Sunlight

Kind of a two part question, curiosity and practicality.

One - why does sunlight skunk your beer? Why won't other kinds of light do it to the beer? This is basically out of curiosity, I just don't understand it.

Two - Does anyone have any experience with skunking their beer by accident? How much exposure does it take? I ask because I used to keep my beer in a completely dark basement, but I'm realizing the basement tends to get too cold. I moved my equipment to a main floor utility room and wrapped it in a hoodie. I don't think its getting much sunlight, but there is a little more in the room than there was in the basement.

From my understanding, all light skunks your beer. Sunlight is particularly bad because there are a lot of UV rays. It reacts with the hops to make your beer taste like crap.

Chemists at the University of North Carolina and Ghent University in Belgium found that when exposed to light, the alpha acids in hops break down into free radicals that then react with sulfur-containing proteins to make a chemical called 3-methyl-2-butene-1-thiol, which is virtually identical to the principal constituent of skunk juice. Any time you see "thiol" in the name of a chemical, you can bet it's going to stink. Humans can detect this particular thiol at concentrations as low as one-billionth of a gram per 12-ounce bottle of beer.

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I think if your beer is wrapped in a hoodie you're safe enough

The question I had was about the UV rays and that whole business. My girlfriend and I have a green iguana and he can't obtain the rays through glass. Wouldn't that be the same with beer? I just bottled an oatmeal stout that I left uncloaked and didn't have a hint of skunk. Any thoughts to this?

hops skunking has to do with the interraction of isomerized alpha acids,and sunlight in fermented beer.

I put a LOT of detailed info on skunking in this thread.

wuilliez said:

The question I had was about the UV rays and that whole business. My girlfriend and I have a green iguana and he can't obtain the rays through glass. Wouldn't that be the same with beer? I just bottled an oatmeal stout that I left uncloaked and didn't have a hint of skunk. Any thoughts to this?

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I would suspect your iguana's enclosure is treated with a UV-blocking material. Most glass, unless treated, is transparent to UV.

Bryan

Sorry, should've specified he's in a wire mesh cage directly next to the window. Only reason I brought that up was the vet said uv rays can't be contracted through windows. Tried tying that into beer to maybe relieve some phobias for people like me who don't have an enclosed area that supplies an adequate constant temp for fermentation.

Thanks Rev, there is more than enough information in that thread.

Warthaug said:

Most glass, unless treated, is transparent to UV.

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Not so. Most glass blocks UV. That's why we have to use expensive, quartz cuvets when using a UV spectrophotometer (to measure the bitterness of beer, for example) or why a photographer can block UV from his camera with a simple skylight (1A) filter.

But the energies of UV are not required for the skunking of beer. Photons in the middle of the visible range have sufficient energy. Thus a brown bottle is effective at preventing skunking in many cases but insufficient to prevent direct sunlight from doing the job or to keep specialty beer that sits in bottles under fluorescent light for weeks fresh. A great demo for a class is to just put a bottle of beer in the sun for an hour or 2.

ajdelange said:

Not so. Most glass blocks UV. That's why we have to use expensive, quartz cuvets when using a UV spectrophotometer (to measure the bitterness of beer, for example) or why a photographer can block UV from his camera with a simple skylight (1A) filter.

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It is so - the reason you use a quartz cuvet in a spectrophotometer is two-fold; firstly, it is transparent to far-UV (less than 250nm) wavelengths (normal glass begins to absorb UV around 300, and adsorption starts getting pretty bad around 250); secondly, quartz also works in the IR which is important for some applications. Quartz is often used in biology labs, since measuring DNA and RNA concentrations requires light in the 230nm-range.

A 1cm thickness of conventional glass, like that used in windows or bottles, will absorb about 50% of UV below 300nm, but is nearly completely transparent above 300nm (visible light ranges from ~380-700nm). Alpha acids peak absorption is around 350nm, so the protection provided by conventional glass is nearly zero. Most modern windows are coated with a UV-blocking material, or are doped with a UV-blocking mineral.

1A filters are specifically designed to block UV; they are not simply glass, but instead are glass sputter-coated with a UV-blocking material.

ajdelange said:

But the energies of UV are not required for the skunking of beer. Photons in the middle of the visible range have sufficient energy.

