Effets des antioxydants naturels issus de Pituranthos scoparius et Hibiscus sabdariffa sur la stabilité oxydative d’émulsions à base de lactosérum et la cristallisation des lipides.

Abstract

Nowadays, antioxidants have a significant impact on both human health and food processing. This thesis sheds the light on the action of these antioxidants on the oxidative stability of a whey-based spread-type emulsion, but also the crystallization of fats in the fatty phase. The crude phenolic extracts of Pituranthos scoparius and Hibiscus sabdariffa and the essential oils of the aerial parts of P. scoparius were studied. These extracts were obtained via conventional extraction techniques (decoction and hydrodistillation) and non-conventional techniques (microwaves and ultrasounds). Spectrophotometric assays were performed for the assessment of the contents of total phenolics in the crude extracts studied. Various chromatographic techniques have been performed for the characterization and identification of the different classes of antioxidants. The antioxidant activity was studied by the tests of reducing power and antiradical activity. Different emulsions were formulated at both laboratory and pilot scales, incorporated by the concentrated crude phenolic extracts of the plants and the essential oil from stems of P. scoparius at different concentrations. Their oxidative stability was evaluated by Rancimat test. The two phases of the emulsion have been studied regarding two aspects: whey protein-polyphenol interactions in the aqueous phase and lipid crystallization in the fatty phase containing palm, soybean and sunflower oils. A Tricine SDS-PAGE was performed in the former and a set of investigation techniques for the study of isothermal crystallization (DSC, p-NMR and PLM) in the latter were applied. The results of spectrophotometric, chromatographic and antioxidant activity corroborate with those of oxidative stability and showed a protective effect against lipid oxidation at low concentrations of plant crude extracts (100 and 300 ppm) and the essential oil of P. scoparius (100 ppm). The sensory evaluation of the spread at 100, 300 and 600 ppm of the essential oil of P. scoparius, with a principal component analysis (PCA), revealed the influence of some sensory attributes on the segregation of emulsions at these different concentrations. The interactions of caseins and β-Lactoglobulin with polyphenols in the aqueous phase, with the formation of characteristic dimers, were revealed by densitometry and LC-MS/MS analysis. The formation of these dimers did not affect in any case the oxidative stability of the emulsions tested. The study of the effect of essential oils of stems and flowers of P. scoparius on lipid crystallization (palm oil alone and a mixture of the oils) showed a change in isothermal crystallization behavior of triglycerides with a delay of crystallization, a decrease in calculated enthalpies of fusion, slower crystallization rates and a drastic decrease in solid fat contents (SFC). This modification is also noticeable and different shapes and sizes of triglyceride crystals (spherulites) were visualized for fats with essential oils compared to control ones. The induction of a dilution effect of the system by the essential oils is discarded and the hypothesis of the implication of major monoterpenes in disturbing the crystallization events (nucleation and crystal growth) is proposed. From this study, it can be concluded that the recovery and industrial exploitation of the two plants studied is strongly encouraged, especially for P. scoparius which is already exploited in the Algerian traditional pharmacopoeia and food preservation. In addition to the antioxidant effects reported for the essential oil, it is suggested the possibility of its use as a natural additive to modify fat crystallization for applications in shortenings and various food emulsions.