A Salt-Penetration Rule for Whole and Split Fish: Salting Times by Thickness, the Spoilage Thickness Limit, and Dry Salting Compared with Brining

Salt preserves fish only where the salt has actually arrived, and the last place it reaches is the centre of the thickest part. This paper models salt uptake as one-dimensional diffusion into a slab of fish flesh, with an effective diffusivity of 3.0e-10 m2/s as a stated assumption, and computes the time for the centre to reach 10 percent salt in the water phase, a stated keeping target. Dry salting, with a saturated surface film at 26 percent, and brining in a stated 15 percent brine are computed as separate cases throughout. Salting time grows with the square of the half-thickness, so splitting a fish open, which halves its thickness, cuts the curing time by a factor of 4.0 exactly in this model. Against a stated spoilage clock of 36 hours for an unsalted centre at 25 degrees C, a whole fish thicker than 2.30 cm dry salted, or 1.69 cm brined, has a centre that spoils before the salt arrives; splitting raises those limits to 4.60 cm and 3.38 cm of original thickness. A 4 cm whole fish needs 109.0 hours dry salted and 201.2 hours in brine, against 27.2 hours dry salted once split. Brining at 15 percent takes 1.846 times as long as dry salting to reach the same centre target, because its surface concentration is lower. The study is simulated, not measured, and its limits are upper bounds on safe practice, for reasons set out in the Limitations.