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True, but your rate of skunking will be far faster with UV, as absorbance above 400nm is minimal.

Bryan

Warthaug said:

It is so - ...

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I suppose you could argue in terms of where UV starts and how much attenuation is required before you consider it attenuated so I think its best to just look at some data. http://www.pbase.com/image/143089605 shows the absorption of a glass microscope slide, a glass cuvet and and a quartz one (plus the absorption of the extract from a beer that is made for bitterness determination). Kind of looks as if glass absorbs UV to me.

Warthaug said:

A 1cm thickness of conventional glass, like that used in windows or bottles, will absorb about 50% of UV below 300nm, but is nearly completely transparent above 300nm

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The microscope slide exhibited absorption of 4.0 per cm (transmission of 0.01%) at 300 nm and the cuvet about 4.4 per cm. At 380 nm, the edge of CIE visible light (at least that's where the color matching functions start) the absorption of the slide glass is 0.45/cm and of the cuvet 0.46 corresponding to transmission of 34%. The cuvet measurement is more meaningful as with the slide the absorptions in the visible are getting down towards the instrument's noise level. A cm is a lot of glass.

Warthaug said:

(visible light ranges from ~380-700nm).

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The color matching functions run from 380 to 780 nm.

Warthaug said:

Alpha acids peak absorption is around 350nm, so the protection provided by conventional glass is nearly zero.

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As the picture shows there is no peak of absorption at 350 nm for an isooctane extraction from beer. The peak associated with iso alpha acid is at 275 nm and that's the wavelength at which the bitterness assay is done (thus this beer has an IBU rating of 50*0.85 = 42.5). This, especially as shown on the plot, makes it clear why a glass cuvet will not do.

But WRT to skunking it is not a question of which bond absorbs the most energy. It is a question of how much energy is required to break the particular bond such that the mercaptan can form. Photons in the range 300 - 500 nm are capable of breaking the requisite bond which is clearly not the bond responsible for the absorption peak. Thus brown bottles give some protection but not complete as anyone who puts a bottle of Bud in the sun for an hour can verify.

ajdelange said:

I suppose you could argue in terms of where UV starts and how much attenuation is required before you consider it attenuated so I think its best to just look at some data. http://www.pbase.com/image/143089605 shows the absorption of a glass microscope slide, a glass cuvet and and a quartz one (plus the absorption of the extract from a beer that is made for bitterness determination). Kind of looks as if glass absorbs UV to me.

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I never said it didn't - I said normal glass begins to absorb UV around 300, and adsorption starts getting pretty bad around 250 - i.e. exactly what our figure shows. There is no debate over where UV starts - 400nm the accepted definition, although IMO, it should be 380 as most people still have ability to see wavelengths of 380-400.

ajdelange said:

A cm is a lot of glass.

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It is, but its also the standard path length for measuring absorbance.

ajdelange said:

As the picture shows there is no peak of absorption at 350 nm for an isooctane extraction from beer.

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However, there is an an alkaline methanol extraction, as shown on the link I provided last post. Since beer is a polar fluid, it makes sense to look at absorption in a polar fluid - although why the alkaline pH I cannot say...

I snipped the rest, but I'd point out again that I never claimed non-UV wavelenghts couldn't skunk, but rather, that the capacity of non-UV wavelengths to skunk is minimal, due to the relatively poor absorption by alpha-acids of wavelengths above the 350nm. Photons can only break bonds if they are absorbed; the absorbance profile of alpha-acids is such that absorption above 350nm will be rare.

Bryan

Warthaug said:

However, there is an an alkaline methanol extraction, as shown on the link I provided last post.

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Actually, the extraction is not into alkaline methanol. In MOA Hops 6 pre isomerized alpha and beta acids are extracted into toluene without pH adjustment. The extract is diluted with methanol to which some NaOH has been added in order to get the absorptions (3 wavelengths are measured) into the linear range of the instrument. Methanol is a polar solvent and it makes sense that one would want to ionize the acids by raising the pH in order to keep them in solution.

Warthaug said:

Since beer is a polar fluid, it makes sense to look at absorption in a polar fluid - although why the alkaline pH I cannot say...

