The same high oil content that makes nuts nutritious and delicious also makes them much more fragile than grains and legumes: that oil readily absorbs odors from the surroundings, and goes rancid when it’s split into its component fatty acids and the fatty acids are then fragmented by oxygen and light. The fatty acids have an irritating effect on the mouth, while their fragments have cardboard, paint-like aromas. Walnuts, pecans, cashews, and peanuts are rich in fragile polyunsaturated fats and are especially susceptible to staling. Fat rancidity is favored by bruising, light, heat, and moisture, so it’s best to store nuts in opaque containers at cool temperatures. Shell-less kernels are best refrigerated. Because they contain very little water and so don’t suffer from the formation of damaging ice crystals, nuts can be frozen for long keeping. Storage containers should be truly air- and odor-tight—glass jars, for example, rather than permeable plastic bags.