Milk fat triacylglycerols and their relations with milk fatty acid composition, DGAT1 K232A polymorphism, and milk production traits.

Milk fat (MF) triacylglycerols (TAG) determine the physical and functional properties of butter and products rich in MF. To predict these properties, it is necessary to understand the variability of fatty acids, TAG, their associations, and their effect on milk productive traits, days in milk (DIM), and genes related to fat synthesis. Therefore, the aim of this research was to study the variability of TAG using MF from individual cows and to investigate the effect of fatty acid composition, DGAT1 K232A polymorphism, DIM, and milk production traits (fat content and morning milk yield) on MF TAG profile in the Dutch Holstein-Friesian dairy cattle population. Large differences in MF TAG profiles were seen among cows. We showed that the variability of TAG is highest for low and high molecular weight TAG [TAG with carbon numbers (Cn) 26-30 and Cn52-54, respectively] and lowest for TAG with Cn38, which was the most abundant TAG. Saturation index (saturated fatty acid/unsaturated fatty acid) and the ratio C16:0/C18:1 cis-9 showed significant effects on TAG Cn34, Cn36, Cn52, and Cn54: TAG Cn34 and Cn36 increased as the saturation index and ratio C16:0/C18:1 cis-9 increased, whereas the opposite was seen for TAG Cn52 and Cn54. Moreover, the DGAT1 K232A polymorphism significantly affected TAG Cn38. We showed that the relative concentration of TAG with Cn38 was higher in cows with DGAT1 KK genotype. Production traits (fat content and morning milk yield) and DIM had no significant effect on TAG profile. This is a relevant observation because considerable increases of milk yield and fat content have been seen in the Netherlands over the last 60 yr. The large differences shown between individual cows in MF TAG profile imply differences in physical properties of MF.

Influence of C16:0 and long-chain saturated fatty acids on normal variation of bovine milk fat triacylglycerol structure.

Fatty acids (FA) are nonrandomly distributed within milk fat triacylglycerols (TAG). Moreover, the structure of milk fat TAG differs with feeding regimens. So far, nothing is known about the variation of milk fat TAG structure among individual cows. A deep understanding of the normal variation of TAG structures and the relationships between milk fat FA profile and its TAG structure could help to better control functional and compositional differences between milk fats from various sources and to increase the knowledge on milk fat synthesis. The focus of the present study was to determine the regiospecific TAG structure of individual samples of winter milk fat from Dutch Holstein-Friesian cows with a wide variation of FA profiles and with 2 diacylglycerol acyltransferase 1 (DGAT1) genotypes: DGAT1 K232A genotype AA and DGAT1 K232A genotype KK. From an initial set of 1,918 individual milk fat samples, 24 were selected. The selected samples had a wide range of FA composition and had either DGAT1 K232A genotype AA or KK. The structure analysis was done with a regiospecific approach. This analysis is based on the acyl degradation of TAG by a Grignard reagent and further isolation of sn-2 monoacylglycerols by thin-layer chromatography. An intra- and interpositional approach was used to study the structural variation. With the intrapositional approach, the amount of an FA at the secondary (sn-2) and primary (sn-1,3) positions was related to its total amount in the TAG. With the interpositional approach, the proportion of C8:0, C10:0, C14:1 cis-9, C16:1 cis-9, and C18:1 cis-9 at sn-2 was positively correlated with the amount of C16:0 in the triacylglycerol; in contrast, saturated C14:0, C16:0, and long-chain saturated FA (C14:0-C18:0) were negatively correlated. These observations suggest that the amount of long-chain saturated FA in TAG influences the positioning of other FA in the TAG. With an interpositional approach, the DGAT1 polymorphism had a significant effect on the proportional positioning of C16:0 at sn-2. These results provide a new direction to controlling functional and compositional differences between milk fats.

Monitoring entering and spreading of emulsion droplets at an expanding air/water interface: a novel technique.

The entering and spreading of emulsion droplets at quiescent and expanding air/water interfaces was studied using a new apparatus consisting of a modified Langmuir trough in which the air/water interface can be continuously expanded by means of rollers in the place of traditional barriers. When sodium caseinate and whey protein isolate-stabilized emulsion droplets were injected under the surface of sodium caseinate and whey protein isolate solutions, respectively, it appeared that the droplets entered the air/water interface only if the air/water surface pressure did not exceed a threshold value of approximately 15 mN/m. This condition was satisfied either under quiescent conditions for low protein concentrations or by continuous expansion of the interface at higher protein concentrations. According to equilibrium thermodynamics, entering of the droplets and the formation of lenses should occur for all the systems investigated, but this was not observed. At surface pressures higher than approximately 15 mN/m, immersed emulsion droplets were metastable. This is probably due to a kinetic barrier caused by the formation of a thin water film bounded by protein adsorption layers between the emulsion droplet and the air/water interface.

Firmness and crystallization of milk fat in relation to processing conditions.

We studied the influence of different processing conditions on the crystallization and firmness of milk fat. Liquid milk fat was quickly cooled with a scraped surface heat exchanger to several temperatures ranging from 10 to 20 degrees C to initiate crystallization, and in some cases subsequently passed through an agitated working unit, in which the crystallizing fat was subjected to high shear. Samples of the crystallizing fat were taken directly after either the scraped-surface heat exchanger or the agitated working unit. Crystallization and development of firmness were monitored by a number of techniques that were applied simultaneously. Crystallization was monitored by temperature measurement, nuclear magnetic resonance, differential scanning calorimetry, and microscopic imaging. Firmness was measured by penetrometry. The development of firmness of milk fat produced by a Votator line at temperatures encountered in practice, ranging from 10 to 20 degrees C, can be explained by the kinetics of crystallization into the beta'-form of high-melting and middle-melting triglyceride fractions of milk fat and subsequent recrystallizations. The new insights allow better control of the firmness by adjusting the processing conditions.

Characterization of the adsorption of conjugated and unconjugated bile acids to insoluble, amorphous calcium phosphate.

Recently we showed that supplemental dietary calcium stimulates the intestinal formation of insoluble calcium phosphate and decreases the ratio of dihydroxy to trihydroxy bile acids in human duodenal bile. Because previous in vitro studies indicated that these effects could be due to differential adsorption of bile acids to amorphous calcium phosphate, we characterized the binding of bile acids to calcium phosphate. Freshly formed, amorphous, calcium phosphate bound and thus precipitated glycine-conjugated and unconjugated bile acids, whereas taurine-conjugated bile acids showed little binding. Glycochenodeoxycholic acid hardly adsorbed to other insoluble calcium phosphates, including hydroxyapatite. Adsorption studies using increasing amounts of glycine-conjugated and unconjugated bile acids showed that binding occurred above a bile acid-specific critical minimum concentration, dependent on bile acid hydrophobicity. The simultaneous use of a fluorescent hydrophobic probe indicated that this binding was due to ionic adsorption of monomers of bile acids, followed by their hydrophobic aggregation on the calcium phosphate surface, probably in the form of a bilayer. Finally, using human duodenal bile we found that amorphous calcium phosphate, but not Ca2+, preferentially bound and thus precipitated dihydroxy bile acids. We conclude that freshly formed, amorphous, calcium phosphate is a prerequisite for adsorption of bile acids and that monomers of glycine-conjugated and unconjugated dihydroxy bile acids have a high binding affinity for amorphous calcium phosphate.