The development of optical sensors for monitoring the food freshness during the storage and transportation helps to increase the food security and the customer satisfaction by preventing the misinterpretation of food-date labelling. In this study, a simple and low cost paper-based optical sensor was fabricated for visual detection of the food spoilage by naked eye. The filter paper was coated with the electrically conductive polyaniline ink. The pH-responsiveness of the coated polyaniline nanofibers allowed for the colorimetric detection of the shrimp spoilage through the transition from the doped green emeraldine acid form state to the dedoped blue emeraldine base state. The combination of the filter paper and the ink of polyaniline nanofibers represent a facile and quick method for the fabrication of colorimetric optical sensors for food freshness monitoring applications