A test portion is evaporated at ambient temperature and the oil contained in the residue is extracted with carbon tetrachloride. The oil contained in the residue is determined either gravimetrically by evaporation of the organic solvent and weighing of the residue or by spectrometric measurement of the absorbance of the organic phase at a wavelength of approximately 3.42 m. The gravimetric method is applicable to products having an oil content equal to or greater than 10 mg/kg. The infra-red spectrometric method, being more sensitive, is applicable to products having an oil content greater than 1 mg/kg.