Glass transition, isotherms and microstructure evaluation of thyme-enriched pumpkin snacks

The enrichment of dehydrated vegetables with essential oils to obtain innovative snacks with health promoting properties may cause microstructural changes. In order to design new functional foods, developing water sorption properties and phase/state transitions, as well as microstructure assessments (by fractal analysis and confocal microscopy) are considered relevant tools. Pumpkin slabs were convectively dehydrated at 70ºC and enriched with free and β-cyclodextrin encapsulated thyme oil. GAB and D'Arcy and Watt (DW) models were applied to simulate the sorption behavior of the samples. Glass transition temperatures (Tg) were achieved by DSC methodology. Microscopy image analysis (fractal and confocal) was performed at the center and contours of the samples by means of ImageJ software (FracLac plugin), using images obtained with a Scanning Electron Microscope (FE-SEM) SUPRA 40 (ZEISS) with a magnification range of 500 to 5000X. The GAB and DW models showed good fits to the experimental sorption data for all samples. Thermograms showed the water plasticization effects and that the Tg were in the range of -53.16 - 29.81ºC. The SEM images showed the effects of the different enrichment techniques on the snacks' microstructure, evidenced by differences in the spatial arrangement of the cell and the intercellular space. Fractal dimensions (FDs) in the control and enriched samples showed lower values in the center compared to the contour of the snacks (shrinkage effect). Samples enriched with thyme oil microcapsules presented the highest FDs. The images obtained through confocal microscopy revealed the presence of lipids in the samples exposed to both free and microencapsulated oil. Lipid aggregates exhibited a distinctive angular morphology characteristic of beta-cyclodextrin microcapsules. The impact of enrichment techniques on the microstructural and thermodynamic properties of snacks highlighted their potential applications and emphasized the assessment of food microstructure as an optimization tool for the design of new functional foods based on bioactive compounds.