Thesis (Ph. D.)--Catholic University of America, 1926. / Vitae. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (p. 44-46).

Thesis (Ph. D.)--Catholic University of America, 1926. / Vitae. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (p. 44-46).

Résumé : La recherche développée vise à étudier les différents tests microbiens disponibles pour la détection des antibiotiques dans le lait, ainsi que leur couplage à la spectrométrie infra-rouge (MIR et SPIR). Dans un premier temps, lapplicabilité de certains tests de détection des antibiotiques est envisagée directement à la ferme. Cette étude met en évidence certaines interférences (désinfectants et alimentation), pouvant conduire à des résultats faux positifs. De plus, les différences de sensibilité observées entre tests posent la question de lhomogénéité du contrôle lors des tests officiels réalisés sur les livraisons de lait. Sur base des conclusions tirées de cette première partie de létude, la spectrométrie infra-rouge est envisagée comme moyen de mise en évidence de linhibition bactérienne résultant de la présence de résidus dantibiotiques. Cette seconde partie a nécessité le développement doutils analytiques permettant le comptage rapide des bactéries utilisées dune part et de réaliser le suivi de la croissance bactérienne par chromatographie liquide dautre part. Ce suivi physico-chimique est basé sur un indice de métabolisme intégrant différents acides organiques, dont le principal est lacide lactique. Ces outils analytiques sont appliqués à des échantillons de laits dopés ou non en antibiotiques, analysés par spectrométrie infra-rouge. La confrontation des résultats analytiques aux spectres obtenus dans le moyen infra-rouge ou dans le proche infra-rouge (caméra SPIR) montre le potentiel de ce mode de détection de linhibition bactérienne, corrélée à la production dacides organiques. Summary : Developed research aims at studying the various microbial tests available for the detection of antibiotics in milk, like their coupling with infra-red spectrometry (MIR and SPIR). Initially, the applicability of certain tests of detection of antibiotics is considered directly with the farm. This study highlights certain interferences (disinfecting and feed), being able to lead to false-positives results. Moreover, the differences in sensitivity observed between tests raise the question of the homogeneity of control during the official tests carried out about the milk deliveries. On the basis of conclusion drawn from this first part of the study, infra-red spectrometry is considered like means of detection of bacterial inhibition resulting from the presence of antibiotic residues. This second part required the development of analytical tools allowing the fast counting of the bacteria used on the one hand and to carry out the follow-up of the bacterial growth by liquid chromatography on the other hand. This physico-chemical follow-up is based on an index of metabolism integrating various organic acids, whose main thing is the lactic acid. These analytical tools are applied to spiked milk samples and to unspiked milk samples, analyzed by infra-red spectrometry. The confrontation of the analytical results to the spectra obtained in MIR or PIR spectrometry (camera) shows the potential of this mode of detection of the bacterial inhibition, correlated with the production of organic acids.

Changes in milk fat composition influence its nutritional quality as well as the technological properties of butter. The impact of feed on fat composition is well known; however, limited information is available on the genetic variability of fatty acids in bovine milk. The overall aim of this PhD thesis was to study the genetic variability of fatty acid profile in bovine milk and fat. This type of research needs a large amount of data. Expensive reference analysis is used to measure the fatty acid contents in fat. The first objective of this thesis was to develop an alternative method that could be faster and cheaper than traditional methods. Calibration equations predicting the contents of fatty acid from mid-infrared spectrum were established. The contents of saturated and monounsaturated fatty acids, omega-9, fatty acids with short, medium and long chain were the best predicted. Thanks to the implementation of this method in the Walloon routine milk recording, more than 20,000 milk samples were analyzed. This database permitted to model the variation of fatty acid contents in milk and fat. From these models, the genetic variability of fatty acid profile was shown. The complexity of models increased throughout this project due to the increase of new available data. Differences across 7 dairy breeds were estimated using single and multi-trait mixed models. Milk fat and delta-9 desaturase activity of Jersey and dual purpose Belgian Blue differed significantly from Holsteins. Therefore, the choice of a given breed could modify the fat composition. Heritability values obtained for studied fatty acids with multi-trait mixed model ranged from 0.05 to 0.42. Higher values were observed for saturated compared to unsaturated fatty acids. Moderate heritability estimates were observed for the activity of delta-9 desaturase (0.20) and the hardness of butter (0.27). These two traits were estimated by specific fatty acid ratios. The heritability observed using a multi-trait random regressions test day model for the content of saturated fatty acids (0.42) was similar to the one observed for the percentage of fat (0.37). Considering the impact of selection on fat content, the selection could have a great impact on fat composition. High genetic correlations were observed between some fatty acids having similarities in their synthesis. Heritability and correlations varied through the duration of the lactation. Due to the large number of fatty acids, the estimation of an index, which includes the proper fatty acid profile, could be interesting for a future selection program. This PhD thesis provides the background required by future studies to estimate the impact of animal selection on milk fat composition.