Cornell University, Ithaca, NY, USA Protein precipitation assays These methods are more closely related to the biological effects of tannins. This method depends on the formation of complexes between tannins and bovine serum albumin embedded in agar. Plant extracts are placed in a well in the agar. They diffuse in the agar and precipitate the albumin if tannins are present. In this case a opaque circle forms.
Suitable standards are necessary to estimate the amount of tannins.
The most commonly used standard is tannic acid and the results are expressed in tannic acid equivalents. This method allows determination of large number of samples with limited laboratory facilities. This a method for condensed tannins or PAs that combines some previous methods in an attempt to eliminate their major problems and reduce the time required for the analysis. Isolation of plant condensed tannins using trivalent ytterbium to prepare the standard. Moreover, the absorption varies with plant species because of the wide variety of tannin types present in nature.