In this paper gravimetric resonant sensors interrogated as passive elements for monitoring relevant quantities in sealed food packages are presented. Resonant Piezo-Layers (RPL) devices are used as gravimetric sensors and contactless interrogated using a separation in time of the excitation and detection phases, exploiting the sensing of the resonator transient response. This technique is to first order independent on the interrogation distance and allows to measure physical or chemical quantities affecting the electromechanical resonant response of the sensor. A 6-MHz RPL sensor sensitized with Poly-Ethylhexyl-Acrylate (PEA) polymer has been successfully interrogated contactlessly from outside a sealed chamber to detect milk spoilage with operating distances of up to 20 mm.