Intensity-modulated optical fibre strain sensor for continuous measurements of below-the-surface food deformation during drying

Measurements of food deformation during drying processes are important for food quality control but remain challenging. A mm-scale optical fibre strain sensor is presented here to address this challenge. The sensor can be embedded in soft foods to provide continuous and distributed measurements of large food deformation below the surface during drying. The sensor performance was investigated by both analytical and numerical models as well as the experimental calibration. To demonstrate its application in foods, multiple sensors were embedded inside fresh banana slices during a 4-h-long air-drying process for spatially distributed deformation measurements, during which the banana wet basis moisture contents dropped from around 80%–40%. The sensor measurements covered the full range of the banana normal strain from 0 to 20% and revealed the shell-hardening characteristics by providing both the spatial and time dependences of deformation. The faithfulness of the fibre sensor measurements was verified by measuring the banana surface deformation using computer vision. To the best of the authors’ knowledge, this is the first senor in the literature that can measure the real-time deformation below the surface of food samples during drying. The presented fibre optical strain sensor has a high potential to contribute to improving both fundamental understanding and process monitoring of food drying processes.