The present invention relates to a method for producing refine-grade sugar syrup when harvesting agricultural sugar-containing crops. The present invention relates to the implementation of a new industrial process that comprises the following steps: carrying out a thermal-acid treatment of the sugar-containing liquor; carrying out an electro-chemical and ultra-filtration treatment of said liquor; electrolysing the liquor; processing the liquor in ion-exchange columns with a sorbent; condensing the liquor using a reverse-osmosis method; and evaporating said liquor to a concentration allowing for an extended shelf-life. The liquor is industrially and safely purified using ultra-filtration, reverse osmosis and ion-exchange membranes during the crop transformation process. The sugar-containing product obtained using the method of the present invention may be used in various industries such as in the production of confectionery, alcohol-free beer and tins, for bread baking or in the crystallisation of refined sugar.