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Determination of total phenols and flavonoids in tomato peel by-product

Tomato (Lycopersicum esculentum) is one of the most consumed vegetable in the world, either as a raw or as a processed product. During tomato processing a considerable amount of waste is generated. Tomato peel, a by-product of tomato processing industry, contains significant levels of dietary fiber and bioactive compounds such as phenolics. The objective of this study was to determine the total phenols and flavonoids in the pectin biopolymer extracted from tomato peel as well as in peel residue after removal of pectin. The dried tomato peel was firstly treated with aqueous acid solution (ammonium oxalate/oxalic acid) under reflux at 90 C for 24 h, and then again after 12 h, to remove the pectin. The resulting two-phase system, e.g. extract of isolated pectin and tomato peel residue after extraction, were used for further analyses. Pectin was precipitated with addition of pure ethanol, while the peels were Soxhlet extracted with two solvents (pure and 70 % ethanol, by volume), for 6 and 12 h. Total phenols in extracts were determined with Folin-Ciocalteu assay, while flavonoids were measured with aluminium chloride colorimetric assay. The results have shown that total phenols are present in both pectin extracts obtained from two different batches of dried tomato peels, while flavonoids were found only in traces. Better extraction yield of total phenols from tomato peel residue was obtained by Soxhlet extraction, using pure instead of 70 % (by volume) ethanol (10.94 mg GAE/g and 9.17 mg GAE/g, respectively) and 6 h of extraction. The best extraction yields of flavonoids (2.80 mg RE/g) were obtained by using pure ethanol as solvent after 12 h of extraction, although extraction with 70 % ethanol provided acceptable recovery. In further work the use of mixed water/ethanol solvents for the extraction of tomato peel, as environmentally acceptable systems, will be investigated. As tomato peel contains considerable amount of phenolics should be regarded as a beneficial source of bioactive compounds. Moreover, the phenolics isolated from tomato peel could be potentially used as a food supplement to improve different physical, chemical, and nutritional properties of foods.

flavonoids; pectin; Soxhlet extraction; tomato peel; total phenols

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