Mongoose40

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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.

foodplusbeer

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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.

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

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wuilliez

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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?

Revvy

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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.

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Warthaug

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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

wuilliez

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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.

Mongoose40

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

ajdelange

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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.

Warthaug

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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.

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

Bryan

ajdelange

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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.


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.


The color matching functions run from 380 to 780 nm.

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.