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Potato Flour
The farina of the potato, properly granulated and dried, is frequently sold as a substitute for arrowroot. It may be successfully prepared at home, and will be found useful for thickening soups, and for making puddings easy of digestion for children and invalids. If kept dry it will remain good for years. Peel and wash some good mealy potatoes-those which have a yellow tint are the best for the purpose. Grate them upon a bread grater into a large pan of cold water, stir them well together, then strain the mass through a sieve. Let the grated potatoes settle until the water is quite clear. Pour the water off, and add fresh, stir up, and let it settle as before; and repeat until the water is quite clear, and the powder remaining at the bottom of the pan is pure white and fine. Spread the potato sediment upon dishes, and dry it in the sun or before the fire. Turn it frequently. Pound it in a mortar, and pass it through a hair-sieve, put it into jars, and cork securely. It will be quite white and flavourless. Time, ten minutes to stand each time between changing the waters. The changes to be repeated until the water that comes from it, after being stirred, is quite pure. A table-spoonful of potato flour mixed with two table-spoonfuls of cold water, stirred into a pint of boiling soup or sauce, and boiled for half a minute, will make the liquid of the consistency of thick cream.
This is a method for making potato starch — what the trade still calls farina — from raw potatoes, plus a ratio for using it as a thickener. The whole thing hinges on washing the starch until the rinse water runs clear, and on drying it to absolute dryness before it goes into a jar.
- The starch must be bone dryThe starch must be thoroughly dry -- residual moisture in the stored powder can cake and turn mouldy. Spread it thin, turn it often, and only cork it when it feels powdery and cool rather than faintly clammy. Failing sun or a hearth, the lowest possible oven setting with the door ajar will serve, though many domestic ovens will not hold a temperature as low as 120-150F.
- Which potatoes"Mealy" means high-starch and floury: russet, Idaho, Maris Piper, King Edward. Waxy new potatoes and salad varieties give a poor yield. The "yellow tint" the book prefers may point to a particular older maincrop, though yellow flesh is not in itself a reliable sign of starchiness.
- Grating and strainingA box grater works but a food processor's shredding disc is faster and keeps the pulp submerged less time in air, which limits browning. Squeeze the pulp in the sieve to drive out the last of the starchy liquid; the pulp itself is spent and can be discarded.
- Ten minutes is optimisticThe recipe allows ten minutes' settling between water changes. Potato starch settles hard and fast, but give it 20-30 minutes for a full recovery, and expect four or five rinses before the water runs genuinely clear rather than faintly grey.
- A hair-sieveA horsehair sieve was the fine mesh of the Victorian kitchen. Any fine nylon or stainless sieve, or a drum sieve, serves. The pounding in a mortar is only to break up the dried cake into powder — the starch grains themselves are already as fine as they will get.
- YieldReckon on roughly 1 to 1.5 oz of dry starch per pound of potatoes, so 5 lb of potatoes gives on the order of a cup of finished flour. This is why the shop-bought packet won the argument.
- Thickening: half a minute means half a minuteThe final ratio — 1 tablespoon starch slaked in 2 tablespoons cold water per pint of liquid — is sound, and the brief boil is deliberate. Potato starch thickens at a lower temperature than cornflour and then thins again if held at a boil, so add it at the end and take the pan off the heat.
- SubstitutingIt is a fair one-for-one swap for arrowroot and gives the same clear, glossy set. Against cornflour, use about three-quarters as much potato starch, and expect a cleaner flavour and a slightly slacker set on cooling.
Potato starch belongs to the nineteenth-century project of finding cheap domestic substitutes for expensive imported thickeners — chiefly arrowroot, shipped from the West Indies and sold at a price that put it firmly in the invalid-cookery and nursery bracket rather than everyday use. Farina was regarded as a raw material: a pure, flavourless, keeping powder that a household could manufacture in autumn from its own potato crop and draw on all year. The emphasis on food "easy of digestion for children and invalids" reflects a medical orthodoxy of the period that held pure starches to be gentle on a weak stomach, and it is the reason arrowroot, sago, tapioca and potato farina cluster together in Victorian books. Commercial potato starch was already an established industry in Germany and the Netherlands by the time this was written.