Actual ratio of triacylglycerol positional isomers in milk and cheese

Bovine Milk Triacylglycerol Regioisomer Ratio Shows Remarkable Inter-Breed and Inter-Cow Variation

Regioisomers (or positional isomers) of triacylglycerols (TAGs) of milk are known to show differential outcome in relation to human absorption. Quantitation of TAG regioisomers remains a big challenge due to the lack of facile chromatographic separation technique. The feasibility of using fragment ion intensity ratio to determine the ratio of co-eluting AAB/ABA-type regioisomer pairs was confirmed in this study. The ability of C30 stationary phase in resolving interfering TAG isomers was demonstrated for the first time. This allowed us to reveal the complexity of using fragment ion intensity to quantify 1,2-olein-3-palmitin (OOP), 1,3-olein-2-palmitin (OPO), 1,2-olein-3-stearin (OOS), and 1,3-olein-2-stearin (OSO) regioisomers in milk samples. A novel algorithm was proposed to consider the contribution of OPO/OOP and OSO/OOS double bond (DB)-isomers and to eliminate the interference of isobaric ions from other isomers, an aspect overlooked in previous studies. This liquid chromatography-mass spectrometry method that requires no pre-fractioning and a moderate chromatographic separation time of 36 min is simple and, thus, suitable for screening a large number of samples for genetic analysis of this trait. Preliminary results using a small cohort of animals showed that OPO/OOP ratio differs significantly between Jersey and Holstein cows, and a large variation was also observed across individual Holstein cows.

Comprehensive Characterization of Bovine Milk Lipids: Triglycerides

A comprehensive survey on triglycerides (TAGs) of bovine milk was conducted by a combination of exhaustive liquid chromatography (LC) separation, high-resolution mass spectrometry (MS) detection, and automated lipid molecular feature extraction. A total of 220 groups (a series of species having the same chemical formula and mass) and 3454 molecular species of TAGs were identified based on the accurate mass of the parent ion as well as MS2 information. Sixty-five different fatty acids (FAs) were found across these TAG species; C4:0, C6:0, C8:0, C10:0, C12:0, C14:0, C16:0, C18:0, and C18:1 were the most frequent FAs, whereas C11:3, C11:4, C27:0, C27:1, C28:0, and C28:1 were rare FAs in TAG molecules. The number of species identified represents only a small portion of total TAG molecules that can be theoretically synthesized from 65 FAs. Each TAG group contains on average 15-16 isomeric species (species with different FA composition), but positional isomers do not seem to be widespread. As the isomeric species cannot be completely resolved chromatographically, quantification of TAG was conducted at the group level. The most abundant TAG groups in bovine milk include TAG 34:0, TAG 36:0, TAG 38:1, TAG 38:0, and TAG 40:1. This study provides the most comprehensive milk TAG inventory so far that can be used as a reference for studying milk lipids.

Quantification of Triacylglycerol Positional Isomers in Rat Milk

The absolute amount of triacylglycerol (TAG) positional isomers was analyzed in rat milk fat, a representative of non-ruminant milk fat, using a HPLC-UV-atmospheric pressure chemical ionization-MS/MS system equipped with an octacosyl silylation column or polymeric ODS column. TAGs consisting of two oleic acids (O) and one palmitic acid (P) were the most abundant. In particular, β-OPO, a TAG binding P at the β-position (sn-2) and two Os at the α-positions (sn-1/3), was prominent. The β-OPO content decreased over time, while a TAG consisting of two Ps and one capric acid, a medium-chain fatty acid, increased. TAGs consisting of two Ps and one docosahexaenoic acid were present in small amounts and decreased with time. These results indicated that the recombination of fatty acids in TAGs in milk fat occurs in the mother, and is thought to depend on the infant's stage of growth, in response to their nutritional needs. It was also demonstrated that medium-chain fatty acids were mainly located at the α-position (sn-3), while Ps were mainly located at the β-position (sn-2). Therefore, the combination and binding positions of fatty acids of TAG are considered very important in infant nutrition.