To improve the particle size regulation produced by a coffee grinder (64) associated with an espresso coffee dispensing machine (10), a method is described comprising the steps of;acquiring, via a sensor (42), a data or an electrical signal indicative of the shape of the thread of liquid coffee (C) while it percolates from the outlet (14) of the machine,;driving an operator of the grinder to regulate the particle size thereof as a function of the detected signal or data.