RESUMEN
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: ß6'-galactosyl lactose (ß6'-GL), α6'-galactosyl lactose (α6'-GL), ß4'-galactosyl lactose (ß4'-GL), α3'-galactosyl lactose (α3'-GL), and ß3'-galactosyl lactose (ß3'-GL). This composition differs from that of commercial galactooligosaccharide products, which comprise mainly ß-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, ß3'-GL was the main isomer in colostrum and α3'-GL was the main isomer in transitional and mature milk. ß6'-Galactosyl lactose and ß4'-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 ß3'-GL, the values were 5 and 2 times higher, respectively. Absolute quantitative analysis revealed that ß3'-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.