An Erub Tabshilin is prepared before Yom Tob in situations where Yom Tob is immediately followed by Shabbat, in order to allow cooking on Yom Tob for Shabbat. The Erub Tabshilin must consist of two foods – one baked, and one cooked. The baked food is traditionally a piece of bread, or a piece of Masa on Pesah. As for the cooked food, it was for many years customary to use a hardboiled egg for this purpose. The reason is that the food must be able to remain fresh until Shabbat, and before refrigeration, there weren’t many foods that could remain fresh for this long. An egg was therefore used because it could be stored and eaten for a number of days.
Nowadays, however, Hacham Ovadia Yosef ruled, it is preferable to use a more "Hashub" (significant) food for the Erub Tabshilin. Since food can now be refrigerated and kept fresh, one should try to use a food such as a piece of fish or meat for the Erub Tabshilin, rather than an egg. Certainly, if one uses an egg, it suffices to allow cooking on Yom Tob for Shabbat, on condition that it was not peeled. The Gemara in Masechet Nidda (17) teaches that eating an egg that has been left unpeeled overnight can be injurious to one’s health. Since the food of the Erub Tabshilin must be edible, an egg that is left overnight without a peel is not suitable. Thus, although it is preferable to use a more significant food, one may use an egg, as long as it is not peeled before it is eaten on Shabbat.
In general, any food that is boiled, roasted, poached, etc. is suitable for the Erub Tabshilin. In fact, it once happened that a person had only tomato sauce, and he used it as the cooked food for the Erub Tabshilin, as it is made from cooked tomatoes. Pickled foods are suitable, as well, and thus one may designate a jar of pickles for the Erub Tabshilin together with the bread or Masa. The exception to this rule is food that is not ordinarily eaten at a meal with bread, such as farina and the like. Such foods, according to Hacham Ovadia Yosef, should not be used for the Erub Tabshilin.
There is a debate among the Halachic authorities as to the status of milk with regard to Erub Tabshilin. Hacham Ben Sion Abba Shaul (Israel, 1923-1998), in his Or Le’sion (vol. 3, p. 215), ruled that pasteurization qualifies as "cooking," and thus milk and dairy products purchased in stores today are considered "cooked" foods. Thus, for example, if a person drinks milk at the Se’uda Mafseket (final meal) before Tisha B’Ab, when only a single cooked food is allowed, he may not eat another cooked food. By the same token, milk and dairy products may be used as the cooked food for the Erub Tabshilin. Hacham Ovadia Yosef, however, disputes this ruling, and claims that pasteurization differs from cooking. Boiling has the effect of eliminating bacteria in the milk, but does not cause a fundamental change in its nature. "Cooking" for the purposes of Halacha requires transforming the food in some way, and thus pasteurization would not qualify. Accordingly, Hacham Ovadia rules that one may drink milk and partake of another cooked food at the Se’uda Mafseket. It stands to reason that he would also disqualify milk for the Erub Tabshilin, since it is not considered Halachically "cooked."
Summary: The Erub Tabshilin must consist of one baked food – customarily a piece of bread or Masa – and one cooked food. It is best to use a significant food, such as a piece of fish or meat, though strictly speaking, one may use any food that has been cooked, roasted, poached or pickled. An egg may be used as long as it is not peeled before it is eaten. Pasteurized milk does not qualify as a cooked food. Regardless, as mentioned, it is preferable to use a significant food such as fish or meat.
