FLOW-INJECTION SPECTROPHOTOMETRIC DETERMINATION OF SULFITE USING A CRUDE EXTRACT OF SWEET-POTATO ROOT (IPOMOEA-BATATAS (L.) LAM.) AS A SOURCE OF POLYPHENOL OXIDASE

A flow injection (FI) spectrophotometric procedure is proposed for the determination of sulfite in white wine, white vinegar and juice sampl es, based on its inhibitory effect on the activity of polyphenol oxida se. As enzymatic source of polyphenol oxidase (PPO; tyrosinase, cathec ol oxidase; E.C. 1.14.18.1) a crude extract of sweet potato root (Ipom oea batatas (L.) Lam.) was used directly in the carrier. This enzyme c atalyses the oxidation of cathecol to o-quinone which can couple to ea ch other producing melanin-like pigments with a strong absorption at 4 10 nm. The enzyme activity did not vary for at least 5 months when sto red at 4 degrees C and decreased by only 4-5% during an 8 h working pe riod at 25 degrees C. Sucrose, glucose, fructose, aspartame, sodium cy clamate, saccharin, benzoic acid, boric acid, citric acid, tartaric ac id, fumaric acid and acetic acid do not interfere when present in amou nts similar to those observed in commercial products. Ascorbic acid ca uses a strong positive interference, which was eliminated by passing t he sample solutions through a glass column packed with cucumber (Cucum is sativus L.) pieces, which presents ascorbate oxidase. Recovery of s ulfite from three samples ranged from 97.6% to 104.8% and there was a direct relationship between sulfite concentration and absorbance decre ase within the 4.0 x 10(-5) to 60 x 10(-5) mol l(-1) sulfite concentra tion range. Detection limit was 2.2 x 10(-6) mol l(-1). The throughput was 26 samples h(-1), and the results for five samples (n=5) were in good agreement with those obtained using an iodimetric manual method ( correlation coefficient, r=0.9991). (C) 1997 Elsevier Science B.V.