Tanning animal hide is the av melachah of me’abeid and is forbidden on Shabbat.1

Me’abeid in the Mishkan

Rams’ hides and the hides of the tachash were tanned to be used as coverings for the Mishkan. The tanning process involved soaking the hides in a salty mixture2 to draw out their moisture to prevent decomposition. Afterwards, they were beaten down to make them smooth and flexible.

Pickling

For most of us, the practical application of this melachah relates to food preparation.3 A significant component of the tanning process involves preserving the hide by soaking it in a salty solution. In ancient times, it was common for people to keep food fresh by salting or pickling. Being that this so closely resembles the forbidden act of tanning, the Sages extended the prohibition to activities which involve preparing a brine for pickling foods.

Based on this, brine and saltwater may not be made in large quantities, because it appears that it is being made to pickle something on Shabbat. Even in small amounts, they may only be made prior to the meal at which they will be used, and the mixture may not comprise more than two-thirds salt.4 This becomes relevant on Passover in years when the Seder night falls on Shabbat eve and saltwater is prepared to be used at the Seder. It is preferable to prepare the saltwater before sunset. If one forgot to prepare in advance, only a minuscule quantity of saltwater should be prepared in the evening.5

Additionally, salt may not be sprinkled on certain vegetables or foods that are commonly pickled in a salty brine.6 For example, salting a cucumber salad is forbidden because pickles are made from cucumbers. If one adds other ingredients immediately afterwards, however, it is permissible to salt the cucumbers, because then the salting of the vegetables does not resemble pickling or preserving.7 Therefore, one may salt a salad if they will add vinegar and oil right afterwards. Dipping a whole bunch of these vegetables into salt is also forbidden; one may only do so one at a time and for immediate consumption. Foods which are not usually pickled, and which are not greatly affected by salt (other than adding taste), may be salted, even many at once, as long as it is done for a meal that will begin shortly.8

An Alternative View

Thus far we have explained that actions associated with pickling and preserving foods are forbidden because of their similarity to tanning. This is indeed the view of Rashi.9 Maimonides,10 however, does not relate pickling to me’abeid; rather, the process of pickling foods can be viewed as a form of cooking and is rabbinically forbidden as an extension of the melachah of bishul - cooking.

The views of both Rashi and Maimonidies are recorded in later halachic writings and in the Code of Jewish Law.