interaction with yeast. Avoiding contact of beer with light is essential to achieving a quality product.

Let’s take a closer look at how light actually affects beer, if it affects its taste and aroma, and how this happens:

How does it affect Taste?

Ultraviolet or fluorescent light (but not LED light) has the required wavelength (350-500 nm range) to penetrate and react with hop components immersed in the wort, generating by-products that affect the flavor and aroma of the final beer.

The main hop components affected by light are called isohumulones, which impart the characteristic bitterness and aroma to the final beer.

When these components interact with ultraviolet light or fluorescent light, a chemical reaction takes place that converts the isohumulones into thiol compounds, including 3-methyl-2-butene-1-thiol (MBT). This is the one we are most interested in, as it imparts a characteristic “skunky” odor that is usually found in beers stored in translucent or green glass containers.

How long does it take until it affects the taste?

If the conditions are given, i.e., if the bottle or container allows the hops to interact with light, then the reaction is practically instantaneous and irreversible.

The degradation depends on the intensity of the light and the proximity of the light source to the beer. The detection threshold for MBT or these undesirable compounds is one part per million, a relatively low threshold that allows us to identify them even in small doses.

Even though we might talk about this interaction of beer with light producing undesirable effects, not everyone seems to give it the same level of importance since many well-known companies have chosen to still bottle and market their beer in crystal clear or green bottles that do little-to-nothing as far as protecting the beer from light goes.

The off flavor is present in beer, but the brand’s reputation is so strong that many allude this characteristic flavor to the product itself, without distinguishing the defects in quality.

Paying attention to how beer is fermented and stored in order to prevent the side effects of exposure to light mainly depends on how much homebrewers want to keep it from happening. Those who focus more on craft or artisanal production tend to be more detailed-oriented than some of the big brands and generally focus more on creating a quality product.

Effects of light during fermentation

The undesirable effects caused by interaction with ultraviolet or fluorescent light in beer not only occur in the storage or transport stages of beer but can also occur much earlier in the process.

One of the most significant stages is fermentation, in which light can react with hop components as well as with the yeast to create all those undesired flavors.

Effects on Yeast

Yeast life can be affected once it’s exposed to ultraviolet light, but it depends on the intensity and time of exposure. Yeast, which are single-celled fungi, can have their DNA irreversibly or permanently affected when irradiated with ultraviolet light.

Thus, when the DNA is modified, the functionality of the cell is likely to change, generating uncertain and difficult to predict results in the fermentation process.

Effects on Hops

During boiling, a natural chemical transformation occurs to the hops. When exposed to a high temperature (above 90° Celsius), the alpha acids (humulones) present in the wort are converted into isohumulones, which is what gives the beer its particular bitter character and hoppy flavor.

It is only when the alpha acids are converted to isohumulones that the chemical transformation responsible for generating those undesired off-flavors in beer can take place, which is the so-called “chemical photodegradation of beer”.

This photodegradation involves a chemical transformation between isohumulone and ultraviolet light, giving rise to thiol compounds, such as MBT. Thus, it is of special importance to protect beer from the boil onwards, mainly in the fermentation and storage stages where there is a higher content of isohumulones and where it’s more likely that beer can get exposed to light.

Depends on the type of fermenter

It is advisable that the vessel where fermentation takes place (fermenter) is covered with a material that prevents light from passing through the walls and coming into contact with the wort. Stainless steel fermenters are ideal in this respect.

Amber-colored fermenters prevent 85% of ultraviolet rays from entering, which is why they are usually recommended. Plastic- and translucent glass fermenters allow light to get in as well, which is not very advisable.

However, this can be remedied by covering the fermenter with a blanket or lining it with dark-colored plastic that prevents light from entering and getting in.

The typical plastic fermentation buckets, which are generally white, don’t really suffer too much from this issue, which makes them a great fermenter to get started with since not only does it protect your beer, but it’s also extremely affordable.

Where to store both the fermenter and the beer bottles?

The most important thing to avoid photochemical degradation of the beer is that light does not come into contact with the beer once it is in the fermenter or in later stages of storage, such as once it’s in the bottle.

Here’s what I recommend doing: either we use a fermenter made of a material that prevents contact with light, such as stainless steel or amber-colored plastics, or if this is not possible, we cover it with a blanket or store the fermenter in a dark place in our home, such as a large closet.

The same goes for bottles, you should always store them in a closet, refrigerator, in a box, etc., and preferably using amber colored ones and avoid those that are translucent or green since they simply let UV light through

Other Alternatives

I briefly touched on this a moment ago, but the simplest way of all is to use the fermenter that you want and simply cover it with a blanket, a shirt, plastic bag, etc.

If you’re trying to keep UV light from affecting already bottled beer, then the simplest way is to put them in a closet, pantry, etc.

Types of Beer Most Affected by Light

The higher the hop content in the beer, the greater the impact of light on the final product, so IPA beers, APA beers, Neipas, etc., are the ones that will be most affected and which should be taken care of the most.

In turn, beers with lower hop content will be the least affected, and similar to how bottle color affects how much light can reach the beer, the color of the beer is also important. Blond beers are more susceptible to UV light than dark beers since they allow light to penetrate into the interior of the container more easily, whereas dark beers only allow penetration on the surface.

On a side note, craft beers generally have a higher hop content than commercial ones and do not have any added antioxidants or other additives that protect them from light, so they are much more susceptible to deterioration.


Beers are irreversibly deteriorated by UV or fluorescent light, and it’s even worse for craft beers since they don’t have some of the added antioxidants that commercial breweries use to keep the beer fresh for longer. For this reason, it is very important that the container where the beer is fermented and stored is protected from sunlight and is also made from a material that keeps the beer as isolated from light as possible.

Scroll to Top