It’s that time of year again: fall has arrived. The temperature is dropping, the leaves have all turned, and the holiday season is fast approaching. When I think of fall, I remember all of the fun fall activities that I used to do as a kid. My parents would take my brother and I to the pumpkin patch every year and we would choose our pumpkins, argue about whose was better, go on a hayride, and then go home to carve our pumpkins. The other thing that always comes to mind when I think of fall is the food that I associated with fall. I think of hot apple cider, chili, and soup. There is something very cozy and comforting about having a hot meal on a crisp fall day. And one of my family’s favorite fall meals is French Onion Soup, a delicious soup of sweet onions in a rich broth topped with tons of cheese.
French Onion Soup is something that I have eaten many times in my life, but rarely have I stopped to consider the chemistry involved in my dinner. But how is it that the onions become so much sweeter than they ordinarily are? Think about it: raw onions have a sharp taste and a distinct crunch. But onions in French Onion Soup have been caramelized, and as a result they become softer and much sweeter. In the recipe it says that the onions need to be caramelized by cooking them on medium heat for about a half an hour. But what is caramelization? What chemical reaction causes onions to brown and become sweet? And why aren’t cooked onions crisp and crunchy like raw onions? What happens to onions when they are cooked?
One of the chemical processes that occurs while foods are cooking is called pyrolysis. Pyrolysis is a type of non-enzymatic browning in which chemical compounds are broken down by heat without the use of a protein to catalyze or speed up the reaction. When caramelizing onions, pyrolysis is responsible for breaking down sugars using heat, meaning that the high heat applied to the onions during cooking causes the sugars present to break down into smaller units.
All living things break down sugars for energy, including the plants that we eat. In an average medium-sized onion, there are 9 g of sugars. These sugars can be many different from monsaccharides (one single sugar molecule) to disaccharides (two sugars linked together) to polysaccharides (many sugars linked to each other). Plants can make the monosaccharide glucose through photosynthesis. Plant cells can also break down sugars for energy, like all other living things. And polysaccharides are found mainly in the structural components of plants, like the cell walls.
When onions are being cooked, the heat from the pan raises the temperature within the cells of the onion. Once the temperature is high enough, the poly- and disaccharides in the onion are broken down into monosaccharides by breaking the bonds that link the monosaccharides to each other. This reaction of breaking down larger sugars into single sugar molecules is what causes sautéed or caramelized onions to brown and develop a sweeter flavor.
Pyrolysis also explains why onions become softer when they are cooked. Much of the structure of plants is made up of starches, which are polysaccharides. When these are heated, they also break down into monosaccharides. So cooking vegetables breaks down the structural sugars present as well, making them softer than raw vegetables.
So the next time that you are sautéing onions for French Onion soup, or browning any vegetable for that matter, think about what is happening to your food at the molecular level. The heat that the stove supplies is breaking down the larger sugars naturally found in vegetables down into monosaccharides, making them taste sweeter and making them softer.
- French Onion Soup recipe: http://www.foodnetwork.com/recipes/tyler-florence/french-onion-soup-recipe2/index.html
- Sugar Content of an Onion: http://whatscookingamerica.net/onion.htm
- Caramelization Temperatures: http://www.food-info.net/uk/colour/caramel.htm
- Pyrolysis: http://www.cpeo.org/techtree/ttdescript/pyrols.htm
- Pyrolysis (Wikipedia): http://en.wikipedia.org/wiki/Pyrolysis
- Kinetics of Non-enzymatic Browning in Onion Slices
During Isothermal Heating: http://www.sciencedirect.com/science/article/pii/026087749590054F
- Formation of Sucrose Pyrolysis Products: http://pubs.acs.org/doi/abs/10.1021/jf60161a013
- Effects of pH on Caramelization and Maillard Reaction Kinetics in Fructose-Lysine Model Systems: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.2001.tb08213.x/abstract