TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The natural polyphenol is hydrolysable tannin comprising hydroxybenzoic acid(s) or hydroxycinnamic acid(s). The hydroxybenzoic acid and hydroxycinnamic acid are esterified on a central nucleus. The hydroxybenzoic acid is gallic acid (oligomers) and/or elagic acid (oligomers). The hydroxycinnamic acid is caffeic acid, ferulic acid, and/or synaptic acid. The central nucleus comprising unit(s) of a monomer is carbohydrate, preferably sugar and/or acid, and more preferably glucose, glycerol, and/or quinic acid. The natural polyphenol is condensed tannin chosen from flavonoid, stilbene and phloroglucinol. The condensed tannin is of formula (1 or 2).A,B'=carbon atoms connected by single or double bond;D'=H, hydroxide, or hydroxide esterified with gallic acid or ellagic acid;E=H, hydroxide, O-glucose, or another condensed tannin corresponding to (1 or 2) to produce dimeric, trimeric, tetrameric or higher condensation products;andR=H, hydroxide, O-glucose, 1-3C O-alkyl, or O-gallic-acid group.The natural polyphenol is provided for addition into the liquid fuel as such or under a formulated form. The formulated form is solution, dispersion and/or emulsion. The polyphenol is provided for addition into the liquid fuel in the form of a solution in an adequate solvent. The adequate solvent is ester, ether, polyether, alcohol, ketone, aliphatic hydrocarbon mixture, and/or aromatic hydrocarbon mixture. The adequate solvent is high-boiling ester, ester of glycerol, and/or ester of glycol, preferably fatty acid methyl ester, fatty acid ethyl ester, and/or fatty acid propyl ester. The adequate solvent is the liquid fuel itself or its analogues, or a solvent comprising fuel. The polyphenol is provided for addition into the liquid fuel in the form of a concentrated liquid solution having a polyphenol concentration of 5-60% (m/v), preferably 20-40% (m/v). The polyphenol is used in combination with another additive chosen from emulsifiers and miscibility enhancers. Preferred Composition: The concentration of polyphenol provided for addition into the liquid fuel is such that the final concentration of polyphenol after addition to the liquid fuel is 0-5% mass/volume, preferably 1-2500 ppm, more preferably 5-1000 ppm. Preferred Property: The polyphenol has purity greater than 80% or is portion of a plant extract.