Quantitative and qualitative variability of the caseinolytic potential of different strains of Pseudomonas fluorescens: implications for the stability of casein micelles of UHT milks during their storage.

Pseudomonas fluorescens grows at low temperature and produces thermo-resistant protease(s) that can destabilize UHT (Ultra High Temperature) milk during its storage. The consequences of contamination of microfiltered milk with 9 strains of P. fluorescens on the stability of the corresponding UHT milk during storage had been investigated in this study. The strains were classified in two groups according to their ability to destabilize UHT milk. For the group of highly destabilizing strains, sedimentations of UHT milks, low values to phosphate test and the presence of aggregates were observed. Zeta potential and hydration of casein micelles decreased, whereas non casein nitrogen (NCN) and non protein nitrogen (NPN) contents increased. The analyses of NCN fraction by liquid chromatography coupled to mass spectrometry indicated that the different casein molecules were hydrolyzed in a similar way for the destabilizing strains suggesting that the same enzyme was implicated. For the group of slightly or not destabilizing strains no visual and biochemical alteration were found. This study showed that destabilization of UHT milk by P. fluorescens was highly variable and strain-dependent.

The supramolecular structure of milk fat influences plasma triacylglycerols and fatty acid profile in the rat.

BACKGROUND: The digestion fate of milk fat depending on its supramolecular structure for a given dairy product composition has rarely been studied. AIM OF THE STUDY: To highlight differences of lipid digestion, we measured (i) the plasma triacylglycerol and cholesterol concentrations and (ii) the total plasma fatty acid profile of fasted rats force-fed with different dairy preparations; the three creams and the unemulsified preparation had a similar composition with different and controlled fat suprastructures. METHODS: All preparations, manufactured in the laboratory from a given milk batch, contained 205 +/- 3 g . kg(-1) fat that was either fed (i) unemulsified consecutively to the skim milk phase, or as a cream with the following fat globule structures: (ii) native milk fat globules of approximately 4 microm covered with the native milk fat globule membrane (MFGM), (iii) small native milk fat globules of approximately 2 microm selected from the latter by microfiltration and covered by the MFGM, or (iv) fine homogenized fat droplets of approximately 1 microm covered mainly with caseins. RESULTS: The plasma triacylglycerol appearance was delayed for the creams compared with the rapid onset for the unemulsified preparation. At 90 and 180 min after feeding, the plasma triacylglycerol enrichment was significantly lower for the homogenized cream than for the unemulsified preparation. At 120 min after feeding, triacylglycerol enrichment was significantly lower for each cream than for the unemulsified preparation. At 180 min after feeding, the plasma relative enrichment in C12, C14, C15, C16 and C18:1 n-9 fatty acids was significantly lower for the homogenized cream than for unemulsified fat and regular cream. CONCLUSIONS: Global lipid digestion based on plasma triacylglycerol enrichment and relative enrichments in some fatty acids was decreased with small homogenized milk fat droplets compared to unemulsified milk fat. These data show that dairy products with the same composition but varying in fat supramolecular structure result in different kinetics of lipid digestion, which could be of health concern.

The dispersion state of milk fat influences triglyceride metabolism in the rat--a 13CO2 breath test study.

BACKGROUND: Milk fat, which has different structures in the various dairy products, is a major and controversial lipid source in the Western diet. However, information about the digestion fate of milk fat depending on its supramolecular structure for a given composition is scarce. AIM OF THE STUDY: In this study, 13CO2 breath tests were performed with fasted rats force-fed different dairy preparations of similar composition but differing in fat suprastructure in order to highlight differences of general lipid metabolism. METHODS: Each preparation consisted of a NaCl solution, anhydrous milk fat labelled with a 13C mixed triacylglycerol, casein (as native phosphocaseinate powder with some lactose), and dipalmitoylphosphatidylcholine. Milk fat was either fed (i) unemulsified consecutively to the aqueous phase, or emulsified as (ii) coarse droplets of approximately 10 microm covered mainly with the phospholipid, or (iii-iv) fine droplets of approximately 1 microm covered mainly with casein, force-fed either in the liquid state or in a semi-crystallized state. 13C abundance in expired air samples was measured by isotope ratio mass spectrometry; results were expressed as 13C enrichment and were submitted to an ANOVA analysis. RESULTS: The 13CO2 excretion curves of the unemulsified preparation and the coarse emulsion were similar and presented a sharp peak, both significantly different from the fine emulsion curves characterized by a nearly linear cumulative recovery. The crystalline state of the fine emulsion droplets and the viscosity of these emulsions did not affect significantly their excretion curves. The lipid metabolization (indicated by the 13C recovery) was significantly slower for the fine droplets coated with casein than for the large droplets coated with the phospholipid and the unemulsified fat. For the latter, a single 13C peak rapidly appeared, while for small droplets coated with caseins, 13C excretion was continuous up to 6 h. CONCLUSIONS: Global lipid metabolism based on oxidation to CO2 was decreased with smaller compared to larger emulsified milk fat particles with different coatings. These data support the concept that dairy products with different fat suprastructures are digested and metabolized differently.

Native fat globules of different sizes selected from raw milk: thermal and structural behavior.

The aim of this study was to characterize differences in the thermal and structural behavior between different sized native milk fat globules. A novel microfiltration process permits the selection of native small fat globules (SFG, 1-3 microm) and large fat globules (LFG, >5 microm) in raw milk, that were analyzed by X-ray diffraction (XRD) coupled to differential scanning calorimetry (DSC). There were no major differences in triglyceride crystalline structures between SFG and LFG, after eliminating thermal history and the influence of cooling rates. The three main 3L and 2L crystalline structures appearing under slow cooling existed regardless of globule size. The supercooling increased for the SFG, mainly due to heterogeneous nucleation in winter milk, and also to compositional variations in spring milk. Differences appeared regarding stabilized crystalline forms at 20 degrees C and subsequent cooling: the SFG contained less 2L triglyceride structures than the LFG. These results can be important in dairy manufactures using tempering periods.