When working with a product such as coffee, we tend to find balancing our drinks easier when milk is sweeter. One of the unsolved mysteries discussed in the final paragraph of Professor Abbott’s latte page is why steamed milk tastes sweeter than unsteamed milk. In our dialogues with the professor, he expressed his potentially myth-busting opinion to us:
Lactose itself is not especially sweet, and one theory has been that steaming the milk helps break it down to sweeter glucose and galactose. But if this were true then the milk would taste just as sweet when it cooled back to room temperature — and if you do the test for yourself you will see this is not the case. Another explanation is that the lactose changes its shape (‘α and β conformations’) at higher temperatures. But the sweetness difference between the two shapes, and the change in the amount of each shape makes this explanation unlikely. The most likely explanation seems to be the tongue’s physiology, because fructose also tastes sweeter in hotter drinks.
Lactase is an enzyme which splits lactose into its component parts of glucose and galactose. Lactose is not an intensely sweet sugar; it is 70% less sweet than sucrose. However, its glucose and galactose are approximately twice as sweet as lactose (see chapter 10.3.3 of this link). If the glucose and galactose are separated, the milk tastes much sweeter. When lactase is added to milk, the hydrolysis caused by this enzyme substantially increases the perceived sweetness of milk. Hydrolysis is the chemical breakdown of a molecule, a process involving water. The lactase enzyme is found naturally inside the casein micelles.
The hydrolysis of lactose.