The present invention relates to a method for improved demineralization of fermentation broths, including the steps of providing a solution comprising one or more oligosaccharides, carrying out a first membrane filtration and preferably being a microfiltration or ultrafiltration, a second membrane filtration of the permeate of the first membrane filtration and a first nanofiltration step; with a sub-step of concentration and/or a sub-step of diafiltration. Moreover, the present invention is directed to an improved purification of fine chemicals from a fermentation broth.