Qualitative and quantitative mass spectrometry comparison of characteristic galactosyl lactose isomers from goat milk at different lactation stages

Galactooligosaccharides are composed mainly of galactosyl lactose, which is important for infant growth and as a functional food additive. Although galactosyl lactose is abundant in goat milk, its complex structure has hindered the separation and analysis of its isomers. In this study, 5 isomers of goat milk galactosyl lactose were separated by HPLC: β6′-galactosyl lactose (β6′-GL), α6′-galactosyl lactose (α6′-GL), β4′-galactosyl lactose (β4′-GL), α3′-galactosyl lactose (α3′-GL), and β3′-galactosyl lactose (β3′-GL). This composition differs from that of commercial galactooligosaccharide products, which comprise mainly β-configuration oligosaccharides. The isomers were then qualitatively and quantitatively compared at different lactation stages using online HPLC-mass spectrometry. Relative quantitative analysis showed that the total content of the 5 galactosyl lactose isomers was highest in transitional goat milk. Specifically, β3′-GL was the main isomer in colostrum and α3′-GL was the main isomer in transitional and mature milk. β6′-Galactosyl lactose and β4′-GL tended to increase and then decrease during lactation. Moreover, α3′-GL content was 2 times higher than in colostrum and 10 times higher in transitional milk than in mature milk; in contrast, for β3′-GL, the values were 5 and 2 times higher, respectively. Absolute quantitative analysis revealed that β3′-GL was the most abundant isomers in colostrum (32.3 mg/L), and α3′-GL was the most abundant in transitional milk (88.1 mg/L) and mature milk (36.3 mg/L). These findings provide an important quantitative basis for understanding the relationship between structure and function of galactosyl lactose in goat milk, as well as its exploitation as a functional food.