Just in time for winter, we decided to develop a new seasonal variation of one of our Modernist Cuisine traditions: Pressure Caramelized Sweet Potato Soup. The recipe for this magical soup incorporates black peppercorns to give it a nice zip, and hints of sweet onion and Makrud leaves complement the caramelized sweet potato stock.
The charm of this soup is twofold: the elevated temperature of pressure-cooking coupled with an alkaline environment ensure that caramelization reactions will flourish.
Vegetables are made up of cells with strong walls that soften at higher temperatures than the cells in meat do. Vegetables are composed mostly of water, however, and their temperature normally won’t exceed the boiling point of water (100˚C/212˚F) until they are dried out. Vegetables in a fully pressurized cooker don’t dry out as they quickly become tender under higher temperatures (120˚C/250˚F). And because the air is sealed in, you don’t need to add much water, so juices are extracted without becoming diluted.
Add to this a pinch of baking soda to bring the soup to a more alkaline pH of about 7.5 and you have ideal conditions for Maillard reactions to commence. The result is a gorgeously colored soup that is the concentrated essence of caramelized sweet potato.
We like to finish our soup with purple sweet potato confit, roasted chestnuts, and toasted marshmallows. The purple sweet potatoes add a brilliant dash of color, and toasted marshmallows add a touch of tradition and whimsy. This soup is the perfect way to begin special dinners this holiday season.