One difficulty with potatoes is that they are difficult to store. Anyone who has ever lost track of a bag of potatoes knows this. They have an unfortunate tendency to send forth a tangle of roots, and, worse, rot into a foul-smelling puddle. Andean peoples solved this problem by freeze-drying. Exposing potatoes to the intense cold of the high mountains transforms them into little fists of stone, immune to decay. The technique also neutralizes the poisonous glycoalkaloids present in some of the bitter varieties, allowing these to be eaten safely. If the potato-rocks are trampled underfoot like petrified grapes, it is possible to reduce them to a dry powder that lasts for years. This dried substance, chuño, captivated Spaniards when they first encountered it in the 16th century, and they invariably described in some detail how it is made. Europeans were however slow in adopting it themselves; it was left to industrial manufacturers in the 20th century to bring us Smash and other commercially produced instant mashed potatoes.
By the 1840s some 40 percent of the population subsisted almost entirely on potatoes, or potatoes with a bit of buttermilk if they possessed enough land to pasture a milch cow. Poor men in rural Ireland ate between three and five kilos of potatoes a day and little else. The varieties grown were as limited as this diet. While a single valley in the Andes might contain over a hundred different types of cultivated potato, most of the potatoes grown in 19th-century Ireland were a yellow-fleshed variety known as Irish Lumper. Monocultures are extremely vulnerable to disease, since a single pathogen can devastate the entire harvest. When Ireland’s potato crop failed in 1845, and again in 1846 and 1848, over a million people died.