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La texture des produits laitiers est un paramètre clé influençant les choix des consommateurs. Dans les systèmes acides de type yaourt, l’ajout d'agrégats protéiques permettrait le développement de produits aux textures innovantes ne contenant que des ingrédients laitiers. Le but de ce travail est de mettre en évidence et de comprendre les mécanismes de texturation d’émulsions laitières acidifiées modèles, par des agrégats protéiques de différentes composition, morphologie et taille, en les comparants avec les systèmes classiques supplémentés en protéines sériques. Pour cela, des gels laitiers gras acidifiés par la GDL ont été fabriqués en faisant varier la concentration des différents d’agrégats. L’ajout de protéines sériques et d’agrégats fractals permet d’augmenter la fermeté des gels et de diminuer leur synérèse. Alors que l’ajout d’agrégats sphériques et denses et d’agrégats mixtes (caséinesprotéines sériques) ne modifie pas la texture. Les agrégats ont un rôle dans la phase continue. Le développement de la surface interfaciale en augmentant la pression d'homogénéisation et la fraction de matière grasse peut permettre l’adsorption d’agrégats à l’interface des gouttelettes de matière grasse et augmenter la fermeté. La nature des agrégats influence les interactions s’établissant avec le réseau de caséines au cours de l’acidification. Parmi les interactions formées, des ponts disulfures entre les protéines sériques ou les agrégats fractals et le réseau ont été mis en évidence. La grande variété de texture obtenue dans ces systèmes permet d’envisager la création de produits innovants et plus particulièrement enrichis en protéines pour cibler certains types de population. / The texture of dairy products is a key parameter influencing consumer choices. In yogurt-type acid systems, the addition of milk protein aggregates would allow the development of products with innovative textures containing only dairy ingredients. The aim of this work is thus to highlight and understand the texturizing mechanisms of acidified models dairy emulsions through protein aggregates of different compositions, morphologies and sizes, by comparing them with conventional systems supplemented with whey proteins. For this purpose, fatty dairy gels acidified with GDL were manufactured by varying the concentration of different types of aggregates. The addition of whey proteins and fractal aggregates increases the strength of the gels and reduces their syneresis. Whereas the addition of compact spherical and mixed (casein-whey protein) aggregates does not modify the texture. Aggregates have a role in the continuous phase. The development of the interfacial surface by increasing the homogenization pressure and the fat fraction can allow the adsorption of aggregates at the interface of the fat droplets and increase the firmness. The nature of the aggregates influences interactions formed with the casein network during acidification. Among the interactions formed, disulphide bridges between whey proteins or fractal aggregates and the network were highlighted. The large variety of texture obtained in these systems demonstrates that it is possible to create innovative and specific products, especially protein-rich ones, to target certain types of population.

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In this thesis, aspects of metabolism of lipids in dairy cows were studied, particularly 18:1 trans fatty acid (tFA) concentrations in plasma and lipoprotein fractions, and transportation of FA in epithelial mammary gland cell cultures. Two in vivo studies were conducted to elucidate which lipoprotein fractions were involved in bovine plasma transport of tFA by infusing oils that induced different plasma tFA profiles. Fatty acid profiles of plasma and lipoprotein fractions [high (HDL), low (LDL), and very low density (VLDL) lipoprotein fractions] were analysed. Results showed that VLDL fraction was the main transport mechanism of tFA in plasma and was more responsive to supply of tFA. MAC-T cell cultures were used to study specific aspects of lipid uptake without interference from the cow’s digestive system and general metabolism. Treatments consisted of adding long-chain FA solutions to lactogenic medium at varying concentrations. After incubation, cystosolic triacylglycerol (TAG), protein and DNA contents were determined. Results demonstrated that cytosolic TAG accumulation is stimulated by addition of long-chain FA, which affects FA profile of cytosolic free FA. Overall, results from this thesis enhance knowledge on plasma transport and mammary uptake of 18:1 tFA in dairy cows. The information strengthens understanding of some aspects of milk fat synthesis and metabolism of FA in dairy cows fed with different sources of tFA. Findings from this thesis are important because saturated FA and tFA in milk fat have caused concern among human health researchers and more recently milk fat has gained appreciation as a functional food due to the health-promoting potential of some FA (vaccenic and conjugated linoleic acids) found specifically in ruminant-derived products.

In the blue cheese Stilton the starter mould Penicillium roqueforti grows and sporulates during the ripening period and is considered to be responsible for the unique blue cheese aroma. However, the sporulation of the mould, which results in the formation of blue veins, takes place in a fraction of the Stilton matrix which overall is very heterogeneous. Most blue cheeses develop a secondary microflora of yeasts which may affect their aroma. The aim of this study was to investigate the yeast flora of Stilton, the aroma profile of the cheese and the role of the yeasts in the aroma production. The approach in this work was to study individually the different sections of Stilton (the blue veins, the white core and the outer crust) as previous studies have demonstrated each section has a differing bacterial flora. In addition to the classical microbiology, a series of molecular techniques (Denaturing Gradient Gel Electrophoresis, Restriction Fragment Length Polymorphism and Terminal RFLP) were compared and applied for the screening of the local fungal communities in the cheese. The results showed that the two approaches were complementary. It was concluded that the structure of the fungal community was different for each section of the cheese. The aroma profiles of the three different sections of Stilton were studied using solvent extraction Gas Chromatography-Mass Spectrometry (GC-MS), a headspace GC-MS technique (SPME GC-MS) and direct headspace analysis (Atmospheric Pressure Chemical Ionisation [APCI]-MS). The different sections of Stilton presented different aroma profiles. Overall, the blue and the outer crust had similar profiles. These two sections contained higher amount of ketones while the white contained higher amounts of alcohols and aldehydes. Yeast isolates and the starter Penicillium roqueforti were cultivated alone and in combination in a cheese model and the aroma production was studied with SPME GC-MS analysis. The co-culture of the starter Penicillium roqueforti and individual yeast isolates resulted in aroma profiles different from those that were produced by the mould or the yeasts individually. The model of Penicillium roqueforti with Yarrowia lipolytica resulted in an aroma more similar to blue cheese than produced by the mould alone. Sensory analysis (Flash profile technique) was used in order to compare the aroma of this model with the aroma of blue cheeses and the perception of the combined culture was found to be similar to Stilton cheese, whereas that of the mould alone was not. Yeasts are a significant part of the microflora of Stilton and they are able to affect the aroma production. Selected isolates of Yarrowia lipolytica could be used in combination with Penicillium roqueforti for the production of blue cheese aroma e.g. as a starter culture.