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POTATO FLOUR (another way.)
This flour is obtained by grating any quantity of good mealy potatoes, and steeping them in a vessel of water for six or eight hours. Change the water, and stir every three hours during the day. Pour off this water at night, covering again with fresh water as before. In the morning strain the flour into a hair sieve. Have ready a clean vessel in which to stand the sieve, and wash the flour through it. Allow it to settle, then pour off the water, and spread the sediment on dishes to be put into a slow oven. The flour will dry quite white, and keep good in jars, closely sealed up, for a considerable time.
What this produces is potato starch rather than flour in the milled sense: the fine white sediment that settles out of the wash water, not the dried potato itself. Everything depends on repeated rinsing and a genuinely low drying heat.
- The hinge: rinse until the water runs clearThe repeated water changes are not idle soaking; they carry away sugars, proteins and cell debris that otherwise turn the starch grey and give it a raw-potato smell. Keep changing the water until it pours off almost colourless, and keep the vessel cool while it stands.
- "Slow oven"Conventionally read as around 250-300F, with the door left slightly ajar so moisture escapes. Err to the low end: while the sediment is still damp the starch can gelatinise and the powder will dry yellowish and gritty rather than chalk-white.
- Hair sieveA fine horsehair sieve; a nylon or fine stainless mesh strainer, or a doubled square of muslin, does the same work. The point is to hold back the grated pulp while the starch slurry passes through.
- GratingA food processor with the grating disc, or the fine side of a box grater, replaces the labour. Finer shreds release more starch, so err toward fine.
- Choose the potatoes carefully"Good mealy potatoes" means a high-starch floury variety - russet, Maris Piper, King Edward - not a waxy salad potato, which yields very little. Discard any green-tinged or sprouting tubers; do not rely on the washing to remove the bitter glycoalkaloids.
- YieldThe book gives none. Expect roughly 1 to 1.5 oz of dry starch per pound of raw potatoes, so 5 lb of potatoes produces only a few ounces - which is why potato starch was worth buying even when the method was well known.
- Drying and storageThe powder must be bone dry before it goes into jars; break up the dried cake and sift it, and if it feels at all cool or clammy to the touch return it to the oven. Sealed dry it keeps very well, which is all the original claims when it says "a considerable time".
- Using itIt thickens at a lower temperature than wheat flour or cornflour and thins again if held at a boil or reheated, so add it as a slurry off the heat and serve promptly. Weight for weight it tends to thicken more strongly than cornflour, so start with rather less and add more if needed.
Potato starch was a product of the same nineteenth-century interest in cheap domestic starches that gave British kitchens arrowroot, sago and "farina," and its extraction was known in Europe by the late eighteenth century, though factory-scale production seems to be largely an early nineteenth-century development - fécule de pomme de terre was a staple of French confectionery and sauce work, and German and Dutch factories exported it in quantity. The washing-and-settling technique is generally taken to be among the oldest in the starch trade and is unchanged in principle today: starch grains are heavier than water and insoluble in it cold, so they simply sink while everything else can be poured away. It is also the reason the same procedure appears in older books as a way of making starch for laundering collars - the kitchen and the wash-house wanted exactly the same white powder.