This work investigates the drying of yacón (Smallanthus sonchifolius), a source of fructooligosaccharides and inulin, two prebiotic compounds. Experiments were performed using an in-house device consisting of a magnetic suspension balance coupled to a drying chamber and an in-situ shadowgraphy system. This allowed the continuous acquisition of mass, sample volume and exchange surface area to be register versus time at different temperatures (50–70 °C) and relative humidities (10%–30%), which was never done with yacón. Once corrected by the actual area of the exchange surface, the drying kinetics (kg.s-1.m-2) depicts two phases, a very long constant drying rate flux and a falling drying rate flux. Even though yacón samples lost up to 91% of their initial volume, an iso-volume deformation was observed: the reduction of sample volume equals the volume of removed water. Thanks to these two experimental facts, an extended, original, van Meel model is proposed to account for this huge change of sample shape in a predictive way.