One of the thirty-nine areas of forbidden activity on Shabbat is "Me'abeid," or tanning, the process by which salt and other chemicals are applied to leather to harden it and make it suitable for use.
This prohibition has two common modern-day applications. Firstly, Halacha would forbid manually stretching a new shoe to soften the leather on Shabbat. It is permissible to wear a new shoe on Shabbat even though the leather will naturally soften as a result of being worn, but one may not stretch a new shoe with his hands to soften the leather on Shabbat.
A second area of application involves food preparation. Although the Gemara establishes the rule of "Ein Ibud Be'ochlin," that the prohibition of Me'abeid applies to leather and not to foods, Tosefot explain that this refers only to the level of Torah prohibition. On the level of Rabbinic enactment, the prohibition of Me'abeid applies even to the preparation of food. Accordingly, certain restrictions apply to adding salt on Shabbat to food if salt causes a chemical reaction in that food. One may add salt to a food if the salt will only enhance its taste, but not if the salt will somehow transform the food. Adding to salt to radishes, for example, produces moisture on the surface of the radishes that diminishes their sharpness. Therefore, one may not add salt to a radish unless he does so just prior to the meal, and he salts one radish at a time. Even just before serving, one may not salt an entire plate of radishes. Peeled cucumbers, by contrast, do not undergo any transformation as a result of adding salt, and therefore there are no restrictions on salting cucumbers on Shabbat.
Summary: One may not manually stretch new shoes to soften the leather on Shabbat. One may not add salt to foods such as radishes, which experience a chemical reaction as a result of contact with salt, unless one does so just prior to his meal and salts one radish at a time.
