Effect of drying method and length of storage on tannin and total phenol concentrations in Pigeon pea seeds

Total phenols and condensed tannins in Pigeon pea seeds were determined by flow injection spectrophotometry. Folin-Denis and vanillin assays were used for total phenol and condensed tannin, respectively. Three Pigeon pea accessions were cultivated in a greenhouse. After harvesting, one access was separated into two aliquots. One aliquot was freeze-dried (−196 °C), and the other was dried in a forced air oven at 50 °C, like the other access samples, to evaluate drying temperature interference. The seeds were initially analyzed and then stored at around −10 °C, 16 °C or at ambient temperature (25 ± 10 °C). The stored samples were analysed every 30 days, for a 90 day period. A significant difference (P<0.05) occurred between the drying methods for condensed tannin as well as for total phenols, regardless of the storage conditions. The experiments for comparing total phenols, obtained at different storage times, showed significant differences (P<0.05) for total phenols, indicating that tannin quantification has to be done immediately after harvesting to avoid formation of protein complexes or polymerisation, which lead to erroneous results. In accordance with results, freeze-drying proved to be best for preserving the characteristics of the sample. In order to compare the tannin and phenol contents within accessions, fresh seeds should be used.