The effect of lactulose thermal degradation products on β-lactoglobulin: Linear-, loop-, and cross-link structural modifications and reduced digestibility

The thermal degradation products of lactulose and the interaction between lactulose and β-lactoglobulin (βLg) were investigated in a thermal model system. Lactulose was thermally degraded into fructose and galactose, which were further degraded into methylglyoxal, glyoxal, 3-deoxyglucosone, and 2, 3-butanedione via heating. After incubating with lactulose, the structure of βLg was changed, which manifested by the formation of new band with doubled the molecular weight of βLg in the mobility spectrum and the changes in the internal fluorescence spectrum. Furthermore, the lysine and arginine residues of βLg were confirmed to be the modification sites of the thermal degradation products of lactulose, and the modification types of linear-, loop-, and cross-linked were detected. The digestibility of βLg incubated with lactulose was significantly decreased due to the modification of trypsin digestion sites and the formation of cross-linked conjunctions. Therefore, the adverse effects of lactulose application in thermally processed foods should be concerned.