Microwave-assisted isomerisation of lactose to lactulose and Maillard conjugation of lactulose and lactose with whey proteins and peptides

Lactose was isomerised to lactulose by microwave heating and purified by a methanolic procedure to a product with approximately 72% lactulose content. Afterwards, lactose and the lactulose-rich product (PLu) were conjugated with either whey protein isolate (WPI) or its antioxidant hydrolysate (WPH) through microwaving. Lactose had a higher Maillard reactivity than PLu, and WPH was more reactive than WPI. The browning intensity of WPI-sugar systems was however higher than that of WPH-sugar pairs. Atomic force microscopy showed larger (up to ≈103 nm) particles for WPI-PLu conjugates compared to WPH-PLu counterparts (up to ≈39 nm). The Maillard conjugation progressively increased the radical-scavenging activity of WPI/WPH-sugar pairs with increasing conjugation time and improved the foaming properties of WPI and WPH. The WPI/WPH-sugar conjugates showed higher solubility and emulsification index than unreacted counterpart pairs. For native WPI, β-lactoglobulin was not degraded by in vitro gastric digestion, whereas for WPH-PLu conjugates degraded completely.