The Great Clinitest Debate

Initially, I had a series of posts here that was a long drawn-out debate about the usefulness of Clinitest (a test normally used by diabetics to measure urine sugar content) as a brewing tool. Dave Burley doggedly continues to hail the merits of Clinitest, while I (equally doggedly) continue to write rebuttals when Dave posts in favour of the Clinitest. Mind you... I haven't said that Clinitest cannot be used or is not accurate, but rather that it's accuracy across a wide range of beer types was unproven.

Since then, Steve Alexander has posted an article to the Homebrew Digest that truly does a much better job of explaining the limitations of Clinitest for determining the end of fermentation. Therefore, I've moved the long, convoluted series of posts to a secondary page and am republishing (with permission) Steve's excellent post here. For a peek at the WRONG way to debate the usefulness of Clinitest in brewing, click here.

Oh... I just found another interesting post, this one from Andy Walsh on the topic of Clinitest. In this article, Andy reports tests that he did (and paid for most of the reagents himself!) on Clinitest, measuring the reaction of the test to various carboyhydrates. To read this post, click here.

From: "Steve Alexander" <>
To: "Posting Address Only - No Requests" <>
Subject: Clinitest Utility as a Measure of Completion of Fermentation
Date: Mon, 17 Aug 1998 16:36:02 -0400
Message-ID: <01bdca1e$a641c640$>
Status: RO

Clinitest Utility as a Measure of Completion of Fermentation

Clinitest kits cost about $20US recently priced. The bottles of (refill) tablets are available for approximately $12US. They are intended to measure glucose or keto-sugars that pass into the urine of uncontrolled diabetics, but current practice in diabetes care requires use of the much more direct method of measuring blood sugar levels with analytic meters and reagent strips that are glucose specific. Therefore Clinitest may require special order at a pharmacy. Clinitest is an implementation of the 'Benedict' test for reducing sugars.

Clinitest measures the reducing end concentration of all reducing sugars by comparison with a color charts. The suggested '5 drop' test will permit a 1/4% resolution of glucose concentrations from 0 to 1% with an added point for 2% and greater. Note the Clinitest percentage readings are referenced to glucose so a 1% reading, (which I will call 1% reducing sugar glucose equivalent(RSGE) concentration) corresponds with a 1 gram glucose per 100ml of water concentration, but in the more general case of solutions containing other sugars indicates a total reducing sugar concentration of 55.6 mMol. The table below lists the available Clinitest color chart readings (RSGE) versus the equivalent Molar concentration of reducing sugars.

        reading          reducing conc.
        ----------       -----------------
        negative          0 mMol
        1/4%             14 mMol
        1/2%             28 mMol
        3/4%             42 mMol
        1%               56 mMol
        2+%             111 mMol or greater

From [1] in 1.043 SG wort the following sugar and dextrin concentrations are given, and I calculate the fermentability, reducing sugar concentration and the reducing concentration after 'extreme fermentation', meaning the removal of all fermentable sugars: Note especially that certain sugars such as sucrose are not reducing sugars, yet contribute to fermentability. From several sources I have estimated that the non-reducing dextrins represent no more than 5% of dextrins.

sugar          conc.   fermentable  reducing conc    r.c.after extreme ferment
Glucose        40.0 mM     y            40.0                   0.0
Fructose       13.9 mM     y            13.9                   0.0
Sucrose         5.8 mM     y             0.0                   0.0
Maltose       162.0 mM     y           162.0                   0.0
Isomaltose      3.5 mM     n             3.5                   3.5
maltotriose    21.0 mM     y            21.0                   0.0
other trisacc.  1.0 mM    var            1.0                  0-1.0
dextrins       10.2 mM     n           9.7-10.2             9.7-10.2

totals        257.4 mM               251.1-251.6          13.2 - 14.7

After removal of all fermentable sugars, which is not entirely realistic, the final reducing sugar concentration does correspond with a 1/4% RSGE . The wort above is medium-high fermentability due to a mash schedule with an initial rest at 63C/145F for 60 minutes. The fermentable sugar to total extract ratio is 70.0% which falls in the middle of the range of data in reference [2].

If a less fermentable wort of 65% fermentable extract (see ref[2], col 8], 68.3C/155F mash) was created, we should expect that there would be a loss in fermentable sugars of 5% extract, and a corresponding gain in non-fermentables. The loss of fermentables would decrease the reducing concentration by about 12.1 nM and the increase in nonfermentable adds to reducing sugars reducing by about 4.7mM, assuming a proportional mix of fermentable/non-fermentable sugars. So for a less fermentable wort after extreme fermentation, we should expect that the reducing sugar concentration might be roughly (add 4.7) 19 mM. In higher gravity wort typical in homebrew, the non-fermentable sugar concentrations should increase in proportion with the extract per volume (OG points).

In practice fermentations do remove that vast majority of fermentable sugars, but certainly do not remove all fermentable sugars. Here the limits and flocculation of the yeast and the fermentation conditions become important. Reference [3] give values of residual fermentable sugar in commercial unprimed beers (5 ales, 7 lagers) that correspond with reducing concentrations over a range of 0 nM to 52 mM, with a mean value of 18.6 mM. These residual fermentable sugar levels correspond to 0% to 0.94% (mean value 0.33%) RSGE. Three beers of twelve contained only trace fermentables, while five of twelve contained greater than 0.4% RSGE (22 mM).

In the tables of reference [3] there is no clear relationship between initial SG (OG) and the residual fermentable sugars in beer.


In beers made from wort with widely varying mash temperatures and so fermentability and widely varying SG values, we would expect a non-fermentable reducing sugar concentration of roughly 10 to 30 mM. Residual fermentable reducing sugar concentrations in commercial examples vary from trace to 52 mM. This forms an expectation that values for normally fermented beers should fall in a range of 10 mM to 80 mM (0.18% to 1.4% RSGE) for 'real world' beers with the greatest part of the variation due to residual fermentable sugars.

A Clinitest reading of 0.25% RSGE confirms completion of fermentation. At levels above the "2% or greater" color chart value Clinitest indicates incomplete fermentation. Intermediate readings of 0.5%, 0.75% and 1.0% RSGE available with Clinitest may be useful for assessing completion of fermentation, but only in the hands of an experienced brewer capable of estimating wort fermentability and yeast performance by other means.


[1] Proc.EBC Congress, Interlaken, 1969, pp 205, 'Dextrins in Brewing', Bent Stig Envoldsen, Tuborg Breweries Ltd.
[2] Malting and Brewing Science, 2nd ed, 1981, Chapman Hall, Pub,vol 1, pp 288-289, reporting results of Hall quoted by Harris, 1962.
[3] Malting and Brewing Science, vol2, pp 777, 'representative analysis of beers', table 22.1 and v2, pp 784, table 22.5 'sugar content of commercial beers'.

Copyright 1998 Steve Alexander
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