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 samples, based on its inhibitory effect on the activity of polyphenol oxidase. As enzymatic source of polyphenol oxidase (PPO; tyrosinase, cathecol oxidase; E.C. 1.14.18.1) a crude extract of sweet potato root (Ipomoea batatas (L.) Lam.) was used directly in the carrier. This enzyme catalyses the oxidation of cathecol to o-quinone which can couple to each other producing melanin-like pigments with a strong absorption at 410 nm. The enzyme activity did not vary for at least 5 months when stored at 4°C and decreased by only 4–5% during an 8 h working period at 25°C. Sucrose, glucose, fructose, aspartame, sodium cyclamate, saccharin, benzoic acid, boric acid, citric acid, tartaric acid, fumaric acid and acetic acid do not interfere when present in amounts similar to those observed in commercial products. Ascorbic acid causes a strong positive interference, which was eliminated by passing the sample solutions through a glass column packed with cucumber (Cucumis sativus L.) pieces, which presents ascorbate oxidase. Recovery of sulfite from three samples ranged from 97.6% to 104.8% and there was a direct relationship between sulfite concentration and absorbance decrease within the 4.0×10−5 to 60×10−5 mol l−1 sulfite concentration range. Detection limit was 2.2×10−6 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).