Implications of soluble tannin-protein complexes for tannin analysis and plant defense mechanisms

Factors which establish whether tannin and protein interact to form soluble complexes or precipitates were identified. The ratio of tannin to protein in the reaction mixture influenced solubility of the tannin-protein complexes. At protein-to-tannin ratios larger than the optimum ratio, or equivalence point, soluble tannin-protein complexes apparently formed instead of insoluble complexes. Several other factors influenced the amount of protein precipitated by tannin-containing plant extracts, including the length of the reaction time and the conditions of the tannin extraction. The analytical and ecological significances of soluble complexes were considered. A titration method which allows simultaneous determination of the equivalence point and assessment of the protein-precipitating capacity of any plant extract was developed. It was postulated that in vivo, tannin and protein may not only form insoluble complexes with antinutritional effects, but may also form soluble complexes which have unknown metabolic effects.