When milk is heat-processed, some of the lactose will isomerize to lactulose, a disaccharide. Lactulose
can decompose by the same routes that most sugars decompose. These are complicated processes with many products and pathways, so sugar decomposition is rarely, if ever, fully understood.
- It can hydrolyze to its constituent monosaccharides fructose and galactose.
- Lactulose or its released monosaccharides equilibrate with their ring-opened isomers and the alpha-hydroxyl can air oxidize to produce an alpha-dicarbonyl. These alpha-dicarbonyls are prone to further oxidation to generate formic acid. Over time, as the solution becomes more acidic from the formic acid build-up, further oxidations and eliminations (dehydration) can take place to produce a mixture of colored products.
- In addition to degrading to smaller molecules, sugars (and the dehydration and oxidation products mentioned above) can also oligomerize to produce a variety of viscous, colored products. Caramelization (see the "Chemistry" section in the link) is a term applied to this chemical polymerization and coloration process.
If you would like to read more about lactulose and its decomposition, take a look at this reference, especially section 7.2.1 starting on page 236.