The Torah states three separate times (Shemot 23 and 34; Devarim 14): “You shall not cook a kid in its mother’s milk.” Our Sages (Chullin 114a) expounded that each of the times this prohibition is mentioned comes to teach us another law: The first time it is mentioned teaches us about the prohibition to cook milk and meat together. The second time teaches us about the prohibition to eat milk and meat cooked together. And the final time teaches us about the prohibition to benefit from milk and meat that was cooked together. Thus, although we do not find any prohibition to merely cook any other foods prohibited by the Torah, such as blood or meat of a non-kosher animal, regarding milk and meat, a prohibition exists even with regards to cooking, even if one has no interest in eating it.
Rabbeinu Bechaye writes that when milk and meat are cooked together, this causes spiritual blockage of the heart and it is for this reason that Hashem has commanded us to abstain from eating milk and meat together so that they do not block their hearts with forbidden foods and so that their material composition remain pure to focus on the Torah’s ways and their minds capable of achieving the knowledge of Hashem. He concludes by saying that any reasons offered for the prohibition of milk and meat mixtures are insufficient and cannot be understood as the primary reason behind the Mitzvah, for this Mitzvah is considered a tenet similar to the Mitzvot of the Red Heifer and the goat-offering on Yom Kippur. Indeed, our Sages tell us that in the future, Hashem shall reveal to the Jewish nation why the Torah commands us to observe the Mitzvot of milk and meat mixtures, the Red Heifer, and the goat-offering on Yom Kippur.
If a kitchen sink becomes clogged and it is quite possible that there are meat and cheese residue and particles there that are causing the clog and one would like to pour boiling water from an electric kettle down the drain in order melt the residue and allow the water in the sink to flow freely again, some say that this is forbidden because of the prohibition to cook milk and meat together, for the boiling water causes the food particles stuck in the drain to cook. Nevertheless, the great Rishon Le’Zion, Moreinu Hagaon Harav Yitzchak Yosef Shlit”a writes about this matter extensively and rules that there is room for leniency in this regard for several reasons. Nonetheless, since there are cleaning goods and caustic materials in every household nowadays and these items can cause the food to become completely putrid and no longer fit for consumption at all, it is preferable to spill some of these cleaning items down the drain first in order to cause the foods to become putrid and only then proceed to pour boiling water down the drain. This is because any of the Torah’s prohibitions regarding forbidden foods apply only when the food item is tasty and fit for consumption; however, when the food becomes putrid and unfit for consumption as a result of such caustic materials and the like, the prohibition of milk and meat mixtures no longer applies and there is no longer concern for any prohibition.