May one prepare Tehina on Shabbat from a paste, or does this violate the prohibition of "Lash" – making dough?
"Lash" is one of the 39 forbidden tasks forbidden by the Torah on Shabbat. The principal here is the restriction of forming a dough-like substance from a solid and liquid. So it would be forbidden on to add water to flour on Shabbat to make dough, and certainly to knead the batter after adding the water. Now Tehina is prepared by adding water and some other ingredients, such as lemon juice and salt, to a pre-made paste purchased in the grocery store. At first glance, this process does not constitute "Lash" at all, since it transforms a hard substance into a liquid substance. The problem however, lay in the fact that before the mixture becomes a liquid, it first hardens and thickens, which is precisely the prohibition of "Lash." Should we therefore, forbid preparing Tehina because of this interim stage, at which point the paste solidifies and hardens? The work Shemirat Shabbat Ke-hilchata (by Rabbi Yehoshua Neuwirth) indeed forbids preparing Tehina on Shabbat for this very reason.
However, Rabbi Moshe Halevi, in his work Menuchat Ahava, rules leniently, and permits preparing Tehina from a paste on Shabbat, provided that one do so just before the Shabbat meal. He arrives at this ruling based on a number of factors. Firstly, the Rashba (Rav Shlomo Ben Avraham Ibn Aderet 1235-1310) maintains that the prohibition of "Lash" does not apply just before a meal, since it is then considered part of the eating process itself. Additionally, the Chazon Ish (Rabbi Avraham Yeshaya Karelitz, 20th century) was of the opinion that "Lash" does not apply to foods upon which "Lash" has already been performed ("Ein Lisha Achar Lisha"). Since the paste in the jar had already been mixed with water, Halacha does not, according to this view, forbid adding water a second time. Thirdly, the process of preparing Tehina ultimately results in a substance of very soft consistency, and the hard form is but a temporary stage. Therefore, the act of "Lash" performed in this case is what is called a "Melacha She’eina Mitkayemet" – an act whose result does not endure, and is thus permissible.
Due to all these factors, the Menuhat Ahava rules that one may prepare Tehina on Shabbat, so long as he does so immediately prior to the meal. Additionally, he requires that one prepare the Tehina in a manner different from the standard procedure. Namely, rather than beating it very hard to expedite the process, one should instead mix it gently with a spoon, and in this manner it is permissible, so long as one prepares the Tehina just prior to his meal.
