Softening and deforming: your roasted chestnuts.

As winter settles, roasted chestnut’s vendors have appeared in the streets. We know: preparing chestnuts can be burdensome because of the burr (the spiky shell) and the stiff hull around the nut. The soft skin, under the hard hull, usually comes off with it, or can be eaten if not, and is less of a problem.

Removing the burr is fairly easy: it opens when the chestnut falls from the tree. What is more annoying is the hard hull.

To free the nut from the hull, the way to go is to boil it, make an incision, and roast it. During boiling, the hull softens, whereas during roasting, it cracks open. Indeed, the hull contains essentially ca. 40% of lignin, an organic polymer similar to that found in algae and bark, ca. 20% of cellulose, another stiff organic polymeric material found in wood, ca. 20% of other cellulose-like or starch-like polysaccharides (i.e. alpha or beta- glucans), and some other small organic molecules, like crosslinkers, to bind everything together.

As anticipated, when placing an organic material in water, it softens. Here one needs to use heat in addition to the water to help destroy some of those crosslinks, and to move the water between the stiff components: with high lignin and cellulose content, the swelling is barely noticable.

After making the incision, the chestnuts with their hulls are roasted, meaning placed at a high temperature, but in a dry environment. During the drying, the hull shrinks. As it is constrained everywhere, except where the incision has been made, it shrinks more at the cut side and enlarging it.

Although this hull is a waste products, some researchers are investigating its use for reinforcing polymers like polypropylene (, or even for filtering arsenic from water (! (for other interesting waste materials, see this older post).

Tips from Switzerland: the double paper bag with integrated waste storage for the hulls (, and from Japan: the plastic nail to help you open the chestnuts instead to destroying your fingers.