When light strikes beer, the beer can become skunked.  This flavor and aroma is described as tasting like onion and smelling like the nasty stuff that shoots out of a pissed-off striped mammal.  The skunky flavor comes from the compound 3-methylbut-2-ene-1-thiol, or MBT for short.  MBT is formed by the degradation of iso-α-acids in the presence of a photosensitizer (like riboflavin) and some source of sulphur.  Simply put if you expose your beer to light, a photochemical reaction happens and produces the skunky stuff.  It doesn’t take much MBT either.  Sensory tests demonstrate the olfactory threshold for MBT is only a few parts per billion.  So how do brewers prevent MBT formation?

The simplest way to stop the reaction is keep out the light, hence brown bottles.  So why brown and not green bottles?

Green bottles vs. brown/amber bottles

Glass is made from silica (sand) and other substances.  The other substances added during the glass production determines the color of the glass.  To make a green bottle, you add metals such as iron, chromium or copper to the molten glass during production.  Brown bottles are made adding nickel, sulfur and carbon.  The other substances “block” different light wavelengths.  Green bottles are more effective at blocking certain wavelengths, while brown bottles work better for other wavelengths.

Bottles obviously do not block out all light.  If the bottle did reflect all light, the bottles would be solid white and you could not see the beer inside the bottle.  The bottle could also be another color and completely opaque to block all incoming light.  Beer makers do not use opaque bottles due to cost, although some brewers are experimenting with aluminum “bottles”.  The question here is: which wavelengths of light do you want to block from your beer?

Light with more energy accelerates the reaction which forms MBT and thus causes more damage to beer.  Lower wavelength light has more energy, so a bottle which is more effective at blocking lower wavelength would be better at protecting beer.  As you’ve probably guessed, brown glass is more effective at blocking lower wavelengths.  Brown glass is better than olive glass which is better than green glass which is better than clear glass.  Clear glass offers no protection at all.  That’s why many brewers use brown glass for their beers.

Why would you use green or clear beer bottles?

Many beers still use clear or green glass.  The knee-jerk response for an explanation from many beer lovers is “only the crappy macro swill comes in clear glass, it ships skunked”.  Well yes, but there are many good beers in clear glass too – Newcastle Brown Ale and Old Speckled Hen are notable examples.  There are many good beers in green bottles too.  So why are these beers not light struck?

Blocking light is the easiest way to prevent skunking beer, but it’s not the only method nor is it the best method.  Remember MBT forms from the degradation of iso-α-acids in the presence of a photosensitizer.  Brown bottles remove the light, but you could also remove the unstable iso-α-acids.  Many larger breweries use a hop extract which is modified so it cannot become light struck.  This process is costly, but it works and it’s more reliable because even beer in brown glasses will form MBT within a few hours if exposed to direct sunlight.  Another method patented by Busch adds 1,8-Cineole (Eucalyptol) to the beer to prevent MBT from forming.  Using these techniques, it doesn’t matter what color bottle is used for the beer.

Love in the can

The best package for beer is cans.  In the past there was some resistance to cans, again because “only the crappy macro swill comes in cans”.  Now that many craft brewers are discovering cans will keep their product safer much longer, this resistance is quickly disappearing.  Unfortunately, canning beer is still out of reach for the homebrewer.

If you’re planning on bringing your homebrew beer to the pool or beach, your best option is to use a keg.  It’s as close to a can as you can reasonably get.  If you are stuck with bottles, make sure to use a beer cozy and drink fast.