Analysis of tannins in cowpeas and tea and their effect on protein and mineral metabolism in rats

Cowpeas [Vigna unguiculata (L.) Walp] of Coronet, Mississippi Silver (MSC), and White Acre (WA) cultivars were harvested at 3 maturities. MSC had the greatest amount of tannins; WA, the least amount. Testa of cowpea had the greatest amount of tannins while whole cowpea and its cotyledons had the least amount. Measurements were made on cowpeas to determine effects of cultivar and maturity on color, cotyledon:testa ratio, hydration coefficient, tannin content, and tannin polymerization. Extraction efficiency of tannin increased as amount of sample decreased. A major portion of tannins may be removed from cowpea by cooking in water and discarding the liquid.

Tannins were extracted from MSC and black tea and added to diets that were fed to rats. Diets contained 0.057, 0.171, and 0.570 mg tannins/g diet. Ingestion of tannin from MSC or tea did not affect PER, growth rate, in vivo apparent digestibility, nitrogen excretion, liver weight, or nitrogen content of liver. However, the source of tannin affected absorption of calcium, magnesium, and iron. Diets with tannins isolated from MSC caused higher concentration of plasma calcium and sodium, but not magnesium, than the control diet; tea tannin produced higher calcium concentration only. Source and level of tannins had no effect on ash and calcium contents of the rat's left femur. MSC-derived tannins increased the concentration of magnesium in the femur when compared with control diet or tea-derived tannins.