Effect of pH, Temperature, and Time on Inactivation of Tannins and Possible Implications in Detannification Studies

Abstract

The tannins from quebracho and leaves of Acioa barteri after purification, and commercially available tannic acid, were stored in buffers of different pH values (6−11) at 0, 20, and 37 °C. The recovery of tannins was monitored at different time intervals (up to 18 h) using Folin−Ciocalteu and butanol−HCl−Fe3+ reagents. The recovery of tannins decreased with increase in time of storage, increase in pH, and increase in temperature. The decrease in the recovery of tannins was accompanied by a decrease in their protein precipitation capacity, and also there was no defined peak for the assay medium following the butanol−HCl−Fe3+ method for the purified quebracho and A. barteri tannins when stored at pH 11. These observations suggested that the decrease in recovery of tannins was due to inactivation of tannins. A. barteri tannins were found to be most susceptible to pH-mediated inactivation. The stirring of quebracho tannins increased the rate of inactivation at pH 11. The results suggested that alkaline pH inactivates the tannins. The rate of inactivation is higher at higher temperatures, and it can be further increased by oxygen present in the air or possibly by any other oxidizing agent. These observations were exploited to detannify tannins in some agroindustrial byproducts by using hydrogen peroxide (a strong oxidizing agent) in the presence of sodium hydroxide. This treatment was found to be very effective; the decrease in tannins reached as high as 99% in some cases. The results of the present study also highlighted a problem related to quantification of tannins in samples treated with alkalis. During extraction of tannins from such materials, tannins were inactivated due to alkalinity in the extraction medium, thereby giving an underestimate of the actual tannin levels. The alkalinity in the extraction medium results from the residual alkali present in the treated materials. This problem was overcome by addition of hydrochloric acid in the extraction medium so that the pH during extraction did not rise above 7. The results of the present study also suggest that for studying the nutritional, physiological, and biological effects of tannins (i) the pH of the tannin-containing solution should never at any stage be alkaline, (ii) the solution should be kept at low temperature, and (iii) the exposure to oxygen or to any other oxidizing agent should be avoided. Keywords: Inactivation of tannins; pH; temperature; detannification; quantification of tannins