Q- Is it permitted to pour vinegar or any salad dressing containing vinegar and salt into salads, avocado, cabbage, tomato, lettuce, peppers, onions, endives, anise, carrots, corn, etc on the Sabbath right before the meal?

Can you use lemon juice?
 
A. The prohibition involved in preserving food (according to Rashi – Shabes 37a) is the melochoh of Meabeid or tanning, that was required to process the hides needed to cover the Mishkan, (according to Rambam Ch. 22, and Shulchan Aruch 321- 3, it is cooking or Bishul).

The Talmud (ibid. 75b) indicates that there is no tanning in foods, as they are not permanent and are unlike hides or other perennial objects. Yet the sages imposed rabbinical restrictions on food preservation, salting or pickling, which were necessary prior to the availability of refrigeration.

Mishnah Berurah (ibid. 14) indicates that in certain cases and conditions, salting and preserving vegetables and salads is permitted: such as:

     a)- when this kind of vegetables are not ordinarily preserved or are not suitable for pickling,

     b)- when they are steeped in salt or vinegar one by one and eaten immediately,

     c)- when the vegetables are cooked, as they are then not normally preserved, and

     d)- if you add prior a bit of oil or mayonnaise, as this weakens the preservation capacity of salt and vinegar. This would then permit the use of salad dressing.

Other spices, such as paprika, pepper, onion and chilli powder are permitted (ketzos Hashulchan ibid.)  Equally so, is the squeezing of a lemon on a salad, or the adding of wine or alcohol liquor, as these are not used in tanning .
 
Rabbi A. Bartfeld as revised by Moireinu Horav Shlomo Miller Shlit”a