Two foods, belonging to different protein sources (dairy vs plant), namely Emmental cheese and tofu, were treated by the instant pressure drop (DIC) process inserted between two drying stages to obtain expanded and crispy snacks. This process consisted of a short heating step (t = 38-52 s) using steam under pressure (p = 0.26-0.62 MPa) and instantaneous pressure drop toward a vacuum. This pressure drop provoked auto-vaporization of the superheated liquid, which resulted in volume expansion, and cooling of the food. The expansion was visualized using a rapid camera at a frequency of 1000 images/s. The pressure and temperature histories were registered simultaneously. Three expansion ratios were evaluated: (i) r the ratio of food volumes after and before DIC process measured using sand pycnometry, (ii) r c calculated from dimensions measured by a caliper, and (iii) r(1) calculated from dimension reading from images. The shrinkage s = (r(1)/r(C)) -1, and the deformation d = (r(C)/r) -1 of the samples were then evaluated. Using response surface methodology, the optimal processing conditions for maximum expansions r = 2.5 and r(C) =4.5 were found for Emmental and tofu, respectively as p = 0.5 and 0.62 MPa, t=40 and 38s. The creation of mist, inherent to the pressure drop of steam, made the visual observation impossible at the beginning of pressure drop. The samples were already expanded when the mist disappeared. Taking the mist presence as the upper limit of expansion time, the expansion took dozens of milliseconds at a pressure drop rate of 1.25 MPa/s. The temperature in the reactor decreased at a rate of 750 degrees C/s. (C) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.