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With MOA Beer 23A (bitterness) the extraction is into non polar isooctane and it makes sense, therefore, to want to keep the pH low in order that the acids are not ionized. Thus hydrochloric acid is added in this method

Warthaug said:

I'd point out again that I never claimed non-UV wavelenghts couldn't skunk, but rather, that the capacity of non-UV wavelengths to skunk is minimal, due to the relatively poor absorption by alpha-acids of wavelengths above the 350nm.

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And yet the texts say it is light in the 300 - 500 nm band that is responsible.

Warthaug said:

Photons can only break bonds if they are absorbed;

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But photons can be absorbed without breaking bonds* and that is probably what is happening here. The double bonds of carbonyl groups (of which there are 3 in iso humulone) are set into vibration by UV at wavelengths around 275 nm such that compounds that contain them have large extinction coefficients and I'd guess that it is that phenomenon which explains the peak absorption at 275 in an extraction of iso alpha acid. But it is not one of those bonds which is broken when skunking occurs but rather the single bond between the carbonyl carbon and the isohexenoyl side chain. Apparently photons of up to 550 nm wavelength have sufficient energy to break that bond.

Warthaug said:

... the absorbance profile of alpha-acids is such that absorption above 350nm will be rare.

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Rare, perhaps, relative to the number of more energetic photons that are absorbed by the carbonyl bonds but yet likely enough to be absorbed to cause skunking.

*Of course some of these bonds do get broken by the high energy photons. The absorption of an extract at 275 nm does decline over time if the sample is left in the spectrophotometer. That can only mean that whatever is absorbing the 275 nm photons is decreasing in concentration.

ajdelange said:

And yet the texts say it is light in the 300 - 500 nm band that is responsible.

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I never disagreed, however the effect of >400nm wavelengths would be minimal (in comparison to UV/violet). Your budweiser example was a perfect illustration of this - assuming brown glass absorbs 100% of UV (I doubt that is the case, but bear with me), it takes an hour or so in direct sun for the beer [using the term loosely ] to begin to skunk noticeably. In a clear glass/bottle, the same beer noticeably skunks in a few minutes in direct sun.

Of course, the brown also absorbs visible light, so the comparison is not perfect. A comparison of UV-filtered vs. unfiltered light would be ideal, but I've never seen one and don't care to try on my own (I use pewter/clay mugs for a reason...).

ajdelange said:

But photons can be absorbed without breaking bonds* and that is probably what is happening here.

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In terms of the absorbace charts, absolutely. Any photon that is absorbed will, by definition, excite an electron. Until it returns to ground state that electron is susceptible (compared to the ground-state) to chemical reactions. Most of the time, it will return to ground-state, but once in a while it'll undergo the reaction which skunks the beer.

However, not all excitations are made equal - an absorable lower-energy (longer-wavelength) photon will excite an electron to a lower energy state than an absorbable higher-energy photon. The stronger the excitation, the more likely a chemical reaction will occur, both because more excited electrons are more easily be pulled out of its bond, and because it takes longer for more excited electrons to return to ground state. As such, if the same bond undergoes excitation by a UV vs visible photon, the probability of it undergoing a chemical reaction (i.e. skunking) is higher in the case of an absorbed UV photon.

ajdelange said:

The double bonds of carbonyl groups (of which there are 3 in iso humulone) are set into vibration by UV at wavelengths around 275 nm such that compounds that contain them have large extinction coefficients and I'd guess that it is that phenomenon which explains the peak absorption at 275 in an extraction of iso alpha acid. But it is not one of those bonds which is broken when skunking occurs but rather the single bond between the carbonyl carbon and the isohexenoyl side chain. Apparently photons of up to 550 nm wavelength have sufficient energy to break that bond.

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Fair enough; I'm not so familiar with the spectra of alpha-acids, that I know which bonds absorb the most strongly at which wavelengths. But we do know the "skunkifiable bond" is UV sensitive, so unless its absorbance was greatly increased in visible vs. UV wavelengths (which I doubt, otherwise there would be a strong absorption peak in the >400nm range), we would still expect UV to be far more likely (on a per-photon basis) to damage that bond than visible.

Bryan

My head hurts