The release of the water causes a breakdown in the structure of the onions, which is why they start softening. ... During the caramelization time, the larger sugar molecules in onions are broken down into smaller, simple sugar molecules. This is why caramelized onions have a sweeter taste than their raw counterparts.

What causes onions to caramelize?

So, for the caramelization of onions to take place, these large sugar molecules have to be transformed into the smaller, sweeter versions of themselves. This chemical process occurs when the onion comes in contact with heat, causing the temperature within the cells of the onion to rise.

What happens to onions when they caramelize?

When onions caramelize, one of the main things that happens is the sucrose, or natural sugar hidden within their cells, is transformed into other, simpler forms of sugar, including glucose and fructose. Those sugars taste sweeter than sucrose, which accounts for the increase in sweetness as the onions caramelize.