Diacetyl analysis on a UV/Vis spectrometer

[netta]

Hey,

I tried to find diacetyl measurement of a relatively simple method. I have previously measured the concentration of the GC, but the possibility does not exist anymore. Now, however, I have a UV / Vis spectrometer in use, if any of you had a good tip for me, I would be very happy.

 

[bierhaus15]

The ASBC UV/SPEC method 25D is the gold standard. If you don't have ASBC membership, some googling will turn up the method. There is also the sensory method, which is very easy to do and costs nothing if you are just looking for a yes/no.

 

[Vale71]

Bierhaus beat me to it. Here is some informational material from Hach (who also make a VDK kit for their spectrophotmeters).

https://www.hach.com/asset-get.download.jsa?id=50398969601&usg=AOvVaw3hb1VQeMxjYbSbMqzVKyfX

 

[ajdelange]

The problems with measuring VDK's by the methods described in the MOAs is that it responds to both and that it is not very sensitive at the levels you would want to measure i.e. around the threshold. If you have sarcina sickness you won't need a GC or Spec to determine that that is the case and I really don't see much value in knowing that the beer is super disgusting as opposed to super duper disgusting. I found in running this method (with a Hach Spectrophotometer) that I had to use a huge cuvet (5 cm) to get readings which were a sigma or two above the noise and even then it was pretty shaky. Again I emphasize that this was at the level of trying to determine whether my diacetyl was detectably above or below taste threshold. I gave up on the method pretty much for these reasons, because my CO2 flow meter disappeared and because the wet chemistry part of the ASBC MOAS is a big PITA.

 

[Vale71]

Hach does give 0.015 to 0.5 mg/kg as a measuring range for their current kit, which more than adequately covers the range around the detectability threshold.

 

[ajdelange]

A TNT method for VDK's. I can't believe it. Where was this when I needed it? Hours of distillation and evaporation in a water bath - no more.

For the OP who may not be familiar with Hach's TNT. It stands for Test 'N Tube. In this series of tests you are furnished with a barcoded glass tube which contains some dry reagents. You pipet in the proper amount of sample and in some cases one or two other reagents (included with the kit) and wait a prescribed time for color development to complete. Pop it into the instrument. It reads the bar code and configures the machine. The concentration of the analyte appears on the screen. Push a button and the reading gets logged to your server if you have configured one.

 

[Qhrumphf]

At work I've seen beers pass a lab VDK test (not sure what the method is but I know it involves distilling) but still fail a sensory panel (after the good old heat-hold-chill force test) near unanimously.

 

[ajdelange]

If you smell/taste and don't note diacetyl and then heat the beer any remaining acetolactate will oxidize to diacetyl and, if there is enough of it, be organoleptically detected. Thus the heat/chill/sniff test is really a test for potential diacetyl (aceto lactate) in the package. It is a perfectly sensible thing to do, of course, because that's where most diacetyl is formed and, barring sarcina sickness, what the brewer really needs to be worrying about.

If you heat the beer and then do a lab check for diacetyl a beer that flunked the panel test would doubtless flunk the lab test or at least the lab test would show higher levels of diacetyl than it did without the heating. And that sounds like a very sensible thing to do if you are worried about diacetyl formation in the package.

 

[Qhrumphf]

If you smell/taste and don't note diacetyl and then heat the beer any remaining acetolactate will oxidize to diacetyl and, if there is enough of it, be organoleptically detected. Thus the heat/chill/sniff test is really a test for potential diacetyl (aceto lactate) in the package. It is a perfectly sensible thing to do, of course, because that's where most diacetyl is formed and, barring sarcina sickness, what the brewer really needs to be worrying about.

If you heat the beer and then do a lab check for diacetyl a beer that flunked the panel test would doubtless flunk the lab test or at least the lab test would show higher levels of diacetyl than it did without the heating. And that sounds like a very sensible thing to do if you are worried about diacetyl formation in the package.

Honestly I just think the acceptable standard for allowed VDKs as measured in the lab is set too high and the pass/fail threshold in the lab needs to be lower. Either that or the test isn't being performed correctly. Apart from sensory, not my rodeo.

 

[ajdelange]

Lab tests aren't intended to tell you whether the sample has passed or failed but rather to simply inform you of the level of VDKs detected in your sample. It is up to you to decide how much diacetyl is acceptable to you and/or your customers. That amount clearly depends on style and demographic.

The concept of potential diacetyl puts a new twist on the question. I had never considered that since I am a home brewer and my "product" is never stored under conditions that promote additional conversion of aceto lactate. Were I to get back into it (tempting to try this new TNT test) I think I would test straight from the fermenter and after warming to get a fuller diacetyl picture. Kind of like when your doc has the lab do free and bound PSA.

 

[Qhrumphf]

Right. By "pass" or "fail" I mean it is respectively below or above the acceptable threshold set by the company. Which if it passes that threshold and we're still flagging it down on sensory, that threshold for the lab test needs to be lowered. That's all.

 

[ajdelange]

Gotit.