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Old 06-20-2012, 06:28 PM   #1
bmurph
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Default Do hop oils affect the optical properties of beer?

So I went to bottle a pale ale after 3 weeks fermentation (total) and 2 weeks dry hopping. The fermentation looked as normal as any other brew, but the final gravity reading I got from my refractometer read about 1.028 (OG: 1.054). The weird thing, though, is that the beer tastes as good as I could expect it to, not overly sweet or anything. I no longer own a hydrometer to check the gravity that way (it broke a while back), so I'm wondering if the oils from the dry hopping (1 oz Citra in a 5 gal batch) might be affecting the reading. I know I should have checked before dry hopping, but there hadn't been any visible signs of fermentation for a few days so I figured what the hell. Any thoughts?

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Old 06-20-2012, 07:21 PM   #2
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The hop oils doubtless do have optical properties different from those of sucrose dissolved in water but their having an appreciable effect on the RI of beer is doubtful. What is responsible for the high reading you are seeing is alcohol. Refractometer readings on fermented beer are pretty unreliable for this reason. Professional brewers do use refractometry to check beer alcohol content but their translation between RI and ABV is based on a calibration using one of the reliable (pycnometry, densitometry, GC) alcohol determination methods for similar beers. There are formulas which attempt to estimate ABV from refractometry without calibration. As such they could/do also estimate AG/ADF. The results will be ballpark at best but that may be all you are looking for.

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Old 06-20-2012, 07:30 PM   #3
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http://seanterrill.com/2012/01/06/re...er-calculator/
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Old 06-21-2012, 09:00 AM   #4
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Thanks, I feel a lot better about this beer now. Only, what if I did have a stuck fermentation? One of these online calculators would have me believe the beer is fine...

I bought the refractometer thinking it would make sampling the beer much simpler, but this seems to complicate things. Guess I didn't do my homework. I think this current beer is ok based on its taste, but can I rely on that or do I need to go back to the hydrometer to be safe?

Does anyone use a refractometer alone, and if so, how can you differentiate between alcohol content and sugar content?

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Old 06-21-2012, 01:02 PM   #5
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Properly used a refractometer can be of benefit in brewing. But yes, many do rely on them exclusively which is, IMO, a mistake unless you are willing to accept approximate results. This is not intended to imply that approximate results mean sloppy practice. Approximate results can be fine especially if repeatable. It really isn't necessary to know exactly what the ADF or ABV are but in the case of the former to detect the point at which it isn't decreasing any more i.e. at which fermentation is complete. But you make an interesting point. A seasoned brewer would probably understand that a refractometer reading of 6 Bx from a wort that read 12 at pitching was caused by incomplete fermentation rather than alcohol if he didn't observe a healthy rocky head, gas evolution etc. but an inexperienced one might not. Best to check with a hydrometer.

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Old 06-24-2012, 01:46 PM   #6
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Refractometers are for OG only. It works by the refraction of light by dissolved sugars. Other dissolved compounds will also refract the light. For example alcohol. Alcohol actually refracts light in the opposite direction that sugar does, which is why a Refractometer can only be used accurately for OG. Don't throw out your hydrometer when you get a Refractometer... You will need it for FG!

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Old 06-24-2012, 04:05 PM   #7
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Refractometers work best for OG but even there they can be innacurate by as much as 1°P or more even for OG. They are calibrated based on sucrose in water. There is little sucrose in wort.

Alcohol and sucrose actually pull RI in the same direction. That's why OP's reading was higher than he expected it to be.

Refractometers can be (and are) used on fermented wort but prior to doing so it is necessary to prepare a calibration curve by comparing RI readings to accurate (desitometer, pycnometer) assays of alcohol and extract.

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