Journal of the Science of Food and Agriculture J Sci Food Agric 85:1647–1650 (2005) DOI: 10.1002/jsfa.2163 Estimation of the degree of polymerization of condensed tannins of some wild fruits of Zimbabwe (Uapaca kirkiana and Ziziphus mauritiana) using the modified vanillin-HCl method Maud Muchuweti, ∗ Ashwell R Ndhlala and Abisha Kasiyamhuru Biochemistry Department, University of Zimbabwe, PO Box MP 167, Mt Pleasant, Harare, Zimbabwe Abstract: Methanol and glacial acetic acid were compared for their effect in the vanillin assay for tannins. The time taken for the development of colour was followed using a spectrophotometer. By reacting vanillin and condensed tannins in the presence of acetic acid, the time for the development of the chromogenic substance was reduced approximately threefold compared with the time taken in the presence of methanol. The degree of polymerization was followed by reacting a catechin standard, the tannin of the Uapaca kirkiana and Ziziphus mauritiana, with vanillin in acetic acid medium at 30 ◦ C and measuring absorbance at intervals. The values obtained suggested between 4 and 10 monomer units of catechin per polymer of tannins for the different portions of the two fruits.  2005 Society of Chemical Industry Keywords: polymerisation; Uapaca kirkiana; Ziziphus mauritiana; chromogenic; vanillin INTRODUCTION The vanillin-HCl method has been widely used for the quantitative determination of condensed tannins in plant materials. 1 The assay is sensitive, relatively simple and is specific for flavan-3-ols, dihydrochalcones and complex condensed tannins. Flavanols, unlike the majority of natural phenolics, react with vanillin in acidic medium to yield a coloured product with absorbance maximum at 500 nm. The vanillin method for estimating tannin is most attractive in that it is specific, but the drawback is that it suffers from a reported lack of repeatability between samples, days and laboratory. 1–3 Tannins are phenolic plant secondary compounds and are widely distributed in the plant kingdom. 4–6 Methanol is the usual solvent for the extraction of tannins from many plant materials but acetic acid can also be used. By reacting tannins with vanillin in glacial acetic acid, the degree of polymerization of purified tannins can be determined. Butler et al reported that, in glacial acetic acid, the reactions of tannins and catechin with vanillin are kinetically similar. 4,7–11 Generally the concentration of phenolic compounds decreases with decrease in flesh firmness. Bashir et al 12 reported a decrease in total phenolics of white and pink guavas as the fruit ripens. The decrease in phenolics as the fruit ripens was also reported in bananas and mango. The decrease in total phenolics as fruit ripens results in a better taste than the astringent taste of unripe fruits. This decrease was associated with the increase in polymerization of condensed tannins and hydrolysis of the astringent sugars, such as arabinose ester of hexahydrodiphenic acid. 12 Goldstein and Swain, 13 also suggested that the decline in tannin content and the associated loss in astringency of fruits on ripening is due to an increase in the degree of polymerization. The tannin content reaches a maximum early in the maturation process and declines to levels that are different for different plants. 13,14 The Uapaca kirkiana fruit is a fleshy round berry, up to 40 mm across, with a tough reddish brown skin surrounding juicy yellow-brown pulp, in which several hard white-ridged seeds are embedded. The ripe fruit is edible during the period November to February. The unripe Uapaca kirkiana fruit has an astringent taste and this may probably be due to the presence of tannins in this fruit, which will depolymerise on ripening. The pulp is honey-sweet with a slight flavour or oranges. It is eaten fresh, the tough seeds and skin being discarded. 15 Zizphus mauritiana fruits are yellow, 10 mm in diameter, ripening to brown and progressing to black as they dry. They are pleasant tasting and thirst ∗ Correspondence to: Maud Muchuweti, Biochemistry Department, University of Zimbabwe, PO Box 167, Mt Pleasant, Harare, Zimbabwe E-mail: muchuweti@medic.uz.a.zw (Received 28 June 2004; revised version received 11 November 2004; accepted 20 December 2004) Published online 31 March 2005  2005 Society of Chemical Industry. J Sci Food Agric 0022–5142/2005/$30.00 1647