Re: Why does beer give a positive Benedict's test result?
Date: Thu Oct 1 13:03:43 1998
Posted By: Dick van Wassenaar, Analytical PROTEIN Biochemist, Unilever Research Laboratory
Area of science: Biochemistry
Benedict's test on Beer.
As you may know beer contains mainly water and some alcohol [produced by
the enzymatic conversion of the sugars by fermentation] but apart from
these substances a large number of other biomolecules. These molecules
[including derivatives of ethanol and higher alcohols like 2-methylbutanol]
are there because they are synthesized during the beer production process
or are being produced during storage. Staling of beer, resulting in a bad
taste is due to the deterioration of some of the components naturally
present in beer. These reactions are initiated by light and catalyzed by
e.g. riboflavin, one of these components in beer. Ultimately the beer
contains degradation products like aldehydes, ketones, [furfural, acetone,
trans-2-butenal and trans-2-nonenal] oxidized lipids, and so on.
The basic application of the Benedict test is to detect easily oxidized
compounds like reducing sugars, hydroxyketones and aldehydes. Under
appropriate conditions these compound can reduce Copper(II) ions to
Copper(I) ion-oxide which appear as a yellow or red precipitate.
The reason that beer gives a positive a positive reaction in the Benedict
test is that the beer contains these compound that react with the reagent.
These compound may be remaining sugar and the other components like aldehydes
and ketones mentioned before.
For further information on reaction schemes see: Journal of Chemical
Education, Vol. 64, November 1987, page 984-985 and
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