Beer is a beautiful and complex drink. Several hundred different chemical compounds have been identified within a typical beer. Of these myriad compounds, those derived from hop additions during the brewing process are much loved by homebrewers, and are vital to the organoleptic qualities of beer. Compounds derived from hops play several roles in beer, but in particular they are crucial as a source of aroma (from the essential oils) and bitterness (from the hop resins).
   
Although the flavor and aroma chemistry associated with hop oils and hop polyphenols is rather complex, the chemistry associated with providing bitterness to beer is well understood. The most important compounds associated with hop-derived bitterness are the α-acids. In a pure state, the hop α-acids are weak acids that occur as pale-yellowish solids. The naturally occurring α-acids exhibit very poor solubility in water. α-acids typically comprise between 2 and 15% of the dry weight of the hop, depending upon the specific hop variety and the hop storage environment. Hops with higher α-acid content have greater bittering potential.
   
When energy is applied to these α-acid molecules during the boiling of the wort, the atoms within the α-acid molecules rearrange themselves in a process called isomerization. The α-acid molecules are isomerized to form iso-α-acids. There are three different α-acids in hops. These α-acids are molecularly similar, but differ from one another in their side-chain structure. Each iso-α-acid exists in two forms, cis and trans, which differ in the orientation of the side chains relative to the rest of the components within the molecule. The six iso-α-acids differ in the quality and intensity of their bitterness. So hops provide both flavor and aroma from their essential oils and bitterness from their iso-alpha acids. Under certain storage conditions, they can also supply another, unwanted, character to beer.

The photochemical effect of light on beer
The flavor of beer is directly related to the ingredients used to make the beer. Beer flavor is also affected by the details associated with the brewing process. Beer flavor continues to change after packaging and storage, and can be dramatically altered by exposure to heat and light. Exposure to heat will increase the oxidation rate of the compounds in beer and produce a flavor and aroma that is often described as “wet paper,” “cardboard” or “sherry-like.” Exposure to light will produce offensive flavors and aromas that are described as “catty,” “skunk-like” or “lightstruck.”
  
There are many opportunities for beer to be exposed to light during storage. Commercial beer could be exposed to light when sitting on a display shelf or inside the refrigerator at a grocery store. Beer could also be exposed to light inside your refrigerator at home. Beer may also be exposed to sunlight if allowed to sit outside — for example, at a picnic or sporting event.
   
When light (especially UV light) hits beer, it provides the energy necessary to drive a chemical reaction that cleaves an iso-α-acid into two pieces.  The smaller of the two pieces gets modified — losing a CO residue and gaining an SH+ — to turn it into 3-methylbut-2-ene-1-thiol. The “thiol” part of the name indicates that there is sulfur present. Sulfur compounds often have strong, offensive aromas —  think rotten eggs or, in this case, skunk spray. The flavor threshold of 3-methylbut-2-ene-1-thiol is so low that a concentration of even a few parts-per-billion is enough to irreversibly spoil the beer and impart the characteristic “lightstruck” flavors and odors. And for the record, 3-methylbut-2-ene-1-thiol is very similar to one of the main chemical components of skunk spray.   
   
Visible light is a form of electromagnetic radiation that has a wavelength of between 380 nm and 780 nm. Visible light with wavelengths from 380–450 nm appears purple or violet. Light with a wavelength shorter than 380 nm is called ultraviolet (UV) light. At wavelengths from 620 nm to 780 nm, visible light appears red. Light with a wavelength longer than 780 nm is called infra-red light. The detrimental impact of light on beer is greater for higher energy light. Waves of light with a shorter wavelengths — or, equivalently, a higher frequencies — carry more energy. Light with a wavelength of 380 nm is higher in energy than light with a wavelength of 500 nm, and light with a wavelength of 500 nm is higher in energy than light with a wavelength of 600 nm.

How packaging protects beer from lightstrike
Beer can be entirely protected from being “lightstruck” by storing it in opaque containers such as cans or kegs. Beer that is packaged and stored in bottles, however is susceptible to developing “lightstruck” flavors. The level of protection provided by bottles depends upon the color of the glass. Glass that allows fewer high-energy wavelengths of light to pass through is better for protecting the flavor of beer. Figure 1 (this page) shows how light that is emitted from a standard, tungsten-filament light bulb is filtered by clear, green, blue and brown glass. Figure 2 (facing page) shows the same thing for sunlight, and Figure 3 (page 64) shows the same spectra for fluorescent light bulbs.
   
Note that the emission spectra from the three sources are very different. The sun’s spectrum is shifted to higher-energy, shorter wavelengths compared to the tungsten bulb. The sun puts out a broad spectrum of wavelengths, but the spectrum of light that reaches the Earth (and the spectrum shown in Figure 2) is the result of most UV light, and many bands in the infra-red, being filtered out by our atmosphere. The frequency spectrum of an incandescent bulb is based on how a tungsten filament glows when a current passes through it. The fluorescent bulb has well defined emission line “spikes” due to the fact that fluorescent bulbs work based on the excitation of mercury vapor.
   
The data presented in Figures 1, 2 and 3 show that brown glass filters the largest amount of high energy light, while blue, green and clear glass filters much less. Note that green and blue glass is much less effective than brown glass for light in the frequencies associated with skunking (the shorter wavelengths near the left hand side of the graph). The photochemical reaction occurs very quickly; a well-hopped beer in clear glass can become noticeably offensive with just 30 seconds of exposure to sunshine.
    
Another approach at protecting beer from lightstrike
Apart from storing beer in light-proof containers, the photosensitivity of beer can be reduced by chemically altering the iso-α-acids so that the chemical precursor to the photochemical reaction responsible for producing the “lightstruck” flavor is not present within the beer.
   
Iso-α-acids can be converted to reduced iso-α-acids by hydrogenation or by reaction with sodium borohydride. Three major types of reduced iso-α-acids can be produced: dihydro, tetrahydro, and hexahydro.
   
Although the chemically reduced iso-α-acids are as photoreactive as the ordinary iso-α-acids, 3-methylbut-2-ene-1-thiol cannot be formed from these compounds subsequent to photocleavage. As a result, the lightstruck flavor developed from reduced iso-α-acids has distinctly different, and much less obnoxious, organoleptic features than the lightstruck flavor resulting from photocleavage of the ordinary iso-α-acids. Substitution of iso-α-acids by dihydro-, tetrahydro- and hexahydro-iso-α-acids allows brewing of light-stable beers, which can be bottled in clear or green glass. Additionally, the bittering properties of dihydro-, tetrahydro- and hexahydro-iso-α-acids are comparable to that of ordinary iso-α-acids as shown in Table 1 (page 64). These modified hop extracts are used in some commercial beers, but are not available to homebrewers.

Conclusions
The best way to keep beer from developing lightstruck flavors and odors is to prevent it from coming into contact with light. Packaging beer in opaque containers such as cans or kegs will guarantee that no photochemical reactions occur. If beer is to be packaged in bottles, then brown bottles are definitely the best choice. If beer must be packaged in bottles that are green or clear. keep it away from light as best as possible. Hops themselves can be exposed to light, as they are throughout growing season, because only iso-α-acids get cleaved by light.

    Chris Bible is a frequent feature story contributor to Brew Your Own and writes the “Advanced Brewing” column in every issue.