The invention relates to a method and a device for the early detection of the presence of micro-organisms in food, particularly in milk and derivatives thereof. One of the main advantages of the invention lies in the fact that the aforementioned detection method can be performed inside the commercial packaging thereof without the need for said packaging to be opened. The presence of micro-organism is detected, before they can produce drastic changes in the physical properties of the product, according to changes in the propagation of elastic waves (velocity, attenuation and harmonic distortion) through the product and, moreover, different types of micro-organisms can be distinguished. The inventive detection method is performed under dry conditions and requires an environment with controlled humidity and temperature.