Ask The Brewmasters

This message was posted by a user wishing to remain anonymous

Hi everyone, 

We are struggling with repeatable results while running the ASBC method for TA. Our pH meter is in spec., sodium hydroxide new and affective, following method, burette in working order... etc.. Issue we're facing: We can not repeat result between different lab techs, testing the same sample. Has anyone experienced this same issue and was there an easy/direct resolution?

Thanks in advance. Cheers, y'all!

Assuming the sample temperature is consistent between the two techs? Can't remember if the ASBC method mentions a stir plate, but I'd definitely make sure that you're using one! Happy to chat more, just wanted to throw a couple questions out.

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Justin Alexander
Analytical Chemist
New Belgium Brewing Company
Wellington CO
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Hi,
 Are you degassing samples before titration? Dissolved CO2/carbonic acid will impact your titrations and could be a source of error if different operators collect/handle/degas samples differently or if the samples sit out for various amounts of time as may be the case with serial testing of a single sample by different operators. In the case of serial testing, it should be moving to less TA over time if it is a degassing issue. It's an issue I had when first getting into TA measurements. 

Cheers,
Kevin

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Kevin McCabe,PhD
Founder
Double Strand Consulting
Mosier OR
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Anonymous:

How much of a difference between analysts are you seeing?  Is it one mL of titrant or five mL?  Is it a large enough difference to worry about?  How reproducible is an individual analyst?  Ask a pair of analysts to repeat the assay a number of times and then use a t-test to determine if the difference between analysts is larger than one would expect by chance alone.

One source of variation could be the beer degassing step.  It's important to remove the carbonic acid from the beer before the titration with base is started.

How variable is the starting pH of the decarbonated beer between analysts?

Hope that helps,
Matt

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Matthew Cottrell
Heavy Seas Beer
Baltimore MD
(302) 430-3489
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This sounds interesting.
I have not encounted such a problem with wort from different malt cereal or beer with my laboratory procedure.
If your instrumentation and reagents are in good conditions. Then kindly repeat the analysis in house again. Standardization procedure; 3x sample analysis maybe in the morning. 
Repeat again in the afternoon and evening. Compare the means at different time of analysis. Repeat same procedure the next day.  Compare means within day and between day.  Use a statistical tool to know the coefficient of variation. If its less than 5% then you are okay. This should be your in house standardization procedure before carrying out TTA.

Please note there are  two equations thus
1. TTA expressed a lactic acid%
2. Total acid not expressed as latic acid%.
Which equation did both lab use ?

If the same equation was used then i will say this...

Variation among lab is possible, well what was the percentage difference between both analysis and statistical inference. Is the sample wort or  beer? This would allow one to know the cause of this problem. Was the sample kept in a sterile vial before transporting to another lab and free from possible continous fermentation via contamination?

You might trust your pH meter but calibrate it again.

Another possible error one can do during such analysis is with reading. If the analysis was done continuously without refilling the burette prior to the next analysis one might forget to carryout a substraction of previous reading from current  value
Please cross check your equation again. 
I hope this will help.

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Hellie Gonu
Kasetsart University
Bangkok
+66627053924
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I read through all of the response, and any and all of these could be your problem.    The tool you need to use is a Gage R&R, or MSA if you prefer.    Without doing this your only option will be trial and error on all of the things that it might be until you find the right one.   If you are not familiar with a Gage R&R it is a statistical analysis of an intentional study of a method or a process that will allow you to pull apart the variance contribution that is coming from operator vs. equipment.    Here is a link that will explain it more fully.

After you have done this, you will be able to focus on the right area for correction and have confidence that you will get it corrected.    You can even quantify how much improvement you have made by repeating the study at the end.

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Chris Mehr
Director of Quality
Revolution Brewing
Chicago, IL
cmehr@revbrew.com
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I have also struggled with this seemingly simple assay.  I recommend you calibrate you Sodium Hydroxide titrant with a primary standard like potassium hydrogen phthalate.  The ASBC also recommend this, under "master the method", at the bottom of their list of "tips and tricks".  But I feel they do not emphasize the importance of this enough.  Sodium hydroxide is highly hygroscopic, so if you prepare your titrant by weighing our crystals or pearls of pure reagent, some of that weight will be water.  Also, the titrant will gradually react with CO2 in the air, and lose strength.  So to know the true molarity of your titrant, you should first titrate a sample of 0.1M KHphthalate.  When I first tried this I was surprised to discover that what I thought was a carefully-prepared 0.1N NaOH solution was really only about 0.08N.

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Cullen Dwyer
QA Manager
Wachusett Brewing Co
Westminster MA
(978) 874-9965x160
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