Impact of potato starch structural transitions on microstructure development during deep-frying

Abstract

Starch is an important constituent in several deep-fried foods, however, the impact of starch structural transformations on microstructure development during deep-frying is not well understood. In this study, the impact of deep-frying duration (0–180 s) on the physical state of potato starch (30–60% moisture content) and its microstructure was studied using time domain proton NMR and X-ray micro-computed tomography, respectively. Starch gelatinization was shown to occur within the first frying time interval assessed (0–15 s) during which starch chains were in increased contact with water and in a rubbery state. In this state, polymers were mobile (assessed by their T2 relaxation time) and not sufficiently rigid to maintain the expanded shape attained from frying-induced water evaporation. As a consequence, permanent changes to the microstructure were minor. Few air-filled pores were formed and oil absorption was limited to the pores which existed prior to frying. With further frying, starch was shown to progressively transition to the glassy state thus becoming more rigid and capable of forming a semi-flexible network. During this transition, sample expansion occurred, where the rigidity of chains could preserve the expanded structure resulting from the violent evaporation of steam. Microstructural changes were limited once a significant portion of starch had transitioned into the glassy state and polymers were rigid enough to resist expansion. The findings of this study provide a basis for understanding how the physical states of starch influence pore development, oil absorption, and oil distribution during deep-frying.