The label-free detection of bovine milk in goat milk through a miniaturized optical biosensor is presented. The biosensor consists of ten planar silicon nitride waveguide Broad-Band Mach-Zehnder interferometers (BB-MZIs) monolithically integrated and self-aligned with their respective silicon LEDs on the same Si chip. The BB-MZIs were transformed to biosensing transducers by functionalizing their sensing arm with bovine k-casein. Measurements were performed by continuously recording the transmission spectra of each interferometer through an external spectrometer. The amount of bovine milk in goat milk was determined through a competitive immunoassay by passing over the sensor mixtures of anti-k-casein antibodies with the calibrators or the samples. The output spectra of each BB-MZI recorded during the reaction were subjected to Discrete Fourier Transform in order to convert the observed spectral shifts to phase shifts in the wavenumber domain. The method had a detection limit of 0.04 % (v/v) bovine milk in goat milk, dynamic range 0.1-1.0 % (v/v), recoveries 93-110 %, and intra- and inter-assay coefficients of variation less than 12 and 15 %, respectively. The proposed biosensor compared well in terms of analytical performance with a competitive ELISA developed using the same monoclonal antibodies. Nevertheless, the duration of the biosensor assay was 10 min whereas the ELISA required 2 h. Thus, the fast and sensitive determinations along with the small size of the sensor make it ideal for incorporation into portable devices for assessment of goat or ewe's milk adulteration with bovine milk at the point-of-need.