Résumé
Fibers are macronutrients naturally presents in fruits and vegetables and contribute to the nutritional quality of raw or processed food. During heat treatments, fibers, and particularly soluble fibers are subjected to degradations that impact the textural property of the dish but also its nutritional properties (composition and bioaccessibility of micronutrients). This study was aimed to understand the behavior of Jew's mallow soluble fibers (Corchorus olitorius) during the cooking process of a traditional Tunisian dish: Mloukhiya. A simplified recipe of Mloukhiya was made by mixing three main ingredients (dried powder of Corchorus olitorius, tomato paste and olive oil) and was submitted to a cooking treatment (150 min) with added water. Quantification of soluble fibers as well as methylation degree (ratio of methanol and uronic acids) were monitored at different cooking times along with viscosity and microstructure analyses by epifluorescence and confocal microscopy (Zeiss LSM 880). The chemical analysis showed a decrease (40%) of soluble fibers and a slightly increase of their degree of methoxylation was observed. We suggested that these polysaccharides also underwent a 0-eliminative depolymerization during cooking process. At initial time, the dish had a high viscosity and its structure was an "oil in water" emulsion stabilized by the presence of fibers. As a function of time, the viscosity substantially dropped, due to the depolymerization phenomenon. Microscopic observations by epifluorescence and confocal confirmed the destabilization of emulsion due to polysaccharides degradation with a coalescence of oil droplets (Image Jc software show a decrease of oil droplet percentage from 25 % to 15 % during the first hour of heating). These observations can be linked with a previous work showing the increase of bioaccessibility by better micellarization or better diffusion in oil of fat-soluble micronutrients during cooking1. By impacting soluble fibers chemical structure, heat processes modulates the microstructure of a complex emulsion from a vegetable-based dish and the extractability of fat-soluble micronutrients. (Résumé d'auteur)