In the realm of Jewish law, the interaction between different food types is a subject of careful consideration. One significant area of discussion concerns the consumption of fish and meat.
The Gemara states that eating fish and meat together is problematic, as it is considered "kasha le tzara'at," meaning it can cause a skin condition akin to psoriasis, and also leads to bad breath. This is categorized as a matter of "sakana," or danger. Therefore, if one consumes meat and then fish, or vice versa, it is an obligation to wash one's hands thoroughly. Unlike certain leniencies found in other dietary laws, such as those pertaining to milk and meat where merely checking one's hands might suffice, the combination of fish and meat, even chicken and fish, is treated with strictness due to its dangerous nature. This applies even if one used a fork, as residues can still remain. The established practice is to wash the hands, gargle with wine or water, and consume a piece of bread to cleanse the mouth. The Ben Yishai further elaborates on this, suggesting that one should wash both the inside and outside of the mouth, and eat bread soaked in a beverage, to ensure no residue of fish remains, especially on areas like the mustache or beard.
The Rambam offers guidance on the order of consuming different types of meat within the same meal. He advises that if one wishes to eat fowl and beef, the fowl should be eaten first, as it is lighter and easier to digest. Similarly, eggs should precede fowl, and mutton should precede beef. The general rule for health, according to the Rambam, is to consume lighter foods before heavier ones. Applying this principle, it would be healthier to eat fish before meat, as fish is a lighter food. The Ruach Haim Palachi questioned why one would eat meat before fish, given this health principle. However, some individuals prefer to eat the heavier main course first to ensure they have room for it, saving lighter items for later.
Maran, in Siman 173, notes that Sephardic Jews are accustomed to being careful in this regard. The Ben Yishai and the Ramah both state that the danger of combining fish and meat exists primarily when they are cooked together, but not necessarily when eaten one after the other. Based on this, the custom of Ashkenazi Jews not to meticulously clean the mouth or wash hands between fish and meat was explained, though a recommendation to eat and drink something between them was still given for cleansing. The Hukmat Adam also states that while some wash hands, it is not strictly necessary, but one should drink something in between. However, according to the Sephardic tradition, it is imperative to wash hands and cleanse the mouth between eating fish and meat, due to the principle of "hamira sakanta me'esura"—danger is more stringent than prohibition. This is a common concern on Friday nights, at weddings, or in restaurants where fish appetizers are often followed by meat entrees.
Regarding the consumption of fish and dairy, this topic presents a significant debate. While meat and milk are forbidden by Torah law, and fish and meat are dangerous, the combination of fish and dairy, such as bagels with cream cheese and lox, or a tuna melt, is a long-standing custom. The controversy stems from differing textual versions (girsot) of the Beit Yosef. Some texts of the Beit Yosef include a warning against fish and cheese, while others do not. Rav Hida considered the inclusion of fish and milk to be a "ta'ut sofer," a scribal error, arguing that the Gemara does not discuss this combination. Therefore, according to him, there is no issue. However, others point out that while the Beit Yosef mentioned it, he did not include it in the Shulchan Aruch, which might suggest a leniency. Yet, the Ben Yishai and other sources like the Zohar and Benei Baria treat this combination as problematic. A personal anecdote recounted the strictness of Hacham Baruch who, upon seeing fried fish with marinara sauce and cheese served in a Yeshiva, ordered 500 servings to be discarded, considering it a real "sakana."
The Ashkenazi poskim, including the Ramah and the Shach, generally ruled leniently on fish and dairy, as did the Chida. They noted that Ashkenazi Jews have consumed this combination for centuries without apparent harm. Conversely, several Sephardic poskim, such as the Kanhak and the Knesset Gedolah in his She'erit Yehuda, ruled stringently, viewing it as a health hazard. Hacham Yitzhak Lamporinti in Sifat B'Had Yitzhak agreed that eating fish immediately after drinking milk, or vice versa, can cause serious, even fatal, disease. Maran, the Rishon LeTzion, maintained that there was likely no mistake in the text of the Beit Yosef, and therefore, fish and milk are forbidden. He and other rabbis, however, permitted eating fish with butter, seeing a distinction. The great rabbis of Baghdad, including Zeb Chesedek and Abdullah Samekh, permitted butter with fish, while the Ben Yishai forbade it. Ultimately, the custom in some communities, particularly among Sephardim, is to be strict, considering fish and dairy a "sakana." However, when it comes to butter, there is more room for leniency, and many rabbis permit it.
The discussion also extends to sitting at the same table where one person is eating fish and another is eating meat. This is generally permitted because the inherent danger (sakana) acts as a natural barrier, preventing one from inadvertently partaking in the other's food. This differs from milk and meat, where sitting at the same table without a physical separation is forbidden due to the concern of accidental consumption. The case of Avraham Avinu serving milk and meat to his guests is often raised, with explanations such as the Malbim suggesting the meat was "parve" (neutral), like tofu, or that there was no "marit ayin" (appearance of impropriety) in that context. However, when it comes to "sakana" items, like fish and meat, the danger is paramount. The custom for fish and dairy remains a point of divergence, with some communities strictly avoiding it, while others, particularly Ashkenazim, permit it, relying on the lenient opinions and the lack of historical evidence of harm. Nevertheless, if one is at a restaurant and is served a dairy appetizer followed by a fish main course, or vice versa, the strict opinion would require washing hands and cleansing the mouth in between, or at least consuming bread and a beverage, as a measure of caution against potential "sakana."