Prerequisites for pore formation in foods

Puffing of starchy food materials during intensive heating processes is governed by a complex interplay between heat and mass transfer, phase transitions, and viscoelastic deformation of the biopolymer matrix. In this work, this pore formation is investigated using a cell model consisting of an initially small gas cavity surrounded by a deformable starch matrix. The model accounts for heat transfer, moisture transport, vapour generation, and viscoelastic relaxation, and is used to systematically explore pore growth and stabilisation under a wide range of processing conditions. Via the simulations we aim to derive the prerequisite for stable pore formation, i.e. crust formation, that can be used for later evaluation of oil-less frying or intensive drying processes. These prerequisites are expressed in terms of dimensionless numbers, related to process conditions and material properties. The simulations demonstrate for pore formation requires superheating of the matrix, which leads to gas overpressure inside the pore, as expressed by the condition the external temperature exceeds the boiling point. However, stable pore structures are obtained only if a glassy crust forms sufficiently rapidly, meaning that the final state of the matrix should fall in the glassy region. The dynamics of pore growth are further controlled by the Deborah number, relating the viscoelastic relaxation time to the characteristic time scale for moisture removal. Optimal and stable puffing occurs for Deborah numbers of order unity, whereas excessively small or large values lead to unbounded pore growth or premature arrest of expansion, respectively. By analysing the trajectories of the matrix state in a starch state diagram containing the boiling and glass transition lines, the mechanisms underlying pore growth, stabilisation, and collapse are elucidated. The identified criteria provide physically based guidelines for assessing and optimising oil-free frying and drying technologies such as air frying, microwave-assisted vacuum drying, and superheated steam drying.