The Science Behind Caramelised Onions

For the longest time, I thought caramelised onions were caramel sauce with onions. I was proven wrong at the ripe age of 12. How I went that long without realising this is a mystery to me.

The caramelisation of onions is non-enzymatic browning because we use heat to make the onions brown — not oxygen. This is an example of my favourite kitchen reaction, the Maillard reaction!

If you notice, caramelised onions are much smaller than regular onions. I used half a kilo of onions (about three cups) and after caramelisation, I was left with only about a cup of onions. This is because onions are approximately 90% water. By heating the onions, firstly, we are releasing the water from them. Secondly, heating the onions above 100 degrees C causes pyrolysis (aka devolatilisation), which is what causes the browning!

Caramelised onions are also noticeably sweeter than raw onions (hence their name). This is because the sugar molecules inside the onion break free and the sulphuric components, which are released when cutting onions and give onions their characteristic, also "mellow down." Apparently, if you use baking soda, the process occurs much faster — I will have to try that out soon.

That's about it! I burnt some of the onion pieces, but let's pretend we can't see them. They still tasted delicious, which is all that matters.

If this post interested you, I highly recommend going through this blog post.

If you want to caramelise your onions, this video has helpful tips for beginners.