Ok, I did the UV experiment to see if the valve lock bags SM's sells
does indeed block UV "B". "B" is the specific wavelength I blocked,
since that is what my meter is most attuned to but it does pick up C and
A to a lesser extent.....just not quite as accurate
I put a source of 248 W/M2 on the meter and then read it through one of
the bags. (sorry that's all my lizard lamp was putting out tonight) I
cut the bag in half so I was only reading through one sheet of the
plastic since realistically that's what your beans will see.
It was reading from 37 to 52 W/M2 through the bag. This means that some
UV can pass through but only about 20% of what is at the source.
Whether this is due to the properties of the plastic (ie like it can't
pass through glass.. or the semi opacity of the bag killing light
passage was not known)
I therefore took a regular light meter, the one I use to measure
sunlight to check the efficiency of my solar panels. I exposed that
regular meter to the UV and it was only reading like 27 W/M2 (remember
this meter is a general light meter and NOT tuned specifically to the UV
spectrum) I put the bag over it and it read 25.
I then exposed the meter to a light source from an incandescent light of
1000 W/M2 and 'looked' at it through the bag.. it was reading in the 990's.
This means that the opacity of the bag and whatever properties was
blocking less than one percent of the light pass through.
The valve bags SM's sells DO block UV to some extent, from my far from
superior experiments, Im saying 50 to 75 percent of them ballpark
figure. With this, IF UV does contribute to off tastes, oxidation
etc etc, the bags do help offset it a bit.
For what it's worth. Hope this helps.