Ascorbic acid is essential for human health due to its potent antioxidant properties; however, excessive intake can lead to adverse effects. This has created a high demand for simple and cost-effective analytical techniques to determine this analyte outside of laboratory settings for food quality control. In this study, a portable colorimetric sensor was developed for the quantitative on-site determination of ascorbic acid. The technique is based on the reduction of phosphorus heteropoly acid by ascorbic acid, resulting in the formation of a blue-colored product. The heteropoly acid was immobilized on a polymer template in the sensor's indicator zone with menthol used as an environmentally friendly extractant to enhance the device's stability and extend its shelf life. The colorimetric sensor demonstrated reliable detection with a low limit of detection of 5 mg kg−1, calculated based on a blank test using 3σ. Under optimized conditions, the linear detection range was 15–150 mg kg−1. The disposable sensors were applied to determine ascorbic acid in fruit and vegetable juices and purees for baby food, with recovery rates between 88 % and 110 %. This easy-to-produce and ready-to-use technique presents significant potential for mass on-site screening of ascorbic acid in food samples.