  We present a mm-scale optical fiber strain sensor to continuously measure the
food deformation below the surface during the drying process. The sensor has a
soft buffer sleeve, which enables a large measurable deformation range and
minimizes the influence on the native food deformation. The sensor performance
was investigated both theoretically and experimentally. To demonstrate the
application in foods, we embedded the sensor inside fresh banana slices during
a 4-hour-long air-drying process, where its wet basis moisture contents dropped
from around 80% to 40%. The fiber sensor measurements covered the full normal
strain of ~20% of the banana slices and revealed the shell-hardening
characteristics in both the spatial and time dependences, by the comparison
with the surface strains measured by the computer vision. Our work provides an
unprecedented strain sensor that holds a high potential to contribute to
improving both fundamental understandings and process monitoring of food drying
processes.
