Substances derived from milk, such as cheese and whey, have traditionally been considered to fall under the prohibition,[34][35] but milk substitutes, created from non-dairy sources, do not. However, the classical rabbis were worried that Jews using artificial milk might be misinterpreted, so they insisted that the milk be clearly marked to indicate its source. In the classical era, the main form of artificial milk was almond milk, so the classical rabbis imposed the rule that almonds must be placed around such milk; in the Middle Ages, there was some debate about whether this had to be done during cooking as well as eating,[36] or whether it was sufficient to merely do this during the meal.[37]