The Color of Glass

We were recently approached by a member of the Williamsburg community with a specific question about the color of glass bottles and the direct impact that color has on the beers contained within. After answering we realized that this is a fairly common question out in the beer world, so we wanted to share the conversation with everyone who might be wondering about the same. 

Question:  

"What is the rationale behind [the] commentary about avoiding beer in bottles that are not brown. I suspect it has something to do with light affecting the taste of the beer, which would explain clear bottles, but I would have thought that green bottles would be similar to brown bottles in their opacity (or perhaps even black bottles, like Guinness, I think). 
 
So do you know of any actual studies about the effects on beer by the color of the bottle that the beer comes in?" 

Answer:  

There is absolutely scientific evidence to support the claim that beer should be bottled in brown glass, instead of green or clear. Isomerized alpha acids are the bittering agent derived from hops added during the kettle boil. When light in the blue-ultraviolet spectrum hits beer, riboflavin in the beer takes in that energy and will transfer it to the iso-alpha acids. Those chemical compounds can then break down to form 3-methyl-2-butene-1-thiol (MBT), which has the overwhelming aroma of a skunk. The threshold for MBT is just 10 parts per trillion, so clearly it doesn't take much to affect beer aroma. The reaction can also be very quick, with some sensory reports stating that a draft beer (stored in a light impermeable keg) can take on light struck flavor in the time it takes to finish a pint outside on a sunny day. 

Brown glass blocks the majority of the light spectrum that causes this reaction, though not all of it. Green and clear glass simply do not block these wavelengths. Obviously cans and kegs are much better than any color of glass at protecting the beer from light struck off-flavors. The only exception to this rule is when certain types of processed hop extracts are used instead of traditional leaf or pellet hops. The beer brewed with these hop extracts will no longer be in danger of skunking due to the chemical processing of the alpha acids during the production of the extract. Beers from large brewers often use these types of light stable hop extracts to protect beers packaged in clear or green bottles. 

*Source:  Master Brewers Association of the Americas Practical Handbook for the Specialty Brewer- Volume 1- Raw Materials and Brewhouse Operations. Edited by Karl Ockert. 

Hopefully that answered our friend's question, and some of your own as well!