Microstructural characterization of vacuum-fried matrices and their influence on starch digestion

During vacuum frying, starch gelatinization may be limited due to the water boiling point depression, which reduces starch digestibility. In addition, relevant structural changes are induced, which could affect the accessibility of amylolytic enzymes to the site of action. To differentiate the effect of structural changes in the food matrix over the effect of starch gelatinization on starch digestibility, we characterized the microstructure using x-ray micro-computed tomography (micro-CT), confocal laser scanning microscopy (CLSM) and environmental scanning electron microscopy (ESEM), and we also studied the starch in vitro digestibility in two different systems: vacuum (9.9 kPa, T_{water-boiling-point} = 45 °C) and traditional atmospherically-fried dough (170 °C) with a similar degree of starch gelatinization (60 %). Vacuum-fried matrices had a much higher oil content (∼51.3 % dry basis) than their atmospheric counterparts (∼20.3 % dry basis), as confirmed through micro-CT and CLSM. The quantitative analysis of micro-CT images showed that vacuum-fried samples had less air porosity (36.6 % air-filled pores) than atmospheric fried ones (49 % air-filled pores), whereas, no differences were found with respect to total porosity (p < 0.05). Vacuum-fried samples exhibited significantly lower rapid available glucose levels (38 %) and higher unavailable glucose levels (52 %), compared to those found in atmospheric-fried samples (43 and 43 %, respectively). Since all matrices contained a similar starch gelatinization degree, this difference may be attributed to the high oil content within the matrix, linked to a homogenous arrangement of ungelatinized starch throughout the inner matrix and the presence of external cracks on the surface, resulting in a weaker structure.