Microstructural characterization of deep-fat fried formulated products using confocal scanning laser microscopy and a non-invasive double staining procedure

• Microstructure of fried gluten or potato-flakes based products was characterized.
• A non-invasive double staining procedure allowed confocal microscopy observation.
• Oil and pore structure were characterized qualitatively and quantitatively.
• A direct relationship of porosity and oil uptake was found in gluten-based products.
• Gluten-based products were less porous but retained, in general, more oil.

The aim of this work was to characterize the inner microstructure of different fried formulated products to understand its relationship to oil absorption. Two product categories, which were either based on potato-flakes or wheat gluten, were analyzed. A non-invasive double staining procedure to perform a two channel observation using confocal scanning laser microscopy was set-up. The dough was directly stained during preparation with fluorescein-5-isothiocyanate (FITC), laminated and fried in oil stained with Nile Red. Thereafter, samples were observed without further intrusion. Pore size, porosity and oil were quantified using image analysis. A direct relationship between porosity and oil absorption was found in gluten-based products, but, no relationship was detected in potato-flakes-based ones. Further, gluten-based products were less porous but retained, in general, more oil. Parameters such as pore tortuosity and/or connectivity, as well as oil wettability, might explain such differences. In addition, the developed microscopy technique developed herein may be used in other food systems.