Simmered Dishes.
"Sa, shi, su, se, so" — sugar, salt, vinegar, soy, miso — is the order Japanese cooks season a simmering pot. Fick's law of diffusion explains the first two.
A nimono simmers ingredients in just enough seasoned dashi to come part way up them, over gentle heat, under an otoshibuta (落とし蓋): a lid of wood or paper that sits directly on the food, smaller than the pot. It holds the food under, keeps the small amount of liquid circulating over the top as it bubbles up and falls back, slows evaporation, and stops delicate pieces tumbling and breaking. Vegetables are cut to even sizes with their corners bevelled (面取り) so they do not crumble; daikon is often pre-cooked in rice-washing water to make it tender and mild.
Seasoning goes in by さしすせそ: 砂糖 (sugar), 塩 (salt), 酢 (vinegar), 醤油 (soy), 味噌 (miso). The first two are physics. Salt and sugar enter the food by diffusion (III.8), and a sucrose molecule, twelve atoms of carbon with its oxygens and hydrogens, diffuses about three times more slowly than the small ions of salt. Salt added first also draws water from the surface and firms it, making sugar's slow entry slower still. So sugar goes in first, salt after; vinegar, soy and miso, whose aromas fade with heat, go in last. And because diffusion continues as the pot cools, a nimono left to cool in its liquid and gently reheated tastes more seasoned through — 味が染みる, "the flavour soaks in". The workshop simmers daikon.
Below: the average seasoning taken up by daikon pieces of the thickness you choose, simmered for the minutes you choose at 90 °C, for salt and for sugar (course diffusion bench, Fick's law in a slab, diffusivities from the literature halved for plant tissue), as a share of the liquid's concentration.
- Use a drop lid.
- Season in the さしすせそ order.
- Explain diffusion of salt and sugar.
- Let nimono rest to season through.
Sugar first, then salt, then soy — then wait
In the diffusion bench a 30 mm piece of daikon simmered 30 minutes takes up about 0.24 of the liquid's salt on average but only 0.14 of its sugar — salt is ahead by about 1.8 times, close to the square root of their threefold difference in diffusivity, as Fick's law predicts early on. Add sugar ten minutes early and it catches up. Thickness matters more than time: by the square law (II.2), a piece half as thick seasons through in a quarter of the time, which is why nimono vegetables are cut to even, moderate sizes.
Daikon nimono for 4: peel 600 g daikon, cut into 3 cm rounds, bevel the edges and score a cross on one face; simmer in rice-washing water (or water with a spoon of raw rice) 15 minutes until a skewer goes in easily, then rinse. Put in a pan with 500 mL dashi and 1 tbsp sugar and 2 tbsp mirin; cover with an otoshibuta (a circle of baking paper with a hole works) and simmer gently 10 minutes; add ½ tsp salt and 2 tbsp usukuchi soy, simmer 20 minutes more. Turn off the heat and leave to cool in the liquid at least an hour (refrigerate if longer, XII.7); reheat gently to serve, with a little yuzu zest.
Simmer two batches of daikon, adding sugar and soy together in one and sugar ten minutes earlier in the other. Taste the centres.
Terms
- 煮物
- Nimono: food simmered in seasoned dashi.
- 落とし蓋
- Drop lid resting on the food.
- さしすせそ
- The seasoning order: sugar, salt, vinegar, soy, miso.
- 味が染みる
- "The flavour soaks in" as a nimono rests.
Sources
- CRC Handbook of Chemistry and Physics: diffusion coefficients in water at 25 °C — sodium chloride 1.61 × 10⁻⁹ m²/s, sucrose 0.52 × 10⁻⁹ m²/s. In plant tissue both are lower; the course halves them.
- S. Tsuji, Japanese Cooking: A Simple Art, Kodansha, 1980 (rev. 2006).
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.