Off-flavours

Beer is a fascinating, diverse and delicious beverage, but if it’s not properly stored, it can develop some not-so-delicious off-flavours.

Here we’ll learn more about the three most common ways the flavour of beer can be ruined after it leaves the brewery and what can be done to prevent this from happening. (As a note, none of these off-flavours are health risks, but they certainly are unpleasant!) 

Oxidation

Beer is best consumed fresh, because with time, all beers will develop signs of oxidation, or staling.

Signs of oxidation include:

• Hops fade first; both their bitterness and their bright, fresh aromas and flavours decrease.
• Next, the “malt shift” follows, introducing overly-sweet caramelly or honey-like flavours, along with a waxy flavour that sometimes described as lipstick-like.
• Finally, a wet paper, cardboard, or envelope-like aroma and flavour develops, a sign that beer is truly past its best.

The rate at which these changes occur will vary, depending on a beer’s ingredients, brewing process or style. Hop-forward styles, like American IPA, can fade in flavour more quickly. While styles with a full body and high alcohol content can age quite well if properly cellared. (More on this at the end of the article.)

Now let’s go into what actually causes oxidation:

This off-flavour results from beer being exposed to oxygen during the brewing or packaging process. There is always going to be some level of oxygen exposure, but brewers aim to limit it as much as possible.  

When exposed, oxygen binds chemically to various malt components. The oxygen is then carried through the brewing process in this bound form. Over time these bonds break down, freeing atomic oxygen back into the beer where it can oxidize the fatty acids and alcohols, leading to flavour changes. 

So how do we prevent the effects of oxidation?

These effects are unavoidable; with time, all beers will develop signs of staling, hence why all beers (should) have an expiration date. This is the date by which the brewer believes the beer will no longer represent the brewery-intended flavour due to the effects of oxidation.

But we can help to slow their development… by keeping beer cold.

A beer’s shelf life can easily be shortened by exposure to warm temperatures. Refrigerated storage at 3°C (38°F) is best for all beers at all times, as non-refrigerated storage accelerates beer aging.

It’s important to note that temperature changes within a reasonable range (from refrigerated storage to room temperature storage, for example) will not inherently damage beer’s flavour. But beer should never be allowed to reach temperatures in excess of 25°C (77°F), as these conditions lead to rapid flavour degradation.

(As a note: oxidation may also result from issues during draught dispense, but we will not explore them here.)

Lightstruck/ skunky

A lightstruck, or skunky, flavour is caused by beer’s exposure to light – including sunlight, fluorescent light, and most LED lights – and is most noticeable in the aroma of a beer.

(If you’re not familiar with the smell of a skunk, this aroma is also described as smelling like freshly brewed coffee, grass, or marijuana.)

What causes this lightstruck or skunky aroma?

It’s formed by a reaction between certain wavelengths of light (both blue and UV) and hop’s bittering compounds (the iso-alpha acids) and may be evident after just a couple of minutes of light exposure.

So how do we prevent it?

Cans, kegs, and bottles in closed case boxes that completely shield beer from light give maximum protection from skunking.

But bottled beers are subject to skunking:

• Brown glass blocks most of the wavelengths of light that cause skunking, and therefore offers superior protection to clear and green glass
• Green glass blocks very little of the light that causes skunking
• Clear glass offers no protection against skunking

Some brewers that use clear or green glass bottles as a marketing choice have come to accept this aroma as part of their beer’s character. Consumers are very tolerant of this flavour, but technically it’s still considered an off-flavour.

There is a way for brewers who package their beers into clear or green glass bottles to avoid the development of this off-flavour, and that is by using specially modified hop extracts that do not react with light. This is what Miller uses in its Miller Lite product, for example, so even though it’s in a clear glass bottle, you will not detect the lightstruck aroma and flavour.

Dirty draught lines

Beer lines at bars and pubs are subject to the same sanitation and infection issues as breweries.

What does beer taste like when poured through a dirty draught line?

Draught line infections can lead to buttery or sour flavours in beer:

• Pediococcus and Lactobacillus bacteria commonly infect the beer lines themselves, producing buttery flavours and a spoiled or sour milk taste.
• Acetobacter can be found in dirty beer faucets and gives beer a vinegary taste.

What causes draught line infections?

As beer runs through the line from the keg to the faucet, a thin film of protein and other compounds sticks to the inside of the tubing and can become quite difficult to remove, creating a breeding ground for spoilage bacteria, like Pediococcus and Lactobacillus.

Acetobacter growth usually begins in or on spill trays, bar tops, or used bar rags and will eventually spread to beer faucets. Serving staff submerging the faucet into a beer while pouring will increase the growth rate of these bacteria.

So how do we prevent this?

With regular line cleaning – every 2 weeks at a minimum. And it’s not just about cleaning the beer lines: the couplers, FOBs and faucets need to be cleaned regularly, too.

This will keep the beer that’s running through these systems tasting the way the brewer intended.

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When oxidation is not always an off-flavour

As mentioned above, beer styles with a full body and high alcohol content can age quite well if properly cellared. So what makes them unique?  

Over time, bound oxygen is freed and released back into the beer where it can oxidize the fatty acids and alcohols present. While the oxidation of fatty acids contributes those wet paper, cardboard, or envelope-like aroma and flavours, oxidation of alcohols can actually prove quite pleasant.

As alcohols are oxidized, a port- or sherry-like character becomes evident. Leathery or tobacco-like oxidation character can develop, as well. And the beer will dry out, becoming less sweet and more vinous, or wine-like.

In stronger beers, these positive oxidation flavours can often mask its less pleasant effects.

Additionally, many of these beers are bottle conditioned, which further protects the beer. The live yeast acts as a scavenger for oxygen – using it as a nutrient and preventing oxidation – keeping the beer tasting fresh for longer.

That said though, storage temperature is still important here. Strong beers age best just above cellar temperature, from 55–65°F (13–18°C).

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