Pareve Frankfurters in a Dairy Restaurant
Question: Is a person who is eating parave frankfurters allowed to eat cheese or drink chocolate in the same meal?
Answer: At first glance, it might seem that this should be prohibited in order to avoid "marit ayin," an erroneous conclusion drawn by somebody who observes another person doing something. In fact there are cases where the sages prohibited eating a permitted food because of the fear that it would be replaced by a similar food that was forbidden. The earliest source for this is in the Talmud, where it is forbidden to eat fish blood that has been put into a dish (although in principle this is permitted) because of its similarity to animal blood (Keritut 21b). This is permitted only when the blood clearly comes from fish (if, for example, the scales are visible). This ruling appears in the Shulchan Aruch (Yoreh Dei'ah 66:9).
Recent rabbis have used this as a source to forbid eating meat together with milk made from almonds, as was ruled by the RAMA (Yoreh Dei'ah 57:3). This last case involves eating meat while drinking milk whose source is not animal, which is exactly like the current question. Thus, it would seem that parave frankfurters and cheese should not be eaten together.
However, Rabbi Moshe Klein in Responsa Mishneh Halachot (5:96) suggests that the cases brought above are rare – "it is not a common practice to use milk made from almonds in cooking, and since fish blood is not usually used for cooking these cases should be avoided."
The author then goes on to make a novel ruling:
"The only items which were prohibited because of marit ayin are things which in the majority of cases are not performed in a permitted way, or things which are not often found in a permitted way. But for things which are done in a permitted way in a majority of the cases the principle of marit ayin does not apply, since anybody who sees what happens assumes that it was done in a permitted way."
Based on this reasoning, Rabbi Klein allowed using margarine in a meal with meat (even though margarine is very similar to butter) and eating potato with mayonnaise (in spite of its similarity to cream).
The author's reasoning that it is not common to use almond milk and fish blood can evidently be extended to a broader viewpoint – that the fear of marit ayin is relevant only when the simplest explanation of what the people see is that the action is prohibited. This formulation appears in Tiferet Yisrael (Kalkelet Hashabbat 34) and is quoted by Rabbi Ovadia Yosef in Yavia Omer (volume 6, Yoreh Dai'ah 8), who allows the use of soya milk at the end of a meat meal. This is because "the matter has become common and everybody is aware of the existence of vegetable milk, and we therefore do not fear marit ayin... since this should not be feared unless there is more of a proof of a prohibition than of a permitted action." (We might note that this specific case is simpler than what was discussed above, since the "milk" is not used together with the meat.)
Summary: Since parave frankfurters are quite common, it seems that one need not fear the principle of marit ayin, and one is permitted a priori to eat them together with dairy products.
