One evening, while learning the fine art of peanut shell cracking, The Five-Year-Old naturally wanted to know more about the substance she was working with.
The Five-Year-Old: “Mommyo, are peanut shells made of wood?”
Not exactly, but peanut shells have quite a bit in common with wood. Both wood and peanut shells are made up primarily of cellulose and lignin.
Cellulose is a plant fiber mainly used in nature to build the primary cell walls of green plants (in other words — it’s the stuff that makes the plant grow). Not surprisingly, cellulose is the most abundant organic compound on Earth. Some 33% of all plant matter is cellulose (cotton contains 90% cellulose and wood 40-50%). Industry extracts the cellulose from plants to manufacture everyday products like paper, paperboard, nylon, and cellophane. Experiments are also underway to use cellulose as the foundation for alternative biofuels. Although our bodies can’t digest the cellulose in the plants we eat, we still manage to make good use of it. (Hint: You may be more used to thinking of the plant cellulose in your diet as fiber.)
Lignin works in conjunction with cellulose to provide strength and support to the plant’s cell walls (and to conduct water). The more lignin a plant has, the woodier it feels. Lignin gives the branches, twigs, and trunks of woody plants their shape and structure. It provides the strength and durability to make woods like teak, oak, and pine useful to the furniture and construction industry. Unfortunately, it’s also the stuff that makes wood burn.
An answer on a Yahoo! message board says that the lignin to cellulose ratio for peanut shells is 79:100. By way of comparison, the lignin to cellulose ratio for a California Sequoia tree is 70:100, and for an oak tree 62:100. I was unable to find a secondary source to verify these ratios (at least in terms that I could understand), but if they are true, the relatively high concentration of lignin in peanut shells would explain their woody feel.