Enzymatic hydrolysis during in vitro digestion of triacylglycerols in a model system and milk: Isomerisation of diacylglycerols and monoacylglycerols

It is generally agreed that sn-1 and sn-3 fatty acids (FA) are released from triacylglycerols (TAG) during digestion, while sn-2 FAs remain attached to the glycerol backbone to form sn-2 monoacylglycerols (MAG). Isomerisation of diacylglycerols (DAG) and MAGs, in which the sn-2 FA migrates to the sn-1 or sn-3 position, can lead to the release of the sn-2 FA by pancreatic lipase, resulting in the formation of free glycerol. The main objective of this study was to gain a deeper understanding of the degree of DAG and MAG isomerisation, followed by enzymatic hydrolysis during in vitro digestion of a model system and milk. The digestion of TAG-C14:0/C18:1n-9cis9/C14:0 demonstrated that 25.8 mol % of the sn-2 FA, C18:1n-9cis9, was hydrolysed during digestion. Milk digestion yielded a significantly higher proportion of free FAs and a lower proportion of MAGs than predicted if only sn-1 and sn-3 FAs from TAGs were released. In addition, our results showed a 10 % formation of free glycerol during digestion, which means full hydrolysis of TAGs occurred. The estimated degree of acyl migration was highest for C12:0 (33 mol %) and implied a greater extent of acyl migration for medium-chained and unsaturated FAs than for longer-chained and saturated FAs.