A brewing unit includes a seat capable of a rotation movement, in which a piston is slidably received. A counter-piston co-operates with the seat and the piston to define a brewing chamber. Moreover, cam members are provided to control movements of the seat, the piston, and the counter-piston. The seat, the piston and the counter-piston can assume with respect to one another at least one position of charging of a product for preparation of the beverage, at least two distinct brewing positions which correspond two different volumes of the brewing chamber, and a position for discharging the spent product.