1. A method of automatically assessing the quality of frothed milk foam in a milk-based beverage prepared by a dispensing machine, the method comprising: providing a beverage containing frothed milk within a container, the beverage having a free surface of frothed milk having a surface texture; defining a plurality of reference classes representative of a foam quality level based on a surface texture of the frothed milk on the free surface, wherein the surface texture is described by air bubbles on the free surface, said classes being characterized by a plurality of numerical parameters comprising size and number of the plurality of bubbles, and wherein each reference class is described by a respective plurality of numerical ranges of the plurality of numerical parameters, a numerical range corresponding to a respective numerical parameter of the plurality of numerical parameters for each reference class, whereby each reference class is defined by a respective plurality of numerical ranges of the plurality of numerical parameters; acquiring a digital image of the free surface of the beverage as a primary image; digitally processing the acquired primary image to obtain a processed image of the free surface of the frothed milk, wherein a plurality of bubbles representative of the surface texture is identified in the processed image, each bubble being defined by a contour line that defines its size; in the processed image, calculating a plurality of values of the corresponding plurality of numerical parameters which characterize the plurality of bubbles that have been identified in the processed image; checking a membership of each numerical parameter value that has been calculated in the processed image to the plurality of numerical ranges of said numerical parameter, the numerical ranges being associated with their respective reference classes, and according to the result of the step of checking, assigning a current reference class of the plurality of reference classes, to thereby assign a reference class of foam quality to the acquired image of the beverage containing frothed milk.